CEE_rb19 Civil Eng Research

School of Civil & Environmental Engineering • Nanyang Technological University

ISSN 0219-0370 NO. 19, 2006

http://www.ntu.edu.sg/cee/ http://www.ntu.edu.sg/cee/research/bulletin/index.asp

IN FOCUSIN FOCUSIN FOCUSIN FOCUSIN FOCUSNew Frontier in

Civil Engineering Research

Civil engineers play a pivotal role in the unprecedentedchanges of the world today. As in the tradition, theyoften endeavor to push the boundaries of what is

possible in the profession in order to improve the livingstandards of human society. In line with these changes, R& D works in civil engineering have to strive to make animpact environmentally as well as globally. From thetraditional research areas in environment, water resources,structures, construction, geotechnics and transportation, newfrontiers are continuously being explored to meet thechallenges of the information era.

The goal of research activities conducted in the School ofCivil and Environmental Engineering is to advance the stateof knowledge in important fields of civil engineering and toachieve international recognition and leadership in niche areasof research. Under the School’s strategic plan, future CEEresearch activities will be built around two anchor blocks,namely

• Homeland Security• Environmental Engineering

Core competencies in traditional, technical disciplines willbe consolidated to leverage on the thematic-research on“Homeland Security” so that the existing research strengthwith governmental organizations and private industries canbe further enhanced. Researchers from other disciplines couldbe invited to participate so that it can be broadened to areasbeyond traditional civil engineering.

Another important theme is “Environmental Engineering”.Research context will be broadened to include infrastructuresystems and engineering, urban planning and management,facilities management, Public-Private Partnership types ofprocurement for public assets and facilities, environmentalmanagement and industrial ecology – in line with marketdevelopment trends such as globalization.

An important and strategic inclusion to the School in thepast year is Maritime Studies, which is a multidisciplinarystudy in various aspects of the maritime industry. In thecontext of international trade, maritime transport is thedominant mode for shipping of the vast majority of products.The maritime industry plays a vital role in facilitating tradeand hence economic activities. As such, Maritime Studiesis largely related to the business aspects of these economicactivities. It combines management, economics,organizational, logistic, legal, financial and technologicaltopics in the maritime industry.

The inclusion of this discipline into the family poses achallenge to the traditional civil engineering family.However, Maritime research presents vast opportunities atvarious levels, such as shipping company, port, communityand shipboard levels. Research topics cover a broad rangeof subjects such as shipping network design, port planning,shipping operations and management, maritime security,shipping market and freight analysis, maritime policy andregulation.

Management of research

In order to optimize resources to achieve research objectivesof the School, it is imperative that an efficient managementscheme be in place to support research activities. To thisend, the School has set aside resources coordinated by theVice-Dean (Research) to identify and solve administrativeproblems to ensure the highest efficiency for researchers. Inthe School, research activities are structured in groups withshared interests. Eight major research groups have beenidentified in the School as follows:

CONTENTSCONTENTSCONTENTSCONTENTSCONTENTS

ADDITIONAL COPIES AND ENQUIRIES

For general enquiries about this publication,

and to request for additional copies, please write to :

The Dean

School of Civil and Environmental Engineering

Nanyang Technological University

Nanyang Avenue

Singapore 639798

Tel : 65-67905264

Fax : 65-67910676

Email : [email protected]

Published by Dean, School of Civil and Environmental Engineering, Nanyang Technological University, Singapore Printed by PHOTOPLATES PTE LTD

IN FOCUS 1

CONSTRUCTION

�� Evaluation of Guarantee as a Put Option using Monte

Carlo Simulation – The Case of the Second Link 10

�� Creating and Sustaining Competitiveness of Small and

Medium-sized Construction Enterprises in China 12

�� Problems of Overseas Ventures for Singapore

Construction SMEs 14

�� Competitive Strategies and Competitive Advantages of

Large Chinese Construction Enterprises 16

� Research Findings on International Strategic Management

of Chinese Construction Firms 18

�� A Simple Flow Control Tool for a Real-time Evacuation

System Augmented with Virtual Reality 19

ENVIRONMENT

�� Selection of Microbial Seeds for Fast Granulation in

Aerobic Treatment of Wastewater 21

�� Stabilization/Solidification of Industrial Waste Sludges

using MSWI Fly Ash 23

�� Occurrence of “G-Bacteria” in Aerobic Granules 26

�� Distribution of EPS and Cell Surface Hydrophobicity in

Aerobic Granules 28

�� Size-Dependent Diffusion Limitation in Aerobic Granules 30

�� Biological Treatment of Municipal Wastewater by Upflow

Anaerobic Sludge Blanket Process 33

�� Treatment of Lipid-Containing Food Waste in Hybrid

Anaerobic Solid-Liquid (HASL) System 34

�� Characterisation of Fouling on Spent Ultrafiltration

Membrane Surface from a Commercial Membrane

Bioreactor 36

�� Ultrasonication of Sludge for Anaerobic Digestion 38

GEOTECHNICS

�� Hydrofracturing In-Situ Stress Measurements in Singapore

Granite 41

� Performance–Based Acceptance Criteria for Quality

Control of Reclaimed Sandfill 42

� Soil Improvement for Tree Stability 44

�� Numerical Study on Crack Propagation and Shear Behavior

of Jointed Rock Mass 46

MARITIME STUDIES

�� A Measurement and Comparison of Cost Competitiveness

of Container Ports in Southeast Asia 48

STRUCTURES

�� Allowable Free Span of Rigid Risers 49

�� An Experimental Study on Fatigue Behaviour of Partially

Overlapped Tubular K-Joint 51

�� Failure Assessment Diagram of Damaged Square Hollow

Section T-joints 54

�� Analysis of Foam Claddings for Blast Alleviation 56

�� The Smoothed Particle Hydrodynamics by Considering

Material Inhomogeneity 58

�� Ground Motions Recorded in Singapore during the Nias,

Northern Sumatra Earthquake (MW

= 8.7) of

28 March 2005 60

� Electromechanical Impedance Modeling of PZT Active

Sensors for Health Monitoring of Cylindrical Shell

Structures 62

� Krylov Precise Time-Step Integration Method 64

� Fuzzy GA for Optimal Vibration Control of Smart

Cylindrical Shells 66

� Numerical Characterization of Structural Response to

Above-Ground Explosions 68

�� Generic Beam-Column Model for Response to Impulsive

Ground Shocks 70

�� Reinforced Concrete Frames Subjected To

Explosion-Induced Ground Motions (EIGMS) 72

SPATIAL INFORMATION

�� Three-Dimensional Geological Map of Singapore with

3D GIS Software 74

TRANSPORTATION

�� Functional Framework for an Optimum Liner Service

Network Planning System 76

�� Survey on Performance of a GPS-Based Vehicle

Tracking and Communication System for Container

Transport in Logistics 77

�� Investigation of a Car-Following Model Using a Desired

Spacing Criterion 79

�� Enhancement of Expressway Incident Detection through

Algorithm Fusion 81

�� Road Traffic Accident Models for Signalised Four-Legged

Junctions 83

WATER

�� Structure of Secondary Flows in Open Channel wuth

Longitudinal Bedforms 84

�� Which is the Best Time of Concentration Formula for

Overland Flow? 86

�� Effects of Meteorological and Hydrogeological Factors

on Gross Recharge Percentage at Unconfined Sandy

Aquifers with an Equatorial Climate 88

�� Field Column Study of Organic Removal Efficiency in a

Shallow Vadose Zone 90

�� Measuring Shear Stresses on a Hollow Fibre Membrane

with Particle Image Velocimetry (PIV) 92

�� Propagation of Tsunami in Malacca Strait – A Solitary

Wave Approach 93

RESEARCH REPORTS

�� Abstracts of Research Reports 96

�� Abstracts of PhD Theses 102

PUBLICATIONS 106

EDITORIAL BOARD

Chiew Yee Meng - ChairmanAshraf Hefney (Geotechnics)

Carmel Heah (Language Editor)Charles Cheah (Construction Technology & Management)

Francesco Cavallaro (Language Editor)Karina Gin (Environment)Lee Chi King (Structures)

Li Shu Yun (Language Editor)Lloyd Chua (Water)

Ma Guowei (Mechanics)Susan Khoo (Language Editor)Tan Yan Weng (Transportation)

Tor Yam Khoon (Surveying)

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The research interests of all faculty members are found in

one of these areas tabulated above. Each member, either

individually or collectively as part of a group of interested

members, is able to explore and conduct research within

these interest groups. In many research activities conducted

in the School, collaborations with overseas Universities and

organizations are not uncommon. In this Research Bulletin,

a collaborative research programme with Hehoi University

spearheaded by the Maritime Research Centre (MRC) is

reported. The two projects conducted under this programme

are:

(1) Flow structures and mixing processes in vegetated flows;

and

(2) Secondary flow structure and transverse dispersion in

compound channels of various geometries.

Experimental investigation of flow structures and

mixing processes in vegetated flows

Aquatic vegetation (Figure 1) strongly affects river and

coastal hydrodynamics. Vegetation as an integrated

component of a river channel generally increases flow

resistance, thus reducing efficiency in delivering flood.

However, the understanding of vegetation effect on river

eco-systems is changing in recent years. The conventional

‘flood control’ ideology has gradually evolved into a

philosophy of ‘flood management’. The functioning of

vegetation is not only limited to stabilization of stream banks,

but also providing habitat for other aquatic organism and

performing an essential role in nutrient cycling and water

Major Research Group Areas of Research Focus

Structures and Mechanics Computational Mechanics

Dynamics and Seismic Engineering

Structural Connections

Protective Technology

Construction Technology and Management Concrete and Building Technology

Construction Management

Wind Engineering

Environmental Engineering Waste Minimization, Recycling and Resource Recovery

Membrane Technology

Biotechnology in Wastewater Treatment

Water Resources Sediment Transport and Coastal Management

Integrated Urban Stormwater Management

Transportation Transport Planning and Modeling

Traffic Management and Control

Pavement Materials and Technology

Geotechnical Engineering Foundations of High-Rise Buildings

Land Reclamation

Underground Space Development

Tropical Soil Engineering

Surveying Digital Spatial Data Acquisition and Analysis

Maritime Studies Maritime Logistics

quality. Therefore, studies of hydrodynamics of vegetated

flows are of fundamental relevance to both hydraulic

engineering and river ecology.

Figure 1. Two examples of vegetated flows

submerged vegetated flow

non-submerged vegetated flow

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2006

Vegetated flow for the submerged case is characterized by a

strong shear layer that develops at the top of the canopy due

to the vertical discontinuity of the drag. The shear layer

enhances the flow turbulence significantly, and coherent

vortices of canopy-scale are thus generated. These coherent

vortices dynamically dominate the vertical turbulent exchange

and they play an important role in the fate and transport of

sediment, nutrients, contaminants, dissolved oxygen, and

fauna in aquatic systems.

This research project conducted by MRC is to acquire detailed

experimental information of the modeled vegetated flows,

including turbulence characteristics and mean flow structures.

The experiments are conducted in a 14 m long, 30 cm wide

and 60 cm deep tilting flume at the Hydraulics Laboratory.

Three kinds of cylindrical PVC sticks are used to model

rigid vegetations. Velocity measurements are conducted using

a two dimensional Particle Image Velocimetry (PIV) system,

as shown in Figure 2(a). The advantage of PIV is that

velocities can be determined simultaneously in a flow field

without disturbing the flow. Image sequences are recorded

using a CCD camera to capture the seeding particles

illuminated by a laser sheet in the vertical streamwise plane,

as shown in Figure 2(b). A pair of images of illuminated

seeding particles is taken at a frequency of 15 Hz, as shown

in Figure 3. The images are stored in a PC disk for post-

processing using a PIV analysis software (DaVis 6.2,

LaVision). The instantaneous velocities in an image plane

Figure 2 (a) Typical sketch of the experimental setup; and

(b) Experiment setup

(a)

Figure 3. Image of seeding particles illuminated by the laser

sheet and the computed velocity vector.

(b)

are obtained by a cross correlation method from a pair of

images. The standard FFT method is used for the calculation.

Time-mean velocities, turbulence characteristics can be thus

calculated using the instantaneous velocity sequence of 300

frames. Figure 3 shows an example of the mean velocity

vectors.

Through this study, a better understanding of the mechanisms

and features of the shear layer and coherent structure at the

top of canopy can be obtained. It also facilitates finding the

quantitative relationship between flow structures and

vegetation configurations, such as the vegetation size,

vegetation density and arrangement pattern. More importantly,

it helps to examine the physical process of mixing in the

shear layer and to assess the role of aquatic vegetation in

controlling sediment and pollutant fate.

Secondary flow Structure and transverse dispersion in

compound channels of various geometries

Many rivers consist of a main channel with adjacent

floodplains, as shown in Figure. 4. In such two-stage rivers,

the flow is within the main channel for the majority of the

time. During flood events, however, the flow spills over into

the floodplains. Since flood control is the focus of much

engineering work, the knowledge of the hydrodynamic

behavior of compound channel flow has important

applications in river management. For example, a good

estimation of the water level for a given flood event is based

on the correct prediction of the conveyance capacity, velocity

distribution, and boundary shear stress distribution. Moreover,

the knowledge is also required in engineering applications

related to bank protection, sediment transport and scour

analysis.

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The variation in flow depth and roughness height across the

channel leads to strong interactions and exchange processes

between main channel and floodplains. Therefore, compound

channel flows differ fundamentally from simple open channel

flows. Turbulent structure in a compound channel is

characterized by a large shear layer generated by the

difference of velocity between the two flow components in

the main channel and the floodplain. In the high shear-layer

region, not only vortices with vertical axis exist, but also

vortices with longitudinal axis develop. The latter vortices

are also called secondary flows since they are imposed on

and continue downstream with the primary flow. These

vortices are responsible for the transfer of high momentum

from the main channel to the floodplain.

Many experimental investigations have been carried out to

examine the distributions of mean velocity in compound

channel flows. However, an accurate measurement of the

secondary velocity components in open-channel flows is very

difficult, because their order of magnitude is only up to 5%

of the primary mean velocity. The mechanism of the

interaction between the main channel and the floodplain flows

are not well understood. Tominaga and Nezu in 1991

conducted a detailed measurement of the three-dimensional

turbulence structures in compound open channel flows. From

their research, it has been found that the secondary flows

play an import role in the momentum exchange and thus the

primary velocity distribution. The case observed by them

was restricted to the compound channel with a vertical

connection between the main channel and flood plains.

However, some relevant studies also demonstrate that the

features of secondary flows are strongly affected by the

geometry of the channel cross section. Secondary flows may

have different structures for different geometries, and thus

the momentum exchange and mean velocity distribution also

vary.

A series of experiments is conducted in an 18 m long, 1.6

m wide and 60 cm deep tilting flume at the Hydraulics

Laboratory. Six kinds of connections between the main

channel and the floodplain are designed for the experiments.

Figure 4. Sketch of a typical compound open channel

Figure 5. Flume of compound channel with the connection of

(a) 90° slope and (b) 45°slope.

(a)

(b)

They include 30°, 45°, 60°, 90° slopes with respect to

horizontal plane, and also a concave and a convex slope.

The height difference of these slopes is 15cm. Figure 5

shows two examples of the compound channels. From these

experiments, detailed flow data in compound channels of

various geometries under different flow conditions are

obtained, and thus the particular turbulence structures related

to different geometries can be clarified. It further helps to

evaluate the effects of secondary flows on transverse

exchange of momentum and mixing process, which are of

vital importance in hydraulic engineering and river ecology.

IN FOCUS


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2006

Research Centres

The main events hosted by the School’s research centres during the period from July 2004 to June

2005 are summarised below.

Centre for Advanced Construction Studies

(CACS)

� A course on Fire Engineering Design of Steel and

Composite Structures was conducted A/P Tan Kang Hai,

Prof Ian Burgess from the University of Sheffield from

22 to 24 July 2004.

� A course on Some Pointers for Better Contracting of

Construction Projects was conducted by Adj Prof Wong

Yui Cheong from 17 to 18 September 2004.

� A seminar on Common Mistakes in Steelwork Design

conducted by A/P Chiew Sing Ping was held on

30 October 2004.

� A/P David Chew and Ast/P Charles Cheah paid a visit to

five top-ranked Chinese universities: Tsinghua University,

Harbin Institute of Technology, Tianjin University,

Nankai University and Southeast University in December

2004.

Centre for Transportation Studies (CTS)

� A course on Urban Traffic Management and Control was

conducted by Prof Henry Fan, A/P Lum Kit Meng, A/P

Tan Yan Weng and A/P Wong Yiik Diew on 12 to 16 July

2004 for transport professionals from Singapore.

� A course on Traffic Impact Assessment was conducted by

A/P Tan Yan Weng from 14 to 17 September 2004 for

transport professionals from Singapore.

� The course on Urban Transport Planning and Design

organised under the Singapore-Republic of Korea Third

Country Training Programme was conducted by Prof Henry

Fan, A/P Tan Yan Weng and A/P Wong Yiik Diew from 1

to 5 November 2004. This course was attended by 15

participants from 9 countries. A second run of this course

was held on 6 to 10 June 2005 and was attended by 20

participants from 9 countries.

� The course on Urban Transport Planning and Design

organised under the Singapore Technical Co-operation

Programme was conducted by Prof Henry Fan, A/P Lum

Kit Meng, A/P Tan Yan Weng and A/P Wong Yiik Diew

from 22 November to 3 December 2004. This course was

attended by 20 participants from 16 countries.

Environmental Engineering Research Centre

(EERC)

� Public Seminar on Aerobic Granulation: An Innovative

Biotechnology for Wastewater Treatment was conducted

by Prof Tay Joo Hwa on 25 May 2005.

� Special course on Anaerobic Processes in Environmental

Applications was conducted by Prof Lee Tu-Chung on 28

July, 2 & 4 August 2005.

� External Short on Introduction to Land Contamination

and Remediation was conducted by Ast/Prof Lim Teik

Thye in June 20.

� The NTU-Stanford Symposium on the Environment, entitled

“Technological Challenges for Water Resources and the

Environment”, was held in NTU on 31 May and 1 June

2005. The Symposium showcased research conducted under

Singapore Stanford Partnership (SSP), Clean Water

Programme (CWP), EERC and Institute of Environmental

Science and Engineering (IESE), as well as research

conducted by NTU’s affiliates and international partners,

including Stanford University, National Taiwan University

and The University of Hong Kong. Topics covered included

use of Singapore’s reclaimed lands as recharge and recovery

aquifers, water reclamation, water quality monitoring, water

resources management, membrane technologies, solid waste

management, advanced treatment technologies and

environmental biotechnologies.

� A delegation from the University of Warwick led by Dr

Anthony Price (Associate Dean, School of Engineering)

visited CEE (PTRC & EERC) on 25 May 05.

� A delegation from Arizona State University led by its

President Michael M Crow visited NTU on 8 Jun 05 and

was briefed on activities in EERC.

� A delegation from Massachusetts Institute of Technology led

by its President Prof Susan Hockfield visited NTU on 30

Jun 05 and was briefed on activities in EERC.05

Geotechnical Research Centre (GRC)

� A seminar on Liquefaction and Displacement of Earth

Structures Induced by Earthquake & Introduction to the

Drafted International Standard: ISO23469, Seismic Actions

for Designing Geotechnical Works was presented by

Dr Liu Hanlong from Hohai University, China on ??

� A seminar on New soil improvement techniques was

presented by Dr Liu Hanlong from Hohai University,

China on >??

� A seminar on In Situ Soil Testing: From Mechanics to

Interpretation was presented by Prof Hai-Sui Yu from

University of Nottingham on??

� A seminar on Fibre Optics Inclinometer Instrumentation

and Reading for Soil Measurements was presented by Mr

Philip Ho and Mr Poh Kong Beng from Singapore (name)

?? on ??

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� A seminar on Is Soil/Ground and Engineering Material

or an Entity? Or should a Geotechnical Specialist be a

Doctor or and Engineer? Was presented by Prof Madhira

R Madhav, Prof Emeritus from JNT University,

Hyderabad, India on ??

Maritime Research Centre (MRC)

A. Activities

Industrial Visits

MRC visited the following companies to explore possible R

& D projects:

1. OOCL, Chulia Street

2. Prosper Marine

Seminar

� The second Joint NUS-NTU-MPA Maritime Industry

Seminar organized by MRC was held on 28 July 2005 at

NTU.

R & D Projects

MPA-NTU JOINT PROJECTS

Approved projects

No new approved project

Projects Submitted For Consideration

1. Developing A Maritime Piracy Detection System Using

Microwave Radar Sensor – Saman S Abeysekera and Tan

Hock Lye (EEE)

Support to Post-graduate students

1. Supports 8 visiting students on short term attachment

(1- Lu Sheng-Qi, 2- Fang Shi-Long, 3-Liu Jianmin, 4- Yan

Jing, 5 – Xiao Yang , 6- Liu Tao; 7 – Ding Bin; 8- Nguyen

Thi LanTruc )

Support to Undergraduate programmes – MPA’s grant

(MIAP)

Processing application for MIAP – IA attachment

Support to Current NTU staff and Alumni – MPA’s grant

(MIAS)

Working with industry on potential collaboration

Protective Technology Research Centre

(PTRC)

� A seminar on Analytical Methods Needed for Analysis of

Reinforced Concrete Structures Exposed to Nearby Blast

Loads was presented by Mr John Crawford, PE, the

President of Karagozian & Case, and Dr Shengrui Lan, a

Principal Engineer in Karagozian & Case, Burbank,

California, USA on 4 February 2004.

� A seminar on Carving Crazy Horse: Art and Engineering

of Blasting Massive Rock Monuments was presented by

Prof Charles Dowding, Northwestern University, USA on

23 March 2004.

� A seminar on Construction Vibrations was presented by

Prof Charles Dowding, Northwestern University, USA;

Prof Pan Tso-Chien, PTRC, NTU; and A/P Leong Eng

Choon, CEE, NTU from 25–26 March 2004.

� A presentation was made to Mr Larry Lynn, Independent

Consultant & Former DARPA Director via Temasek

Laboratories at NTU on 16 June 2004.

� A seminar on Advances in Geophysics and New

Applications in Geotechnical Engineering was presented

by Prof Michael Asten, Monash University, Australia on

13 July 2004.

� A short course on Geophysical Methods for Site

Characterisation was presented by Prof Michael Asten,

Monash University, Australia; Dr Wang Hou, Director,

Ryobi Geotechniques Pte Ltd and A/P Leong Eng Choon,

CEE, NTU on 16 July 2004.

� A presentation was made to Temasek Society, MINDEF via

Temasek Laboratories at NTU on 27 August 2004.

� A seminar on Research on Protective Technologies in

Australia and the RNSA Collaboration was presented by

A/P Priyan Mendis, Reader on Civil Engineering, University

of Melbourne on 1 September 2004.

� The IES Prestigious Engineering Achievement Awards

2004 was won by a PTRC and DSTA’s research project on

the development of the underground ammunition storage

space. The award was received by Prof Pan Tso-Chien,

Director of PTRC and Mr Ong Yew Hing, Deputy Director

for Defence Construction of DSTA at the IES 38th Annual

Dinner & Dance held on 5 November 2004 at the Ritz

Carlton.

� A short course entitled Blast Effects Design and Analysis

of Structural and Mechanical Systems was conducted by

Mr John Crawford and Dr Shangrui Lan from Karagozian

& Case, USA on 6-8 December 2004.

� A seminar on Earthquake Response Control using Tuned

Liquid Dampers was presented by Prof Pradipta Banerji,

Dean for Alumni & International Relations and Professor in

Structural Engineering from Indian Institute of Technology

Bombay on 12 January 2005.

� A seminar on Damage Detection in Plate Structures using

Ultrasonic Guided Waves was presented by Prof Pradipta

Banerji, Dean for Alumni & International Relations and

Professor in Structural Engineering from Indian Institute of

Technology Bombay on 13 January 2005.

� A seminar on The Mathematics of Natural Catastrophe

was presented by Dr Gordon Woo, Catastrophe Risk Analyst

from Risk Management Solutions, Inc, UK on 14 January

2005.

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2006

� A focus seminar on The Great Banda Aceh Earthquake

& Tsunami on 26 December 2004 – Vulnerability of

Buildings and Infrastructure was presented by Prof Pan

Tso-Chien, A/P Li Bing, A/P Lu Yong, Ast/P Au Chi Kit,

MAE during the CoE Tech Week.

� A seminar on Importance of Maintenance Technologies

for Concrete Structures was presented by Prof Taketo

Uomoto, Director and Professor, International Center for

Urban Safety Engineering, Institute of Industrial Science,

The University of Tokyo on 29 March 2005.

� A seminar on Use of GIS in Disaster Mitigation was

presented by Prof Kimiro Meguro, International Center for

Urban Safety Engineering, Institute of Industrial Science,

The University of Tokyo on 29 March 2005.

� A seminar on Tsunamis, Disasters and Defense Works was

presented by Prof Nobuo Shuto, An International Authority

in Tsunamis and Coastal Engineering on 3 May 2005.

� A Memorandum of Understanding was signed between

the PTRC and the International Center for Urban Safety

Engineering (ICUS), Institute of Industrial Science (IIS),

University of Tokyo at CEE/NTU on 29 March 2005 to

collaborate in the area of Research, Education and Training.

� Director of PTRC, Prof Pan Tso-Chien, was interviewed

by the Channel News Asia on 7 April 2005, The New Paper

on 13 April 2005 and The Straits Times on 20 April 2005

for (i) his inputs and views on the development of the regional

tsunami warning system and how PTRC/CEE/NTU can

contribute towards this development, (ii) PTRC/CEE/NTU’s

research capabilities in earthquake engineering and (iii)

PTRC/CEE/NTU’s likely contributions to the national centre

for earthquake engineering research which will be established

within a year as announced by DPM.

Commendations

The School takes pride in the outstanding research achievements of the following staff

members:

� Two of our faculty members, Assoc Prof Cheng Nian

Sheng and Ast/Professor Lloyd Chua Hock Chye,

together with Professors B Mutlu Sumer and Jorgen

Fredsoe from the Technical University of Denmark, have

been selected by the Environmental and Water Resources

Institute as the recipient of the 2005 Karl Emil Hilgard

Hydraulic Prize.

The Karl Emil Hilgard Hydraulic Prize was instituted

in 1939. It is given to the author(s) of a paper dealing

with a problem of flowing water, either in theory or

practice, which is judged to be of superior merit. It is an

annual award and the winners are selected by the American

Society of Civil Engineers (ASCE).

The 2005 award-winning paper entitled Influence of

Turbulence on Bed Load Sediment Transport by Sumer

BM, Chua LHC, Cheng NS and Fresoe J, was published

in the Journal of Hydraulic Engineering, ASCE, in August

2003. The recipients received this prestigious award at

the World Water and Environmental Resources Congress

held in May 2005 in Anchorage, Alaska.

� The team of CEE: A/P Edmond Yat-Man Lo, A/P

Volodymyr Ivanov, A/P Shuy Eng Ban , A/P Karina

Yew-Hoong Gin, P/O Saurav Kumar, P/O Kong-Chue

Wong, P/O Woon-Kang Yee, P/O Yuehua Chen, altogether

with the teams of PUB and EnviroPro Ltd. were given

The Innovator Award by The Enterprise Challenge (TEC),

Prime Minister’s Office to recognize their innovativeness

and enterprising spirit in TEC project ‘Pilot Project on

the Application of Enhanced Engineered Wetland

Technology for the Removal of Nutrients from Stormwater

at Kranji Reservoir’.

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Research Projects and Activities

A list of recently approved research projects is summarised below. Readers are welcome to email

the respective investigators for more information regarding their work.

PROJECT TITLES PRINCIPAL INVESTIGATOR

Advanced Membrane Technology Centre (AMTC) Wong Fook-Sin (IESE) ([email protected])

- Infrastructural Funding. (NTU-IESE-AMTC Collaboration Project) Tay Joo Hwa ([email protected])

Ren Jizhong

Hassan Mahmud

Strength of overlapped rectangular hollow section K-joints subject to Chiew Sing Ping ([email protected])

chord face failure. (NTU-CIDECT collaboration with Prof Jaap Lie Seng Tjhen ([email protected])

Wardenier, Delft University of Technology, The Netherlands) Lee Chi King ([email protected])

Conversion of metal sludge into construction materials. Chu Jian ([email protected])

(NEA-NTU collaboration Project) Lim Teik Thye ([email protected])

Chui Peng Cheong ([email protected])

Ballast Water Treatment System. (MPA-TEC collaboration Project) Tay Joo Hwa ([email protected])

Leslie Loke (IESE)

Toh Ah Cheong (MPA)

Ministry of Home Affairs-NTU Joint Collaboration Project. A Case Pan Tso Chien ([email protected])

Study of Integrated Petroleum Service Station.(Ministry of Home

Affairs-PTRC, NTU collaboration Project)

A Case Study on Collapse of a Smaller RC Structure (Ministry of Pan Tso Chien ([email protected])

Home Affairs-PTRC, NTU collaboration Project)

Biological Treatment of Municipal and High-Strength Wastewaters Tay Joo Hwa ([email protected])

(Glow Tec Bio Pte Ltd collaboration Project) Stephen Tay ([email protected])

Greener and cheaper way to treat wastewater Tay Joo Hwa ([email protected])

– EERC (PUB-NTU collaboration Project) Wah Yuen Long

Show Kuan Yeow ([email protected])

Tay Tiong Lee ([email protected])

Greener and cheaper way to treat wastewater-IESE (PUB-NTU-IESE Tay Joo Hwa ([email protected])

collaboration Project) Wah Yuen Long

Wong Fook Sin ([email protected])

Chua Hwee Chua

AG Fane ([email protected])

Nanomaterials and Membrane Research Darren Sun Delai ([email protected])

(PUB-NTU collaboration Project)

Development of 3D-GIS (geographical information system) for Tor Yam Khoon ([email protected])

subsurface infrastructure archiving, managing, planning and design. Zhao Jian ([email protected])

(Mindef-NTU-DSTA collaboration Project) Zhao Yingxin (PM-Protective Structures

Design Centre)

Feasibility of Establishing the Tuas/Jurong Industrial Estate as a Wang Jing Yuan ([email protected])

Model Eco Industrial Park (EIP) through Integrated Waste

Minimization. (NEA-NTU & JTC collaboration Project)

Dynamic Response of Piles and Pile Shafts Li Bing ([email protected])

(Mindef-NTU-DSTA collaboration Project) Teh Cee Ing ([email protected])

Use of Recycled Rubber Tyres in Civil Engineering Applications Ting Seng Kiong ([email protected])

(NTU-NEA collaboration Project) Wong Sook Fun (EERC)

([email protected])

The Use of Flow Cytometry for Pathogen Detection (Project Spring). Karina Gin Yew Hoong ([email protected])

- EERC

IN FOCUS


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2006

PROJECT TITLES PRINCIPAL INVESTIGATOR

Seed Funding for Strategic Research at Research Technoloplaza Lawrence Koe ([email protected])

(research manpower). - EERC

Conversion of Food Waste into Biogas and Fertilizer. Wang Jing-Yuan ([email protected])

Application of Real Options in Infrastructurre Projects. (Ct&Cm) Charles Cheah Yuen Jen

([email protected])

Augmented Reality (AR) Enhanced Rescue and Positioning Syatem Chen Po-Han ([email protected])

for Indoor Environment Hsu Wen Jing (SCE)

Henry Duh Been Lirn (MPE)

Bacterial Coaggregation: Drivers, Players and Implications for Stephen Tay Tiong Lee ([email protected])

Biological Wastewater Treatment. (En&Wr) Tay Joo Hwa ([email protected])

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CONSTRUCTIONCONSTRUCTIONCONSTRUCTIONCONSTRUCTIONCONSTRUCTIONEvaluation of Guarantee as a Put Option using

Monte Carlo Simulation – The Case of the Second LinkLiu Jicai ([email protected])

Cheah Yuen Jen Charles ([email protected])

Introduction

An infrastructure project structured under the Build-Operate-

Transfer (BOT) arrangement can be a risky proposition. To

alleviate the concern of the private sector and attract long-

term investors’ participation in financing the project, host

governments often grant various forms of support. However,

as has been reported widely, some governments do not

explicitly account for the contingent liabilities and

implications associated with these support packages (Mody

and Patro, 1995). Despite their prevalence, the cost of these

financial incentives to governments and their value to private

recipients are not well understood (Mason and Baldwin,

1988).

The importance of balancing risk and value has been

highlighted in Cheah (2004). Governmental support such as

subsidies and guarantees would add direct value to the

transaction. Besides, value can also be created by structuring

operating options and flexibilities during the course of design

and execution of a project. Without proper evaluation of

these elements, the matching of risk and value cannot proceed

in a guided manner.

The techniques of evaluation may range from a simple rule-

of-thumb learnt from past experience, to sophisticated

methodologies, such as Monte Carlo simulation and real

option analysis. The latter will be illustrated here in a case

study of the Malaysia-Singapore Second Crossing. All these

methods are commonly complemented by sensitivity and

scenario analyses in order to examine the variations of the

outcome, subject to changes in the initial inputs.

Brief background of the Malaysia-Singapore

Second Crossing

Peninsular Malaysia (West Malaysia) is bordered on the south

by Singapore, which has a close relationship in foreign trade

with Malaysia. For more than 70 years, a causeway that was

built in the 1920s served as the only physical connection

between the two countries. With fast economic growth in

the past two decades, the scale of transportation between the

two countries increased quickly – up to 55,000 vehicles use

the causeway daily and severe congestion is observed even

today. In the early 1990s, the government of Singapore and

Malaysia recognized the need for a second connection. A

new bridge was then proposed to improve land-based

transportation service and serve as the second crossing for

inter-country travelers. With its spacious three-lane dual

carriageway, the new bridge has a capacity of 200,000

vehicles a day (or 73 million vehicles per year), which is

four times the load of the old causeway.

UEM, a leading Malaysian construction group, was successful

in securing the mandate from the Malaysian government

and setting up a special purpose vehicle (which was named

as “LINK”) to implement this project. In July 1993, a

concession agreement was signed between the Malaysian

federal government and UEM. UEM then seconded to LINK

all rights, liabilities and obligations under the agreement to

receive tolls in return for designing, constructing, operating

and maintaining the Malaysian side of the second crossing

(approximately 1.7 km) and the expressways that connect to

other parts of the neighborhood. The Singapore government

will be responsible for constructing the Singapore side of

the crossing (approximately 0.3 km).

Valuation of guarantee as a put option

The guarantee that the Malaysian government provided for

the toll revenue is effectively a form of “put options” written

to the sponsor of the project. Government guarantees are

sometimes granted without much consideration of their real

worth. In retrospect, it would be meaningful to assess the

potential value of the guarantee granted in a project of such

nature, which could be taken into consideration during the

negotiation stage.

Effectively, the guarantee comes from the fact that if the toll

revenue from actual traffic volume (CFia) in each year i

reaches the level that has been projected (CFip) in the pro

forma cash flow model, the government would not have to

pay any subsidy. Otherwise, the government would have to

make up for the shortfall in revenue, just like it actually did

by paying the sponsor RM20 million in 1999. For the purpose

of evaluating this form of guarantees, the government’s

obligation to pay in each year (SFi) would depend on the

relative value between CFia and CFip, as shown in Figure 1.

Therefore, the total subsidy (SD) that the government would

provide over the entire concession period can be estimated

by using the following equation:

(1)

Due to the uncertainties in initial traffic volume and growth

rate, the value of SD is not deterministic but instead follows

a certain distribution. This distribution can be obtained by

simulating cash flows based on the distributions of the initial

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Figure 1. Evaluation model for revenue shortfall

Conclusions

In all forms of public-private partnerships, project participants

should take a holistic view of risk and value in process of

negotiation. Certain forms of governmental support, such as

guarantees and subsidies, can be interpreted as options, since

the obligations are triggered when some pre-specified

conditions are met. The value of these options has to be

properly accounted for, so as to strike a better balance

between risk and benefit. The real option approach is a

promising tool to value some of these options. Although

there are many ways of modeling real options, the approach

presented in this paper makes use of Monte Carlo simulation

of a cash flow model. In the case of the Malaysia-Singapore

Second Crossing, it is demonstrated that the guarantee

granted, which ultimately resulted in subsidy payments, could

indeed be substantial.

References

[1] Cheah, C.Y.J. (2004) Public-Private Partnerships in

Infrastructure Development: On Value, Risk and

Negotiation. CIB W107 Globalization and Construction

Symposium, November 17-19, Bangkok, Thailand.

[2] Mason, S.P. and Baldwin, C.Y. (1988) Evaluation of

Government Subsidies to Large-scale Energy Projects.

Advances in Futures and Options Research, 3, 169-181.

[3] Mody, A. and Patro, D. (1995) Methods of Loan

Guarantee Valuation and Accounting. CSF Discussion

Paper No. 116, World Bank, Washington, D.C.

Figure 2. Frequency distribution chart of total subsidy SD

traffic volume and growth rate. During each trial, values of

the initial traffic volume and growth rate are sampled and

used to derive the gross revenue in each period. Since the

cash flow model is constructed in a spreadsheet environment,

the conditions set out in Figure 1 can be easily incorporated

using logical functions in the relevant cells and hence the

value of SFi for each period can be derived accordingly.

Finally, the cell that contains Equation (1) represents the

forecast cell for the simulation.

Following the above procedure, the distribution of SD can

be derived, which is shown in Figure 2. With a median value

of RM54 million, it is noted that the value of the guarantee

is quite significant (relative to the basic NPV value of

RM326.6 million). This supports the argument that a proper

evaluation of guarantees can be important.

Sensitivity analysis of option values

In practice, it is always prudent to examine how sensitive

the estimated option values are to the various input

parameters. This is particularly important in a negotiation

context. Table 1 shows the results of sensitivity analyses of

SD subject to variations in the standard deviations of initial

traffic volume and growth rate. From Table 1, it can be seen

that the value of SD is more sensitive to the standard deviation

of the initial traffic volume assumed. It should be noted that

the standard deviations of these two variables determine the

volatility of the project cash flows, which is a key determinant

of the value of the option evaluated. The findings essentially

underline the differences in risks that confront the government

and the project sponsor.

Table 1. Sensitivity of total subsidy (in RM millions)

to standard deviations of initial traffic volume and growth rate

Standard Deviation

of Growth Rate

3% 6% 9% 12%

0.55 26.7 28.8 29.2 31.5

1.10 46.6 54.0 60.8 66.5

1.65 60.2 77.0 92.4 95.1

2.20 85.6 104.8 121.1 123.1

2.75 112.2 129.2 162.6 168.4

3.30 140.1 169.3 180.1 191.8

Sta

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Creating and Sustaining Competitiveness of Small andMedium-sized Construction Enterprises in China

Yan Shigang ([email protected])

Chew Ah Seng David ([email protected])


its capacity to achieve its targets. Such targets would typically

take full account of competition. To create and sustain the

competitiveness of construction SMEs, new markets and new

strategies are urgently needed. Arising from the SWOT

analysis, a conceptual framework is postulated for the

development strategies for Chinese construction SMEs, which

comprises strategic alliances, innovation and differentiation

strategies (Figure 1).

Introduction

In the Chinese construction industry, construction SMEs have

enjoyed rapid development since the late 1990s with

progressive adoption of reforms and opening-up policies.

The SMEs have grown to be an important force in promoting

the development of the Chinese construction industry. As

the SMEs assume a greater role in the Chinese construction

industry, this research will examine the theories concerning

their development. The competitive environment pertaining

to construction SMEs will be analyzed through questionnaire

surveys and interviews with founders, managers and engineers

of construction and related enterprises. Finally, overall

competitive strategies for Chinese construction SMEs will

be discussed and a conceptual model is postulated to create

and sustain their competitiveness in China.

SWOT analysis of Chinese construction SMEs

SWOT analysis is concerned with the analysis of an

organization’s internal and external environment with the

aim of identifying internal strengths in order to take advantage

of its external opportunities and avoid external threats

(Langford and Male, 2001). In consideration of the variety

of Chinese construction SMEs with regard to size and ability

to attain national resources, the authors contend that the

SWOT analysis concept could be used to investigate Chinese

construction SMEs’ market position. Although Chinese

construction SMEs are not identical, they share many things

in common such as organizational culture, ongoing enterprise

reform, and the need to face competition from domestic and

foreign contractors.

The strengths, weaknesses, opportunities, and threats facing

Chinese construction SMEs are pilot-studied using a survey

of 15 construction enterprises based in Beijing, Tianjin,

Shanghai, Guangdong province and Hebei province. Among

the 15 construction enterprises, there were six large state-

owned enterprise (SOE) contractors, seven SME contractors,

and two SME design institutes. The respondents consisted

of consultants, designers, project managers and senior

managers. Interviews were also conducted with founders,

managers and engineers of construction and related SMEs.

The interviewing and survey results are summarized in Tables

1 and 2.

Development strategies to create and sustain

competitiveness for construction SMEs

The competitiveness of a firm may be generically defined as

Table 1. The strengths and weaknesses of SMEs

in Chinese construction industry

Rank Strengths Weaknesses

1 Flexible and market- Limited funds

driven

2 Effective decision- Low level of employees’

making education

3 Flexible talent Low competitiveness in

mechanism obtaining large projects

4 Low running expense Difficult to access

and cost financing from banks

5 Efficient company Lack of risk management

structure

Table 2. The opportunities and threats of SMEs

in Chinese construction industry

Rank Opportunities Threats

1 Improvement in Intense competition among

market mechanisms domestic enterprises

2 Policies and regulations Lack of differentiation in

to support SMEs production and service

3 Cooperation with Uncertainty in economy

foreign-funded enterprises

with WTO accession

4 A continuing construction Uncertainty in politics

market boom

5 A broadened approach Entry of foreign

in financing construction firms in

domestic market

Figure 1. A conceptual framework for development strategies

of Chinese construction SMEs

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• Strategic alliance Strategic alliances refer to a form of

lateral working relationship between a firm and its

competitors in one or more aspects of marketing (Li,

1998). The premise of this strategy is based on resource

dependence and interorganizational exchange

perspectives, which hold that organizations seek to

reduce environmental uncertainty by exchanging

resources for mutual benefits. In order to take advantage

of opportunities and cope with challenges facing the

construction SMEs, strategic alliance is a promising

strategy.

• Innovation strategy An important feature of the Chinese

construction industry is the increasingly fast pace of

change in product and service markets. Coupled with

rapidly declining product life cycles in the information

technology, there are growing competitive pressures for

construction SMEs to respond quickly to the market

environment. Thus the capacity for innovation is a critical

factor for construction SMEs’ success. Innovation can

be accrued to construction SMEs externally and/or

internally. External innovation would entail developing

new products and/or services. Internal innovation can

encompass new processes, organizational structures and

culture.

• Differentiation strategy Differentiation is concerned

with creating the perception that something is seen by

buyers as being unique (Male and Stock, 1991).

Differentiation strategy is attractive when buyer

preferences and/or requirements are diverse. Given the

limited resources in terms of finances, human expertise

and production process, it is appropriate for construction

SMEs to adopt a differentiation strategy, either in general

differentiation or focus differentiation. General

differentiation is a particularly important strategy for

most SMEs in a huge and rapidly growing China

construction market, which is likely to remain buoyant

in the foreseeable future. For those specialty construction

SMEs with few employees, focus differentiation may

be the only viable strategy for their target market.

Successful differentiation would allow SMEs to demand

a price premium on their products, increase sales and

gain clients’ loyalty.

Conclusion

Due to the country’s institutional changes and its accession

to the WTO, China’s construction SMEs are in a transition

period. To maintain their competitiveness in the rapidly

changing environment, new strategies are needed. Based on

the pilot survey and interviews, the strengths, weaknesses,

opportunities, and threats of construction SMEs have been

identified. Drawing from the SWOT analysis, a conceptual

framework for the development strategies for Chinese

construction SMEs are postulated by the authors, which

include strategic alliances, innovation and differentiation. It

should be noted that the strategies suggested in this

framework serve mainly as general approaches for Chinese

construction SMEs. Research into the different grades of

construction SMEs in China are needed in future to formulate

more specific development strategies to meet their objectives.

References

[1] Langford, D. and Male, S. (2001). Strategic management

in construction, Oxford: Blackwell Science.

[2] Li, H.Y (1998). Market strategies and new venture

performance in China’s transition economy, Ph.D.

dissertation, City University of Hong Kong.

[3] Male, S. and Stock, R. (1991). Competitive advantage

in construction, Butterworth- Heinemann, Stoneham,

Mass.

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Problems of Overseas Ventures forSingapore Construction SMEs

Wong Yun Tau ([email protected])


Introduction

Since the 1997 Asian financial crisis, Singapore domestic

construction industry has yet to achieve positive growth.

The situation was further aggravated with the SARS outbreak

and the threats of terrorism. For the past few years, Singapore

domestic construction demand remained lukewarm at around

S$11 billion per annum. On the contrary, Singapore

construction companies secured more jobs in overseas

markets. Total construction export value increases more than

eight-fold from S$0.3 billion in 2000 to S$2.5 billion in

2004 (BCA, 2005a). However, most of the successful

Singapore construction companies in overseas are

Government-Linked Companies (GLCs). Although Singapore

construction small and medium enterprises (SMEs) represent

over 90% in number for the whole construction industry,

their contribution has been disproportionately low. Little

attention was focused on their problems in construction export

and reasons for their failures overseas. As a result, this

research was initiated to fill the literature gap.

Research motivation and objective

Venturing overseas is not new to Singapore construction

firms. Several companies have been active abroad for some

decades. However, few of the companies that have

successfully established their early presence overseas are

able to sustain this momentum. Some of them have actually

been downsized or divested. Neither their pitfalls nor success

factors in overseas ventures were captured in theory and

practice. Their mistakes keep recurring, with learning

experiences remaining confidential and not shared industry-

wide. Hence, this creates the main impetus and motivation

to carry out this research, which aims to identify the problems

of Singapore contractors in overseas ventures (Wong et al,

2005). Drawing from this research, it is envisaged that an

essential framework could be formulated to assist Singapore

contractors to succeed overseas.

Research methodology

In order to gain an in-depth understanding of Singapore

contractors’ performance overseas, this research has adopted

the case-study approach and survey questionnaire to collect

qualitative and quantitative data. With the help from BCA,

interviews were arranged with eight Singapore contractors,

comprising seven construction SMEs and one large contractor.

All the interviewees’ positions in this study are at least of

senior managerial level and above. They are personally

involved and responsible for their company overseas

development and operation. Additionally, these eight

Preliminary findings

Preliminary analysis has successfully identified four major

problems for Singapore contractors in construction export.

Table 2 shows the four most critical problems cited by the

interviewees. In the following section, selected findings in

each problem will be highlighted for discussion. In addition,

distinctive comments in italics from interviewees would also

be included to add qualitative insights into the issues being

studied. Each comment is attributed to the interviewee

identified with company indicated in brackets.

companies have ventured into several countries in their

overseas expansion program. The countries range from

Southeast Asia to East Asia and even the Middle East. Based

on their number of years of overseas experiences, these eight

companies were subsequently grouped into two clusters-

amateur companies and veteran companies, as shown in Table

1, to facilitate the analysis of their overseas problems.

Table 1. Classification of Companies A to H

Veteran companies Amateur companies

A C G H** B D E F

Years of

overseas 15 <15* 10 16 <5 <5 <5 <5

experiences

* Ceased overseas operation since 1997 ** Large contractor

Table 2. Four critical problems for

Singapore contractors in overseas ventures

Veteran companies Amateur companiesCritical

TotalProblems

A C G H B D E F

Limited size √ √ √ √ √ √ √ 7

and resources

Difficulty √ √ √ √ √ √ 6

with external

stakeholders

Lack of √ √ √ √ √ 5

government

incentives

Lack of √ √ √ √ 4

support from

Singapore

companies

• Limited size and resources

As an example, amateur companies which are new to

overseas ventures found the lack of financial resources

as key disadvantages in meeting financing requirements

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of overseas projects. “While we are technically competent

for the job, yet we are financially incapable to undertake

the project” (F). Similarly, veteran companies’ main

constraint to grow and expand overseas was their limited

budget. “Business opportunity is not an issue; what

bothers us is insufficient money to finance more than

one overseas project at any time” (A).

� Difficulty with external stakeholders

Both veteran and amateur companies encountered

environmental problems from the external stakeholders

caused by the protectionist measures in the host

countries. However, that does not mean that they are

not performing well in the new markets. “Compared to

China, India is developing at a slower pace and is further

plagued with resource problems; but these are good

problems which show that the country is booming and

generating greater construction demands for us” (A).

� Lack of government incentives

Out of the eight interviewees, five of them thought that

the Singapore government should emulate other countries

in devising more attractive incentives to promote

construction export. Very few of them actually found

trade missions organized by government agencies to be

effective and useful. “We have ventured overseas well

before government starts its construction export

promotion and we might have better connection than

BCA” (A).

� Lack of support from other Singapore companies

Compared to firms from Taiwan and Hong Kong,

Singaporean firms are generally self-centered and

unwilling to share. Though result-oriented, they are not

united, unlike the Japanese who tend to engage their

own people for any related works overseas. The

Singapore developer/consultant selfdom takes the

initiative to hire Singapore contractors for works

overseas. Instead, they would usually engage foreign

contractors with the (mis)conception that foreign product/

services are always better. “Singapore firms are simply

lacking confidence in their compatriot’s service and

product quality overseas” (A).

Conclusion

The Singapore construction industry is filled with many

construction SMEs who have limited financial capability to

venture abroad. There is little government assistance scheme

to assist them in exporting their service. This situation is

further aggravated by the unwillingness of fellow Singaporean

firms to support and partner each other in overseas ventures.

In the light of this predicament, government agencies, such

as BCA, should review their current strategy to achieve the

goal of increasing the overseas market-share of Singapore

contractors. This on-going research, upon completion, will

culminate with a framework and suggestions to realize that

objective.

References

[1] BCA, 2005a, Singapore contractors clinched $2.5billion

overseas contracts in 2004 – record high in 10 Years. Media

Release, online publication, URL: http://www.bca.gov.sg”http://www.bca.gov.sg

[2] Wong, Y.T., Chew, D.A.S., and Cheah, C.Y.J., 2005,

Strategic framework for Singapore contractors forming JVs

in developing countries: A preliminary study. Proceeding of

the First International Conference on Construction and

Engineering Management, 16-19 October, 2005, Seoul,

Korea, 6 p.

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Competitive Strategies and Competitive Advantages ofLarge Chinese Construction Enterprises

Kang Jian ([email protected])



Introduction

The concept of competitive advantage has been widely

studied in the academic field and applied to various industries

in many western countries. However, practical cases and

empirical findings related to the Chinese construction industry

remain lacking. To help construction enterprises in China

identify and structure critical sources of competitive

advantage, this research aims to build a conceptual model

based on the findings gathered from the case studies of 12

large Chinese construction companies. The relationships

among variables within the model are subsequently verified

and refined through statistical analyses of survey results,

which consist of 85 valid responses.

Essentially, the conceptual model combines two main streams

of strategic management theories – industrial organization

(IO) and resource-based view (RBV) theories. According to

Kale (1999)’s definition, competitive advantage refers to the

ability of a firm to outperform its rivals on some performance

criteria, such as profitability and market share. In the model,

which is shown in Figure 1, it is postulated that there are

two major sources of competitive advantage: competitive

strategies and imperative resources and competencies (iRCs).

This article presents the findings related to competitive

strategies and the analytical results of Hypothesis No. 1.

Measurement of variables

The variables examined in the questionnaire are grouped

into three categories: performance, competitive strategies and

iRC variables. The questions designed to test each variable

are developed based on the interview results and case

findings. A survey question, which is designed to examine

the issue of cost leadership, is presented here as an illustration.

In that question, cost leadership was measured by: (1) cost

of material and equipment procurement; (2) manpower cost;

(3) cost control during the construction process; (4)

administrative cost; (5) subcontracting cost. The respondents

were asked to choose one or more items or nominate areas

Figure 1. Components of competitive advantage

and related hypotheses

Selected analytical results

A regression analysis is conducted to test the relationship

between competitive advantage and competitive strategies

and the results are presented in Table 2. Competitive

advantage, which serves as the dependent variable, is

represented by the growth rates of sales and net profit. The

measure of “overall performance” is given by the average of

sales and net profit growth rates. Although the value of

adjusted R2 seem to be low, the F-statistics are indeed

significant, indicating that the independent variables do

explain the variation of the dependent variable. Specifically,

it indicates that differentiation and market/product

diversification can contribute to the development of

competitive advantage at a 5% level of statistical significance.

in which they thought they had an advantage over their main

competitors. The results were collated using a scale of 0-6,

in which 0 means none of the items was chosen, 1 implies

that only one of the items was chosen, and so on. In order

to determine whether a company has indeed implemented a

cost leadership strategy, the results were transformed into

dummy variables (0, 1). The transformation process is largely

judgmental and is based on the study of the whole sample

population. This is because whether a cost leadership strategy

is implemented is purely a relative concept – it should refer

to the other companies in the industry. Here, the mean value

is used for the transformation – values greater than the mean

would be assigned as “1”, while values less than the mean

are assigned as “0”. It should be mentioned that due to the

variability of the design format of each survey question, the

measurement process also varies.

Table 1 shows the mean, standard deviation and value scale

of some of the variables.

Table 1. Measurement and descriptive statistics

of selected research variables

Research Variables Mean Std. Deviation Scale

Performance (Dependent Variables)

Sales growth 3.15 1.418 1-5

Profit growth 2.46 1.323 1-5

Overall performance 2.81 1.272 1-5

Competitive Strategies (Independent Variables)

Cost Leadership .46 .484 0-1

Differentiation .56 .499 0-1

Market/Product .48 .503 0-1

Geography .47 .502 0-1

Functional/vertical Integration .54 .501 0-1

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Discussion of results

As Porter (1980, 1985) suggested, a firm cannot pursue cost

leadership and differentiation at the same time, for it would

face the risk of being ‘stuck in the middle’. The research

results confirm this view for the Chinese construction context

(since the independent variables “differentiation strategy”and “cost leadership” are not concurrently significant). The

reason that the differentiation strategy, instead of cost

leadership, has a stronger relationship with performance is

due to the excessive competition that exists in the Chinese

construction industry. Many under-performing companies still

remain in the industry because of high exit barriers. These

companies, especially small and medium-sized enterprises

within the under-performing group, often lower their bidding

price to acquire new projects. In some of the projects, the

price could even be lower than the project cost in order to

prolong their survival in the industry. On the contrary, large

companies have more resources and competencies to

distinguish themselves from their competitors. Therefore, they

can acquire more projects and profits by adopting a

differentiation strategy instead of a cost leadership strategy.

Other than differentiation, the findings in Table 2 also confirm

that market/product diversification can contribute to strong

performance. A market/product diversification strategy

requires a company to compete in different types of projects,

such as road, tunnel, bridge, subway, airport, railway, and

dam, etc. Diversification can often reduce risks, which is

faced by a focus strategy that targets only one type of market

segments. Furthermore, market/product diversification fully

utilizes the experiences, resources (such as manpower,

equipment) and competencies (such as project management

competencies) developed in one type of projects and

synergize these resources and competencies with applications

to other types of projects.

Table 2. Results of regression analysis for Hypothesis No. 1

Variables Sales Growth Profit Growth Overall Performance

(Model 1a) (Model 1b) (Model 1c)

Cost Leadership .149 .013 .062

Differentiation .346** .384** .358**

Geographical

Diversification .129 .159 .130

Market/Product

Diversification .227** .217** .229**

Functional/

Vertical Integration .019 .044 .004

Adjusted R2 .205 .227 .212

F-Value 5.331*** 5.703*** 5.166***

*P≤0.1; **P≤0.05; ***P≤0.01

The results do not support the notion that geographical

diversification could influence performance. One possible

factor is the protectionist policies of local and regional

government in China. In order to protect local benefits, secure

short-term profits and lighten the pressure of local

employment and fiscal problems, many local government

and authorities have chosen to lower the entry barriers to

their local construction companies. This would aggravate

the level of competition within the local industry. At the

same time, the local government might elevate the entry

barriers to companies from other regions, thus making it

difficult for them to secure new projects in the region.

The results also do not support the notion that vertical

integration could influence performance. This means that

vertical integration may not be a viable strategy for large

construction companies in China. One possible explanation

is that although vertical integration could reduce operational

risk and transaction costs (such as backward integration into

the manufacturing of equipment and materials and forward

integration into real estate development), it cannot utilize

resources and competencies developed in the existing

construction business to venture into other similar projects.

For example, a few categories of iRCs – Guanxi resource

with original clients, project management competencies and

technological and innovative capabilities, have limited

“expansion value” into the upstream and downstream

businesses.

Finally, this article only presents one of the regression models

that verifies the relationships between competitive advantage

and competitive strategies. There are also other models

developed to examine the relationships between (1)

competitive advantage and iRC variables; (2) competitive

strategies and iRC variables. These models are also tested

and interpreted in conjunction with established strategic

management theories.

References

[1] Kale, S (1999) Competitive advantage in the construction

industry: firm specific resources and strategy, Ph.D.

Thesis, Illinois Institute of Technology.

[2] Porter, M E (1980) Competitive strategy: techniques for

analyzing industries and competitors. New York: Free

Press.

[3] Porter, M E (1985) Competitive advantage: creating and

sustaining superior performance. New York: Free Press.

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Research Findings on International StrategicManagement of Chinese Construction Firms

Liu Guozhi ([email protected]) Chew Ah Seng David ([email protected])


Introduction

Chinese construction firms (CCFs) have made significant

achievements in international construction in recent years. 49

CCFs were included in the 2005’s Top 225 International

Contractors based on their international revenues in 2004. Up to

2004, CCFs had completed total international turnovers of

US$114.03 billion and realized total international contract values

of US$156.29 billion. For the first eight months of 2005, CCFs’

contracted and transacted values for international construction

are US$15.92 billion and US$11.75 billion respectively (Ministry

of Commerce, China, 2005). Their international market share in

Asia was 60 percent of the total in 2003, against 23 percent in

Africa, 10 percent in Europe, and 7 percent for the rest of the

regions (China International Contractors Association, 2004).

Strengths and weaknesses of Chinese

construction firms

Compared to international competitors from the advanced

economies, CCFs’ strengths and weaknesses are now analyzed.

CCFs’ strengths mainly lie in low costs and strong government

support. Abundant natural and human resources have led to lower

construction costs. Construction tools, machinery and equipment

made in China are much cheaper than those made in the advanced

countries. CCFs’ overhead could have been much lower than

that of the international competitors if their organizational

structures were optimized. Furthermore, a few specialized CCFs

have their own proprietary technologies for certain kinds of

projects. Such technologies and other innovative materials may

not be superior to their international rivals; nonetheless they are

attractive to most clients in the developing countries due to cost

advantage over competitors from the advanced economies.

Conversely, CCFs’ weaknesses are reflected in the following

aspects:

• Fierce competition among a large number of equal-sized

firms. Few giant conglomerate CCFs are developed.

• Management of CCFs is still not fully market-oriented.

• Over capacity and overstaffing. Idle rates of CCFs’

equipment and labour resources remain high.

• Heavy social responsibility burden. CCFs are obliged to

provide social security for retired staff.

• Low profits and tight cash flow. Such problems undermine

their ability to secure financing for large projects.

• Low international revenues. For example, Hochtief AG’s

international revenues in 2004 alone exceed that of all 49

top CCFs’ combined revenues.

• Limited technology and management know-how, especially

for large and sophisticated projects.

• Lack of experienced and versatile personnel for international

business management.

Summary of research findings

A survey was conducted to validate a number of hypotheses

regarding the strategic issues for CCF’s international strategic

management. The survey was carried out in two rounds, with 92

qualified answers received for the first round and 59 qualified

answers received for the second round. A total of 16 senior

managers from different CCFs were also interviewed to verify

these hypotheses. The findings are briefly summarized as follows:

• Integration of cost leadership and differentiation is deemed

the best and ultimate generic competitive strategy for CCFs

internationally.

• Both market entry modes and contracting modes to be

adopted by CCFs should vary from market to market.

• Three most important measures to build CCFs’ core

competencies are: restructuring the organization; cultivating

and recruiting talents; and improving technology and

management.

• Building relationship with banks/financial institutions and

public listing are two most important channels for CCFs to

enhance financing capability.

• Three most important factors that influence marketing are:

employing sufficient information channels; using

experienced marketing professionals; and top management’s

instruction to make detailed SWOT analysis.

• Merger and acquisition (M&A) among strong CCFs in

different specialized trades are advantageous; M&A among

construction-related firms (like material manufacturers,

designers and developers) are also important.

• Leadership of top managers is pivotal to CCFs’

internationalization strategy formulation; top managers’

personal ability and international experience are two

important aspects of leadership.

• Control by CCF’s headquarters over foreign subsidiaries

should be reduced initially to allow localization to take

place; control should gradually be raised to synergize CCF’s

global strategy.

• Localization is important for CCF’s internationalization,

especially during the early stage of internationalization.

• At the maturity stage of CCF’s internationalization, the top

three factors for preventing business decline are: effective

management to reduce operational cost; technology

innovation; and multinational collaboration for economies

of scale.

• CCFs should focus on core business at early stage of

internationalization; they can gradually diversify their

business at maturity stage; related business diversification

is more important than non-related business diversification.

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• The Chinese government’s policies and support are important

to CCF’s internationalization. Top four important aspects

are: providing business information and consultancy services;

providing financial support; overseeing CCFs’ restructuring

and M&A; and strengthening supervision and coordination

roles of industry associations and government agencies.

Conclusion

The research reviewed CCFs’ international performance and

showed that they have made significant achievements in

international construction in recent years. A number of large

CCFs with strong competitiveness are discussed. Strengths and

weaknesses of CCFs are analyzed and compared with other

international competitors. Based on two rounds of questionnaire

surveys and personal interviews of senior managers of CCFs,

key research findings regarding the international strategic

management of CCFs are summarized, which form the core of

this research.

References

[1] Department of Foreign Economic Cooperation (2005), Briefstatistics of foreign economic cooperation (1991-2005),Ministry of Commerce, China.

[2] China International Contractors Association (CHINCA),www.chinca.org.cn available at 18th Dec 2004.

A Simple Flow Control Tool for a Real-time EvacuationSystem Augmented with Virtual Reality

Chen Po-Han ([email protected])

Feng Feng ([email protected])

Introduction

In a real-time system, especially for emergency evacuation,

computation tasks or functions should be able to be finished in

a reasonably short time. Users of our system are supposed to be

rescue team members. When they are evacuating a large group

of people, how to assign evacuees to different exits to shorten the

evacuation time to a largest degree is a big challenge. With some

flow control functions provided, the system should be able to

provide real-time evacuee division and dispatching suggestions

within a short time of computation. Existing evacuation software’s

flow control algorithms are not effective enough for our system

in terms of their theoretical computation complexity and actual

computing speed, especially when the network is large in terms

of arc and node numbers.

The flow computation methods of some existing evacuation

software have been reviewed. As the tasks of our system are

unique from existing evacuation systems, the assumptions of our

flow control computation are evaluated. The characteristics of

the target buildings of our system, such as shopping malls, campus

buildings, and large office buildings are analyzed. On the basis

of all these observations, a fast flow control algorithm, slimmed

for our VR augmented emergency evacuation system has been

developed. Its application to a shopping mall has also been

implemented.

Research objectives

The flow division and dispatching algorithm of our emergency

evacuation system is designed for rescue team members, who are

supposed to be able to find the people to evacuate. These people

are unfamiliar with the building and therefore stuck in a dangerous

area. After the people, or so called evacuees, are found, group

division and dispatching suggestions can be given according to

the layout of the building and the total number of evacuees at the

location. Each group of evacuees will be led by one rescue team

member to head for the designated destination, which is usually

an exit door. The objective of this research is to develop a suitable

algorithm for this situation.

Inside building structures: wide doors and

narrow doors

Through observations, we find that it is a typical phenomenon

that all exit doors except for one or two, have widths much

smaller than most of the passageways inside a modern building.

They can be called Narrow Doors (ND). With reasonable

transformations, the doors, whose widths are bigger than most of

the passageways, can be converted to a series of ND. As an ND

is the bottleneck of any evacuation route, no multiple-path finding

is needed for a source-target pair. One shortest path is enough for

this one-source one-target evacuation.

If, additionally, the interactions between intercepting evacuation

groups are not considered, our problem can be converted to a

simpler transshipment problem. In fact, this interaction omission

is quite reasonable. One reason is that human behaviors in

emergency can’t be predicted so fine, which is much likely to

make the utility of a high-accuracy flow control strategy not able

to justify the huge computing resource consumption. Another

reason is that in addition to the evacuation groups led by the

security team members guided by our system, it is supposed that

there are other people evacuating by themselves present in the

building, which will make high-accuracy computation result

useless. Another interacting strategy may be more suitable for

our case. When people, especially large groups of people, intercept

with each other, and this interception invalidates the existing

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evacuation grouping and dispatching, the intercepting people can

be merged into one group. After this mergence, a new one-source

multiple-target calculation can be applied. We believe this merge-

and-recalculation strategy is more suitable in a real emergency

scene.

Solution to the one-source multi-door problem

The definition of our one-source multi-door problem is as follows:

There are in total Nt evacuees at the source waiting to be evacuated.

They are going to be divided into k groups. Each group has Ni

evacuees (i=1, 2, 3…, k). The path group i takes to an ND is Pi,

and the length of Pi is Li. V is the walking speed of evacuees. Wi

is the dynamic capacity of Door i, which Pi goes to. Ti is the total

evacuation time of group i. Then:

(1)

Ti=Li/V+Ni/Wi (2)

In an optimal solution, all EGs should finish evacuation at the same

time, so T1=T2=…=Tk=X.

So

X=Li/V+Ni/Wi (3)

Ni=(X-Li/V)*Wi (4)

Substitute Ni in (1) with Ni in (4), we get:

(5)

(6)

The subset of exit doors is not selected without any limitation. A

reasonable exit door subset should make X-Li/V>=0 for all doors

in the subset. This makes sure that in the subset there is not a

door such that the length of the shortest path to this door is too

long so that even if it is assigned only 1 person, it still will need

longer time for the person to reach this door than the total

evacuation time of all other groups. If we allow Ni to be negative,

the resulting door set is clearly not optimal.

Through this mathematical description, our problem is converted

into a simple transshipment problem, for which a SIMPLEX

solution can be applied according to certain standard procedure

[1]. In our coding of the algorithm, to avoid the high computation

complexity of the SIMPLEX algorithm, another slimmed

algorithm is applied and the detailed code explanation will be

given in our future work on the project.

A shopping mall case

In order to demonstrate our flow control algorithm, part of a

floor of the Takashimaya building in Singapore has been converted

into a network with nodes and arcs, with dynamic capacity of

doors defined. A division and dispatching calculation request can

be initiated at any location in the network. The relevant information

to the calculation, such as the location, the number of people and

the walking speed should be input to the program.

Then the program will give the group division and dispatch profile

in a message box, and the routes each group should follow are

shown in different color. The dispatching profile is also stored in

an output file in .txt format for reviewing. The output is in the

form of Fig.1.

When the number of groups is limited to a certain number, a

variation of the algorithm is applied and has been coded into our

program. The input needs an additional parameter, the number of

systems. The output is similar to the no limitation situation. The

only difference is that the number of groups is always within the

given limitation.

Conclusions and future work

Our research has defined and given a reasonable solution for a

(b) Evacuation routes

Figure 1. Output of the program

(a) Evacuation profile

common emergency evacuation flow control problem. Through

the testing of an example network with the coded algorithm, the

solution is efficient enough in terms of computing time and

accuracy level. Therefore, the algorithm can be readily transplanted

onto a VR platform. This algorithm and the tools provided on its

basis are supposed to be an efficient aid for evacuation rescue

teams.

Some parameters, such as the dynamic capacity of doors with

different width and the walking speed of the people, are arbitrarily

given at this stage through dialogue input. They should be under

further research through some behavior literature reviewing or

experiments. Human behavior under emergency environments

has a large impact on the validity and utility of an evacuation

flow control strategy. For a well performing real-time evacuation

system, careful reviewing on this aspect is necessary.

References

[1] Winston W.L. (2004). Operations Research, 400-403. ©2004

Brooks/Cole, Thomson Learning Inc. ISBN 0-534-42362-0.

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ENVIRONMENTENVIRONMENTENVIRONMENTENVIRONMENTENVIRONMENTSelection of Microbial Seeds for

Fast Granulation in Aerobic Treatment of WastewaterWang Xiao Hui ([email protected])

Volodymyr Ivanov ([email protected])

Tay Tiong Lee, Stephen

Tay Joo Hwa ([email protected])

Introduction

Bio-granulation is a new technology which can be used in

municipal and industrial aerobic wastewater treatment.

Microbial granules have advantages over activated sludge

such as fast settling and higher resistance to toxic substance,

diverse microbial community, removal of particles [1] and

ability to perform aerobic and anaerobic processes [2].

However, the formation of microbial granules from activated

sludge requires several weeks [2], whereas activated sludge

system can be started up in several days. Therefore, one of

the aims of this research was the selection of microbial

cultures and the development of microbial seeds to shorten

the granule-forming period from several weeks to a few

days. Another aim was screening of these microbial cultures

on their bio-safety so that only safe microbial cultures will

be used in the study and commerical development of the

seeds for fast microbial granulation.

Material and Methods

Reactors set-up and operation

Lab-scale column sequencing batch reactors (SBRs) were

used, each with a working volume of 2.5 L. Control process

was started with 100 mL of activated sludge with a

concentration of mixed liquor volatile suspended solids

(MLVSS) of 3000 mg/L. The experimental process was

started using 45 mL of Pseudomonas veronii cell suspension

and 10 ml of activated sludge suspension. The experimental

and control sequencing batch reactors were operated with a

cycle of 3 h, including 5 min of influent filling, 169 min of

aeration, 1 min settling and 5 min of effluent discharge.

Effluent was discharged from the middle port of the reactor

at a volumetric exchange ratio of 50%, giving a hydraulic

retention time (HRT) of 6 h. Air was supplied through a

dispenser at the reactor bottom with an airflow rate of

2.5 L/min. Synthetic wastewater was composed of glucose,

2 g L-1, (NH4)2SO4, 0.2 g L-1, Nutrient Broth (Difco, Detroit,

USA), 0.15 g L-1; KH2PO4, 0.1 g L-1, MgSO4 .7H2O, 0.04 g

L-1; CaCl2.2H2O, 0.04 g L-1; FeSO4.7H2O, 0.02 g L-1and trace

element solution, 1 ml L-1. Chemical oxygen demand (COD)

loading of the reactors was 8.5g COD L-1 day-1.

Biosafety screening of strains

Although the strains which have higher aggregation abilities

could be used as microbial seeds for granulation

enhancement, the human and environmental safety assessment

has to be carried out before application. Among the identified

five microbial strains, Aeromonas caviae, is know to be a

human pathogenic bacteria, the species Klebsiella

pneumoniae is a urinary tract pathogen. The release of the

strains of these species into environmental engineering

systems might potentially cause health problems for human

and animals. Therefore, the strain K. pneumoniae is not

suitable for environmental engineering application because

of bio-safety issues. The species of Pantoea agglomerans,

previously named as Enterobacter agglomerans, is listed in

Biohazard group 1, a member of the phytopathogenic genus

Erwinia. Although this organism is usually relatively benign,

it does have a risk for nosocomial infection. Therefore, only

strain of Pseudomonas veronii might be considered as suitable

one for the treatment of wastewater because until now there

has been no published data on its pathogenicity. Therefore,

strain Pseudomonas veronii was selected for the further trials

Results

Isolation and identification of strains

Totally, 19 strains with different types of colonies were

isolated from microbial granules. Five microbial strains with

an aggregation index of higher than 50% were selected for

16s rRNA gene identification. The aggregation index of each

culture is shown in Table 1.

Table 1. Properties of isolated cultures

Isolated strain No. of bases used Aggregation

to establish identity index

Aeromonas caviae

strain A 1503 67.4

Enterobacter agglomerans

strain B 1349 71.9

Pantoea agglomerans

strain C 1491 83.9

Klebsiella pneumoniae

strain D 1338 65

Pseudomonas veronii

strain E 1408 51

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Figure 1. The granules formed from the Pseudomonas veronii

strain and activated sludge.

as a starter culture for the formation of microbial granules

in aerobic wastewater treatment.

Enhanced formation of granules

The mixture of Pseudomonas veronii cells and activated

sludge was used in experiment. The control was inoculated

with activated sludge collected from a local municipal

wastewater treatment plant.

Granules started to form in experimental bioreactors after

eight days of cultivation (Figure1), while only microbial

flocs could be seen at that time in the control. It was shown

that addition of strains with higher aggregation abilities than

that of activated sludge, diminished the duration of

granulation process from several weeks to 8 days. This

evaluation is based on the value of SVI, which was lower

than 70 mL g-1 of the typical value of matured granules

(Figure 2). The COD removal efficiencies for both control

and experiment reactors were stable at 85% to 98%.

Cultivation of cells from the dispersed granules on solid

medium showed that 40 ± 6% of colonies from the granules

formed after 10 days of cultivation in the experimental reactor

were colonies of Pseudomonas veronii. This indicates that

the added strain dominated in formed aerobic granules.

Discussion

Bioaugmentation of activated sludge with specialized

bacterial strains (microbial seeds), has been practiced since

the 1960s. Their application in wastewater treatment solves

operational problems, such as shock loading in treatment

plant. Bioaugmentation could be a powerful tool and cost

effective method to improve several aspects in wastewater

treatment processes such as improved flocculation and

degradation of recalcitrant compounds.

Due to selection of microbial strain with high self-aggregation

aerobic granules were formed in eight days of cultivation,

instead of several weeks required for the formation of the

granules from activated sludge. Another important aspect of

selection and screening of the seeds for microbial granulation

was isolation and testing of safe strain of the species

Pseudomonas veronii for fast microbial granulation in aerobic

wastewater treatment.

References

1. Ivanov V, Tay J-H, Tay ST-L, Jiang H-L. Removal of

micro-particles by microbial granules used for aerobic

wastewater treatment. Water Sci Technol 2004;50:147-

154

2. Ivanov V, Tay J-H, Liu Q-S, et al. Microstructural

optimization of wastewater treatment by aerobic granular

sludge. In: Bathe S, Kreuk Md, McSwain B,

Schwarzenbeck N, eds. Aerobic Granular Sludge.

London: IWA Publishing, 2005:43-52

Figure 2. SVI change during granule formation.

: control; ■ : experiment.

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Stabilization/Solidification ofIndustrial Waste Sludges using MSWI Fly Ash

Chui Peng Cheong ([email protected])

Cao Yali ([email protected])

Introduction

Municipal solid waste incineration (MSWI) fly ash and heavy

metals-bearing sludge are hazardous wastes that must be

solidified and/or stabilized (S/S) before landfill disposal. Most

metal-bearing sludges are high in Al2O3 and SO3 chemical

composition and when combined with MSWI fly ash will

result in the formation of Friedel’s salt and ettringite hydrate

in the S/S matrix [1] which provides a better fixing

effectiveness for heavy metals. The objective of this study

was to use MSWI fly ash as a binding material for co-

disposing heavy metal-bearing sludges. The co-disposal

effectiveness of the mix was evaluated by analyzing its

physical and leaching properties as well as hydrate

composition.

Experimental Details and Results

Materials

The MSWI fly ash used in this study was collected from the

Senoko Incineration Plant. The three industrial waste sludges

(labeled as A, B, and C) were collected from different local

chemical and electronics plants. The sludge was first dried

at 103oC and subsequently ground to less than 9.5 mm in

size. The target heavy metals selected for this study were

copper, lead, manganese, nickel, and zinc, based on their

presence in the raw sludges. Each of the three types of sludges

was mixed with different proportions of ordinary Portland

cement (OPC) and MSWI fly ash as indicated in Table 1.

The mixes were prepared at a water to solid ratio of 0.3. The

sludge-cement-fly ash matrix was cast in 50 mm cubes and

left to air-cure after demoulding.

Compressive Strength of Fly ash – Cement Matrix

The US-EPA recommended value of 0.3 MPa was adopted

as the target compressive strength of the solidified matrix

for landfill disposal [2]. The compressive strengths at 3 and

7 days for the various sludge-cement-fly ash matrices were

tested and are given in Table 2. There was a general increase

in strength with increase in cement in the matrix. Sludge

samples with fly ash and cement were found to exhibit a

higher compressive strength than those with cement alone.

Samples from sludge B, however, registered extremely low

compressive strength and experienced extreme deformation

during the tests. The failure of the stabilization process was

due to its greasy nature, which inhibited mixing and hydration

of the fly ash and cement.

Leaching of Heavy Metals

The US-EPA Toxicity Characteristic Leaching Procedure

(TCLP) Method [2] was used to determine the leaching of

Table 1. Mix proportions for each type of metal sludge

(% by weight)

Mix OPC (%) MSWI FA (%) Sludge (%)

1 0 0 100

2 0 50 50

3 5 0 95

4 5 45 50

5 10 0 90

6 10 45 45

7 15 0 85

8 15 40 45

heavy metals from the different matrices. Tables 3a, b, and

c list the concentrations of various heavy metals in the

leachate of mixes using the three raw sludges. As shown, the

leached concentrations for Pb, Zn, and Ni from the raw

sludges far exceeded the limits allowed for landfill disposal

as specified by the National Environment Agency (NEA),

Singapore. However, they were within the legal limits when

stabilized by cement and/or fly ash.

XRD analysis

The results from X-ray diffraction analysis for matrices A2

and A8 given in Figure 1 shows the formation of three new

reactive products, in addition to the remaining phases from

the raw MSWI fly ash such as KCl, NaCl, CaSO4, and

Ca(OH)2. For matrix A2, the ettringite (AFt) phase and

Friedel’s salt phase were formed after 7 days of hydration.

The main peaks for AFt were at 9.73, 5.61, and 3.88 Å, and

that for Friedel’s salt phase were at 7.86, 2.86, and 3.86Å.

In addition, there was also a C-S-H phase formed in matrix

A8, besides AFt and Friedel’s salts. This C-S-H phase peaked

at 2.97 and 1.84 Å. The C-S-H phase is an important phase

responsible for the strength development of cement-based

materials. The inclusion of cement was responsible for the

formation of the C-S-H phase in matrix A8, which resulted

in a higher compressive strength than matrix A2.

Figure 1. XRD of matrixes after 7 days of hydration

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Table 2. Compressive strength of different mix proportions (MPa)

Sludge A Mix 3-day 7-day Sludge B Mix 3-day 7-day Sludge C Mix 3-day 7-day

A1 * * B1 * * C1 * *

A2 0.12 0.36 B2 * * C2 * *

A3 0.16 0.22 B3 0.03 0.03 C3 * *

A4 0.61 0.99 B4 0.06 0.07 C4 0.29 0.67

A5 0.17 0.15 B5 0.05 0.04 C5 * *

A6 1.31 2.05 B6 0.03 0.03 C6 0.18 0.52

A7 0.90 1.31 B7 0.05 0.04 C7 0.1 0.14

A8 1.94 2.05 B8 0.03 0.03 C8 1.11 1.76

* Samples disintegrated upon demoulding due to very low strength.

Table 3a. TCLP results of sludge A samples

TCLP concentration of mix samples (mg/L)

A1 A2 A3 A4 A5 A6 A7 A8

As 5 <0.030 0.038 <0.030 <0.030 <0.030 <0.030 <0.030 <0.03

Cr 5 0.057 0.151 0.02 0.095 0.048 0.134 0.03 0.104

Cu 100 60.2 0.099 1.254 0.084 0.292 0.104 13.88 0.107

Fe 100 0.184 0.051 0.062 0.041 0.031 0.033 0.063 0.036

Pb 5 5.022 0.12 0.322 0.071 0.067 0.078 3.17 0.044

Mn 50 0.144 0.221 0.225 0.06 0.124 0.045 0.816 0.067

Ni 5 0.591 <0.002 0.17 0.015 0.043 0.017 0.742 0.005

Zn 100 1.077 0.104 0.092 0.039 0.022 0.064 0.413 0.018

Table 3c. TCLP results of sludge C samples


C1 C2 C3 C4 C5 C6 C7 C8

As 5 <0.030 <0.03 <0.03 <0.03 <0.03 <0.03 <0.03 <0.03

Cr 5 <0.001 <0.001 <0.001 0.125 0.028 0.162 0.085 0.148

Cu 100 0.333 0.012 0.116 0.286 0.22 0.242 0.243 0.266

Fe 100 0.045 0.067 0.021 <0.001 0.01 <0.001 <0.001 <0.001

Pb 5 1.198 9.404 0.185 0.092 0.076 0.057 0.053 0.016

Mn 50 0.093 0.009 0.077 0.078 0.026 0.025 0.012 0.024

Ni 5 20.6 0.067 9.787 0.25 1.544 0.046 0.531 0.048

Zn 100 0.268 1.602 0.019 <0.003 <0.003 <0.003 <0.003 0.01

ElementNEA

Standard

Table 3b. TCLP results of sludge B samples


B1 B2 B3 B4 B5 B6 B7 B8

As 5 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030

Cr 5 0.295 0.018 0.009 0.378 <0.001 0.708 <0.001 0.539

Cu 100 2.055 0.158 5.463 0.976 4.515 0.905 2.376 0.58

Fe 100 2169 5.239 1.72 0.678 0.499 0.386 0.237 0.12

Pb 5 0.527 2.693 0.14 <0.001 0.053 <0.001 <0.001 <0.001

Mn 50 33.3 0.172 37.5 0.143 28.8 0.147 20 0.092

Ni 5 3.23 0.065 4.184 0.02 2.981 <0.002 1.95 <0.002

Zn 100 2522 18.27 3799 9.746 2577.5 10.18 1747 5.57

ElementNEA

Standard

ElementNEA

Standard

ENVIRONMENT


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FTIR analysis

The micro-surrounding of structures for reaction products

can be characterized by the Fourier transform infrared (FTIR)

spectrum. As shown in Figure 2, the two bands at 601 cm-

1 and 876 cm-1 were related to the symmetric and asymmetric

vibrations of Al-OH bands in the Al(OH)6 octahedra structure

of ettringite and Friedel’s salt [3]. The band at 1622 cm-1

was an H-O-H vibration (v2H2O) of the interlayer water for

the Friedel’s salt phase whereas the broad bands at 3406 and

3539 cm-1 were due to vibration of OH ions, (vOH) in the

structural water [3]. Chloride was not absorbed in the band

range 400–4000 cm-1. The peaks located at 1142 cm-1 and

1118 cm-1 could be attributed to v3-SO4, which confirmed

the presence of ettringite. The band at 937 cm-1 for matrix

A8 was due to Si-O stretching band [4], which proved the

presence of the C-S-H phase.

Conclusions

MSWI fly ash is a suitable material for stabilizing heavy

metal-bearing sludges. The matrices of heavy metal-bearing

sludge and MSWI fly ash have strong fixing capacities for

such heavy metals as Zn, Pb, Cu, Ni and Mn. The addition

of MSWI fly ash can improve the strength of the matrix.

Specimens with only 5% to 15% cement content were

observed to be sufficient to achieve the target compressive

strength of 0.3 MPa required for landfill disposal. An

optimum mix comprising 45% fly ash, 5% cement and 50%

of the industrial sludge could provide the required

solidification and stabilization. The main hydration products

of the optimum S/S matrix are ettringite, Friedel’s salt and

C-S-H. These hydration products play an important role in

the fixing of heavy metals.

References

[1] Andres A, Ibanez R, Ortiz I, Irabien JA. 1998,

Experimental Study of the Waste Binder Anhydrite in

the Solidification/Stabilization Process of Heavy Metal

Sludges. Journal of Hazardous Materials, 57:155-168.

[2] US-EPA Method 1311, Toxicity Characteristic Leaching

Procedure.

[3] Myneni SCB, Traina SJ, Waychunas GA, Logan TJ. 1998,

Vibrational Spectroscopy of Functional Group Chemistry

and Arsenate Coordination in Ettringite. Geochimica et

Cosmochimica Acta, 62:3499-3514.

[4] Stoch A, Zdaniewicz M, Paluszkiewicz CZ. 1999, The

Effect of Polymethylsiloxanes on Hydration of Clinker

Phases. Journal of Molecular Structure, 511–512:319-

325.

[5] Birnin-Yauri1 UA, Glasser FP. 1998, Friedel’s salt,

Ca2Al(OH)6(Cl,OH)2H2O: Its solid solutions and their

role in chloride binding. Cement and Concrete Research,

28:1713-1723.

[6] Yousuf M, Mollah A, Lu F, Cocke DL. (1998), An X-

ray Diffraction (XRD) and Fourier Transform Infrared

Spectroscopic(FT-IR) Characterization of the Speciation

of Arsenic (V) in Portland cementTtype-V. The Science

of the Total Environment, 224:57-68.

Figure 2. FTIR of matrices after 7days of hydration

The Al-OH bending vibration of pure Friedel’s salt occurred

at 780 cm-1 and 601 cm-1[5]. For sludge-fly ash matrices,

however, there was no trace of Al-OH bending vibration

besides the vibration of AlO6 octahedron at 601 cm-1. In

addition, a new Al-OH bending vibration at 670 cm-1 was

detected [5]. According to the results of Mollah’s work [6],

this shift of wave is related to the replacement of bivalent

heavy metals for cations in the structure of ettringite and

Friedel’s salt phases resulting in the changes of the Al(OH)6

octahedra.

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Occurrence of “G-Bacteria” in Aerobic GranulesA. M. Maszenan ([email protected])

J. H. Tay ([email protected])

Stephen T. L. Tay ([email protected])

Introduction

Microbial granulation is an attractive novel process for

biological wastewater treatment, where the microbial cells

are not freely suspended in the bulk liquid as in conventional

activated sludge systems but instead are organized into highly

organized structure known as granules. These granules are

capable of removing biodegradable organic matter, nitrogen

and phosphorus and toxic pollutants and can be fully formed

in sequential batch reactors within three weeks of start-up

(Tay et al., 2002). These aerobic granules also have several

advantages over conventional activated sludge systems,

including a strong and compact microbial structure, good

settling ability, high biomass retention and the ability to

accept a high organic loading rate (Moy et al., 2002).

Furthermore, aerobic granules have demonstrated high toxic

tolerant to phenol (Jiang et al., 2002). Studies conducted so

far have centred on operational conditions on the formation

of aerobic granules, and microbiological studies employing

culture independent techniques such as clone library revealed

a large diversity of bacterial population in aerobic granules

at different growth stages (Yi et al., 2002).

Recently in our routine microscopic examination of aerobic

granules microbial population, cocci with a distinctive tetrad

arrangement was observed (Figure 1a). Some of these “G-

Bacteria” have also been observed to form clusters (Figure

1b). Cocci with a distinctive tetrad arrangement were first

observed by Cech & Hartman (1990) in an alternating

aerobic-anaerobic sequencing batch reactor fed with glucose

as the sole carbon substrate, and they called them “G-

Bacteria”. Pure culture studies based on extensive isolation

and characterization have revealed that “G-Bacteria” is very

diverse with several new genus being described (Maszenan,

2000). In this study, a similar approach was employed to

study the “G-Bacteria” in aerobic granules so that their

possible roles in aerobic granules could be elucidated.

Methods Isolation and cultivation

Aerobic granules appeared in the reactor after 2 weeks of

operation with an organic loading of 6 g COD/L- day,

operated at a cycle sequencing time of 4 h, i.e. 6 cycles per

day at an aeration rate of 2.5 L/min. Aerobic granules were

gently disrupted with sterile glass bead for 15 minutes.

Isolates of cocci in tetrad arrangement or cluster were

obtained by micromanipulation technique using a Skerman

micromanipulator on glucose sulphide agar, and incubated

at 25οC for a week. The purity of cultures that grew was

checked microscopically and only those fitting the

morphological description of “G-bacteria” were recovered

and stored in GS medium in 15% glycerol at -80οC.

Figure 1. Morphology of “G-bacteria” obtained from aerobic

granules sludge biomass from a laboratory-scale SBR.

(a) Light micrograph of tetrad and cluster cells in granular

sludge biomass (b) Scanning electron micrograph of strain

GR1-019 showing cocci and cluster arrangement.

Scale bar in (a) represent 5 µm and in (b) represent 3 µm

Biochemical and Enzymes characterization

Substrate utilization capabilities for isolates were obtained

with both GN and GP Biolog microplate systems and API

20E (BioMerieux). Enzyme profiles were determined using

API ZYM strips (BioMerieux) following the manufacturer’s

instructions. Gram stain was performed according to the

modified Hucker method and confirmed with an alternative

method employing 3% KOH. Electron microscopic

examination was performed as detailed in Maszenan (2000).

Phylogenetic determination

The almost complete 16S rRNA gene of isolates were

obtained using the ABI model sequencer (Applied

Biosystems, Perkin-Elmer) and ABI PRISM BigDyeTM

Terminator Cycle sequencing Chemistry (Applied

Biosystems, Perkin-Elmer), and assembled with BioEdit

version 5.10. Phylogenetic analysis performed using several

methods described in the PHYLIP package including

DNAdist and neighbour-joining software. Trees were

generated using TREECON after bootstrap and transversion

analysis.

Results and discussion

Based on 16S rDNA sequencing, a large proportion of the

isolates obtained belonged to the genus Micrococcus, which

is a Gram-positive bacteria with High G+C content (Table

1), and none of the “G-Bacteria” belonging to the genera

Amaricoccus, Tessaracoccus, Friedmanniella (Maszenan,

2000) which were previously isolated from activated sludge

systems were isolated in aerobic granules.

Interestingly, a Gram-positive isolate designated strain GR1-

015 was isolated in aerobic granules and found to be closely

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affiliated to the genus Tetrasphaera. “G-Bacteria” belonging

to the genus Tetrasphaera was isolated from an activated

sludge system designed for enhanced biological phosphorus

removal, and has been show to accumulate polyp in pure

culture studies. Another isolate of Gram-positive bacteria

isolated in this study is strain GR1-019 which was found to

be closely related to the genus Frankia (Figure 2). Strain

GR1-019 also stains positive for polyp which suggests it

too can accumulate polyp as storage polymers. In addition,

strain GR1-019T also stains positive for capsule stain with

the Indian ink method. The ability to synthesize storage

polymers such polyp and glycogen in the feast-famine cycle

of alternating aerobic-anaerobic reactor is a trait which

ensure “G-bacteria” can compete with other bacterial

populations in activated sludge systems. Furthermore capsule

and exopolysaccharide protect “G-Bacteria” from predation

and also enhances bacterial adhesion which ensures “G-

Bacteria” are retained in the system used to cultivate aerobic

granules.

References

[1] Cech J. S. and Hartman P. (1990). Glucose induced

breakdown of enhanced biological phosphate removal.

Environmental Technology 11, 651-656.

[2] Jiang H.-L., Tay J.-H. and Tay S. T.-L. (2002).

Table 1. Taxonomic affiliation of “G-Bacteria” isolate

obtained from aerobic granules.

Taxonomic Number Strain

affiliation of isolates number

Gram-positive Micrococcus 28

bacteria Arthrobacter 1 GR1-007

Kytococcus 1 GR1-001

Frankia 2 GR1-019,

GR1-58

Tetrasphaera 1 GR1-015

Endocytic

bacterium 1 GR1-013

Gram-negative Haloanella 1 GR1-022

bacteria Flavobacterium 1 GR1-045

γ-Proteobacteria 1 GR1-056

Moraxella 1 GR1-051Figure 2. Phylogenetic tree based on analysis of 16S rRNA

gene sequences of strain GR1-019 and representatives of high

G+C bacteria. All sequences used in analysis were obtained

from GeneBank. Bootstrap values, expressed as a percentage

of 100 replications are shown at the branching points.

Scale bar indicates 10 substitutions per 100 nucleotides.

Aggregation of immobilized activated sludge cells into

aerobically grown microbial granules for the aerobic

biodegradation of phenol. Letters in Applied

Microbiology 35, 439-445.

[3] Maszenan A. M. (2000). The occurrence,

characterization and biodiversity of “G-Bacteria” in

activated sludge systems. Ph D thesis. La Trobe

University, Australia.

[4] Moy B. Y.-P. Tay J.-H., Toh S.-K. Liu Y. and Tay S. T.-

L. (2002). High organic loading influences the physical

characteristic of aerobic sludge granules. Letters in

Applied Microbiology 34, 407-412.

[5] Tay S. T.-L., Ivanov V., Yi S., Zhuang W.-Q. and Tay J.-

H. (2002). Presence of Anaerobic Bacteriodes in

aerobically grown microbial granules. Microbial Ecology

44, 278-285.

[6] Yi S., Tay J.-H., Maszenan A. M. and Tay S. T.-L. (2002).

A culture-independent approach for studying microbial

diversity in aerobic granules. Water Science and

Technology 47, 283-290.

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Distribution of EPS and Cell SurfaceHydrophobicity in Aerobic Granules

Wang Zhiwu ([email protected])Liu Yu ([email protected])

Tay Joo-Hwa ([email protected])

Introduction

Aerobic granulation as a promising biotechnology forwastewater treatment has attracted intensive researchattention. Most research on aerobic granulation has beenfocused on the cultivation and physical characteristics ofaerobic granules. So far, the structure of anaerobic granuleshas been studied extensively, and extracellularpolysaccharides (EPS) have been found to contribute to thebuild-up of the matrix structure and the stability of anaerobicgranules (Liu et al., 2004). The failure of microbialaggregation has been found to be correlated to the metabolicblocking of EPS synthesis, while EPS deficiency would resultin a weak structure of anaerobic granules. Compared toanaerobic granules, little is currently known about the internalstructure of aerobic granules as well as the EPS and cellhydrophobicity distribution in aerobic granules. Thus, thisstudy investigated the distribution of EPS and hydrophobicityin aerobic granules as well as the essential role of EPS inaerobic granulation.

Materials and methods

Aerobic granules were cultivated in a sequencing batchreactor (SBR) and were fed with sodium acetate as the solecarbon source. EPS were extracted from the ground granulesaccording to the cold aqueous technique, and the extractedEPS were analyzed using calorimetric method. In this study,the sliced aerobic granule was stained with 300 mg L-1calcofluor white (Fluorescent Brightener 28, Sigma) for 1hour in 20 mL Phosphate Buffered Saline. The samples werethen rinsed with 2 mL of Milli-Q water to decrease thebackground fluorescence and were visualized withEpifluorescence Microscope BX-FLA-3 (Olympus, Japan).In addition, intact and sectioned aerobic granules werevisualized by Image Analyzer (IA) (Olympus ImagingAnalyzing System SZX9, Japan). Hydrophobicity of theaerobic granule was determined and expressed as thepercentage of cells adhering to the hexadecane after 15 minpartitioning.

Results

The heterogeneous structure of aerobic granules

Figure 1 shows that the aerobic granule has an opaque outerlayer with a depth of about 800 (m from the granule surfaceand a relatively transparent inner core. It appears that thehighly transparent center part of the granule was not void,and was instead filled with jelly-like substances. Five aerobic

Figure 1. Cross section view of the aerobic granule in brightfield (A) and dark field (B) visualization modes, Bar: 500 µm

granules were sliced and the respective density of the outerand the inner layer was then measured. Results indicate thatthe density of the outer layer of granule was higher than thatof the core part of the aerobic granule. Hence, the core partof granule was composed of transparent and jelly-likesubstances, while the outer shell of the granule mainlyconsisted of dense materials leading to an opaque structure.

EPS and hydrophobicity distribution in aerobic granule

The outer shell of the aerobic granule was separated fromthe core part and the corresponding EPS content as well ashydrophobicity was determined. Figure 2 shows that the EPScontent in the core part of the granule is nearly 5 timeshigher than that in the shell part of aerobic granule. To localizethe EPS distribution, the sliced aerobic granule was stainedby calcofluor white, and was then visualized by epifluorescentmicroscopy. Using a fresh granule as the reference (Figure3), it was found that the fluorescent dye was mainly attachedto the outer shell of the granule, while the fluorescence wasvery weak in the center of the granule. The fluorescenceintensity profile in the direction of the granule radius furthershows that most calcofluor white stained EPS are situated inthe outer shell of the granule with a depth of 400 (m belowthe granule surface. This may imply that the (-linked EPSwould be mainly located in the outer shell of the granule. In

Figure 2. Hydrophobicity (black) and EPS (gray) distributionin the shell and core part of the aerobic granule

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fact, calcofluor white has been commonly used to label the(β-linked EPS (deBeer et al., 1996). In addition, Figure 2shows the respective hydrophobicity of the shell and coreof aerobic granule. It seems that the granule outer shell hasa much higher hydrophobicity than the granule core.

Discussion

Figure 1 shows that aerobic granules have a heterogeneousstructure consisting of a dense outer shell and a loose-structure core, while the outer shell of the aerobic granuleis mainly composed of (-linked EPS which are morehydrophobic (Figures 2 and 3). According to their physico-chemical properties, EPS can be classified into bound EPSand soluble EPS, while generally, only soluble EPS areconsidered to be biodegradable (Laspidou and Rittmann,2002). This may imply that the EPS detected in the core ofaerobic granule would be easily biodegradable.

Previous research showed that substrate could only penetrateto a depth of 800 (m below the aerobic granule surface(Tay et al., 2003). This may indicate that a nutrient deficientsituation would be encountered in the core part of aerobicgranules, as shown in Figure 1. There is evidence that thenutrient deficiency condition would induce the productionof soluble EPS. It has been reported that compared to abalanced culture, 10 times more soluble EPS was secretedin nutrient deficiency culture (Aquino and Stuckey, 2003).This provides a plausible explanation for the enhanced

Figure 3. Cross section view of aerobic granules, A: freshgranule; B: granule stained by calcofluor white, Bar: 100 µm

production of soluble EPS in the granule core (Figure 2).It is a reasonable consideration that soluble EPS would beless important in constructing and maintaining the structuralstability of the aerobic granule, i.e. EPS property anddistribution in aerobic granules instead of their quantitywould play a crucial role in improving the stability ofaerobic granules. In addition, it appears from Figures 2 and3 that the high hydrophobicity of the outer shell of aerobicgranule result from the insoluble (-linked polysaccharidesaccumulated in the shell. In fact, insoluble (-linked EPShas been found to serve as the backbone of the biofilmstructure.

References

[1] Aquino S.F., Stuckey D.C. (2003) Production of solublemicrobial products (SMP) in anaerobic chemostatsunder nutrient deficiency. J Environ Eng-Asce 129:1007-1014.

[2] deBeer D., OFlaharty V., Thaveesri J., Lens P.,Verstraete W. (1996) Distribution of extracellularpolysaccharides and flotation of anaerobic sludge. ApplMicrobiol Biotechnol 46: 197-201.

[3] Laspidou C.S., Rittmann B.E. (2002) A unified theoryfor extracellular polymeric substances, solublemicrobial products, and active and inert biomass. Water.Res 36: 2711-2720.

[4] Liu Y.Q., Liu Y., Tay J.H. (2004) The effects ofextracellular polymeric substances on the formation andstability of biogranules. Appl Microbiol Biotechnol 65:143-148.

[5] Tay J.H., Tay S.T.L., Ivanov V., Pan S., Jiang H.L.,Liu Q.S. (2003) Biomass and porosity profiles inmicrobial granules used for aerobic wastewatertreatment. Lett Appl Microbiol 36: 297-301.

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Size-Dependent Diffusion Limitationin Aerobic Granules

Li Yong ([email protected])Liu Yu ([email protected])

Introduction

Aerobic granulation technology has attracted intensiveresearch attention in recent years. Aerobic granules areaggregates of self-immobilized bacteria with a size up toseveral millimeters. Previous research looked mainly intothe effect of operating conditions on aerobic granulation insequencing batch reactor (SBR), as reviewed by Liu and Tay2004. However, to date little information is known aboutdiffusions of substances in aerobic granules. This study wasaimed at simultaneously modeling diffusion of both dissolvedoxygen and substrate in aerobic granules, while alsopredicting the change in the bulk substrate concentration. Inorder to optimize the design and operation of the aerobicgranulation process, a sound understanding of mass transferin granules is essential. Thus, this study is expected to offeruseful information on the optimization of aerobic granularsludge SBR.


Mature aerobic granules were sorted into different sizes usingthe wet-sieving method. The sorted aerobic granules weretransferred to beakers with a working volume of 800 mL.Synthetic wastewater was used in the batch experiments.Initial granule and substrate concentrations were maintainedat 6250±200 mg volatile solids (VS)/L and 465 mg COD/L,respectively. The experiments were conducted at 25°C, anddissolved oxygen was kept at saturation level. Samples wereanalyzed for COD and VS by standard methods and dissolvedoxygen was monitored by a dissolved oxygen meter (ModelYSI 5000).


Models development

Mature aerobic granules are considered to have an equilibriumor stable size when growth and detachment forces arebalanced (Liu and Tay 2002). To develop one-dimensionalmodels for aerobic granules, it is assumed that (i) an aerobicgranule is isotopic in physical, chemical and biologicalproperties, such as density, diffusion coefficient; (ii) anaerobic granule is ideally spherical; (iii) no nitrification andanaerobic degradation occur in the process; and (iv) aerobicgranules response to the change in bulk substrateconcentration so quickly that the response time can beignored. The mass balance equations between the two layerswhose radiuses are respectively, r and r+dr can be writtenas:

(1)

in which v is the substrate conversion rate, s is substrateconcentration and Ds is the diffusion coefficient. In fact, thesubstrate conversion rate can be expressed by the Monodequation:

(2)

in which ρx is biomass density and µ and µmax are the specificgrowth rate and the maximum specific growth raterespectively. Ks is the half constant and s is the substrateconcentration Yx/s is the growth yield. Substitution of Eq. 2into Eq. 1 gives Eq. 3:

(3)

The derivative at the center of granule is zero and theconcentration of substrate at the surface of the granule isequal to its bulk solution.

As Eq. 3 is a non-homogenous equation, in this study wedeveloped a numerical method to completely solve Eq. 3based on the Finite Difference Method (FDM). For thispurpose, the radius is thus divided into n grids, i.e.

(4)

This numerical scheme is applied to all situations withoutmaking any assumption and therefore the accuracy issignificantly increased. Dissolved oxygen is an essentialelement for the growth of the aerobic granule. It should bepointed out that Eq. 3 indeed can also be applied to oxygenif the set of parameters for substrate are replaced with theset of parameters for oxygen.

After the substrate concentration is determined, the substrateutilization rate (v1) of a single aerobic granule can becalculated as follows:

(5)

Summation of the substrate utilization rates of all individualaerobic granules gives the total substrate utilization rate (vall):

(6)

in which V is reactor volume. At time dt, the change insubstrate concentration in the reactor can be described as

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(7)

At an initial substrate concentration, Sbulk0, the bulk substrateconcentration at any time t can be calculated as:

(8)

As discussed earlier, Eq. 3 can not be solved analytically,and the method based on the finite difference principle isthus applied to solve this equation in the present study. Ateach time step, the state is considered pseudo-static, whichmeans the bulk substrate concentration is constant at eachtime step. Then the change in the bulk substrate concentrationis a process of mapping, i.e. the substrate concentration isdetermined by the previous time step. This model is validfor different substrates so long as the parameters are replacedwith the specific substrate parameters. In this study, Eq. 3was applied to organic substrate and dissolved oxygen undervarious operating conditions.

Diffusion of acetate and oxygen in different-size aerobic granules

The initial acetate concentration and granule concentrationwere respectively kept at 465 mg COD/L and 6250 mgVS/L, and the initial DO was maintained at 8.3 mg/L. AsFigure 1a shows, for aerobic granule with a radius of 0.1mm, both oxygen and acetate can diffuse across the entireaerobic granule, while for aerobic granules with a radius of0.3 mm, diffusion limitation appears (Figure 1b). For aerobicgranules with a radius larger than 0.5 mm, oxygen limitationbecomes obvious against the acetate diffusion. These resultsshow that the penetration depth of DO tends to decreasefrom 280 to about 100 ?m as the granule radius increasesfrom 0.5 to 1.0 mm. After the DO concentration drops tozero, the aerobic activity in granule is seriously suppressed,and this eventually leads to a flat acetate diffusion profile.In large-size aerobic granules acetate is not a limiting factor,i.e. microbial growth or activity would be determined by theavailability of

DO in the aerobic granule.

It should be realized that most previous research on aerobicgranulation investigated the profiles of DO and substrateseparately. The oxygen profile inside an aerobic granule wasnot linked to substrate utilization. As shown in Figure 1,diffusion of substrate and oxygen in aerobic granule is adynamic process, and is interrelated one another. Theintegrated approach as developed in this study may offer amore reasonable and powerful tool for the study of masstransfer of various substances in aerobic granule.

Profile bulk substrate concentration in aerobic granularsludge reactor

It is possible to simulate the bulk substrate concentration in

Figure 1. Profiles of substrate (dash) and oxygen (solid) inaerobic granules with various radiuses (a) radius of 0.1mm

(b) radius of 0.3 mm (c) radius of 0.5 mm (d) radius of 1.0mm.

(a)

(b)

(c)

(d)

the reactor during an operation cycle. According to Eq. 8,the substrate conversion rate is proportional to biomassconcentration and is inversely related to the yield coefficientat a given granule radius. Figure 2 shows the bulk substrate

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controlled in order to maximize their metabolic activity. Thebreakpoints in Figure 2 indeed represent the turn from oxygenlimitation to substrate limitation. Under the condition ofoxygen limitation, the substrate removal rate is mainlydetermined by the availability of dissolved oxygen, i.e. thesubstrate removal is independent of the bulk substrateconcentration. This may explain why a pseudo zero-orderreaction kinetics is observed for substrate in Figure 2. Suchan observation is consistent with experimental results reportedby Yang et al. (2004). However, under the substrate limitationcondition, Figure 2 shows that the substrate removal tendsto decrease with the decrease of the substrate concentration,i.e. it is a function of the substrate concentration. It is obviousthat the performance of aerobic granular sludge SBR ismainly controlled by the availability of dissolved oxygen.

Conclusions

A one-dimensional model for aerobic granules was developedand was successfully applied to aerobic granular sludge SBR.The simulation model was completely solved without addingin any extra assumption in numerical analysis. This allowsto synchronously simulate diffusion of acetate and dissolvedoxygen under various conditions. It was shown that diffusionof acetate and oxygen in aerobic granule is a dynamic process,and is interrelated. Simulation on the overall performance ofaerobic granular sludge SBR further indicated that dissolvedoxygen would be a main factor that limits metabolic activityof aerobic granule, i.e. smaller aerobic granules exhibitedhigher metabolic activity in terms of the substrate removalrate. It becomes clear that in aerobic granules larger than 0.5mm, the dissolved oxygen would be the bottleneck whichlimits the substrate utilization rate. It is expected that theproposed model can offer an effective and useful tool forpredicting and optimizing the performance of an aerobicgranular sludge reactor.

References

[1] Liu, Y., Tay, J.H., 2002. The essential role ofhydrodynamic shear force in the formation of biofilmand granular sludge. Water Res. 36, 1653-1665.

[2] Tay, J.H., Liu, Q.S., Liu, Y., 2002.Characteristics ofaerobic granules grown on glucose and acetate insequential aerobic sludge blanket reactors. Environ.Technol. 23, 931-936.

[3] Yang, S.F., Liu, Q.S., Tay, J.H., Liu, Y., 2004. Growthkinetics of aerobic granules developed in sequencingbatch reactors. Lett. Appl. Microbiol. 38, 106-112.

(a)

(b)

Figure 2. Substrate-time profiles in aerobic granular sludgereactors with different granules sizes. -: simulation; ∆:

experimental data (a) radius of 0.5mm (b) radius of 1.0mm.

concentration profiles in the reactors with various-size aerobicgranules.

In addition, two sets of experiments were also conductedusing aerobic granules with different radiuses (Figure 2),and experimental conditions were designed to be similar tothe simulation conditions, i.e. the initial substrate CODconcentration was 465 mg/L and the dissolved oxygen waskept at 8.3 mg/L, while the granule concentration wasmaintained at 6250 mg VS/L. It can be seen in Figure 2 thatsimulation is in good agreement with the experimental data.

Figure 2 shows that under the similar operation conditions,acetate can be removed fast in the reactor with small aerobicgranules, e.g. the substrate removal rate by 0.5 mm granules(Figure 2a) is 3 times higher than that by 1.0 mm granules(Figure 2b). These seem to imply that the reactor with small-size aerobic granules would be more efficient and have ahigher treatment capacity than the reactor with large-sizegranules, i.e. the size of aerobic granules should be properly

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Biological Treatment of Municipal Wastewater byUpflow Anaerobic Sludge Blanket Process

Lua Choon Hau ([email protected])

Tay Joo Hwa ([email protected])

Show Kuan Yeow ([email protected])

Introduction

Upflow anaerobic sludge blanket (UASB) process characterized

by low energy requirement, low sludge production, high loading

capacity along with green energy yield in the form of biogas, is

an attractive technology for wastewater treatment. However, its

use is usually limited to high strength wastewater with soluble

substrates. One of the challenges for UASB process is its

applicability to lower-strength wastewaters, i.e., sewage

and domestic wastewater. In this study, the application of UASB

process for municipal wastewater treatment was examined.

Reactor setup and operation

Four lab-scale UASB reactors namely R1, R2, R3, and R4 were

used in the study. The reactors were operated at different hydraulic

retention times (HRTs) of 24, 18, 12 and 6 hours corresponding

to average organic loading rates (OLRs) of 0.28, 0.37, 0.56 and

1.12 kg COD/m3/d.


Experimental results showed that there was a fluctuation in effluent

COD concentrations at each HRT applied during the first 3 weeks

of operation, which was likely due to the sensitive startup period,

and an unexpected high influent COD of 664 mg/L on day 9 fed

to the system (Figure 1a). However, a relatively stable effluent

COD of 49-85 mg/L was attained after startup, resulting in a 63-

78% removal. A gradual improvement in COD removal could

also be observed from day 27 in all reactors (Figure 1b). Figure

1 shows that R1 with a HRT of 24 hrs had the lowest effluent

COD and the highest removal efficiency, while R4 with a HRT

of 6 hours had the highest effluent COD and the lowest removal

efficiency. This is expected because longer liquid residence time

would lead to more complete biodegradation of substrate.

Figure 2 shows the effluent suspended solids concentration and

its removal efficiency. A relatively stable effluent suspended solids

concentration could be obtained in all the reactors even as early

as two weeks after the startup. An average effluent SS

concentration of 26, 28, 29 and 36 mg/L was achieved for reactors

R1 to R4 respectively, at steady-state conditions. The effluent

suspended solids concentration was strongly related to HRT

applied which would influence the washout of biosolids. The

highest liquid upflow velocity at 0.093 m/h with a HRT of 6

hours was relatively low as compared with many full-scale UASB

systems treating industrial wastewater. Low liquid upflow velocity

could provide beneficial effects on removal of suspended solids.

These results indicate that UASB reactor has an excellent

capability for removal of suspended solids.

Figure 1. COD concentration (a) and removal efficiency (b)

Figure 2. Suspended solids concentrations (a)

and removal efficiencies (b)

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removal efficiency of 63% and SS removal efficiency of 63%

were obtained in the reactor with HRT of 6 hours. However,

comparing the difference in COD and SS removal between 6 and

24 hours HRT, it is obvious that the improvement is not significant.

As discussed earlier, an effluent COD of well below 100 mg/L

and SS of less than 50 mg/L was achieved in the reactor with a

HRT of 6 hours. This suggests that municipal wastewater can be

treated by UASB operated at HRTs as low as 6 hours, with

treated effluent conforming to the discharge standards.

Conclusion

The results demonstrate that UASB is a feasible treatment process

for low-strength municipal wastewater. The UASB reactors

exhibited capability for excellent suspended solids and COD

removal conforming to the discharge requirements.

Figure 3. Effect of HRT on COD and SS removal efficiencies

Treatment of Lipid-Containing Food Waste inHybrid Anaerobic Solid-Liquid (HASL) System

Liu Xue-Yan ([email protected])

Olena Stabnikova ([email protected])

Wang Jing-Yuan ([email protected])

Introduction

The hybrid anaerobic solid-liquid (HASL) system, which was

developed as a modified two-phase anaerobic digester, proved

to be highly efficient for the treatment of solid food waste [1,

2]. The efficiency of anaerobic digestion depends on the

composition of treated matter. Lipids (characterized as oil, grease

and fat), which are present in high quantity in Asian food, are

potential inhibitors of methane production [3]. It is well known

that fat and vegetable oil are biodegraded to glycerol and long-

chain fatty acids (LCFA), followed by subsequent β–oxidation

of LCFA. LCFA are slowly degraded under anaerobic conditions

and the millimolar concentration can inhibit the growth of

anaerobic microorganisms, including acetogenic bacteria and

methanogens. However, the energy value of lipids is twice as

high as that of carbohydrates. Therefore, theoretically, the

presence of lipids in anaerobically treated matter can increase

the biogas production.


The lab-scale HASL system was operated in a constant

temperature room at 35±1°C. The anaerobic granules, adapted to

high concentration of volatile fatty acids (VFA), were used as the

inoculum for the methanogenic reactor. Food waste was collected

from a canteen at NTU and was shredded into particles with an

average size of 6 mm in a Robot-Coupe Shredder CL50 (Ultra,

Hobart, France). Lipids were prepared by melting the internal fat

of a hog followed by solidifying them at room temperature. 800g

shredded food waste was placed in each acidogenic reactor. Then

lipids were added to these four reactors as follows, % (w/w of

total solids in food waste): 0, 20, 30 and 40 in control and

experiments E1, E2 and E3, respectively.

Chemical oxygen demand (COD) was determined in the well-

mixed samples in triplicates by standard methods. VFA were

analyzed using HPLC (Perkin Elmer, Series 200, Norwalk, CT,

USA). Biogas production was monitored by a wet gas meter (Ritter

TG05, Bochum, Germany). Enumeration of autofluorescent

methanogens was performed by epifluorescence microscope BX-

FLA-3 (Olympus, Tokyo, Japan).


The influence of different lipid contents on the VFA and COD

production in the acidogenic reactors was shown in Figure 1.

The maximum VFA concentrations were 1724, 1610, 1597 and

1449 mg/L in control, E1, E2 and E3, respectively (Figure 1a).

The production of VFA in the acidogenic reactors with 0, 20, 30

and 40% of lipids, were 7.1, 7.3, 7.4, and 6.4 g, respectively,

during 14 days of operation. Change of COD concentrations in

the effluent from the acidogenic reactors showed a similar trend

(Figure 1b). Maxima were reached on 3rd and then decreased

consistently to the end of food waste treatment.

The biogas production in control and experiments was shown in

Figure 2.

A comparison of COD and SS removal is presented in Figure 3.

It can be seen that 78% COD and 73% SS removal efficiencies

were achieved in the reactor with a HRT of 24 hours. Both COD

and SS removal efficiencies decreased with HRT, and a COD

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Figure 1. Change of VFA concentrations (a)

and COD concentrations (b) in the effluent from the

acidogenic reactors in control and experiments.

The total biogas production increased with addition of 20 – 30%

of lipids, but further increase of lipid content in food waste

diminished its positive effect on anaerobic digestion. This was

probably due to a higher biogas production potential of lipids

compared to carbohydrates when lipid content was low (20 –

30%), but the higher lipid content (40%) inhibited methanogenic

activity.

The concentrations of methanogens in the methanogenic reactor

were estimated by direct count under epifluorescent microscope

and shown in Figures 3 and 4.

The presence of lipids in food waste enhanced the energetical

value of effluent from acidogenic reactor causing an increase of

growth of autofluorescent methanogens. However, the content of

lipids higher than 20% inhibited bacterial growth. The growth

curves of methanogens for E2 with the lipid content of 30% did

not differ much from ones for E1 with the lipids content of 20%,

but for E3 with lipid content of 40% the growth curves were the

same as those for control.

Conclusions

The HASL system can be used for the treatment of food waste

containing 20 – 40% of lipids. The presence of lipids in food

waste had two main effects on anaerobic digestion of food waste

in the HASL system: (1) enhanced production of biogas and (2)

inhibited the growth of methanogens. The first effect was prevalent

when the lipid content was 20%. It was balanced by the second

effect when the lipid content was 30%. The negative effect

dominated when the lipid content was 40%.

A lipid content of 40% diminished the VFA production in the

acidogenic reactor as well as biogas production and the

concentrations of methanogens in the methanogenic reactor.

References

[1] Wang JY, Xu HL, and Tay JH. (2002). A hybrid two-phase

system for anaerobic digestion of food waste. Wat Sci

Technol 45:159–165

[2] Wang JY, Zhang H, Stabnikova O, Ang SS, and Tay JH.

(2005). A hybrid anaerobic solid-liquid (HASL) system for

food waste digestion. Wat Sci Technol 52: 223–228.

[3] Ivanov VN, Stabnikova EV, Stabnikov VP, Kim IS, and

Zuber A. (2002). Effects of iron compounds on the treatment

of fat-containing wastewaters. Appl Biochem Microbiol

38:255–258.

(a)

Figure 2. Biogas production in control and experiments

Figure 3. Microphotograph of methanogens

in the methanogenic reactor.

Figure 4. Concentrations of autofluorescent

methanogens in the methanogenic reactor.

(b)

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Characterisation of Fouling onSpent Ultrafiltration Membrane Surface

from a Commercial Membrane BioreactorWilliam Phay ([email protected])

Dickson Lai ([email protected])Harry Seah ([email protected])Darren Delai Sun ([email protected])James O Leckie ([email protected])

Introduction

In recent years, the biological treatment of wastewaters has

seen some important advancement in the way of membrane

bioreactor (MBR), which is a combination process involving

biological processes and filtration membrane separation.

However, the membranes in MBR systems are in constant

direct contact with the harsh raw wastewater in the biological

reactor. The biological suspension of activated sludge

contains biological flocs formed by the conglomeration of

microorganisms and a whole range of soluble, insoluble and

colloidal compounds. As a result, the MBR membrane is

susceptible to solids accumulation and fouling that cause the

flux to decline or the pressure required to maintain a given

flux to increase. This study characterizes the spent

ultrafiltration membranes from the commercial MBR system

at PUB Chestnut Wastewater Treatment Plant, particularly

to examine the surface deposition after 9 months’ contact

with biomass in tropical environmental conditions with

monthly regular chemical cleaning.

Materials and Methodology

All analytical reagents used were obtained from Aldrich

Chemical Company (USA). The ultrafiltration (UF)

membrane used in this commercial MBR study was kindly

supplied by Kubota, Japan. The membrane plate was removed

from the biological activated sludge tank for regular NaOCl

cleaning each month. Before putting it back into the biological

activated sludge, the membrane plate was carefully washed

using clean water to remove free chlorine. A Jeol JSM-

5310LV SEM with energy dispersive X-ray (EDX)

spectroscopy was employed to study the surface morphology.

The samples were gold coated prior to SEM analysis. A

Perkin Elmer UV/Vis spectrometer (Lambda Bio20) was used

to measure the spectra from standard humic acid solutions at

pH 4,7 and 9 and to compare the extracted solution from the

membrane and fiber layers.

Results and Discussion

Figure 1 shows a fouled membrane surface before NaOCltreatment. It can be seen that the UF membrane surface hasan attachment and growth layer. There are generally twokinds of fouling: surface fouling and pore fouling. Porefouling involves the blocking of the membrane pores by

soluble organic and inorganic compounds, andmicroorganisms such as extracellular polymeric substances(EPS) and soluble microbiological products (SMP). Figure 2reveals that monthly regular cleaning is effective in removingthe deposits on the surface of UF membranes (surfacefouling), even though the membrane has been used for 9months. However, close examination of the membrane surfaceindicates a fouling problem, which was observed as darkpatches of brownish material with occasional black patchesin certain localized regions.

Figure 1. Fouled flat sheet membrane before chemical cleaning

Figure 2. Fouled flat sheet membrane after chemical cleaning

Figure 3. Flat-plate UF membrane assembly configuration

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In order to investigate the nature of this fouling problem,investigations using SEM-EDX and UV visible spectrometertechniques were carried out. The UF membrane element, asshown in Figures 1 and 2, is a flat-plate consisting of strongpolymeric UF membrane cast on the outside surface of aporous flat support medium. The flat surface is furthermounted on a flat plastic plate with a spacer material. Figure3 shows the configuration of the flat-plate membrane unit.

Close visual examination of both membrane and fiber layersreveal that the fouling on the membrane’s surface consistsmostly of a light brownish colored material. However, thedarker black patches that were originally observed beforethe dissection of the membrane (Figure 2) could be moreclearly linked to the organic fiber layers found below themembrane surface.

SEM analysis of both the fouled and unfouled regions of theorganic fiber layers were carried out for comparison. Figure4a presents the SEM picture of an unfouled layer of theorganic fiber layer. It can be seen that the organic fiberlayer consists of an intricate weaving of individual fibers.Each individual fiber making up the fiber layer can be easilydistinguished under the SEM and it can be seen that they arerelatively clean. A clear SEM picture of a fouled area of theorganic fiber layer is illustrated in Figure 4b. In this case,the SEM picture clearly shows debris has accumulated insidethe organic fiber support layer.

Figure 4. SEM pictures of (a) unfouled fiber layer and(b) fouled fiber layer

Figure 5. EDX analysis of debris inside the fouled fiber layer

Further investigations to determine the chemical compositionof the debris was carried out using the energy dispersive x-ray (EDX) technology. It was found that the debris materialconsisted of a mixture of different inorganic constituents,which included aluminium, silicon, sulfur, chlorine, calcium,titanium and iron. The details are illustrated in Figure 5.

Although we were unable to evaluate the mineral structure

of the inorganic compounds using XRD, it is believed thatthese inorganic elements will be in the form of salts,precipitate or metal oxides. The sulfur might possibly bepresent in the sulfate form. If this is true, gypsum may bepresent. Of course, other organic materials such as humicacids and bacteria may be present. It is most likely that thebacteria will make use of these inorganic and organicmaterials as nutrients for their growth, hence leading tobiofouling.

Investigation was further carried out on the membranesurfaces where fouling occurred. The SEM pictures showingthe difference between the unfouled and the fouled membranesurfaces are shown in Figures 6a and 6b. From the SEMpictures, it appears that the membrane was constructedthrough a multi-weaving network of fibers to form amembrane with filtration capabilities. Even after themembrane was used for 9 months and was chemically washedmonthly, it can be seen that the membrane’s surface werecoated/spiked with a foulant material.

Figure 7. EDX pattern of clean and fouled membrane surface

Figure 6. UF membrane showing (a) unfouled and(b) fouled surfaces

Additional analysis of the foulant was carried out using EDX.An EDX analysis was performed on the clean membrane’ssurface as well as the fouled membrane’s surface. The EDXresults are illustrated in Figure 7. The EDX pattern obtainedfrom the clean membrane’s surface showed that the onlypossible inorganic element present was chlorine. This wasexpected prior to analysis, since the membrane was washed

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using NaOCl to remove the organic, inorganic and biologicaldeposits. Hence, some of the chlorine may have reactedwith the membrane’s surface. However, in the case of thefouled membrane’s surface, although chlorine was alsodetected, traces of calcium were also detected as shown inFigure 7. It appears that the fouling on the membrane’ssurface is also related to calcium deposits being present.

The brownish coloration found on both the membrane’ssurfaces and fiber layers in water treatment processes areusually associated with humic substances deposits (Fang et al2004; Thomas, et al, 2000). In order to evaluate the presenceof humic substances on the fouling membrane surface andfiber, a Perkin Elmer UV/Vis spectrometer (Lambda Bio20)was used to measure the spectrum from standard humicsubstances solutions at pH 4, 7 and 9 and compared to theextracted solution from the membrane and fiber layers. Thesamples were extracted from the membrane’s surface and thefiber layers using a NaOH solution. The extraction fluid wasthen input in the UV spectrometer for analysis. Similardeclining profiles of the absorption curve for both standardhumic substances solutions and extracted solution from themembrane and fiber layers was discovered. This evidenceindicates that humic substances were present and that thefouling of the membrane and fibers could be partly attributedto the presence of these humic substances.

Conclusion

It is clear that fouling occurs on both the membrane surface

and the organic fiber layer. SEM results of the organic fiberlayer indicate the presence of debris, which has accumulatedbetween the interlocking networks of fibers. The debrisappears as patches ranging from brown to black in colorationin the organic fiber layers. EDX analysis further reveals thatthe debris consists of mineral elements such as aluminium,silicon, sulfur, chlorine, calcium, titanium and iron. It isbelieved that the accumulation of these inorganic mineralsbetween the network of organic fibers may provide a strongpotential nutrient source for bacteria. As for the membrane’ssurface, fouling as seen under the SEM appears to consist ofa coating whereas EDX analysis also revealed the presenceof calcium, in the fouled regions. Tests for the presence ofhumic acid in both the fouled fiber layers as well as on thefouled membrane’s surface using a UV spectrometer givesresults consistent with humic acids.

Reference

[1] Fang, H.J., Sun, D.D., Wu, M., Phay, W. and Tay, J.H.,

(2004) “Removal of humic acid from ultrafiltration

membrane surface using photocatalytic oxidation

process” IWA Membrane Conference, 4-10 June, Korea.

[2] Thomas, M.H., Judd S.J. and Murrer, J. (2000) “Fouling

characteristics of membrane filtration in membrane

biroreactors, In Membrane Technology in Water and

Wastewater Treatment, Ed by Hills P., Northwest Water

Ltd., UK, pp 158-165, Cambridge, UK: The Royal

Society of Chemistry.

Ultrasonication of Sludge for Anaerobic DigestionMao Taohong ([email protected])

Show Kuan-Yeow ([email protected])

Introduction

Anaerobic digestion is a commonly applied stabilizationprocess for municipal wastewater sludge. Besides massreduction, odor removal, and pathogen removal, the mostattractive feature of anaerobic digestion is its ability toproduce a net energy gain in the form of methane leading toa cost-effective operation. However, due to the rate-limitingstage of hydrolysis, anaerobic digestion is a rather slowprocess resulting in long retention times required and largedigester volumes. As most substrates in sludge are enclosed

within cell membranes, the cell-bound substrate has to be

released in the hydrolysis phase before it can be utilized by

methanogenic anaerobes. The demands for higher efficiencyprocesses have prompted the need for pretreatment in orderto improve substrate solubility and digestibility. Amongst

many possible pretreatment methods, ultrasound treatment

exhibits a greater potential for being environmentally and

economically sound.

This paper describes the work on the use of ultrasound pre-

treatment for sludge disintegration to improve the subsequent

anaerobic digestion. The effects of sonication density on

sludge disintegration as well as on digester performance are

presented and discussed.

Method

Secondary sludge samples were collected from a Water

Reclamation Plant in Singapore treating municipal

wastewaters. Sonication treatment of sludge was conducted

with a sonicator equipped with a probe transducer (Autotune

Series, Singma Chemical Co. U.S.A). The sludge was

sonicated under low frequency of 20 kHz at different

sonication densities of 0.18 W/ml, 033 W/ml and 0.52 W/ml

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for one minute. The digestion of sludge was carried out

using four upflow anaerobic digesters operated under identical

conditions. Digester r1, serving as a control unit, was fed

with untreated secondary sludge, while the other three

digesters r2, r3 and r4 were fed with sludge sonicated at

0.18, 0.33 and 0.52 W/ml, respectively. The anaerobic

digestion was performed in a temperature room at 35oC and

operated under hydraulic retention times of 8 days.

Results and Discussion

Sludge particle disruption

The most intuitionistic evaluation of the disintegration effects

of ultrasound can be obtained by microscopic analysis, which

provided a visual appraisal of pretreatment. Figure 1

demonstrates the microscope images of secondary sludge

before and after sonication treatment at density of 0.52 W/

ml for one minute. After sonication treatment, significant

sludge disintegration was observed based on flocs breakage,

boundary disappearance for solid-liquid phase and

intracellular materials release to water. Figure 2a illustrates

a quantitative comparison of size distributions of biosolids

by volume after sonication at different sonication densities.

Compared with the control (untreated sample), the peaks of

curves representing sonicated sludge samples shifted towards

the left, indicating substantial increase of smaller particles.

Figure 1. Microscope images for secondary sludge whenuntreated (1) and sonicated at 0.52 W/mL (b)

Figure 2. Effects of sonication on particle size distribution (a)and mean particle size (b)

The mean particle size for the untreated sludge decreased

from 48 µm to 31 µm, 21 µm and 18 µm after sonication at

0.18 W/mL, 0.33 W/mL and 0.52 W/mL, respectively (Figure

2b).

Soluble organics increase

Table 1 presents the characteristics of untreated and sonicated

sludge. The total Chemical Oxygen Demand (COD) and

soluble COD (SCOD) results reveal that sonication exerted

little impact on the total waste concentration, but it managed

to convert a fraction of the particulate compounds into soluble

form. Compared with the untreated sample, SCOD increased

by 22%, 60% and 95% upon sonication at densities of 0.18

W/ml, 0.33 W/ml and 0.52 W/ml, respectively. Further

evidence for soluble organics increase was the Total Organic

Carbon (TOC), which increased from the original 130 mg/

L to 212 mg/L, 317 mg/L and 458 mg/L after sonication at

respective densities. The significant enhancement in SCOD

and TOC indicates that, sonication disruption of sludge flocs

enabled the release of cellular organic substances into liquid

phase, and transformed parts of the insoluble COD into the

soluble forms amenable to further anaerobic digestion.

Greater SCOD and TOC improvement was achieved at higher

sonication densities, which corresponds to the sludge particle

disruption discussed previously. This can be likely attributed

to larger bubbles initiated by high sonication densities thereby

disintegrating sludge more completely.

Table 1. Characteristics of untreated and sonicated sludge

Digesters r1 r2 r3 r4

Sonication Control 0.18 0.33 0.52

density, W/mL

SCOD, mg/L 745 912 1190 1450

±84 ±102 ±125 ±118

COD, mg/L 8590 2642 27090 28270

±670 ±1120 ±970 ±900

TOC, mg/L 130 212 317 458

±21 ±34 ±25 ±25

Biogas production

Figure 3 demonstrates the accumulative biogas production

of the digesters. During the digester start-up, there was an

acclimatization period for all digesters in which no significant

gas production was observed. After the acclimation period,

gas production increased at a steady rate, indicating digesters

reaching the steady-state period. The steady-state biogas

production in ascending order was 380, 550, 1100 and 1160

mL/d for r1 to r4 (Table 2).

Compared with the control digester r1, the biogas production

increased by 45%, 189% and 205% for digesters r2, r3 and

r4, respectively. The enhanced biogas production

corresponded to the increased soluble organics and reduced

particles established in sonicated sludge, which revealed that

sonication treatment could efficiently improve the conversion

efficiency of organics into biogas.

Organics degradation

Table 2 presents a summary of the performance data of the

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digesters at steady-state conditions. The average COD

removal achieved were 85%, 89%, 91% and 94%; and the

average total solids (TS) removal were 72%, 84%, 89% and

92% for digesters fed with untreated sludge and sludge

sonicated at 0.18 W/mL, 0.33 W/mL and 0.52 W/mL,

respectively. The results show that the digesters fed with

sonicated sludge indicated better performance in terms of

organics degradation and removal of solids, which was in

agreement with the biogas production. It is likely that

sonication pretreatment could disintegrate the bound biosolids

structures and release more readily degradable substances.

This would facilitate the hydrolysis-acidogenesis biochemical

reactions and result in a more efficient conversion

environment for carbon utilization by the methanogenic

microorganisms.

Figure 3. Effects of ultrasound treatment on biogas production

Conclusion

Significant sludge disintegration by sonication was evidenced

by remarkable reduction of biosolids particle size and increase

of soluble organics. The subsequent anaerobic digestion

indicates that digesters fed with sonicated sludge exhibit

greater efficiency in organics removal and enhance biogas

production over the digester fed with untreated sludge. The

extents of sludge disintegration and digester improvement

were in positive relationships with sonication densities within

the range tested. The results demonstrated that sonication

treatment is a suitable pretreatment method to improve the

subsequent anaerobic digestion. Under appropriate operation,

digesters fed with sonicated sludge were efficient and stable

Table 2. Digester performance at steady-state

Digesters r1 r2 r3 r4

Sonication Control 0.18 0.33 0.52

density, W/mL

Gas production, 380 550 100 1160

mL/d

COD removal, 85 89 91 94

%

TS removal, 72 84 89 92

%


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GEOTECHNICSGEOTECHNICSGEOTECHNICSGEOTECHNICSGEOTECHNICSHydrofracturing In-Situ Stress Measurements

in Singapore GraniteAshraf Mohamed Hefny ([email protected])

Chua Heng Choon ([email protected])

Introduction

A total of eight hydrofracturing tests in two vertical Boreholes atdepths ranges between 60m and 120m below the ground surfacewere carried out in the Bukit Timah Granite. Results of the tests aresummarized in this article.

Hydrofracturing test set-up

The hydrofracturing tests in Bukit Timah Granite followed thestandard test-set-up and procedure (Haimson, 1995). The systemincludes: A pair of inflatable high pressure rubber straddle packersat interval of 1 m, an air driven high pressure pump with maximumpressure of 100 MPa and maximum flow rate of 10 l/min, highpressure pipes, a flow meter, a pressure transducer, a pressure metre,switches, an automatic recording system and an impression packerwith an orientator.

The procedures of hydrofracturing testing involved: sealing a section,hydrofracturing the rock, reopening the fracture, recording the keypressures and mapping the fracture orientation.


The in-situ stresses interpreted from the test data are summarizedin Table 1. As can be seen from Table 1, there exist high horizontalstresses in the Bukit Timah Granite of Singapore. The ratio of σv

:σh :σH is approximately 1:2:3. The maximum horizontal in-situstress is in the NNE-SSW direction. It is noted that in general thedirection and magnitude of horizontal stresses, measured at differentdepths in the two boreholes are consistent. Test B2 in BH17 at thedepth of 90m produced a low tensile strength, and hence led tosignificantly large horizontal stresses. It has been observed that atthis test location the reopening pressure in the re-pressurizationcycles is equal to the breakdown pressure. This suggests that thecracks at this test location are pre-existing cracks and not inducedby the test. Therefore, results of this test are not considered duringdrawing the conclusions.

The laboratory determined tensile strength is consistently higherthan that derived from hydrofracturing in-situ tests. This is attributedto the selection of “perfect” specimens of small size for the laboratorytest. It is believed that the results from the in situ tests reflect the

actual tensile strength of the rock, and hence provide more accuratein-situ stress results.

The direction of maximum horizontal stress is correlated with thelocal and regional geological setting. The Jurong Formation hasbeen pushed towards and against the Bukit Timah Granite in NEdirection. The tectonic stress in the NE direction was sufficientlygreat and caused intensive folding of the Jurong Formation in theNE-SW direction. The direction of the maximum horizontal stressis also consistent with that indicated by the World Stress Map(Stephansson, 1995). This map shows that the general direction ofmaximum horizontal stress in Sumatra and southern part ofPeninsular Malaysia is NE~NNE, due to tectonic stresses developedat the convergent boundary between the Euroasian and theAustralian-Indian plates. More detailed discussion can be found inZhao et al. (2005).

Conclusions

Eight hydrofracturing in-situ stress measurements were performedin the Bukit Timah Granite of Singapore in two vertical boreholesat depths ranges between 60m and 120m below the ground surface.Results of the stress measurements indicate the existence of highin-situ horizontal stresses in the Bukit Timah Granite. The ratio ofσv :σh :σH is approximately 1:2:3. The maximum horizontal in-situstress is in the NNE-SSW direction. This direction of maximumhorizontal stress is in correlation with the local and regionalgeological setting.

References

[1] Haimson, B.C. (1995). The Hydrolic Fracturing Method ofStress Measurement: Theory and Practice. Comprehensive RockEngineering, Editor-in-Chief: Hudson, J.A., vol. 3, pp. 395-412.

[2] Stephansson (1995). Rock Stress in the Fennoscandian Shield.Comprehensive Rock Engineering, Editor-in-Chief: Hudson,J.A., vol. 3, pp. 445-459.

[3] Zhao, J., Hefny, A.M., Zhou, Y.X. (2005). Hydrofracturing insitu stress measurements in Singapore. International Journalof Rock Mechanics & Mining Sciences, vol. 42, pp. 577-583.

Table 1. Summary of derived in situ stresses

Borehole Test No. Depth (m) σv (MPa) σh (MPa)σH (MPa), σH (MPa), Direction

by Thf by Tlab of σH

BH8 A1 62 1.59 3.30 4.28 7.67 039BH8 A2 85 2.18 4.69 8.00 14.07 017BH8 A3 94 2.38 4.11 5.75 11.76 006BH8 A4 113 2.90 6.19 10.24 14.76 057

Average 2.26 4.57 7.07 12.04 018

BH17 B1 65 1.66 4.00 6.17 7.84 000BH17 B2 90 2.32 8.04 15.39 22.25 011BH17 B3 109 2.80 5.43 9.09 10.88 017BH17 B4 120 3.07 3.94 6.63 8.65 007

Average 2.51 4.46 7.30 9.12 008

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Performance–Based Acceptance Criteriafor Quality Control of Reclaimed Sandfill

Chang Ming-Fang ([email protected])

Huang YongQing ([email protected])

Introduction

Sandfill created rapidly by hydraulic filling or other placement

methods at reclaimed sites is usually loosely packed or

heterogeneous and often requires quality verification using,

for example, the cone penetration test (CPT). One often faces

difficulty in the specification of the required minimum cone

resistance (qc). Current practice of quality control of sandfill

relies heavily on past experience of similar developments. An

effective acceptance criterion would require specific values of

penetration resistance that can be related directly to foundation

performance, such as settlement. To improve the current

practice, one needs to evaluate the suitability of qc as a

measurement of compressibility by making reference to

performance of prototype foundations or plates in the plate

load test. An established framework of settlement analysis,

such as that of Schmertmann et al. (1978), can be adopted for

the development of settlement-based acceptance criteria using

suitable field data, such as those collected at the Changi East

Reclamation site in Singapore.

Data at Changi East reclamation site

The sandfill investigated was initially used as a temporary

surcharge. A step-by-step removal of the sandfill allowed

plate load tests to be carried out at different depths thus

providing an opportunity for the investigation of the

compressibility of sand and load-settlement response of model

footing at various levels.

The sand of marine origin was relatively clean with a

carbonate content of 4 and 16 %. The grain size distribution

was found to vary among three test lots, each measured 2 m

x 2 m, with the coefficient of uniformity Cu generally between

2 and 6 for the sand in Lots 1 and 2 and around 2.9 for the

sand in Lot 3. It is classified as SP (poorly graded sand)

based on the Unified Soil Classification System. The sand

in Lot 1 and Lot 2, placed by the sub-aerial hydraulic

pumping, was generally medium dense to very dense and

that in Lot 3, formed by direct dumping from trucks, was

generally very loose to loose.

Both the plate load test (PLT) and the cone penetration test

(CPT) were carried out using recommended standard test

procedures.

From the pressure-settlement curve obtained in the PLT, the

reload elastic modulus Eoc was calculated from the unload-

reload loop where the average vertical strain was generally

within 0.1% to 0.5%. This range of vertical strain is similar

to the strain level in sand presented beneath conventional

foundations under serviceability state, which is typically 0.1%

in overconsolidated sand and 0.25% in normally consolidated

sand (Baldi et al. 1988; Burland, 1989). Table 1 summarizes

the values of Eoc deduced from PLTs in three test lots. The

modulus values are found to be higher for the hydraulically

filled sand in Lot 1 and Lot 2, than for the directly dumped

sand in Lot 3.

Figure 1 shows typical qc profiles in Lots 1, 2, and 3. For the

surcharge sandfill in Lots 1 and 2, which was placed

hydraulically above water, the qc value is generally between

15 MPa to 25 MPa. In contrast, for the sandfill that was

placed above the mean sea level by direct dumping in Lot 3,

the qc value is much lower, typically at 2 to 3 MPa, except for

the thin platform layers at the top of each placement lift.

Schmertmann’s framework of analysis

Among the various methods for settlement analysis of footings

in sand, Schmertmann’s method (Schmertmann, 1970;

Schmertmann et al., 1978) is by far the most common. The

proposed equation for calculating the settlement (s) is

(1)

where CD is the depth correction factor, Cc is the creep factor,

∆q is the net increase in pressure at foundation level, B is the

foundation width or diameter, h is the thickness of each layer,

and Iz is the strain influence factor. The assumed distributions

Table 1: Reload Modulus from PLTs

Test Elevation Eoc (MPa)

Sequence (m) Lot 1 Lot 2 Lot 3

stage1 11.2~12.2 51.29 62.97 60.60

stage2 10.80 77.27 33.45 17.69

stage3 8.80 54.95 62.75 27.82

stage4 6.6~6.8 62.97 53.95 15.94

stage5 5.50 60.05 64.10 20.50

Figure 1: Typical qc Profiles

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of Iz for footings with two different length-to-diameter (L/B)

ratios are as shown in Fig. 2, with the maximum value Izp

expressed as a function of the net applied pressure ∆q and

σ′vp, the effective vertical stress at the depth where Izp occurs.

Schmertmann et al. (1978) recommended that the field modulus

E can be estimated from qc based on the following correlation

factors: α = E/qc of 2.5 for square footings and 3.5 for the

plane strain condition.

To verify the suitability of Schmertmann’s proposed normalized

strain distributions, a simplified elastic analysis was carried

out by assuming that the soil is a homogeneous, isotropic and

linearly elastic, and the footing is rigid. Vertical strain

distributions calculated using the FEM indicates that Iz is

relatively unaffected by the variation of ∆q and Poisson’s

ratio ν. The value of Iz at ground surface Izo is around 0.2 to

0.3. The maximum value Izp is typically between 0.4 and 0.5

occurring consistently about a depth of around 0.5B. The

maximum influence depth zm which increases gradually with

L/B and ceases to increase as L/B ≥ 10 is about 5B.

To simulate the nonlinear stress-strain relationship of soils

and to attain a close fitting of the complete pressure-settlement

curve, one needs to adopt a variable Izp using a more

generalized equation as follows:

(2)

where n is the curve fitting parameter.

Analyses based on PLT and CPT data collected at Changi

East site indicated that n fluctuated between 0.018 and 0.047

with an average of 0.04 for the medium to very dense sand

(RD ≥ 50%) in Lots 1 and 2. For the loose sand present in Lot

3, the matching value of n ranged widely from 0.27 to 0.39,

with a typical value of around 0.3.

It is suggested that appropriate modifications be made on

Schmertamnn’s distributions of normalized axial strain as follows:

(a) using Izo = 0.2 at z = 0; (b) using Eq. (2) in the calculation

of Izp, where n = 0.04 is recommended for overconsolidated sand

with RD ≥ 50% and n = 0.3 for normally consolidated with RD

< 50%; and (c) setting zm = 2.5 (1+log(L/B))B.

Figure 2. Schmertmann’s Distributions of Iz

Figure 3. Proposed Acceptance Criteria forHydraulically Placed Sandfill

Development of acceptance criteria

Using the present proposed framework of analysis, the elastic

modulus Es that prevails in the field can be back-calculated

from the results of PLTs and compared with the corresponding

qc value. The Es/qc ratio generally ranges from 2.2 to 4.8 and

averages around 4 for the medium dense to very dense sand

with RD ≥ 50% in Lots 1 and 2. For the loose to medium

dense sand with RD < 50% in Lot 3, the ratio varies widely

and bears no specific relation with the relative density.

Interestingly, the back-calculated Es/qc ratio is comparable to

the corresponding Eoc/qc ratio.

In developing suitable acceptance criteria, Schmertmann’s

proposed distributions of normalized vertical strain were used,

with n = 0.04 considered for hydraulically filled sand and

both n = 0.1 and n = 0.3 considered for loose or normally

consolidated sand. A modulus coefficient α of 4 was chosen

for hydraulically filled sand, which is usually considered to

be overconsolidated, for the axis-symmetrical (L/B=1)

condition, and 5.6 for the plain strain (L/B=10) condition. For

normally consolidated or dynamically densified sand, α =2.5

and α =3.5, respectively, were chosen for axis-symmetrical

and plain strain conditions, respectively, following the

proposals of Schmertmann et al. (1978). An equivalent qc

value, or qc, calculated by taking the weighed average of

individual qc values in various sub-layers, is recommended.

Figure 3 shows design charts developed based on the proposed

acceptance criteria for a commonly accepted allowable

settlement of 25 mm for shallow foundations resting on

hydraulically reclaimed sand fill for (a) L/B =1 and (b) L/B

=10 cases. Fig. 4 shows similar charts for square foundations

resting on dynamically densified or normally consolidated sand

fill, with n = 0.1 for less conservative and n = 0.3 for more

conservative applications.

As an example, we assume that a 3m x 3m footing is to be

built on the surface of a marine sandfill. The net vertical

pressure is expected to be 300 kPa, and the settlement is to be

restricted to 25 mm. Taking Cc= CD =1 and α of 4 for

hydraulically placed sandfill and 2.5 for dynamically densified

sandfill, the required qc value would be 7.7 MPa and 14.7%

25 MPa, respectively based on Figs. 3 and 4.

*

*

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Soil Improvement for Tree StabilityHarianto Rahardjo ([email protected])

Leong Eng Choon ([email protected])

I Gde Budi Indrawan ([email protected])

Conclusions

Based on a comprehensive field investigation program

comprising plate load tests and cone penetration tests and

Schmertmann’s framework of settlement analysis of shallow

foundation, modified distributions of normalized vertical strain

were proposed to produce an improved matching pressure-

settlement curve for a footing. Acceptance criteria for fill

Figure 4: Proposed Acceptance Criteria for Direct Dumpedor Dynamically Densified Sandfill

quality control have been developed to facilitate the selection

of the required minimum equivalent qc value in both

hydraulically placed sandfill and dynamically densified sandfill

for a specified allowable settlement of foundations.

References

[1] Baldi, G., Bellotti, R., Ghionna, V. N. and Jamiolkowski,

M. 1988. Stiffness of sand from CPT, SPT and DMT -

A critical review. Proc. of ICE Geotechnology Conference

on Penetration Testing, Birmingham, 299-305.

[2] Burland, J. 1989. The 9th Bjerrum memorial lecture: Small

is beautiful, the stiffness of soils at small strain, Canadian

Geotechnical Journal, 26, 499-516.

[3] Schmertmann, J. H. 1970. Static cone to compute static

settlement over sand. Journal of the Soil Mechanics and

Foundation Division, ASCE, 96 (SM3), 1011-1043.

[4] Schmertmann, J. H., Hartman, J. P. and Brown, P. R.

1978. Improved strain influence factor diagrams. Journal

of Geotechnical Engineering Division, 104 (GT8), 1131-

1135.

Introduction

Trees are an important part of Singapore’s urban environment

and provide numerous benefits to human life. However, uprooted

trees in an urban environment can cause damages to properties

and infrastructures and injury or loss of lives.

There are several factors that contribute to tree failures, such as

decaying of tree trunk, wind loading, and root wedging. The

engineering properties of soil as a medium for tree growth also

play an important role in tree stability. Several cases of tree

failures have been associated with low soil strength.

Fine-grained soils have commonly been used for growing trees.

Recently, mixing of fine-grained and coarse-grained soils has

been considered to be an alternative method for increasing soil

strength. The increase in strength is resulted from the contact

between the coarse-grained materials. Furthermore, the pores

developed between the coarse-grained materials are filled with

the fine-grained soils to retain moisture and nutrients. These

pores encourage the tree roots to penetrate deeper and hence

enhance the stability of the tree root and soil system.

The main objectives of this study are to study mechanisms that

lead to the uprooting of trees in Singapore, to study the engineering

characteristics of top soils in Singapore in relation to the capacity

of the soils to support tree stability, and to improve engineering

properties of top soils in Singapore for stability of tree by mixing

top soils with other materials such as granite chips. For this

purpose, the top soils are mixed with granite chips in percentages

of 50% granite chips to 50% top soils and 80% granite chips to

20% top soils based on dry mass.


Figure 1 shows a simplified wind-tree-soil interaction model.

The controlling factors of tree failures were analyzed based on

three main modes of tree failure: shear failures of soil-root

system, tensile failure of roots, and slippage of roots. In the

preliminary assessment, it was assumed that no coupling mode

of failure is present. In all likelihood, tree failure involves a

coupling of the three modes of failure and the contribution of

each mode to tree failure will be difficult to ascertain without

experimental study. Therefore laboratory and field tests are

currently in progress.

Seasonal variations of matric suctions, which contribute to soil

GEOTECHNICS


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Figure 2. Matric suction readings oftensiometers installed around a tree

Figure 1. A theoretical model for tree stability

Figure 3. (a) Granite chips; (b) Top soils

Figure 4. Failed Specimen in a triaxial test

Figure 5. (a) The excavated planting holes; (b) Soil mixing;(c) Soil compaction; (d) Tree planting.

Figure 6. Soil arrangement for pull-out tests

The most optimum soil mixture obtained from the laboratory

tests will be verified in the field by conducting pull-out tests on

trees. For the pull-out tests, each tree was planted in a planting

hole of 2.5 m x 2.5 m (area) x 1 m (depth). The soils were

placed in the planting holes and compacted to a minimum of

90% relative compaction as per local practice.

Currently, twenty planted young Rain trees are left to grow for

several months, after which the pull-out tests will be conducted.

Figure 5 and Figure 6 show preparation and layout of the test

site, respectively.

Summary

Several theoretical modeling and laboratory tests are currently

being conducted to study mechanisms that lead to the uprooting

of trees in Singapore and to study the engineering properties of

the top soil-granite chip mixtures. Results obtained from the

theoretical modeling and the laboratory tests will be verified in

the field by conducting pull-out tests on the planted trees. From

this study, it is expected that engineering properties of the top

soils in Singapore can be improved to support tree stability.

Acknowledgement

The study is funded by a research grant from National Park

Board-Nanyang Technological University Joint R & D.

Assistance from Mr. Heng Hiang Kim, Mr. Christopher Chia,Mr. Tan Hiap Guan, and Ms. Binu Stanley of the GeotechnicalLaboratory is acknowledged.

shear strength, have been measured using tensiometers installed

around trees (Figure 2). Laboratory tests including investigation

of soil index properties, hydraulic properties, and shear strength

have been conducted to obtain the most optimum soil mixture

of top soils and granite chips. Figure 3 and Figure 4 show

photographs of granite chips and top soils used in this study

and the triaxial test set-up for shear strength measurement,

respectively.

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GEOTECHNICS

Numerical Study on Crack Propagation andShear Behavior of Jointed Rock Mass

Zhao Zhiye ([email protected])Zhang Houquan ([email protected])

Introduction

Rock mass is full of discontinuous fractures or joints withdifferent azimuth angles. A comprehensive study on thebreakage and shear behavior of intermittent rock joints canprovide a good understanding on local and general rockstability, leading to improved analysis and design ofunderground structures.

Many experimental and theoretical studies have been carriedout over the years on the failure process of rock mass withintermittent joints under shear loading. In general, rockfails in a tensile mode for overlapping joints and in a mixedmode for non-overlapping joints. The failure patterns ofrock mass are mainly controlled by the joint separationwhile the shear strength of jointed rock depends mostly onthe failure pattern, and the joint separation is an importantgeometrical parameter on the shear resistance of jointedrock (Gehle and Kutter, 2003). Though the general behaviorof rock specimens containing intermittent joints under shearloading has been extensively investigated, the detailedfailure mechanism, from fracture initiation, fracturepropagation, coalescence to failure is still short of reasonabletheoretical explanation, especially for internal fractureswhich are hardly accessible to observation. This paper aimsto provide an insight on the rock fracture process throughnumerical simulations.

Model Setup

This numerical study is based on the Rock Failure ProcessAnalysis code (RFPA2D) developed at the Center for RockInstability and Seismicity Research of NortheasternUniversity, China. The numerical model is shown in Figure1. The geometry of the model is 100 mm×100 mm in sizeand it is discretized into 200×200=40,000 elements. Onaccount of the heterogeneity of rock material, the elementallocal mechanical parameters such as the failure strength σc

and elastic modulus E0 (as listed in Table 1) are assumed tofollow a Weibull distribution. To simulate the shear box, tworectangle strips are added around the specimen with elasticmodulus of 4×105 MPa and failure strength of 2×103 MPa,which are much larger than the values of rock specimen sothat they don’t fail and exert additional stress on the boundary.The bottom and right boundary of the specimen are fixed soas not to move in the vertical and horizontal directions. Aload P which has an equivalent distributed pressure of 0.5MPa is applied in the vertical direction on the top surfaceand an increasing horizontal displacement Q at 0.002 mm/step is loaded on the left surface to simulate shear slip.Element failure is judged by a Coulomb criterion with atension-cut-off; thus, the element may be damaged either in

tension or shear mode. The detailed descriptions of RFPA2D

model can be found in the paper by Tang (1997).

Crack Propagation of Jointed Rock

Figure 2 visually depicts the evolution of minimum principal

stress field during the course of fracture progress where bright

spots indicate smaller minimum principal stress. Figure 3shows the relationship between shear load, associateddamaged element count and loading step. The points (A-F)in Figure 3 correspond to the loading step in Figure 2.

Table 1. Mechanical and geometrical properties

Parameter Value

Homogeneity index (m) 4

Elastic modulus (E0) 60,000 (MPa)

Compressive strength (σc) 200 (MPa)

Poisson’s ratio (µ) 0.25

Friction angle (ϕ) 30°

Edge-notched joint length (b) 10 (mm)

Imbedded joint length (d) 16 (mm)

Imbedded Joint angle (ω) -45°

Figure 1. Numerical model

It can be seen from Figure 2 that cracks are much easier topropagate through the brighter place in the stress field. It ismuch easier to generate tensile damage than shear damagewith smaller minimum principal stress near the crack tip forrock-like materials. Wing cracks firstly initiate from the tip

of the imbedded joints, not from the tip of edge-notched

joints, due to the strong interaction between joints. The shear

load reaches SR0 (point A in Figure 3) at the load step 29.


47

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Figure 2. Rock Fracture process.

At this loading level, the deformation behavior of rock

specimen under shear loading deviates from linearly elastic

one to a nonlinear one.

In general, most damage elements occur at the tip of the

edge-notched joints. It confirms the experimental results by

Gehle and Kutter (2003) that crack initiation is for the most

part audible, but not always visible with the naked eye. When

Figure 3. The relationship between shear load, damaged

element counts and loading step.

the cracks grow into rock bridges and the release of tensile

stress in the middle rock bridge develops wing cracks, tensile

stress is transferred to the field of edge rock bridges. Tensile

cracks initiate from the tips of two edge-notched joints

actively from step 32 and grow into the deep part of bridged

rock towards another distant joint. It can be observed from

Figure 3 that there is a substantial increase of damage

elements around the loading step 40. At load step 114, the

shear load reaches Point E, some joints enter into the closing

stage. A sudden loss of shear load is linked with a cracking

incident of a large number of elements, especially before the

severe damage of inner configuration of rock specimen. After

step 114, shear resistance keeps a higher value by the self-

support effect of shear zone. From the above observations,

we can conclude that the tensile damage is always playing

an important role during the whole fracture process.

Conclusions

The shear behaviors of a rock specimen containing four jointswith different joint azimuth angles have been investigatedthrough a direct shear test using RFPA2D code. The wholeshear failure process is visually represented and the failurepattern obtained is in reasonable accordance with otherexperimental results. In general, two phases of shearing canbe identified. Wing cracks firstly initiate from the tips ofpre-existing joints with an initiation angle, and then propagatetowards another joint through rock bridge. The second phaseof shearing is characterized by friction process and volumeincrease in shear zone.

References

[1] Gehle C, and Kutter HK. (2003). Breakage and shearbehavior of intermittent rock joints. Int. J. Rock Mech.Min. Sci., 40: 687-700.

[2] Tang CA. (1997). Numerical simulation on progressivefailure leading to collapse and associated seismicity. Int.J. Rock Mech. and Min. Sci., 34: 249-261.

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MARITIME STUDIESMARITIME STUDIESMARITIME STUDIESMARITIME STUDIESMARITIME STUDIESA Measurement and Comparison of Cost

Competitiveness of Container Ports in Southeast AsiaLAM Siu Lee, Jasmine ([email protected])

Introduction

Terminal users face a variety of costs associated with the

container terminal and those that are harder to quantify should

not be ignored for they might represent a larger component

of the overall costs associated with using a particular terminal.

Similarly, the competitive advantage of a container terminal

operator goes beyond the elements that can be quantified.

The study aims to examine the competitive advantage, as

measured by cost competitiveness, enjoyed by the four

selected terminal operators in Singapore, Port Klang (which

can be divided into Northport and Westport) and Tanjung

Pelepas located along the Straits of Malacca. The study also

aims to determine the extent of this advantage and to analyse

its impact on the development of competition between the

selected pairs of container terminal operators from the supply

side.

Research Methodology

Simultaneous quantity-setting model is used to derive the

overall costs of using the terminal. The equilibrium quantity

of TEUs handled and profits earned by Terminal Operator 1

(TO1) and Terminal Operator 2 (TO2) are derived as follows:

&

&

where

θ1 denotes the share of transhipment containers handled by

TO2 for θ1 = [0,1].

θ2 denotes the share of transhipment containers handled by

TO1 for θ2 = [0,1].

a and b are coefficients that have to be estimated.

c1 and c2 denote the overall costs per TEU of supplying the

service by TO1 and TO2 respectively.

In the model, c1 and c2 can be seen to epitomise the overall

competitive advantage of the container terminal operator

because the overall costs per TEU of supplying the services

would have to take into account both qualitative and

quantitative factors that are relevant and important to the

terminal user. Through the substitution of container

throughput and financial performance data for each pair of

container terminal operators into the functions of TO1 and

TO2 and solving these equations will yield solution values

for a, b, c1 and c2.

Findings and Discussions

The results suggest that the terminal operators in Port Klang

and Tanjung Pelepas are able to improve their cost advantage

significantly while at the same time keep prices competitive.

The Malaysian terminal operators have also been expending

much effort to improve service connectivity in a bid to attract

more customers. The increasingly cost competitive operators

in Port Klang and Tanjung Pelepas are able to close the gap

with PSA Corporation in Singapore in the overall costs of

using their terminal facilities although PSAC continues to

enjoy a dominant share of the container-handling market in

the region.

The fact that terminal users continue to value those service

aspects that are harder to quantify including responsiveness,

reliability, consistency and reputation can be seen in the

higher price-cost ratios enjoyed by PSAC over the other

terminal operators. This enables PSAC to continue to charge

a higher premium for the services provided at its terminals

in Singapore. However, the results also reflect a declining

trend in the price-cost ratio of PSAC while those of the

Malaysian operators are found to be increasing. As a whole,

the gap in price-cost ratios between Singapore and the

Malaysian container terminal operators has narrowed.

Comparison of the cost ratios between the various terminal

operators suggests that incumbency for the terminal operator

and considerable amount of switching costs for the terminal

user play a significant role in enabling PSAC to retain its

position as the premier hub port in the region even though

the overall costs of calling at PSAC become more expensive

compared to Tanjung Pelepas, Northport and Westport.

Conclusion

As container terminal operators sought to sustain long-term

competitiveness by creating and drawing upon their respective

competitive advantages, the most desirable strategies are those

that sustain the expansion and revitalisation of demand for

their services. The author would like to highlight the

tremendous amount of opportunities available, in both vertical

and horizontal dimensions, to the container terminal operators

to advance and capitalise on their competitive advantages

beyond aggressive price competition.

MARITIME STUDIES


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2006

STRUCTURESSTRUCTURESSTRUCTURESSTRUCTURESSTRUCTURESAllowable Free Span of Rigid Risers

Ong Yean Chau ([email protected])

Gho Wie Min ([email protected])

Introduction

A rigid riser is a pipe attached to the leg or brace of an

offshore steel jacket by means of mechanical clamps. The

riser allows the transition of piping elements at the topside

location of the offshore platform with the expansion loop

joining the pipeline on the seabed floor. It is a vital part of

the offshore pipeline system. The riser pipe between the

mechanical clamps is exposed to the seawater environment

and subjected to hydrodynamic forces. The wave and current

actions on the riser span also create a phenomenon of vortex

induced vibrations (VIV) which cause potential adverse

impact of the integrity of the riser pipe.

Figure 1.1. Rigid Riser attached to Steel Jacket

Discussion

Total Modal Damping Ratio

The stability parameter currently used to represent the effects

of damping for a given modal shape is a function of the

pipeline effective mass, seawater density, pipe outer diameter

and the total modal damping ratio of system.

For the case of a rigid riser span, the rigid riser boundary is

usually protected by a layer of neoprene or ethylene propylene

diene monomer (EPDM) inside a mechanical clamp. By

assuming that the layer of neoprene or EPDM provides no

damping effect on the system, the analysis would yield the

most conservative rigid riser span length. In reality, the layer

of coatings does impose damping effect on the system. As

shown in Figures 1.2 and 1.3, the effect of damping on the

span lengths can be substantial.

Despite the fact that the stability parameter for the pipe and

the modal damping ratio have a linear relationship, the

resulting riser span and modal damping ratio do not exhibit

any linearity until the modal damping ratio is greater than

0.025 or the pipe span length is greater than 12m.

Relative Soil Parameter (β) and Boundary Conditions

To reflect the actual boundary conditions that the pipeline is

experiencing on the seabed, a host of parameters is

determined in the analysis which includes the boundary

condition coefficients, the relative soil parameter (β) and the

effective span length as a ratio with respect to actual free

span length (Leff/L). Some of these parameters require the

use of engineering judgment in the analyses. Being primarily

designed for use as pipeline span analysis on seabed, almost

all the boundary condition parameters involve the

classification of soil. For rigid risers, no guidance was

available to determine the allowable span utilizing the

information in the codes. Particularly, the relative soil

parameter, β, which determines the ratio of the effective

span length relative to the actual span length, is not readily

available in the analyses for rigid riser allowable spans. In

Table 1.1. Pipe parameters and environmental conditions

Pipe Diameter 273.1 mm

Pipe Wall Thickness 25.4 mm

Corrosion Allowance 50% WT

Corrosion Coating Thickness 12.0 mm

Wave Height 15.2 m

Wave Period 12.0 s

Current Velocity at 5 m above Seabed 0.8 m/s

Recent developments in pipeline span analysis based on the

MULTISPAN Project (Mork et al, 1997) provide new design

guidelines for free spanning pipelines on the seabed.

However, the development of guidelines for rigid riser

allowable spans due to VIV has largely been lacking.

Application of developments from pipeline span analysis

directly to riser span analysis may be inappropriate in view

that the load application and the boundary condition between

the seabed pipeline and the riser are not identical.

The motivation for the current study is to determine if the

recent DNV code, namely DNV-RP-F105 [2002], as well as

its predecessor, DNV Guideline No. 14 [1998], can suitably

be applied in the computation of the allowable span length

for rigid risers.

Data for analysis

The following pipe parameters and environmental conditions

are considered.

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Figure 1.2. Stability Parameter vs Modal Damping

order to highlight the impact of relative soil parameter on

the rigid riser allowable span length, the following analyses

are performed and the results are presented in the table below.

Table 1.2. Comparison of boundary conditions and

effect on effective span length

Boundary Vertical Relative Effective

Type Dynamic Soil Span Length

Stiffness Parameter, β Ratio

Loose Sand 10.9 3.79 1.33

Dense Sand 21.7 4.09 1.28

Very Soft Clay 0.8 2.67 1.64

Very Hard Clay 16.3 3.97 1.30

In-line Riser Span VIV

In the event that the riser span is allowed to vibrate, the

impact of these vibrations on the mechanical riser clamps

should not be underestimated. The deflection at the centre

of the rigid riser span caused by the in-line VIV vibrations

creates additional stresses which are normally not accounted

for in the design of mechanical riser clamps. By assuming

pinned-fixed boundary condition between a 10m long rigid

riser span, the resulting lateral force acting on the

mechanical riser clamp can be 20% of the lateral forces

due to the platform deflections and hydrodynamic forces.

This additional lateral force must be considered in the design

of mechanical riser clamps to prevent damage on its

structural components.

Conclusion

The development of guidelines for the determination of

rigid riser allowable spans has largely been lacking. Further

research and experimental studies have to be carried out to

evaluate the effects of damping, relative soil parameters

and pipe span boundary conditions when a pipeline span

analysis is applied in the context of a rigid riser span.

References

[1] Det Norske Veritas Recommended Practice RP-F105

(2002), Free Spanning Pipelines, Norway

[2] Det Norske Veritas, Guidelines No. 14 (1998), Free

Spanning Pipelines, Norway

[3] Mork, K.J., Vitali, L. & Verley, R., (1997). “The

MULTISPAN Project: Design Guideline for Free

Spanning Pipelines”, Proc. 1997, OMAE.

Figure 1.3. Riser Span Length vs Modal Damping

STRUCTURES


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2006

An Experimental Study on Fatigue Behaviour ofPartially Overlapped Tubular K-Joint

Lee Chi King (ccklee @ntu.edu.sg)Lie Seng Tjhen ([email protected])

Chiew Sing Ping (cspchiew @ntu.edu.sg)Sopha Thong ([email protected])

Introduction

Due to the ease of fabrication and the existence of manydirect calculation methods to assess the joint capacity,simple tubular T, Y and non-overlap K joints are widelyused connection type for the construction of offshore steeljackets. When compared with a K-joints with gap, a partiallyoverlap K-joints may have a higher fabrication costs due tothe complex end profile of the overlap brace. However,partially overlap K-joint has a higher residual capacity asa result of its optimized load transfer pattern. In fact,recently, in a case study comparing the costs of three K-joints design options [1], it was found that after havingconsidered the material and fabrication costs, an overlapjoint is the cheapest option, with the fabrication cost actuallybeing significantly cheaper than that of the alternativecanned gap joint. However, in the past, very few researchworks were carried out to study the fatigue behaviour ofpartially overlap circular hollow section (CHS) K-jointsand virtually no information regarding the fatigue strengthof overlapped CHS K-joints is available. The main objectiveof this study is to investigate the stress concentration factor(SCF) and the stress intensity factor (SIF) distributions fora partially overlapped CHS K-joint under cyclic loadings.

Test rig and loading system

The “orange” test rig shown in Figure 1 was used to testa partially overlap CHS K-joint subjected to axial (AX),in-plane bending (IPB), out-of-plane bending (OPB) andthe combination of these three basic load cases. The rig iscapable of applying static loading on a joint specimen todetermine the hot spot stress (HSS) distributions, as wellas cyclic loading. Two 250kN and one 150kN capacityservo-hydraulic actuators were installed to apply the threebasic load cases. The loads were applied along the threemutually perpendicular axes.

Test specimen

The specimen was made up from CHS of structural steelpipes to API 5L Grade B specifications for the chord andS355 for the braces. The material properties of these pipesare shown in Tables 1. The intersecting profile of the braceswas constructed according to the AWS specifications andultrasonic check was conducted to ensure the quality of thewelding. The specimen was installed by fixing the twoends of the chord and the overlapping brace. The externalloads were applied at the end of the through brace.

Strain gauge locations

Strain gauges were placed around the entire chord/throughbrace intersection as shown in Figure 2. At each spot, apair of strain gauges were used to measure the distributionof the strain components perpendicular to the weld toe. Inaddition, in order to detect any secondary bending momentcaused by loading eccentricity and joint flexibility, a pairof strain gauges were also installed at two cross-sectionsalong the through brace with a distance equal three timesof the diameter of the brace.

Figure 1. Three actuators of test rig

Table 1. Material properties of the specimen

Members Yield Tensile Elongationstrength strength (%)

Chord 352 MPa 493 MPa 32.3

Brace 406 MPa 502 MPa 24.4

Figure 2. Strain gauge location

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Static test procedure

A series of load cases consisting of AX, IPB, OPB loadingsand arbitrary combinations of them were chosen and tested.The specimen was first subjected to an incremental staticload on one axis, and the strains were checked againstlinearity and zero drift to indicate shakedown of residualstress. Before starting the actual tests, the applied loads oneach actuator were preprogrammed. The actuators were thenramped to the predetermined loads. The strain gaugereadings were eventually recorded by special software forfurther calculation.

Static test results

After strain gauges values were recorded, the hot spot strainperpendicular and parallel to the weld toe, (ξ⊥) and (ξ//),were obtained by the linear extrapolation method. The strainconcentration factor (SNCF) was calculated as

SNCF = ξ⊥/ξn (1)

where ξn is the nominal strain obtained from the readingsof the four strain gauges attached along the through bracemember. The stress concentration factor (SCF) wascalculated as

SCF = SNCF(1+νξ///ξ⊥)/(1-ν2) (2)

where ν is the Poisson’s ratio of the material.

The HSS distributions on the chord and braces of thespecimen subjected to combined loads: AX and IPB loadsare plotted in Figures 3. It can be seen that peak HSSoccurred along the curve C on the through brace.

Fatigue test investigation

The alternating current potential drop (ACPD) technique[2] was used to monitor the growth of surface crack atexpected crack growth location. 32 ACPD probes wereequally spaced every 10mm around curve C, along theweld toe of the through brace near the heel crown of thejoint where the peak HSS was detected during the statictest (Figure 4). The fatigue crack development shape wasrecorded using a scan interval of 180 cycles. The test andscanning was stopped automatically when the crack hadpenetrated the through brace thickness.

Test conditions and applied loading

The overlapped tubular K-joint was tested in air under asinusoidal constant amplitude loading until failure with loadfrequency of 0.2 Hz throughout the test. The cyclic loadingpattern applied is shown in Figure 5.

Fatigue test results

The ACPD crack development plot obtained is shown in

HSS in chord member curve (C)

HSS in through brace curve (C)

Figure 3. HSS distributions

Figure 6. The crack was initiated at the heel of the throughbrace and propagated towards to saddle symmetrically.However, as the crack propagated, it no longer remainedsymmetric. Figure 7 shows the surface cracks at the heelcrown of the through brace, which has not been reported inprevious studies. In order to check the actual crack shapes,the tested joint was spilt into two parts along the cracksurface (Figure 8). It can be seen that from the ACPD

Figure 4. Probes distributed along weld toe

Figure 5. Cyclic load applied to specimen

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reading and the actual crack surface, the peak HSS positionis corresponding to the deepest position of the crack.

Figure 9 shows the plot of the design S-N curve [3] for athickness of 19mm together with the fatigue life of thisspecimen. It is observed that the S-N curve is onlymarginally conservative when predicting the life of the joint.

Conclusions

A fatigue test was carried out to investigate the behavior ofa partially overlap K-joint under combined AX and IPBloadings. Test results indicated that peak HSS occurred atthe brace weld toe near the heel crown of the intersectionof the through brace and the chord. It should be pointedout that such test results were not observed before in anyprevious study of CHS tubular joints. The fatigue test resultsdemonstrated that the S-N curve proposed in the currentcodes is marginally conservative when predicting life ofpartially overlapped tubular K-joint.

Figure 6. ACPD crack profile

Figure 7. Surface crack

Figure 8. Opened crack surface

Figure 9. S-N curves

References

[1] Tizani, W.M.K., Yusurf, K.O., Davies, G. and Smith, N.J. (1996), “A knowledge based system to support jointfabrication decision making at the design stage- casestudy for CHS trusses”, Proceeding of the 7th

International Symposium of Tubular Structures, Hungary,pp. 483-489.

[2] Dover, W. D., Dharmavasan, S., Brenan, F.P., and Marsh,K. J. (1995), “Fatigue crack growth in offshorestructures.” Engineering Materials Advisory Services(EMAS) Ltd., Chamelon Press, London.

[3] Zhao, X. L., Herion, S., Packer, J. A., Puthli, R.,Sedlacek,G., Wardenier, J., Weynand, K., van Wingerde, A., andYeomans, N. (2000), Design guide for circular andrectangular hollow section joints under fatigue loading,CIDECT, TUV.

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Failure Assessment Diagram of Damaged SquareHollow Section T-joints

Lie S.T. ([email protected])Gho W.M. ([email protected])

Yang Z.M. ([email protected])

Introduction

Generally, the static strength of any tubular structure is themain performance criteria at the design stage. However, alloffshore structures are continuously subjected to severe cyclicloading conditions. Due to these conditions, fatigue cracksfrequently initiate and develop resulting in damage. It hasbeen found that this fatigue damage is the most frequentsingle cause of repairs, representing 25% of all repairs tosteel platforms found in the North Sea. Cracks reduce thestructural strength capacity. It is therefore critical thatinspection strategies are improved so that these cracks canbe detected as early as possible. This will enable engineersto make rational repair/no-repair decisions. The potentialeconomic consequence of this critical decision for structurescan be very great. There has, of course, been considerableeffort in this research field for the past decades, resulting inseveral theories describing the behaviour of such damagedstructures. The most widespread and successful assessmentapproach is the failure assessment diagram (FAD) approach.Based on this approach, API RP579 [1], BS7910 [2] and R6[3] give guidance for assessing the acceptability of defectsin welded structures. Although these codes and standardsprovide guidance in the form of detailed procedures forfatigue and fracture assessments, these are intended forgeneral applications and do not necessarily give optimalsolutions for all types of structures, such as cracked tubularjoints.

Failure Assessment Diagrams (FADs)

For cracked structures, the failures due to brittle fracture andplastic collapse must be considered together. The FAD methodadopts the assessment curve which uses the ratio of the elasticstress intensity factor to the fracture toughness as the verticalaxis and the ratio of the applied load to the plastic collapseload as the abscissa axis. If the service point falls inside theassessment curve, the structure is considered safe, otherwise,the structure is deemed unsafe. The schematic usage of theFAD for the assessment of flawed structure is shown inFigure 1.

The assessment curve is different for different materials andgeometries. In the standard codes, the lower bound curvesare used to assess all types of structures. However, the usageof these standard curves should be validated when they areused to assess the damaged tubular joints.

Figure 1. FAD philosophy for the assessment of flawed structure

Validation of the Standard BS7910 (1999) Level3A Assessment Curve

In accordance with the BS7910 [1], the assessment curvesfor special structures can be constructed using the J-integralapproach. If the specified assessment curve falls outside thestandard BS7910 [1] assessment curve, it means the standardcurve is a lower bound curve. Therefore, the usage of thestandard curve would be safe. On the other hand if thecalculated assessment curve falls inside the standard curve,then the usage of the standard curve would be unsafe. Acritical feature of this process is the choice of the plasticcollapse load by which the applied load is divided to producethe Lr value. In accordance with the experimental andnumerical studies by Lie et al. [4], the proposed plasticcollapse load of cracked SHS T-joint under brace end axialloads is given by

(1)

where n represent the number of cracks, β is the brace to

chord width ratio, η is the brace depth to chord width ratio,

σe0 is the yield stress, and a is the crack depth. While b0,

b1 and t0 , t1 are the widths and thicknesses of the chord and

brace respectively. The first term is the yield load of

uncracked joint, and the second one is the reduction of the

yield load due to the presence of the cracks.

Because there is no available benchmark for the crackmodelling of damaged SHS joints, the actual fatigue crackshape shown in Figure 2 is used in the numerical casestudy.

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Figure 2. The actual crack and the modelled crack usedin the numerical case study

The dimensions of the specimens, the weld sizes and fatiguecrack details are tabulated in Table 1.

Based on these numerical models, the FADs of two SHS T-joints are constructed as shown in Figure 3.

It can be seen that the assessment curves fall outside thestandard curve. Therefore, the usage of the standard curve issafe.

Conclusion

Based on the true crack model, the specified assessmentcurves for the cracked square hollow section (SHS) T-jointsare constructed using the numerical J-integral approach. It isfound that the standard BS7910 Level 3A [2] assessment

Table 1. Dimensions of the specimens

Specimenb0 h0 t0 b1 h1 t1 Cracks

Depth a Length L tw

(mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm)

T1 350 350 15 200 200 161 12.7 275

122 9.0 125

T2 350 350 15 200 200 12 1 13.6 262 12

Figure 3. The specified assessment curvesfor the cracked SHS T-joints

curve gives a low bound curve. Therefore, its usage use forthe assessment of cracked SHS T-joints is still safe.

References

[1] API RP579 (2000), Fitness-for-Service, American

Petroleum Institute, Washington, USA.

[2] BS7910 (1999), Guide on Methods for Assessing theAcceptability of Flaws in Fusion Welded Structures,British Standards Institution, London, UK.

[3] R6 (2001), Assessment of the Integrity of StructuresContaining Defects, Revision 4, British Energy,Gloucester, UK.

[4] Lie, S.T., Chiew, S.P., Lee, C.K. & Yang, Z.M., 2005,“The Ultimate Behaviour of Cracked Square HollowSection T-Joints,” The Fourth International Conferenceon Advances in Steel Structures (ICASS 2005), 13-15,June, 2005, Shanghai, China, Vol. 2, pp. 1099-1106.

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Analysis of Foam Claddings for Blast AlleviationMa Guowei ([email protected])

Ye Ziqing ([email protected])

Introduction

Structural retrofit using metallic foam materials is of specialinterest to engineering communities and government agenciesagainst possible impact and blast load threats. The superiormicrostructure of metallic foam materials endows them withthe ability to undergo large deformation at nearly constantnominal stress. In addition to the advantage of their ultra-light and nonflammable characteristics, the metallic foammaterials can absorb remarkable energy caused by plasticdeformation in the case of compression. It is expected thatthey can be used as sacrificial claddings in the retrofit ofstructures against impacts and blast loads.

In this study, an analytical Load-Cladding-Structure (LCS)model is developed by considering the properties of the blastload, the foam cladding as well as the protected structure.Two non-dimensional parameters for the foam cladding areintroduced to describe the relations between the foamcladding and the protected structure. Based on the LCSmodel, the maximum deflection of the protected structuresubjected to a certain explosive load is predicted. It is seenthat the maximum deflection of the protected structure varieswith the two non-dimensional parameters of the foamcladding.

Analytical model

The protected structure can be converted to an equivalentstructural model, as shown in Fig. 1. The foam layer isassumed to start to deform at the loaded end and a shockwave front forms. As the shock front proceeds, the foambehind the shock front is compacted up to the densificationstrain εD and moves with the same velocity as the rigidcover plate while that ahead of the shock front is not disturbed(undeformed). The mass of the densified part of the foamlayer is conserved, which is written as

(1)

in which u is the displacement of the cover plate of thecladding, y is the deflection of the structure and ρ is thedensity of the foam material.

There is a stress jump across the shock wave front. Based onthe momentum conservation of the shock front, the stress atthe upstream of the shock front is

(2)

In light of Newton’s second law the motion equation of thedensified foam can be expressed as

(3)

The undeformed part of the foam layer (mf – ∆m) movestogether with the equivalent mass (mse). Hence the motionequation of the equivalent structure is

(4)

The initial peak pressure and total impulse of the blast loadingcan be normalized as non-dimensional pressure (p) andimpulse (i),

(5a,b)

To describe the relation between the plateau stress of thefoam and the resistance of the structure, a non-dimensionalparameter of the foam cladding is defined as

(6)

Meanwhile, the foam cladding should have a sufficient blastresistant capacity. It is reasonable to suggest that the foamlayer has such a thickness that the foam layer becomes fullycompacted when the protected structure experiences themaximum deflection. The other non-dimensional parameterof the foam cladding is defined as follows

(7)


The deflection of the foam protected structure can be derivedbased on the LCS model. Figs. 2 (a)~(d) plot the normalizedmaximum deflection of the equivalent structure underdifferent loads. The non-dimensional maximum deflectionof the structure is defined as the resulted maximum deflectiondivided by the critical deflection yc .

Figure 1. Analytical Load-Cladding-Structure model

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It is seen from Figs. 2 (a)~(d) that the normalized maximumdeflection of the foam protected structure varies with thetwo non-dimensional parameters (κ and τ) of the foamcladding. The maximum deflections corresponding todifferent κ and τ form a three-dimensional surface, and itlooks like the terrain of a valley. The lowest maximumdeflection of the structure corresponds to a small κ and alarge τ .

The maximum deflection of the structure is nearly constantfor a fixed κ as long as τ is sufficiently large and the foamcladding is ensured to withstand the blast load without beingfully compacted. The maximum deflection of the structurebegins to increase when τ is lower than a certain value, asshown in the left slope of the “valley” in Figs. 2 (a)~(d).This is because the foam cladding with a τ smallhas a lower blast resistant capacity and it may become fullycompacted and subsequently impacts the structure with aconsiderable velocity. In this case, the foam protectedstructure may experience a larger maximum deflection thana bare structure.

For a fixed and sufficiently large τ , the maximum deflectionof the structure increases with the increase of κ , as illustratedin the right slope of the “valley” in Figs. 2 (a)~(d). Whenκ is very large, the maximum deflection of the foam protectedstructure is nearly equal to the maximum deflection of thebare structure. This is because the foam does not deform and

acts as a rigid body which moves together with the protectedstructure when κ is larger than p.

Conclusion

An analytical Load-Cladding-Structure (LCS) model has beendeveloped to comprehensively investigate the protective effectof the foam cladding. Two non-dimensional parameters, i.e.,κ and τ have been introduced to describe the relationsbetween the foam cladding and the protected structure. Themaximum deflections of foam attached structure underdifferent loads have been predicted. It is found that the foamcladding can reduce the maximum deflection of the mainstructure to a safe range when κ is small and τ is sufficientlylarge. The LCS model is limited by the R-P-P-L foam materialmodel and the equivalent SDOF structural model. However,it can give a preliminary guidance to the design of foamcladdings for structural protection against blast loads.

Reference

[1] Gibson LJ, Ashby MF. (1997). Cellular solids, Structureand properties, 2nd ed. Cambridge University Press,Cambridge, UK.

Figure 2. Normalized maximum deflection of the foam protected structure under different blast loads(a) p=4, i=1.2 (b) p=4, i=2.0 (c) p=10, i=1.2 and (d) p=10, i=2.0

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The Smoothed Particle Hydrodynamics byConsidering Material Inhomogeneity

Ma Guowei ([email protected])

Wang Xuejun ([email protected])

Introduction

Recently, a new class of numerical methods called meshfree

(or meshless) methods has drawn considerable attention. By

constructing the approximation in terms of scattered nodes

other than meshes, it could eliminate the disadvantages of

rezoning during computations and become more adaptive,

versatile and robust for discontinuum problems. The earliest

meshfree method is the Smoothed Particle Hydrodynamics

method (SPH) which was originally used to solve

astrophysical problems in three-dimensional open space. The

method was first introduced by Gingold and Monaghan

(1977) and Lucy (1977).Since then, the SPH method has

been extended to various fields. One of the most significant

advantages of the SPH method is that it can naturally handle

problems with extremely large deformations and cracks.

Another attractive feature is that each SPH particle can carry

material property. This makes it very appropriate to simulate

the inhomogeneous materials. Although it has been noticed

for a long time that the micro-scale material inhomogeneity

can greatly affect the macroscopic behavior and fracture

process, the exact mechanism of how such process is

conducted is poorly understood. In this study, a SPH program

which can consider the material inhomogeneity effect has

been developed to investigate the dynamic failure process of

brittle materials, such as rock and concrete.

Methodology and formulas

The conservation equations of continuum mechanics in SPH

approximations are given as follows,

● the continuality equation,

● the momentum equation,

● the energy equation,

● the moving equation for particles

Explicit time integration is performed using standard Leap-

Frog time integral method for the above equations.

To consider material inhomogeneity, the Weibull statistical

distribution by Weibull (1951) was used to simulate the real

random microstructures. It can be expressed by using two

parameters as,

where η is the scale parameter, β is the shape parameter

describing the scatter of T. By means of the Weibull

distribution, the numerical model reflecting the

inhomogeneous properties of material can be constructed as

the following two characteristic parameters, i.e., the

inhomogeneous index β and the seed parameter η . For any

specific rock sample, the inhomogeneous index can be

determined on the basis of the defect distribution of the

microstructure, and the seed parameter can be obtained from

laboratory tests.

Applications

The SPH method as well as the inhomogeneous model has

been implemented to simulate the dynamic rock failure

process. One application is the dynamic rock Brazilian

splitting test simulation. An elasto-plastic constitutive model

coupled with a tensile damage model was employed to

describe failure behaviors for brittle materials. An

inhomogeneous index value of 5 was adopted to model the

inhomogeneous rock sample.

Figure 1 shows the schematic map of the test setting and

boundary conditions, where a vertical velocity with the peak

value of –6 m/s was applied on the top steel plate. Figure

2 shows the principal stresses, shear stress and damage

distribution at different failure stages. Results show that the

developed method is very promising in simulating the

inhomogeneous dynamic failure problems.

Figure 1. Brazilian dynamic test simulation

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References

[1] Gingold, R. A., and Monaghan, J. J. (1977). “Smoothed

particle hydrodynamics: theory and application to non-

spherical stars.” Monthly Notices of the Royal

Astronomical Society, 181(2), 375-89.

Figure 2. Distributions of principal stresses (MPa), shear stress (MPa) and damage during the failure process from a) to d)

[2] Lucy, L. B. (1977). “A numerical approach to the testing

of the fission hypothesis [close binary star formation].”

Astronomical Journal, 82(12), 1013-24.

[3] Weibull, W. (1951). “A statistical distribution function

of wide applicability.” J. Appl Phys, 28, 293-297.

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Ground Motions Recorded in Singaporeduring the Nias, Northern Sumatra Earthquake

(MW = 8.7) of 28 March 2005Pan Tso-Chien ([email protected])

Karim Kazi Rezaul ([email protected])

Figure 1. Epicentral locations of the main shock and major

aftershocks (M5.0) of the Nias, Northern Sumatra earthquake

of 28 march 2005

Figure 2. Locations in Singapore where tremors were felt

due to the Nias, Norther Sumatra earthquake

Figure 3. Locations of the SAFER array stations

Introduction

Tremors caused by distant Sumatra earthquakes have beenfelt in Singapore for many years and they have shaken manyhigh-rise buildings to a perceptible level. The nationalnetwork of seismic stations in Singapore was therefore setup in 1996 to monitor ground motions resulting from longdistance Sumatra earthquakes [1]. In this paper, groundmotions recorded in Singapore during the Nias, NorthernSumatra earthquake of 28 March 2005 are reported.Discussions on the local seismic hazard are also presented.

Mainshock and aftershocks

According to the United States Geological Survey (USGS)[2] an earthquake with a magnitude of MW = 8.7 occurred inNias, Northern Sumatra, Indonesia, on 28 March 2005, at16:09:36 UTC (29 March 2005, 00:09:36 Singapore Time).The USGS National Earthquake Information Centre (NEIC)[3] reported that the epicentre of the earthquake was locatedat 2.074° N and 97.013° E, in Nias, Northern Sumatra. Theepicentre was about 757 km west-northwest of Singapore asshown in Figure 1. The earthquake caused wide-spreadtremors felt in Singapore as depicted in Figure 2. The mainshock of the earthquake on 28 March 2005 was followed bymany aftershocks. Following the main shock, aftershockswere collected from the Incorporated Research Institutionsfor Seismology (IRIS) [4]. Occurring between 28 March2005 and 8 April 2005, there were about 65 aftershocks witha minimum magnitude of about 5.0 and the distribution ofthe epicentral locations of the larger aftershocks is shown inFigure 1.

Seismic instrumentation

In 1996, the Meteorological Services of Singapore (MSS)installed a network of seismic stations [1], which consists oftwo down-hole arrays (BES and KAP) of strong-motionstations and five teleseismic stations (BTDF, FTC, NTU,PTK and SJA). The two down-hole arrays are located on theKallang formation of Quaternary deposits. The main station,located in the Bukit Timah nature reserve and denoted asBTDF, is a Global Seismic Network (GSN) station, which isequipped with a comprehensive set of sensors to recordground tremors continuously. BTDF is located on a rockoutcrop site. The recorded ground motion time histories aretransmitted to the MSS headquarters to be processed andanalyzed.

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Figure 4. Baseline=corrected acceleogram of EW (top), NS

(middle), and UD (bottom) component in mm/s2 of the main

shock (2005/03/28 16/09/36 UTC) of the Nias, Northern

Sumatra earthquake, recorded in Singapore

The Nanyang Technological University (NTU) has alsoinstalled two additional seismic stations [1], one inside thecampus (denoted as NYC) and the other in the basement ofa high-rise office building in the central business district(denoted as RP). The NYC station is located on a firmresidual soil site, and the building in which the RP station islocated sits on rigid caissons. The seven MSS and two NTUstations form an array called the Singapore Array forEarthquake Response (SAFER array). The locations of theSAFER array stations are depicted in Figure 3.

Recorded ground motions

One NTU station (NYC) and four of the MSS stations (BES,BTDF, NTU, and PTK) were triggered during the Nias,Northern Sumatra earthquake. It should be noted that thesystem at RP site was down during the event, and there wastherefore no records at the RP site. Ground motions from themain shock (Mw = 8.7) on 28 March 2005 (16:09:36 UTC)were successfully recorded. Baseline-corrected accelerationtime histories at different MSS seismic stations due to themain shock are shown in Figure 4.

Discussion

The Nias, Northern Sumatra earthquake generally induced afar-field PGA that is larger than that by the other majorearthquakes have been felt in Singapore so far. It also inducedmore widespread tremors felt in Singapore. With more tallbuildings present in Singapore today, the possibility is notremote that such far-field ground motions when occur at acloser epicentral distance can induce larger response ofbuildings. Therefore, the effects of these large earthquakeson the high-rise buildings in Singapore need to be investigatedfurther.

Conclusion

In this paper, ground motions recorded in Singapore duringthe Nias, Northern Sumatra earthquake of 28 March 2005are reported. Discussion on the local seismic hazard is alsopresented. From the recorded ground motions, it was observedthat the event generally induced a far-field PGA that is largerthan that of the previous major events. The observationsupports the need for continued monitoring and research onthe seismic response characteristics of high-rise buildings inSingapore, where earthquake-resistant design is notspecifically required.

References

[1] Lee CL. (2000). Characteristics of earthquake groundmotions in Singapore. M.Eng. Thesis, 26-39.

[2] United States Geological Survey (USGS).http://earthquake.usgs.gov/eqinthenews/2005/usweax/.

[3] National Earthquake Information Centre (NEIC).http://neic.usgs.gov/neis/eqlists/sig_2005.html.

[4] Incorporated Research Institutions for Seismology.http://www.iris.edu/quakes/eventsrch.htm.

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Electromechanical Impedance Modelingof PZT Active Sensors for Health Monitoring

of Cylindrical Shell StructuresYang Yaowen ([email protected])

Hu Yuhang ([email protected])

Introduction

The advent of smart materials has brought about a revolutionary

impact on the engineered adaptive capabilities of structural

systems. Having the ability of imparting material deformation

in response to an applied electric field, and conversely generating

a dielectric polarization when subjected to mechanical strain,

the piezoceramic lead-zirconate-titanate (PZT) active sensors

can be used for structural health monitoring when they are

bonded to or embedded into an existing structure. Occurrence

or evolution of damage in the structure will alter the electric

impedance of the PZT sensor due to the electro-mechanical

(EM) coupling effect of the PZT material. Thus, by continuous

recording of PZT impedance signature, the health status of the

structure can be monitored. In this article, an EM impedance

model is presented for damage identification and health

monitoring of cylindrical shell structures.

EM impedance of PZT active sensors

The generic impedance model for 2-D PZT-structure interacting

systems developed in [1] is adopted in this study to closely

simulate the real situation. The EM admittance of a 2-D PZT

active sensor can be calculated as

(1)

where j symbolizes the imaginary part; ω is the input angular

frequency; ap , bp and hp are the length, width and thickness,

respectively; vp is the Poisson’s ratio; d31 and d32 are the strain

coefficients; is the complex dielectric

permittivity at constant stress, with being the real dielectric

permittivity and δ being the dielectric loss factor;

is the complex Young’s modulus at zero

elastic field, with being the real Young’s modulus and η

being the mechanical loss factor; is

the spatial frequency of the oscillations with ρp being the

mass density; and coefficients A’ and C’ are calculated by

(2)

where

(3)

with Z11 and Z22 being the direct force impedances, and Z12

and Z21 being the cross force impedances of the structure.

The interacting forces and the velocity responses at the

boundary of a PZT active sensor bonded to the cylindrical

shell have the following relationship

(4)

where and are the velocities of the shell; a and h are

the radius and thickness of the shell, respectively; and x1, x2,

θ1 and θ2 are the coordinates defining the location of the PZT

sensor.

Force impedance of structure

Considering a cylindrical shell with a small damage simulated

by a hole of radius rd at , the potential and kinetic

energy of the shell can be expressed as

(5)

(6)

where

(7)

E, v and ρ are Young’s modulus, Poisson’s ratio and mass

density of the shell, respectively; and is the transverse

displacement of the mid plane which can be expressed as a

combination of a set of shape functions

(8)

where m = (q+1)(q+2) /2–i ; N is the degree of the polynomial

space; cm are the coefficients; and φm are the approximate

functions which satisfy at least the geometric boundary

conditions.

Proceeding the Harmilton’s principle and extending the free

vibration modes into the forced vibration modes based on the

orthogonality, the displacement can be formulated in terms

of the interacting forces and . Expressing ux and vθ as

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functions of and substituting them into Eqn.(4), the force

impedance of the structure can be obtained. Finally, the PZT

impedance, which is the inverse of the admittance, can be

calculated from Eqn.(1).

Experiments and results

To verify the developed impedance model, an experimental

test has been carried out on an aluminium cylindrical shell

specimen. The equipments and the specimen are illustrated in

Figure 1. Two PZT patches are bonded to the cylindrical shell

at the same location but on the opposite surfaces. Through the

multiplex terminal, an electrical field is applied to the two

PZT patches with opposite directions, so that pure bending

vibration is actuated in the shell. The shell specimen rests on

soft papers to simulate free boundary conditions. A hole is

drilled in the specimen to simulate the damage.

For the undamaged specimen, the real part of the admittance

predicted by the impedance model is compared with the

measured results, as shown in Figure 2. It is observed that the

predicted and measured admittance signatures agree well with

each other, especially the locations of the peaks, which are the

most important information for damage identification, are very

close. For the damaged shell, Figure 3 shows the measured

and predicted EM admittance spectra. Again, predicted peaks

are close to the measured ones. Figure 4 shows the trend of

peak shift predicted by the impedance model for the undamaged

and damaged shells and Figure 5 depicts the experimentally

measured peak shift. Similar trend can be observed from

Figures 4 and 5. That is, certain peaks shift to left due to the

reduction of structural stiffness caused by the damage. It can

be concluded from Figures 2 to 5 that the developed impedance

model is capable of predicting the EM impedance of the PZT

patch bonded on the cylindrical shell specimen. By utilizing

Figure 1. Equipments and specimen of the experiment

Figure 2. Measured and predicted admittance

of undamaged shell

the back-calculation algorithm developed in [2], this model

paves the way for damage identification for cylindrical shell

structures.

Conclusions

The predicted admittance spectra agree well with the

experimentally measured results, which not only verifies the

developed impedance model but also validates the feasibility

of the model for the damage identification of the cylindrical

shell structures. Therefore, the utility of the EM impedance

method for structural health monitoring can be effectively

extended to the cylindrical shell structures.

References

[1] Yang YW, Xu JF, Soh CK. (2005). A generic impedance-

based model for structure-PZT interacting system. J

Aerospace Engnr, ASCE; 18(2):93-101

[2] Xu JF, Yang YW and Soh CK. 2004. EM impedance-

based structural health monitoring with evolutionary

programming. J Aerospace Engnr, ASCE; 17(4):182-193

Figure 3. Measured and predicted admittance of damaged shell

Figure 4. Predicted admittance for undamaged

and damaged shell

Figure 5. Measured admittance for undamaged

and damaged shell

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Krylov Precise Time-Step Integration MethodFung Tat Ching ([email protected])

Chen Zhen Lin ([email protected])

Introduction

The equations of motion of a multi-degree-of-freedom system

can be written as

(1)

where [M], [C] and [K] are time-invariant mass, dampingand stiffness matrices, respectively, {r(t)} is the externalforce vector and {x(t)} is the unknown displacement vector.

The transient responses of Equation (1) are usually obtainedby means of time-step integration methods. In 1994, Zhongand Williams [1] proposed the precise time-step integration(PTI) method. Linearly varying loading form was consideredwithin each time step. This method could give very accuratesolutions by just using only one time step over a large interval.

PTI method

In the PTI method [1], Equation (1) is rewritten as where

The solution at t = ∆t is given by where {U0} is the given initial

condition and {Up} is the particular solution depending onthe excitation {R}. A special feature in the PTI method isthat the exponential matrix exp([A] ∆t) is to be computedrecursively from [exp([A] ∆t/2N)] by N matrix multiplicationsusing the scaling and squaring procedure.

As the dimension of the system increases, the computationalcost increases tremendously. Here, the Krylov precise time-step integration method is proposed to reduce thecomputational cost. Also, the computational efficiency canbe further improved by employing the Padé approximationin computing the initial matrix required in the computation.

Krylov subspace method

The main feature of the Krylov subspace method [2] is toseek an approximate solution to the original problem froma subspace. The original matrix problem is approximatedby another problem of smaller size. Hence the computationalcost can be reduced significantly.

Consider a given square matrix [A] and a nonzero vector{v}. The Krylov subspace defined by

is referred as the m-th Krylov subspace associated with thepair ([A],{v}) and is denoted by Km([A],{v}) or simply Km

if there is no ambiguity.

By using the Krylov subspace method, it can be shown that[2]

where β = ||v||2, [Vm] = [v1, v2, ..., vm] is the orthogonal basisfor the Krylov subspace Km, which can be obtained by theArnoldi algorithm, and [Hm] is an m×m upper Hessenbergmatrix given by [Hm] = [vm]T[A][Vm] . Typically, m is muchsmaller than the dimension of [A].

Padé approximations

It is important to note that in the Krylov subspace method,the accuracy of exp([Hm]∆t) will influence the accuracy ofexp([A]∆t) and hence the stability of the Krylov PTI method.As a result, it is necessary to employ an accurate and efficientalgorithm to compute exp([Hm]∆t).

To improve the computational efficiency, exp([Hm]∆t) canbe computed from the (p, p) diagonal Padé approximationwhich is given by

(2)

where

and

To improve the computational efficiency further, the scalingand squaring procedure in the PTI method is used to computeexp([Hm]∆t) with exp([Hm]∆t/2N) computed from the (p, p)diagonal Padé approximation.

Algorithm analysis

If the dimension of the system in Equation (1) is n, the totalcomputational counts to compute the multiplication of theexponential matrix and the initial vector in the original PTImethod and the Krylov PTI method are

and

respectively. As a result, the Krylov PTI method will bemore efficient than the original PTI method if

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(3)

It can be seen from Equation (3) that the range of m increaseswith ∆t. If ∆t=T , then m can be a very large value. On theother hand, the Krylov PTI method will not be efficientwhen the time step size is too small. Fortunately, therestriction of the time step size is relaxed by using the PTImethod. In conclusion, the current Krylov PTI method willbe more efficient than the original PTI method when thevalue of m used in the computation is not beyond the rangegiven by Equation (3).

Numerical example

A numerical example is given here to demonstrate thenumerical accuracy and efficiency of the present Krylovprecise time-step integration method in solving large-scaledynamic problems. The example is run on a PC with aPentium III 1GHz CPU.

Consider an n degree-of-freedom system governed by

where t ∈ [0,100], [M], [C] and [K] are n×n matrices givenby

and {f0} and {f1} are n×1 vectors with repeated triplets ofelements as shown below

{f0} = [1, 2, 0, 1, 2, 0, ...]T

{f1} = [0.01, 0.02, 0, 0.01, 0.02, 0, ...]T

The initial displacement and velocity vectors {x0 } and { }are, respectively

The results given by the original PTI method and the presentKrylov PTI method are given in Table 1. For the presentKrylov PTI method, the order of Padé approximations p ischosen as 4. The values of m used in the computing processare within the range determined from Equation (3). Table 1shows that the Krylov PTI method is much more efficientthan the original PTI method when the dimension of thesystem is larger than 100. As the dimension of the systemincreases, the advantage of the present method becomes moreobvious.

Conclusions

The Krylov subspace method is used to reduce the dimensionsof the computing matrices. To improve the computationalefficiency, equal order Padé approximations are used toevaluate the initial matrix to be used in the scaling andsquaring procedure. The efficient range of m (order of theKrylov subspace) has been studied. The proposed KrylovPTI method is shown to have high efficiency and accuracy.

References

[1] Zhong WX, Williams FW. “A precise time stepintegration method”. Journal of Mechanical EngineeringScience 1994; 208: 427~430.

[2] Gallopoulos E, Saad Y. “Efficient solution of parabolicequations by Krylov approximation methods”. SIAMJournal on Scientific and Statistical Computing 1992;13: 1236~1264.

Table 1. Computational effort to evaluate the result xn, at t=100s for various n

Dimension of system (n) 40 100 150 200 300

Result Xn 565.459645 825.991465 825.813988 852.437004 825.813988

Original PTI Time step ∆t = 100s ∆t = 100s ∆t = 100s ∆t = 100s ∆t = 100s

method [1] Time cost(s) 0.520 16.634 58.484 149.435 516.512

Krylov PTITime step ∆t = 10s ∆t = 10s ∆t = 10s ∆t = 10s ∆t = 10s

method (p=4)m 29 28 29 29 28

Time cost(s) 1.642 9.564 26.187 47.909 106.473

Note: For the Krylov PTI method, N in each time step depends on ||Hm•∆t||∞ and ranges from 11 to 13 in the present calculations.

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Fuzzy GA for Optimal Vibration Control ofSmart Cylindrical Shells

Yang Yaowen ([email protected])

Jin Zhanli ([email protected])

Soh Chee Kiong ([email protected])

IntroductionPiezoelectric materials have been applied in structural

vibration control to take advantage of their fast response,

their flexibility to be used as sensors and actuators(S/As) in

a large variety of applications, and the fact that they provide

broadband frequency responses. The piezoelectric S/As have

to be of suitable size and placement to ensure maximum

effectiveness and efficiency. A lot of research effort has been

concentrated on beam and plate structures for optimal

vibration control. However, despite their wide applications,

the investigation on optimal vibration control of shell

structures has seldom been reported due to the complexity

of the problem.

Fuzzy logic provides an easy way to deal with complex

problems because it can be built with fuzzy models containing

vagueness and impreciseness in knowledge representation.

In this article, a fuzzy rule-based system is developed and

incorporated into a genetic algorithm (GA) to search for the

optimal design of piezoelectric S/As for the vibration control

of cylindrical shells.

Energy-based approach for optimal design

Figure 1 shows a thin cylindrical elastic shell with NP pairs

of collocated piezoelectric S/As bonded on its surfaces. The

most attractive methodology that accounts for transient

vibration responses is characterized by the maximization of

the dissipation energy extracted by the feedback control

system. The more the energy is dissipated by the control

system, the less the energy is stored in the system. When

considering a constant negative velocity feedback and using

the state-space method, the total energy stored in the system

can be written as , where is the

initial state and P is the solution of the Lyapunov equation

, in which

;

;

;

;

Upq (x,β) and ηk(t) are the mode shape functions and modal

participation factors of the transverse displacement,

respectively; G is the feedback gain matrix; N = m•n is the

number of vibration modes under control; ωk = ωpq,

k = n•(p–1)+q, p = 1,2,...,m, q = 1,2,...,n, ωpq is the natural

frequency; ςk is the damping ratio of the k th vibration mode;

is the effective electrode surface of sensor i; rs and ra

denote the distances from the neutral surface of the cylindrical

shell to the mid-surfaces of the sensor and the actuator,

respectively; hs is the thickness of sensor; e31 is the

piezoelectric stress constant; ε33 is the permittivity constant;

and ρ and µ are the mass density and Poisson’s ratio of the

shell, respectively. More details of derivation can be referred

to [1].

The problem can be expressed as a nonlinear optimization

problem as

, subject to (1) variable

limits: , ; (2) static constraints:

and (3) dynamic constraints which avoid

overlap of piezoelectric patches.

Fuzzy GA

In this study, a series of linguistic terms are defined to describe

the relationship between the boundary of the piezoelectric

patch and the nearest vibration nodal line. According to the

interpretation of the fuzzy linguistic terms, the following seven

fuzzy terms are used: negative large (NL), negative medium

(NM), negative small (NS), zero (ZE), positive small (PS),

positive medium (PM), and positive large (PL). They are

defined by the membership functions shown in [2]. The

knowledge and experience that the piezoelectric patches should

not be located across the nodal lines of the vibration modes

of the structure [2,3] is used to establish the fuzzy rule base

(FRB). A typical fuzzy rule “Coordinate offset = NL and

;

;

;

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Derivative = NT” in the FRB can be interpreted as IF (The

coordinate offset of the piezoelectric patch is NL) AND (The

corresponding derivative of the coordinate variable is NT),

THEN (The coordinate offset of the corresponding coordinate

variable is NM).

Different from the standard GA, the fuzzy GA introduces the

FRB for the local modification of the design variables, which

is of great importance to its search efficiency. As shown in Figure

2, the function of FRB comes after a new generation is produced

and before the next iteration is carried out. Based on the results

of the evaluation for the new generation, the FRB is activated

if necessary. If it is activated, each rule in the FRB will be

checked to make a decision on whether certain design variables

need to be modified.

Numerical examples

The geometric optimization of piezoelectric patches for optimal

vibration control based on the proposed approach is carried out,

and the simulation is implemented for a simply supported

cylindrical shell and a clamped-simply supported plate, but only

the results of the shell are presented due to the length limit of

the article. The comparison between the pure GA and the fuzzy

GA for search efficiency is also presented.

Optimal geometric distributions of the piezoelectric patches for

different vibration modes are calculated by both the pure GA

and the fuzzy GA. 10 runs for each mode are carried out. The

best result of the 10 runs obtained by the fuzzy GA for each

mode is schematically plotted in Figure 3, in which the dashed

lines denote the nodal lines of the vibration modes and the areas

filled with diagonal lines represent the piezoelectric patches

bonded on the shell. From Figure 3, it is apparent that the optimal

distributions of the piezoelectric S/As are located within the

Figure 2. Flowchart of fuzzy GA

The results obtained by the pure GA are quite close to those of

the fuzzy GA. However, the fuzzy GA has much higher search

efficiency than the pure GA. Detailed comparisons of the two

GA methods for the generation in which the optimum is found,

the average best fitness and the percentage reduction in iteration

are listed in Table 1. It can be found that with the fuzzy GA,

the iterations of the genetic optimization process can be

significantly reduced.

Conclusions

A fuzzy GA for optimal vibration control of smart shell structures

has been developed to improve the search efficiency of the pure

GA. Numerical results show that the optimal distributions of

the piezoelectric S/As are located within the regions separated

by the vibration nodal lines. Compared to the pure GA, the fuzzy

GA can reduce the required computing cost by 20-40%.

References

[1] Yang YW, Jin ZL and Soh CK. (2006). Integrated optimization

of control systems for smart cylindrical shells using a modified

GA. J of Aerospace Engnr, ASCE; 19(2): in press

[2] Yang YW and Soh CK. (2000). Fuzzy logic integrated genetic

programming for structural optimization and design. J Comput

Civ Engnr, ASCE; 14(3):249-254

[3] Tzou HS and Fu HQ. (1994). A study on segmentation of

distributed piezoelectric sensors and actuators: Part I –

theoretical analysis. J Sound and Vib; 172(2):247-259

Figure 3. Optimal geometric distribution of piezoelectric

patches for different vibration modes

regions separated by the vibration nodal lines of the cylindrical

shell.

Table 1. Efficiency comparison of two GA methods

VibrationAverage generation Average

Iteration modes

to find optimum best fitnessreduction

Fuzy GA Pure GA Fuzzy GA Pure GA

m=1, n=1 54.3 89.5 3.6920×10–3 3.6920×10–3 39.3%

m=2, n=1 89.6 140.9 5.4831×10–3 5.4831×10–3 36.4%

m=1, n=2 85.6 139.7 7.7511×10–3 7.7511×10–3 38.7%

m=2, n=2 123.0 178.4 9.3672×10–3 9.3708×10–3 31.1%

m=1, n=3 150.2 234.2 1.0845×10–2 1.0929×10–2 35.9%

m=2, n=3 219.8 290.1 1.1651×10–2 1.1732×10–2 24.2%

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Numerical Characterization of Structural Responseto Above-Ground Explosions

Lu Yong ([email protected])Wang Zhongqi

Introduction

The response of building structures to nearby explosions is

complicated by the drastic spatial and time variation of the

blast loads. A sound understanding of the response

characteristics is required before any simplified approach

may be adopted for engineering analysis. In this study, the

structural response to above-ground explosions is

characterized using a full coupled numerical simulation

model. The computational domain encompasses the structure,

the surrounding air, the charge, as well as a layer of soil

medium so that the explosion-induced ground vibration effect

can also be evaluated.

Coupled model and material descriptions

A general configuration of building structures subjected to

an above-ground explosion is depicted in Figure 1(a). The

algorithm of coupling the computational fluid dynamics

(CFD) and computational structural dynamics (CSD) is

adopted to facilitate the interaction between the air shock

wave, the structure, and the ground.

Figure 1: Schematic of a generic coupled model

b) Computed pressure fielda) Model configuration

For the modeling of concrete, the RHT model (Riedel et al.,

1999) is employed. The material model uses three strength

surfaces; an elastic limit surface, a failure surface and a

remaining strength surface for the crushed material. The

failure surface Y is defined as

(1)

where , fc = compressive

strength, A = failure surface constant, N = failure surface

exponent, p* = pressure normalized by fc , p*spall = p* (ft / fc).

FRATE(•ε) is the strain rate function. R3(θ) defines the third

invariant dependency of the model as a function of the second

and third stress invariants and a meridian ratio.

The elastic limit surface is scaled from the failure surface,

while the residual strength is defined with a residual failure

surface. The damage in concrete is accumulated via a damage

model relating to the cumulative plastic strain.

For the reinforcing steel, the John-Cook model is adopted.

For an appropriate modeling of the behaviour of soil under

severe explosion loading, a numerical three-phase soil model

(Wang et al. 2004) is used. The model formulation can be

roughly divided into two main parts: the equation of state

and the strength model. Satisfying the continuity

requirements, it follows

(3)

where V = volume of a soil element, V0 = initial total volume

of the element, Vw = volume of water, Vg and Vs = volumes

of air and soil particles, respectively.

The pressure load causes deformation in each phase, as well

as friction between the solid particles and deformation of the

bond between the solid particles. The friction force and the

force due to the bond all exerts on the solid phase. Satisfying

the equilibrium leads to an incremental form of the equation

of state for the multi-phase soil, in which the contributions

from the constituent phases can be obtained from their

independent equations of state or stress-strain relationship.

To include the effect of hydrostatic stress on the shearing

resistance of the soil, the modified von Mises’ yield criterion

is adopted and further modified to take into account the

strain rate, as

(4)

The Jones-Wilkens-Lee (JWL) equation of state is adopted

to model the detonation of the explosive charge.

Numerical investigation

A generic multi-storey reinforced concrete frame is chosen,

as shown in Figure 1. The dimensions of the frame and the

material used are typical for multi-storey building structures.

The explosion scenario is designed such that both local and

entire structural system response can be represented. Thus,

a charge of 1000 kg TNT (equivalent) is selected. The stand-

off distance is 30m measuring from the front face of the

structure, giving a scaled standoff distance equal to

3m/kg1/3.

Figure 1(b) depicts the contour of the pressure at a particular

instant. Figure 2 shows typical blast overpressure on the

front face of the structure. The peak overpressure tends to

decrease gradually at higher positions. At the same time, the

arrival time of the shock front gradually delays as the height

increases.

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Storey 1-mid height Floor level 1

Floor level 2 Roof level

Figure 2: Blast pressure load on the front face of structure

Several target points are arranged inside the soil to record

the stress wave propagation. Typical soil pressure curves are

plotted in Figure 3. The pressure in the soil also attenuates

rapidly with the increase of the distance from the charge

location. Near the charge the pressure exhibits extremely

high value. In fact, as can be seen from the contours shown

in Figure 1(b), a half-spherical crater of radius about 2m is

created beneath the point of detonation.

Figure 4 shows the damage contours of the entire structure.

Severe damage is observed to occur mainly in the elements

on the front face of the structure, due apparently to the strike

of direct air shock wave. The remaining part of the structure

exhibit more or less a uniform distribution of damage among

different elements and it is generally minor. This indicates

that the global dynamic response plays a relatively

insignificant role as compared to the direct shock effect on

the front face of the structure.

The damage to the front face elements shows a distinctive

pattern. Within each storey, the severest damage is

concentrated at both ends and around the middle portion of

the column. For the case analyzed, the first-storey column

above the ground level exhibits the severest damage. From

the computed vibration histories at target points all over the

structure, it is observed that the highest velocity of about 6

m/s occurs at the first storey column on the front face,

followed by the second storey column (about 1.7m/s). The

maximum velocity at the joint locations is generally on a

lower order of about 0.5m/s. The displacement profile of the

front face elements also shows apparent column member

deformations. On the contrary, the global response as

represented by the drifts at the floor (or joint) locations is

relatively small. In fact, the maximum relative drift between

adjacent floors is no more than 10mm, or 0.3~0.4% of the

respective storey height.

In order to evaluate the extent to which the explosion-induced

ground vibration could affect the response of the structure,

two additional analyses are performed for separate air

overpressure and ground vibration effects, respectively. In

the analysis for the air overpressure effect alone, an artificial

Figure 3: Stress wave propagation inside soil

Figure 4: Damage contours (left = combined load;

right = ground shock only)

rigid barrier is placed in front of the basement in the soil to

block the propagation of the stress wave in the soil towards

the structure base. Likewise, for the other case a rigid barrier

is placed in front of the main structure to block the

propagation of the air shock wave towards the structure so

that only the ground vibration is effective. The damage

contour for the structure under the air blast alone is found to

be practically identical to that under the combined loads,

whereas the damage due to the ground vibration effect alone

appears to be negligible (Figure 4).

The above results indicate that, for typical frame structures

subjected an above-ground explosion, the damage to the

building frame is primarily concentrated in the elements on

the front face, due to the effect of the direct air shock wave.

The global dynamic response of the system is relatively

insignificant. Besides, the ground vibration effect appears to

produce only some secondary vibrations.

References

[1] Riedel, W., Thoma, K., and Hiermaier, S., Numerical

analysis using a new macroscopic concrete model for

hydrocodes. Proceedings of 9th International Symposium

on Interaction of the Effects of Munitions with Structures,

Berlin, Germany, 1999: 315-322.

[2] Wang, Z., Lu, Y., Hao, H. and Chong, K., A full coupled

numerical analysis approach for buried structures

subjected to subsurface blast. Computers and Structures,

2004; 83(4-5): 339-356.

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Generic Beam-Column Model for Responseto Impulsive Ground Shocks

Gong Shunfeng ([email protected])Lu Yong ([email protected])

Introduction

Under impulsive ground shock excitation, the response of

beam-column frame structure can exhibit characteristics of

both impact and natural earthquake responses, where the

critical response may be governed by local dynamic

phenomenon while the global dynamic response also

participate to a lesser extent (Lu et al. 2001; Ma et al. 2003).

To accommodate both levels of the dynamic effects in the

analysis, the present study proposes a beam-column model

with concentrated mass to represent the general dynamic

system. The formulation of the model is based on the

Timoshenko beam theory. The nonlinearity is modeled

through three nonlinear response mechanisms, while a general

hysteretic model is adopted to describe the nonlinear cyclic

behaviour.

Model formulation

The generic column-mass-spring system, shown in Figure 1,

is considered to represent the critical column response to

horizontal component of the ground shock. By adjusting the

amount of the concentrated mass at the column top and the

spring constant, a certain global mode contribution can be

readily incorporated.

Figure 1. Simplification of frame into beam-column

with concentrated mass model

The generic beam-mass-spring system, also shown in Figure

1, is considered for the analysis of the dynamic response of

the beam (floor) system under vertical component of the

ground shock. The two end-springs are used to represent the

axial stiffness of the connecting columns. The two

concentrated mass at the ends of the beam can be adjusted

such that the participation of the axial vibration mode along

the column axis can be represented. Besides the analysis of

the beam response, the vertical forces in the spring supports

can provide a representation of the axial forces in the

columns.

Because of the high frequency (short duration) nature of the

ground shocks, it is important to include the shear

deformation and the corresponding effect into the analysis.

For this reason, the formulation is based on the Timoshenko

beam theory. The Timoshenko beam theory takes into account

the rotary inertia and shear deflection that are not modeled

in ordinary Euler-Bernoulli beam theory. The slope of the

deflection curve depends not only on the rotation of cross

sections of the beam but also the shear deformations. The

governing equations of motion for Timoshenko beam

considering base excitation may be written as

(1)

(2)

where M , Q = bending moment and shear force, respectively,

I = moment of inertia, A = cross-sectional area, ρ = material

density, β = rotation of the cross-section due to bending,

w = transverse displacement, and ag = acceleration of ground

motion.

Besides the elastic bending and shear response, the

constitutive models include the nonlinear sectional moment-

curvature relationship, the diagonal shear behaviour (shear

force-shear strain relationship), as well the direct shear-slide

displacement relationship (Figure 2). The skeleton

relationships are derived from the typical procedures for the

respective analysis, while a general hysteretic model is

adopted to describe the hysteretic loops for all these responses

with appropriate parameter settings.

For the skeleton moment-curvature relationship, the typical

fibre model is employed. In this model, a cross section is

divided into fine strips parallel to the axis of rotation, and

considering confined and unconfined concrete zones. The

Figure 2. Primary response mechanisms and

generic hysteretic model

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establishment of the diagonal shear relationship can be

obtained from appropriate analysis model such as the

compression field theory. The direct shear behaviour is

analyzed on the basis of an empirical Hawkins model.

The hysteretic behavior of RC members under cyclic loading

has been a subject of extensive study in the past. In the

present study, the three-parameter Park model is adopted for

all three resistance mechanisms under consideration. The

detailed hysteretic rules governing inelastic loading,

unloading and reloading processes are illustrated in Figure

2. The parameter α controls the stiffness degradation, βcontrols the pinching, and γ controls the strength degradation.

By adjusting these parameters, different hysteretic patterns

suitable to describe the flexure, diagonal shear and direct

shear responses can be achieved.

Numerical procedure and analysis example

The solution of the inelastic beam-column response based

on Timoshenko beam formulation and incorporating the

various nonlinear mechanisms can be obtained numerically.

A computer program is written for this purpose based on the

finite difference technique.

Beam end shear slip Diagonal shear force vs shear strain

As an example, Figure 3 shows an equivalent beam system

under a vertical ground shock. The concentrated mass

(5000kg) at the two ends of the beam approximates the

reactive mass that may be allocated to the beam ends in the

actual frame. A typical response time history and the diagonal

shear vs. shear strain hysteretic loops are also plotted in

Figure 3 to illustrate the analysis results.

The proposal model can be used to analyze the critical beam

and column response to impulsive ground shocks, and it can

also be extended to analyze the beam-column response to

other types of impulsive load and study the corresponding

governing failure mechanisms.

References

[1] Lu, Y., Hao, H., Ma, G. and Zhou Y.X., Simulation of

structural response under high-frequency ground

excitation. Earthquake Engineering and Structural

Dynamics, 2001; 30: 307-325.

[2] Ma, G., Hao, H. and Lu, Y., Modelling damage potential

of high-frequency ground motions. Earthquake

Engineering and Structural Dynamics, 2003; 32: 1483-

1503.

Figure 3. Example analysis results for beam under vertical ground shock

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Reinforced Concrete Frames Subjected ToExplosion-Induced Ground Motions (EIGMS)

PAN Tso-Chien ([email protected])LIM Chee Leong ([email protected])

Introduction

Growing populations, industrial development and commercialdevelopment have narrowed the gap between the demandand supply of surface land. In view of this and ongoingdefence strategies of each country, underground spacedevelopment became important.

Within Singapore, the Protective Technology Research Centrewas established in 1998, via a Memorandum ofUnderstanding between Ministry of Defence, Singapore, andNanyang Technological University (NTU). The researchobjectives included the experimental testing of rockspecimens from geological sites in Singapore, (Zhao et al,1999), the prediction of ground motions generated byexplosion-induced ground motions (EIGMs) (Hao et al, 2001)and the damage assessment of reinforced concrete (RC)buildings subjected to EIGMs (Lu et al, 2001; Pan et al,2004). With the concerted research efforts of Defence ScienceTechnologies Agency (DSTA) and NTU, the first undergroundammunition facilities (UAFs) were constructed in Mandai,Singapore. This development has resulted in the freeing upof surface land and the Institute of Engineers, Singapore,has awarded DSTA and NTU, the Prestigious EngineeringAchievement Awards 2004. Related to the construction ofUAFs, research was undertaken in the establishment of theInhabited Building Distance (IBD), defined as the safetydistance for surface structures around UAFs.

This article summarizes research findings related to theresponse of RC buildings subjected to EIGMs. Here, theresponse results of material-level models and thoseapproximated by simplified structural-level models arecompared.

RC Frames subjected to EIGMs

Earthquake ground motions and EIGMs are similar in thateach induces structural response due to the inertial forces.However, earthquake ground motions and EIGMs differ inthat EIGMs occur within much shorter durations of the orderof microseconds, and EIGMs have higher accelerationamplitudes of the order of 80 g to 150 g. The regulatoryguideline by NATO on UAFs limits the peak particle velocity(PPV) on building sites to be 0.23 m/s in hard rock (NATO,1997). In a separate study, the response results of a 2-storeyRC subjected to EIGMs showed that the PPV limit of 0.23m/s in hard rock may be conservative. Justified relaxation ofthis conservatism would lead to larger allowable IBD, whichwould in turn free up more land (Lu et al, 2001). In thisstudy, the RC frame was represented by a structural-level ofa column over the height of the frame.

In a later study, a material-level model of a 2-storey RCframe was prepared, and the response of the frame whensubjected to an EIGM was determined (Pan et al, 2004). Forthis study, the EIGMs used were generated by scaling asimulated EIGM felt at a 10 m distance from the source ofdetonation of a 100-tonne TNT, with a loading density of 10kg/m3 (Hao et al, 2001). For the material-level model of theRC frame, the damage assessment is presented in Figure 1.Different zones related to the extent and nature of damage,are shown in Figure 1. Also, NATO’s PPV limit of 0.23 m/s is shown in Figure 1. The implications by Lu et al (2001)and Pan et al (2004) agree in that a PPV limited to 0.23 m/s may be conservative. However, an improvement to thedamage assessment of RC frames may include the strengthenhancement of concrete due to high strain rates, which wouldbe inherent during the loading duration of the EIGM.

Here, the objective is to include the strength enhancementeffect due to high strain rate effect for the damage assessmentof RC frames subjected to EIGMs, where the RC frame isrepresented by a structural-level model. Also, anapproximation of the damage of the RC frame by onlyanalyzing the base column of an RC frame is proposed. Theapproximation assumes that the boundary conditions of thebase column are fixed at the top and bottom. It is believedthat this simplified methodology would provide quick damageassessment of an RC frame subjected to EIGMs. Thefeasibility of such a simplified methodology is tested byapplying it on the 2-storey RC frame used by Pan et al(2004), and the results compared with those based on thematerial-level model by Pan et al (2004). The damage resultsdetermined by the simplified methodology are presented in

Figure 1. Damage assessment based on material-level model(Pan et al, 2004)

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A: flexural failure only;B: flexural & shear failureC: shear failure only; D: no failure—: PPV = 0.23 m/s

PPV

(m/s

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Figure 2. In comparison to the damage results determinedby Pan et al (2004), those determined by the simplifiedapproximation makes a distinction between shear failure andflexural failure. The damage results are similar to thosedetermined based on material-level modeling for frequenciesbelow 100 Hz. At higher frequencies, the damage resultsdetermined by the simplified methodology are conservative.This is because the induced shear forces for the assumedfixed-end boundary conditions tend to be larger than thoseof a base column in an RC frame. However, this simplifiedmethodology does provide a quick assessment of the designadequacies for a RC frames subjected to EIGMs.

Further, the damage assessment for a 2-storey and 6-storeyRC frame was performed separately, based on the simplifiedmethodology. The RC frames are designed based on non-seismic designs. Figure 3 shows the lower bound (LB) forthe damage of each RC frame, when subjected to EIGMs.By comparing the trend of structural failure between the 2-storey and 6-storey frame, a relationship between theresistance of RC frames subjected to EIGMs and the heightof the RC frame can thus be postulated. It can be said thata lighter and stiffer RC frame may resist EIGMs of a higherPPV at the same principle frequency, provided that the RCframes are based on the same design code. While such apostulate may seem intuitive, it suggests that maximumallowable PPV for UAFs should then consider the height ofthe RC frame as well.

Conclusion

In this article, the results of a simplified methodology forthe damage assessment of a 2-storey RC frame are presented.The results show that the simplified methodology producessimilar results to those obtained from material-level modeling,for frequencies of explosion-induced ground motions(EIGMs) less than about 100 Hz. Thus, this simplifiedmethodology can produce quick damage assessment ofreinforced concrete (RC) frames subjected to EIGMs. Lastly,based on the damage assessment of a 2-storey and 6-storeyRC frame, it is postulated that a lighter and stiffer RC frame

Figure 2. Damage assessment basedon the simplified methodology

may resist EIGMs of higher PPVs at the same principlefrequency, provided that the RC frames are based on thesame design code. While seeming intuitive, the postulatesuggests that a PPV limit for UAFs should then consider theheight of the RC frame as well.

Acknowledgment

This work was supported by Defence Science and TechnologyAgency through the NTU-DSTA Joint R&D Project titled“Damage Assessment of Lightly Reinforced Concrete Beam-Column Joints under Reversed Cyclic Loading” (PTRC-CSE/LEO/98.02). Also, the authors would like to bid farewell toAssoc. Prof. Zhao Jian, whose research contribution has beensignificant to Protective Technology Research Centre.

References

[1] Zhao J, Li HB, Wu MB, Li TJ (1999) “Dynamic uniaxialcompression tests on a granite”, International Journal ofRock Mechanics and Mining Sciences, 36: 273-277.

[2] Hao H, Wu YK, Ma GQ, Zhou YX (2001)“Characteristics of surface ground motions induced byblasts in jointed rock mass”, Soil Dynamics andEarthquake Engineering, 21: 85-98.

[3] Lu Y, Hao H, Ma GW, Zhou YX (2001) “Simulation ofstructural response under high-frequency groundexcitation”, Earthquake Engineering and StructuralDynamics, 30: 307-325.

[4] Pan TC, Dhakal RP, Lim CL (2004) “Damage assessmentof reinforced concrete building frames subjected toexplosion-induced ground motions”, Technical ReportNo. 2004-01, NTU-DSTA Joint R&D Project on DamageAssessment of Lightly Reinforced Concrete Beam-Column Joints under Reversed Cyclic Loading (PTRC-CSE/LEO/98.02).

[5] NATO, North Atlantic Treaty Organization, AC/258-D/258: Manual on NATO Safety Principles of the Storageof Ammunition and Explosives, Brussels, Belgium, 1997.

Figure 3. LB for 2-storey and 6-storey RC frame

A: flexural failure only;C1: shear failure only; D: no failure—: PPV = 0.23 m/s


PPV

(m/s

)


PPV

(m/s

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SPATIAL INFORMATIONSPATIAL INFORMATIONSPATIAL INFORMATIONSPATIAL INFORMATIONSPATIAL INFORMATIONThree-Dimensional Geological Map of

Singapore with 3D GIS SoftwareZhang Xianhui ([email protected])Tor Yam Khoon ([email protected])

Introduction

Traditional geological maps, which illustrate the distributionand orientation of geological structures and materials on a2D ground surface, are no longer sufficient for storing,displaying and analysing geological information. It is alsodifficult and expensive to update them, especially forcoverage over large areas. Advances in computer technologiesand Geographic Information Systems (GIS) make it possibleto create three-dimensional and interactive geological maps.Using 3DGIS software, the 3D geological map of Singaporewas constructed. The data used consist of, firstly, 136topographical maps of Singapore, in DXF format at a scaleof 1:5000 (Figure 1) and with 3D coordinates of roads,contour lines and other information and, secondly, fourteengeological formations in 2D polygons and in DWG format.The steps involved are: (1) create grid-based Digital ElevationModel (DEM) using the 136 topographic maps; (2) overlaythe 2D geological and reservoir vector data onto the DEMand the road network, contour lines as well; (3) create anintegrated database holding all the 3D models and theirattributes.

Figure 1. Map index of Singapore

The 3DGIS software is a research software developed by theState Key Laboratory of Information Engineering inSurveying, Mapping and Remote Sensing (LIESMARS) ofWuhan University, P.R.China, and it includes four modules,namely, TinModel (Triangulated Irregular Network Model),3DCM (Three Dimensional City Model), 3DDB (ThreeDimensional Database Builder), and 3DViewer (ThreeDimensional Viewer).

Digital Elevation Model of Singapore

The basis of such a 3D geological map is a DEM, which isa digital representation of the terrain surface in 3D. Asbuilding DEM of coastal regions has one extra step than thatof inland regions (Figure 2), the 136 topographic maps ofSingapore are organized into 29 workspaces - 23 of themcomprise coastal areas and the remaining 6 are of inlandareas. The contour lines, roads, coastal lines are extractedfor separate DEM generation using the TinModel module.These DEMs are integrated as a whole DEM of Singaporeusing the 3DDB module. The final gridded DEM data with5-metre intervals for the whole Singapore is obtained (Figure3). The file size is about 574MB.

Figure 2. Flowchart of creating Singapore DEM

Figure 3. Integrated DEM of Singapore

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3D Geological Map of Singapore

The available geological formations comprise 2D polygons.Draping them on the DEM can be achieved in two stepsusing the 3DCM module. The first step is to overlay theboundaries of the polygons onto the DEM surface. Whencreating a 3D model, the software prompts users to definewhether the elevations are relative or absolute.

Selecting a “relative elevation” of 0 m ensures that theboundary of the geological surface has the same elevationsas the DEM. The second step is to populate the interior ofthe geological surface by 3D points using the “InterpolateDEM” function. This function extracts the respectiveelevation data from the underlying DEM.

The centrelines of the roads (expressway, major road andminor road) are available from the 1:10000 scale planimetricmaps. These 2D centrelines can be converted to 3Dcentrelines by selecting the “relative elevation” parameter -the same approach as creating the geological surfaces model.The “convert line to road” function was applied to transformthe 3D centrelines to roads with the stipulated width andtexture.

The 3D contour lines from the 1:5000 scale topographicalmaps can be imported directly using the 3DCM module byselecting absolute elevation. No interpolation is needed.

Integrated Database of 3D Geological Map ofSingapore

In order to facilitate users in viewing and querying the 3Dmodels of the geological surfaces in a seamless manner, arelational database was built to hold the geometric and theattributes information of the geological surfaces, reservoirs,roads and contours models. The geological name, age,

Figure 4. 3D geological map of Singapore

description and the mechanical properties, such as Young’smodulus are linked to the geological model. Reservoir modelhas the name property only. Road model has road name andclassification properties. These databases are created in3DCM using the built-in file-based database managementsystem.

Using 3DDB, the relational databases of separate 3D modelscan be put together to produce an integrated database whichcan be seamlessly and dynamically viewed in 3DViewer.Figure 4 shows the display of the 3D geological map in3DViewer. The blue surfaces on the DEM are reservoirs.Other surfaces show the various surface geology formations.The green and red lines present the expressways and majorroads, respectively.

Conclusions and Recommendations

The processes of creating a 3D surface geological map ofSingapore using the 3DGIS software have been brieflypresented. This map is capable of storing vast amounts ofinformation and attribute data can be easily and convenientlyretrieved. Progress is on-going to build up the subsurfacegeological structures as well as facilities.

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TRANSPORTATIONTRANSPORTATIONTRANSPORTATIONTRANSPORTATIONTRANSPORTATIONFunctional Framework for an OptimumLiner Service Network Planning System

Lam Soi Hoi ([email protected])Lam Siu Lee, Jasmine ([email protected])

Chuang Min-Hsiang, Gary ([email protected])

Introduction

The planning of shipping liner networks requires a largeamount of data and technical knowledge. The existingapproach is normally driven by the use of a variety of datawith mixed certainty and accuracy. The optimum liner servicenetwork planning system being developed is aimed atintegrating the various data sources with an optimisationmodule, which captures the essential relationship in theplanning of the shipping liner network. A knowledge basewill be developed to capture the domain and technicalknowledge used in the design of the network and to interactwith the optimisation module to yield the most desiredsolution. The results will be displayed graphically and instructured pages consisting of charts and tables, as well asprinted reports to satisfy the needs by the users for purposesof validation and actual usage.

The functional framework is presented for an automatedsystem that integrates the experiences from the planners andoptimisation routines to optimally design shipping routes, aswell as providing a rich set of sensitivity analysis informationto aid the planning process.

Overall structure

Figure 1 shows the proposed overall structure. Basically thestructure can be considered as a three-tier structure. Tier one(layer one) is the general user interface that interacts withthe users to obtain the input and produce output in the desiredformat. Tier two (layer two) consists of the major functionsthat do the calculation, decision making, and optimisationfor the scheduling system. This layer also interacts with thedatabase so that it can fetch, add, delete, and update thedatabase when necessary. It also has to ensure the integrityof the database. The final tier (layer three) can be consideredas the maintenance module. This is the only module thatallows the other departments, such as the finance and logisticsdepartments, to interact with the database. They can add andupdate the database when there is a necessity and the changewill be reflected in the previous two layers so that it canensure the integrity of the database.

Figure 1. Overall structure

Major functionalities

After a short random poll survey on the market and potentialusers of the system, it was found that there are several desiredfunctionalities for the system.

1. Proforma Schedule (PFS)

This schedule is the beginning of a service. The userwill assign the desired route with the individual portslisted. With the automated system, the fields will befilled in automatically with preset values which can bealtered by the user if necessary. The system will thencalculate the relative results from the values to comeout with the optimal date and time to assign the ship toindividual ports. From here, the long term schedule forthat service can be generated together with its revenueforecast. The system will be able to handle insertionand deletion of ports within the desired route and updatethe necessary fields once changes are made.

2. Sensitivity analysis

The system will analyse the impacts of factors, such asfuel cost, freight rate, and load factors, on the cost of

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launching a particular service. The system should updatethe information as soon as possible so that the analysisresults can be deemed as near real time. All the factorsaffecting the sensitivity of the service can be assignedand chosen by the user so that it can be more versatile.

3. Setting high/low seasons

As per other companies in the service industry, linershipping also has seasons. The service rates are higherduring the peak seasons and much lower during the off-peak seasons. The system will determine whether theservice starts in the peak or off-peak seasons and decideon the deployment of that service. According to thevessel size assigned, the user can decide on the frequencyof the service.

4. Cash flow tables

As certain services will not have income especiallyduring the first launch or first few days of the services,the system will calculate the cash flows so that themanagement will learn the capital required to launchcertain services and the number of days before actuallyreceiving positive revenue.

5. Performance analysis of services

After the launch of a certain service, the managementwill like to evaluate the performance of that service.The system will compare the generated results with theactual results recorded by the staff. After the comparison,it will generate tables and graphs to show the differencesbetween the expected and the actual performance sothat the user can fine tune the service to obtain optimalperformance.

Conclusion

The optimum liner service network planning system will bedeveloped as an intelligent decision support system, whichis aimed at combining the domain knowledge of plannersand optimisation based on a large set of data, to provide aplanning tool that can greatly enhance the efficiency andeffectiveness of the liner planning process.

Acknowledgements

This is a funded research project through the MaritimeInnovation and Technology Fund.

Survey on Performance of a GPS-BasedVehicle Tracking and Communication System for

Container Transport in LogisticsLam Soi Hoi ([email protected])

Piotr Olszewski ([email protected])Wong Yiik Diew ([email protected])

The application of Global Positioning System (GPS)technology to track vehicles is on the increase in the logisticsindustry. The use of such technology allows for effectivevehicle fleet management and enhances the service qualityand operational efficiency in the industry. However, the useof such technologies in the logistics industry appears to besporadic at present, despite the apparent demand to enhancethe service efficiencies in the industry. Another key issue isthe occurrence of misunderstandings between the dispatcher/controller and a truck driver. A non-voice communicationsystem such as the GPS-based location and text-baseddispatch system used in the taxi industry will reduce conflictand misunderstanding. Using an on-board display system,the control centre of a transport company can communicatevia Short Message Service (SMS) with its drivers wherever

and wherever they are located. Such an advanced fleetmanagement system, which is mainly built upon a vehiclelocation system with advanced communication means, is pilottested involving three freight companies in Singapore,arranged by SPRING Singapore and CDAS (ContainerDeport Association of Singapore). The total fleet of all theparticipating companies consisted of over 130 prime-moversand about 570 trailers.

The GPS-based tracking and communication system consistsof three main sub-systems: GPS-based asset location system,General Packet Radio Service (GPRS) communication andSMS messaging system, and control centre technologies.Their relationships can be found in the following figure.

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Figure 1. Components of the GPS-based Tracking and Communication System

To evaluate the performance of the system, a before-and-after survey covering three companies was carried out in2004 and 2005. The survey covered the characteristics ofcompanies, their measurement of productivity and servicequality. The data collection exercise resulted in capturing43 truck-days of activities during the ‘before’ period and120 truck-days during the ‘after’ period. A general reductionin the number of containers transported, working times anddistance travelled per prime mover (PM) per day wasobserved. Figure 2 shows the changes in productivity (orworking time) and driving distance per container. There arealso reductions in both the average total working time (by8%) and stop time (by 18%) per container in the ‘after’period. This indicates cutting down on idling time between

Control Centre

GPS-based tracking

Server

DigitalCommunication

TransportationAssets

On-board

terminal

Location and

Communication Devices

Figure 2 (a) working time, and (b) driving distance required fortransporting one container

delivery jobs and an improvement in performance andproductivity in terms of time input per container delivered.The average distance driven per container has been reducedby 15%. Mileage driven with unloaded trailer or without atrailer has decreased even more (19.5%). This is importantbecause driving distance directly affects the cost of transportoperations. These findings suggest improved scheduling andjob assignment under the new tracking system.

Although the study shows that after the introduction of thevehicle tracking system some improvements in productivityhave been achieved, there is considerable scope for furthersystem enhancements. The following developments areenvisaged: the development of an optimised fleet managementsystem, with the aim to minimise empty running and tomaximise fleet utilisation, an integrated logistics informationsystem, such as integration with port information to reducetime wastages, and the tracking of other information such astemperatures to support the required end-to-end control oftemperature in the quality assurance of cold chain operationsin Singapore.

Acknowledgement

The authors appreciate the provision of data for analysingthe performance reported in this article by CDAS andcompanies involved in the pilot study.

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Introduction

Car-following theory is an important element of trafficmodelling that has been researched in various forms for over50 years. It attempts to describe the interaction of two ormore individual vehicles in a single lane of traffic and is amain component in the development of microscopic trafficsimulation models. Most car-following models that have beendeveloped were calibrated with freeway data. However, thefocus of traffic management schemes is often associated withconditions other than those found on freeways. There istherefore a need to assess network performance at the urbanarterial level using models more suited for interrupted flowconditions.

The desired spacing car-following model

The car-following model selected for investigation in thisstudy assumes that when a vehicle n approaches and followsa lead vehicle (n – 1) at any time t, the driver tries to adjusthis acceleration so as to obtain a desired spacing after atime-lag of T seconds, Dn(t + T), i.e.

(1)

where xn–1 and xn are the positions of the lead and followingvehicles respectively, and ε is the driver judgement errorparameter (a random value with mean of 1.0), variable ineach time interval. The ε parameter allows for the inabilityof drivers in the real world to attain the desired spacingexactly due to errors in estimating the leader’s movement.The desired spacing is assumed to be a linear function ofvehicle speed:

(2)

where is the speed of following vehicle n at time(t + T), αn and βn are the individual desired spacingparameters of following vehicle n to be calibrated. Theacceleration of the following vehicle at time t, , whenclose-following is given by:

(3)

Investigation of a Car-Following ModelUsing a Desired Spacing Criterion

Tan Yan Weng ([email protected])Neo Aik Wee ([email protected])

Peter Hidas ([email protected])*

The time-lag T (not to be interpreted as the reaction time) isre-calculated for each vehicle at every time update as thefollowing vehicle plans to get to the desired position behindthe lead vehicle. The following approximation for T is used:

(4)

where Bnorm is the normal deceleration with a value less thanzero.

Field data collection

The model was calibrated using field data collected alongNorth Bridge Road between the intersections with Bras BasahRoad and Middle Road. The traffic situation was recordedover several hours during the weekday mid-day and eveningperiods using a digital video camera from the 25th storey ofa nearby building. The road section consisted of four lanes;only one straight-through lane of the approach (third lanefrom kerb) was studied (Figure 1).

Figure 1. Photograph showing view from observation point

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The video images were projected onto a white board to thelargest extent possible. Lane markings were used as physicalmarkers to draw transverse lines across the carriageway;these markers were then used to determine vehicle positionevery 1 second along the study lane. The dataset for modelcalibration comprised 14 platoons with a total of 85 vehicles,while the validation dataset had 3 platoons with 24 vehicles.Vehicles were classified into car, light and heavy goodsvehicle. Buses and motorcycles were excluded from analysis.

Model calibration and validation

Figure 2 shows the pattern of observed spacing plotted againstspeed for one such platoon C5 of four following vehicles(FV1-FV4) behind the lead vehicle. The solid line representsthe phase when the following vehicle undergoes deceleration;the dashed line represents the acceleration phase. There wasa strong, positive correlation between speed of the followingvehicle and its distance to the lead vehicle. Vehicles travellingat high speeds require much larger gaps to stop in the eventof the vehicle in front of them stopping. The slopes of thecurves were relatively steeper for deceleration than foracceleration over the range of speeds examined, i.e. driverstended to allow for a greater safety distance to the vehiclein front when decelerating than when accelerating. Thedeceleration and acceleration curves also tended to intersectat some value of speed.

Figure 3. Observed versus modelled trajectories forvehicles in platoon C5

Figure 2. Observed spacing versus speed relationshipfor platoon C5

The desired spacing parameters, αn and βn, were estimatedfor individual vehicles by linear regression with spacing asthe dependent variable and speed as the independent variable.The αn parameter (slope of the straight line, in seconds)controls the sensitivity of the following vehicle to attain atargeted spacing with changes in speed. The βn parameter(y-intercept, in metres) is the stopped spacing, i.e. the averagedistance between vehicles when they are stationary in a queue.Both parameters were estimated for separate decelerationand acceleration phases. Figure 3 shows an example of theobserved trajectories of four following vehicles in platoonC5 (blue dashed lines) superimposed with those predictedby the model (solid red lines). The agreement betweenobserved and predicted trajectories was very close.

Results of model validation are presented using platoon V1which comprised passenger cars. Figure 4 shows the plot ofobserved positions for vehicles in platoon V1 versus modelledpositions. The data points were spread out about the line ofperfect agreement (thick black line) during both accelerationand deceleration phases. Most of the data points lay withinthe ±2.5m band indicated by the red lines. The ±5m band isshown by the blue lines.

Figure 4. Observed versus modelled positions forvehicles in platoon V1

Conclusions

This study examined the performance of a car-followingmodel using a desired spacing criterion which is assumed tobe a linear function of speed. The desired spacing parameterswere readily calibrated using field data for different vehiclecombinations in the speed range from 0 to 45 km/h. The useof separate desired spacing parameters for acceleration anddeceleration reproduced the hysteresis loops due to thereaction delays in the acceleration phase. Comparisons ofobserved and modelled positions showed that the model wasable to predict the trajectories of vehicles in a platoonreasonably well. Overall, the model was able to predict theindividual spacing between vehicles to within 3% error.Further work is being carried out to extend the model todifferent driving environments and different lead-followingvehicle combinations.

* Peter Hidas is Senior Lecturer at the School of Civil andEnvironmental Engineering, The University of New SouthWales, Australia.

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Enhancement of Expressway IncidentDetection through Algorithm Fusion

Henry S L Fan ([email protected]) Mak Chin Long ([email protected])

Introduction

Many studies focused on the development of a singleAutomatic Incident Detection Algorithm (AIDA) to detectincident occurrences, and the performance obtained had oftenbeen mixed. To address this issue, an algorithm fusion methodwas developed using 160 incident data collected from theCentral Expressway (CTE) in Singapore. There are variousmethods to implement an effective and efficient algorithmfusion procedure including the application of optimizationtechniques (e.g., trial-and-error, simulated annealing, artificialneural networks). However, the effectiveness of theseoptimisation techniques in fusing a large number ofalgorithms has not been documented in the literature. Thisstudy is an exploratory work, and the promising resultsobtained here relative to the performance of existingalgorithms set the direction of future refinement work.


The algorithm fusion process is illustrated schematically inFigure 1. The Dual-Variable (DV) and COmbined DetectorEvaluation (CODE) algorithms are selected because theywere the best-performing algorithms developed for the CTEsite (Mak and Fan, 2005b). The Double ExponentialSmoothing (DES), Standard Normal Deviate (SND),California Algorithm#7 (Algorithm#7) and Minnesotaalgorithms are used because these are commonly used forexpressway incident detection (Mak and Fan, 2005a). Thesealgorithms were originally developed to use loop-based trafficoccupancy measurements for freeways in the United States.All of these algorithms are fused together in this study to

achieve the desired performance for a study site. It is notedthat the desired performance for a fused-algorithm may varydepending on the expectation of expressway operators, sitecharacteristics, etc. This means a fused-algorithm has to bere-calibrated to adapt to local traffic conditions when thereis a significant change in traffic environment. It is hopedthat the methodology proposed here can be adopted elsewhereto improve incident detection ability. The proposed algorithmfusion method involves the calibration of componentalgorithms; selection of fused-false alarm rate (FAR);determination of algorithm weights; and determination offusion threshold values (see Mak and Fan, 2006).

To measure the effectiveness of an AIDA, it is common touse the relationship between incident detection rate (DR)and FAR. DR is the percentage of incidents (in the database)detected by an AIDA. FAR is the number of false alarmsraised as a percentage of all non-incident intervals in thedatabase. In addition, the efficiency of an algorithm indetecting a set of incidents is typically represented by ameasure known as the Mean-Time-To-Detect (MTTD).

Algorithm fusion

Three algorithm fusion options were formulated and aredescribed in the following sections.

Fusion Option I — Enveloping Individual PerformanceCurves: This involved the construction of an envelope forthe performance curves of the algorithms included in thefusion. The resulting curve reflected the detection ability ofthe fused-algorithms. In this study, the performance curves

Figure 1. Schematic diagram of algorithm fusion

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of DV and CODE algorithms were combined, and the resultingcurve served as the lowest level of performance that a fused-algorithm need to achieve (see Figure 2).

Fusion Option II — Selecting Best-Performing Algorithms:Each of the algorithms included were given an equal weightof unity value, and the excluded algorithms were assigned aweight of “0” value. The following groupings at a fused-FARlevel of 1% are used to demonstrate the potential of thisalgorithm-fusion option in selecting a group of algorithmsthat would provide the best detection performance.

Group 1: comprises the SND, DV, Algorithm #7, Minnesotaand CODE algorithms

Group 2: comprises the DV, Algorithm #7, Minnesota andCODE algorithms

Group 3: comprises the DV, Algorithm #7 and CODEalgorithms

Figure 2 shows the performance of these three algorithmgroups (with a total incident score of at least 2) together withthe performance curves of other fusion options. Of the threegroupings in this option, Group 3 achieved the largestimprovement when compared with the Fusion Option I curve.The significant improvement in the low FAR region isconsistent with the objective of this study which is to achievehigh DR at low FAR values.

Fusion Option III — Applying Different Algorithm Weights:This method used the outputs of all the component algorithmsand assigned different weights to each algorithm to reflect thereliability of the decision raised by each algorithm. Thiseliminated the need to select a group of algorithms with goodperformance which was the case in Fusion Option II. Thealgorithm weights were determined based on the reasoningthat each algorithm should be assigned a weight to reflectappropriately its ability in detecting a set of incidents relativeto other algorithms. Here, the DR and FAR values of individualalgorithms at each of the fused-FAR levels were used tocompute the weights for the algorithm studied.

The performance curves of the Fusion Option III algorithms(at the fused-FAR levels of 1% and 1.5%) are compared withthose of other fusion options in Figure 2. One can see there

Figure 2. Comparison of performance ofalgorithm fusion options

were significant improvements in detection performanceespecially for FAR values from 0.2% to 1.0%, illustrating thepotential of algorithm fusion in enhancing the detection ofincidents along the CTE.

Comparison of algorithm fusion options

Fusion Option I is the easiest to carry out, and acts as areference for evaluating the performance of a fused-algorithm.Fusion Option II and Fusion Option III outperformed FusionOption I significantly over a range of FAR from 0.2% to1.0% with DR above 80%. Although Fusion Option II gaveslightly better performance than Fusion Option III, the effortinvolved in selecting the component algorithms and the limitedchoices in specifying the total incident score (fusion detectionthreshold value) may restrict its application. From thatperspective, Fusion Option III is better as it gives promisingresults with an easy-to-repeat calibration procedure and thefusion threshold value can be varied from 0 to 1, therebyproviding a wider range of desired performance.

One can see that at a DR of 90%, Fusion Option I gave anFAR of about 0.90%, while Fusion Option II and FusionOption III yielded smaller FAR values of 0.26% and 0.40%respectively. From the efficiency perspective, there was nosignificant difference among these fusion options. The AverageMTTD value was 137.0 seconds using Fusion Option I, 130.7seconds for Fusion Option II and 132.1 seconds for FusionOption III.

Conclusions

The fused-algorithms using options II and III outperformedthe DV and CODE algorithms which were the best-performingalgorithms developed earlier for the same studied site. At adetection rate of 90%, these fused-algorithms were able toreduce the false alarm rate by more than 55%. Therefore, thealgorithm fusion method described here can provide enhanceddetection performances, and can serve as an alternativetechnique to the commonly practiced approach of eitherdeveloping a new AIDA or applying the existing AIDAs todetect incident occurrences on expressways.

References

[1] Mak, C.L., and Fan, H.S.L. (2005a). Transferability ofexpressway incident detection algorithms to Singaporeand Melbourne, Journal of Transportation Engineering,American Society of Civil Engineers, Vol. 131, No. 2,101-111.

[2] Mak, C.L., and Fan, H.S.L. (2005b). Evaluation of thetransferability of incident detection algorithms developedfor Singapore expressways, Transportation Planning andTechnology, Vol. 28, No. 5, 315-339.

[3] Mak, C.L., and Fan, H.S.L. (2006). Algorithm Fusion forDetecting Incidents on Singapore’s Central Expressway,Journal of Transportation Engineering, American Societyof Civil Engineers, Vol. 132, (forthcoming).

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Road Traffic Accident Models forSignalised Four-Legged Junctions

Aine Kusumawati ([email protected])Wong Yiik Diew ([email protected])

In road safety studies, there is much effort in relating roadaccidents with operational and engineering factors such as trafficvolume, traffic control and road/junction geometry. A popularapproach involves relating observed accident counts (at definedentities) to the underlying traffic volumes by means of regressionmodels with error structure following several discrete statisticaldistributions. The single, independent variable of traffic volumebasically serves as both a metric of exposure as well as a proxyindicator for inherent risk associated with the operatingenvironment. Past studies have revealed that fairly straight-forward accident-traffic volume relationships can model accidentoccurrences quite satisfactorily, and several local relationshipshave been developed in this study.

Accident-traffic volume models, established using regressionanalysis, were employed to study accident occurrences atsignalised junctions in Singapore. The models used 1999-2003annual casualty (defined as fatal and personal injury) accidentcounts provided by Traffic Police Department at a sample of171 four-legged signalised junctions located in the western partof Singapore. Traffic count data at the junctions were obtainedfrom the inductance loops operated by the Land TransportAuthority; these loops have been found to give overall trafficcounts to generally within ±10% accuracy.

Altogether, three functional forms were investigated, as follows:

Model 1: (1)

Model 2: (2)

Model 3: (3)

where,

µ = annual casualty accident count at a site for agiven year (1999-2003)

Q = total daily junction entry volume at the sitefor the given year

Qp = total daily junction entry volume of majorcross-road at the site for the given year

Qs = total daily junction entry volume of minorcross-road at the site for the given year

k, γ, γ1, γ2 = parameters to be estimated

Model 1 assumes that accident occurrence is related to the sumof entering traffic from all legs while model 2 and model 3assume that accident occurrence is related to the cross-productof entering traffic from the major and minor cross-roads. Fourtypes of nonlinear regression models were considered: Poisson,negative-binomial (NB), zero-inflated Poisson (ZIP) and zero-inflated negative-binomial (ZINB). The NB models were foundto be the most suitable models with significant dispersionvalues (parameter α). The results of the NB models are shownin Table 1.

All the three NB models had a good fit based on the PearsonX2 and Scaled Deviance G2 statistics at the 5% significancelevel. The coefficient estimate for variable ln(Qpt) was foundnot to be significantly different from zero in model 3, hencemodel 3 was not favoured. Model 2 was somewhat better thanmodel 1 by virtue of its slightly larger (less negative) log-likelihood value and slightly smaller AIC (Akaike InformationCriterion) value, but model 1 is the recommended model as ithas the distinct advantage of being a simpler model given thatonly the total entry volume is required for modelling andapplication.

Table 1. Negative Binomial models for signalised four-leggedjunctions

Model 1 (NB):µ = exp(– 10.380 + 1.011 x lnQ) or µ = 3.103 x 10-5(Q)1.011

Degrees of freedom 853

Pearson X2 912.793 χ0.05,853 = 922.1

Scaled Deviance G2 889.619 χ0.05,853 = 922.1

Log-likelihood value -1326.715

AIC value 2657.430

Variable Coefficient t-ratio

Constant k -10.380 -12.877

lnQ 1.011 13.323

Dispersion parameter α 0.617 8.839

Model 2 (NB):µ = exp(– 9.688 + 0.510 x lnQpQs) or µ = 6.203 x 10-5(QpQs)

0.510

Degree of freedom 853

Pearson X2 904.195 χ0.05,853 = 922.1



AIC value 2626.973


Constant k -9.688 -13.589

lnQpQs 0.510 14.122


Model 3 (NB):µ = exp(– 8.514 + 0.167 x lnQp + 0.755 x lnQs) or µ = 2.007 x 10-4(Qp)

0.167 (Qs)0.755

Degree of freedom 852

Pearson X2 854.771 χ0.05,852 = 921.0



AIC value 2616.018


Constant k -8.514 -11.236

lnQp 0.167 1.646

lnQs 0.755 9.023


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WATERWATERWATERWATERWATERStructure of Secondary Flows in

Open Channel with Longitudinal BedformsZhi-Qian Wang ([email protected])

Nian-Sheng Cheng ([email protected])

Introduction

The primary motion in open channel flows is commonlysubject to secondary currents, either in the vertical orspanwise directions or both. In an open channel with agenerally flat bed, secondary flows often appear asstreamwise-orientated circulations. Such circulating secondaryflows can be induced by different causes. According toPrandtl”, there exist two mechanisms responsible for thegeneration of streamwise circulations. The first mechanismis associated with the skewing of the mean flow, i.e. thenon-uniformity of mean flow in the streamwise direction.For example, in curved channels or meandering rivers, flowmay circulate in the cross-sectional plane either driven bycentrifugal or transverse pressure gradient. Such a secondaryflow is called Prandtl’s first kind of secondary flow. Thegeneration of this kind of secondary flow is essentially aninviscid process, and thus can occur either in turbulent or inlaminar flow. The second mechanism for the formation ofstreamwise circulation is related to the non-homogeneity andanisotropy of turbulence. Due to the lateral imbalance ofturbulent stresses, streamwise vortices may be formed, andthen these vortices may be stretched and amalgamated in thetransverse direction, resulting in large-scale flow circulations.Secondary flow so generated is called Prandtl’s second kindof secondary flow, and also turbulence-induced secondaryflow. There are a variety of turbulence-induced secondaryflows in natural streams, which are caused by differentfactors, such as asymmetry of channel boundaries, freesurface effects, variations in bed conditions and instabilitiesin turbulent flows. Those associated with longitudinalbedforms were investigated in this study.Secondary flows generated by longitudinal bedforms oftenappear as pairing, counter-rotating flow cells in the crosssection plane. Such secondary flows are usually called cellularsecondary flows. Although the phenomena of cellularsecondary flows and longitudinal bedforms have been widelystudied, the relevant knowledge, especially for flows, is stilllimited. Due to the difficulties encountered in measurement,the characteristics of cellular secondary flows are not wellunderstood yet. To further understand the characteristics ofcellular secondary flows, a series of experiments was carriedout in this study.

Experiments

The experiments were carried out in a straight rectangulartilting flume that was 14 m long, 0.6 m wide and 0.6 mdeep. Six sets of fixed longitudinal bedforms were used for

generating cellular secondary flows. These bedforms arecategorized into bed strips and bed ridges. The bed strips,characterized by lateral periodic variations in the bedroughness, comprised smooth and sediment-roughened plateswhose surfaces were set at the same level across the channel.The bed ridges were characterized by lateral periodicvariations only in the bed elevation. The flow depths wereall chosen to be nearly the same as the average width ofstrips or ridges. This was determined based on typical valuesof the ratio of flow depth to wavelength of longitudinalbedforms reported in previous experimental and fieldobservations. A Dantec LDA system, FlowLite 2D, wasemployed to measure the vertical and longitudinal velocitycomponents. A DOP2000 ultrasonic pulsed Dopplervelocimeter manufactured by Signal Processing (Lausanne,Switzerland) was used to measure the transverse velocity.

Structure of Secondary Flows

For Case S75, the widths of the rough and smooth bed stripswere specifically set to be equal, and thus the average widthof strips, λ, was equal to the flow depth h. The distributionpattern of V varied with the bed configuration. Downflowoccurred over the rough strips while upflow over the smoothstrips. The maximum vertical velocity was about 2.0% ofthe average velocity of the primary flow in the central regionof the channel. The sidewall effect was significant only forthe zone which extended about 2h from the sidewall.

Figure 1 shows a pair of counter-rotating flow circulations.The breadth of the flow cells is nearly equal to the stripwidth, and the height of the flow cells is the same as theflow depth. The mean magnitude of the velocity vectors isabout 0.01Um and the maximum magnitude is about 0.02Um.The two lateral flows meet near the smooth strip and then go

-2.0 -1.5 -1.0 -0.5 0.0

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0.4

0.6

0.8

1.0

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0.01Um

z/λFigure 1. Velocity vectors of secondary flows measured

for Case S75

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upwards, separating near the free surface. The oppositephenomenon can be observed over the rough strip. Thesecondary flow cell delineated in Figure 1 is characterizedby its typical dimension, which actually is defined by theflow depth in the vertical direction and by the strip width inthe transverse direction. Considering the periodic andsymmetrical characteristics of the secondary flow structure,the stream function, ψ(y, z), may be expressed in the sine orcosine form:

(1)

in which Vr, Lr = typical velocity and length scales,respectively; η = y/h; ζ = z/λ; ϕy = the initial phase in thevertical direction; ϕz = the initial phase in the spanwisedirection; and λ = the average strip width.

By differentiating the stream function, the vertical andtransverse velocity, V and W, can be expressed, respectively,to be

(2)

(3)

Figure 2 shows the comparison between the measured andpredicted results of W and V. In the figure, the computedresults are denoted by solid lines, and the crosses representthe measured velocities normalized by 0.02Um . It can beseen that in spite of the simple mathematical formulationproposed for the stream function, the predicted velocitydistributions are reasonably in good agreement with theexperimental results. This formulation can be further modifiedfor characterizing other irregular secondary flow structures.

Secondary Flow Index

The spatially-averaged magnitude of the vertical velocity ofthe secondary flow, |V|m , is adopted here to evaluate theintensity. It is known that bed shear stress difference wouldcause the imbalance of turbulent stresses in the flow field,which would finally lead to the streamwise vortex and thus

cellular secondary flows. Thus, the dimensionless intensitymay be related to the relative shear stress variation as follows,

(4)

where Um = the depth-averaged velocity of the primary flow;and τbxm= the average bed shear stress. Bed perturbation eitherin bed roughness height or bed surface elevation can beregarded equivalent in term of inducing the lateral variationin streamwise bed shear stress. Furthermore, we also assumethat the shear stress variation induced by the elevationperturbation and that due to the roughness perturbation areadditive. With these considerations, Equation (4) can besubstantiated by the experimental data in the following form

(5)

where ; hmax, hmin =maximum and minimum flow depth; bmax, bmin = highest andlowest bed elevation; and nmax, nmin, nm = maximum, minimumand average Manning coefficient. Equation (5) can be alsoused for estimating the maximum vertical velocity and thusfor computing the velocity distributions within the secondaryflow cell.

Figure 2. Comparison between measured and predicted resultsof V and W for Case S75. The predicted values are denoted

by crosses and the computed results are denoted by solid lines

-1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

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1.0W/(0.02Um)

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Which is the Best Time of ConcentrationFormula for Overland Flow?

Tommy Wong Sai Wai ([email protected])

Introduction

The overland time of concentration is an important parameterin many drainage design methods. Combined with the timeof travel in a channel, it becomes the time of concentrationof an entire basin. The latter is commonly used as a basis forthe determination of the design discharge via the rainfallintensity-duration-frequency curves. As such, the overlandtime of concentration has a major influence on the accuracyof the design discharge. Despite its importance, engineersare often bewildered by the array of formulae that areavailable in the literature and are uncertain about the accuracyof these formulae. The article compares estimates from sevenformulae with experimental values from two bays: concreteand grass.

Time of concentration formulae

The seven time of concentration formulae used in thecomparison are:

Izzard’s (1946) Formula

(1)

where to is the time of concentration for overland flow, in isthe net rainfall intensity, Ni is the retardance coefficient, Lo

is the length of overland flow, and So is the overland slope.The units are min for to, mm h-1 for in, m for Lo, and mm-1 for So, and these units apply to all the subsequent timeof concentration formulae [Eqs. (2)-(7)].

Kerby’s (1959) Formula

(2)

where Nk is the retardance coefficient.

Morgali and Linsley’s (1965) Formula

(3)

where no is the Manning resistance coefficient.

Woolhiser and Liggett’s (1967) Formula

(4)

Federal Aviation Administration’s (1970) Formula

(5)

where Cr = runoff coefficient.

Yen and Chow’s (1983) Formula

(6)

Chen and Wong’s (1993) Formula

(7)

where ν is the kinematic viscosity of water (m2 s-1), and Cand k are constants relating the Darcy-Weisbach resistancecoefficient, f, to the Reynolds number, R, as follows:

f = C / Rk (8)

Comparison of time of concentration formulae

The estimates from the seven formulae are compared withthe experimental values for two bays: concrete and grass.Each bay is 25 m long by 1 m wide and with a slope of 2%.Figure 1 shows the comparison for the formulae that accountfor rainfall intensity. Figure 2 shows the comparison for theformulae that do not account for rainfall intensity, and it isapparent that they are only valid for a limited range of rainfallintensities. Further, a objective function, R2, is used to assess

Figure 1. Comparison of time of concentration formulae thataccount for rainfall intensity with experimental data

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the goodness of fit between the estimated and the observedtimes of concentration. The objective function, R2, is definedas:

(9)

where too is the observed overland time of concentration, toe

is the estimated overland time of concentration, and tom isthe mean of all the observed overland times of concentration.The summations are taken over all the observed and theircorresponding estimated values. A perfect agreement betweenthe estimated and the observed gives a value of unity for R2.A poor agreement gives a negative value for R2. Tables 1and 2 show two rankings of the formulae in terms of the R2

values. It is apparent that from both rankings, formulae thataccount for rainfall intensity generally give better agreementwith the experimental data. The tables also show that thetime of concentration formula that gives the best agreementwith the experimental data is the Chen and Wong’s formula[Eq. (7)].

Conclusions

The comparison of estimates from seven time of concentrationformulae with experimental data from the concrete and grassbays shows that for the formulae that do not account for therainfall intensity [Eqs. (2), (5), and (6)], they are only validfor a limited range of rainfall intensities. The formulae thataccount for the rainfall intensity [Eqs. (1), (3), (4) and (7)]generally give better agreement with the experimental data.Finally, the comparison shows that the time of concentrationformula that gives the best agreement with the experimentaldata is the Chen and Wong’s formula [Eq. (7)].

References

[1] Chen CN, and Wong TSW. (1993). Critical rainfallduration for maximum discharge from overland plane.Journal of Hydraulic Engineering, ASCE, 119(9), 1040-1045.

[2] Federal Aviation Administration. (1970). Airportdrainage. Advisory Circular No. 150/5320-5B,Department of Transportation, Washington, D.C., U.S.A.

[3] Izzard CF. (1946). Hydraulics of runoff from developedsurfaces. Proceedings of 26th Annual Meeting HighwayResearch Board, National Research Council, Washington,D.C., U.S.A., 129-150.

[4] Kerby WS. (1959). Time of concentration for overlandflow. Civil Engineering, 29(3), 60.

[5] Morgali JR, and Linsley RK. (1965). Computer analysisof overland flow. Journal of Hydraulics Division, ASCE,91(HY3), 81-100.

[6] Woolhiser DA, and Liggett JA. (1967). Unsteady one-dimensional flow over a plane - the rising hydrograph.Water Resources Research, 3(3), 753-771.

[7] Yen BC, and Chow VT. (1983). Local design storms,Vol III. Report H 38 No. FHWA-RD-82/065, U.S.Department of Transportation Federal HighwayAdministration, Washington, D.C., U.S.A.

Figure 2. Comparison of time of concentration formulae thatdo not account for rainfall intensity with experimental data

Table 2. Rankings of time of concentration formulae in terms ofR2 value for grass bay

Rank Time of Concentration Formula R2

1 Eq. (7)* with C = 1 & k = 0 (Chen and Wong, 1993) 0.75

2 Eq. (4)* with no = 0.04 (Woolhiser and Liggett, 1967) 0.44

3 Eq. (6) with no = 0.04 (Yen and Chow, 1983) 0.36

4 Eq. (3)* with no = 0.04 (Morgali and Linsley, 1965) -1.41

5 Eq. (5) with Cr = 0.3 (Federal Aviation Administration, 1970) -65.86

6 Eq. (2) with Nk = 0.4 (Kerby, 1959) -72.11

7 Eq. (1) with Ni = 0.06 (Izzard, 1946) -409.35

* formula accounts for rainfall intensity

Table 1. Rankings of time of concentration formulae in terms ofR2 value for concrete bay

Rank Time of Concentration Formula R2

1 Eq. (7)* with C = 3 & k = 0.5 (Chen and Wong, 1993) 0.92

2 Eq. (4)* with no = 0.014 (Woolhiser and Liggett, 1967) 0.89

3 Eq. (3)* with no = 0.014 (Morgali and Linsley, 1965) 0.29

4 Eq. (1)* with Ni = 0.007 (Izzard, 1946) 0.00

5 Eq. (6) with no = 0.014 (Yen and Chow, 1983) - 0.02

6 Eq. (5) with Cr = 0.9 (Federal Aviation Administration, 1970) - 0.71

7 Eq. (2) with Nk = 0.02 (Kerby, 1959) - 0.80

* formula accounts for rainfall intensity

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Effects of Meteorological and HydrogeologicalFactors on Gross Recharge Percentage at Unconfined

Sandy Aquifers with an Equatorial ClimateStephen Tan Boon Kean ([email protected])

Shuy Eng Ban ([email protected])

Introduction

A thorough understanding of the rainfall recharge processesand their controlling factors are essential for rechargeestimation and management of groundwater systems.Groundwater recharge is a much-studied process in thetemperate, semi-arid and arid regions, as groundwater is oftena major water source in these regions to meet the expandingurban, agricultural and industrial water requirements.

This paper investigates the effects of various meteorologicaland hydrogeological factors on the recharge percentages, therainfall-recharge relationships and the recharge thresholdvalues at an unconfined sandy aquifer under an equatorialclimate. The meteorological factors being studied includerainfall intensity, potential evaporation, relative humidity andair temperature. The hydrogeological factors are the vadosezone thickness and the soil hydraulic parameters of sandtextures. Groundwater recharge in this paper follows thedefinition of [1], where recharge refers to rainwater thatpercolates downward and reaches the water table in excessof soil moisture deficit and soil evaporation, and is referredherein as gross recharge.

Study Methodology

The numerical model used in the present study is the VS2DI.The model applies the nonlinear total hydraulic potential -dependent Richards equation as the governing equation forfluid flow. The actual bare soil evaporation rates are computedimplicitly by the program as the upward flux driven bypressure potential gradient between the soil and theatmosphere.

In the present study, the van Genucthen’s water retentionmodel was applied, as it had been found to be sufficientlyaccurate for practical purposes, and very useful evenfor porous materials exhibiting sharp curvature nearsaturation.

The values of soil hydraulic parameters used forrepresenting sand texture in the present study are thosevalues calibrated for a reclaimed land site situated atthe eastern coast of Singapore, named as the ChangiReclaimed Land. The calibrated values of the soil hydraulicparameters are: (porosity) θs = 0.375; (residual moisturecontent) θr = 0.052; (fitting parameter) α = 0.002244 1/mm;(fitting parameter) = 2.247; and (saturated hydraulicconductivity) β = 13.5 m/day.

Results and Discussions

Correlation studies

To investigate the impacts of various meteorological factorson the gross recharge percentage, correlation analyses werecarried out between these factors and their correspondinggross recharge percentages. The results of the analyses areshown in Figure 1.

Figure 1. Correlations between gross recharge percentageand meteorological factors for different ranges of

vadose zone thickness

Gross recharge percentage appears to correlate well withrainfall intensity for all the ranges of vadose zone thickness,where its correlation coefficients fall between 0.70 and 0.88.These results indicate that rainfall intensity has the mostsignificant impact on gross recharge percentage for a widerange of vadose zone thickness, among all the testedmeteorological factors. As soil evaporation is a function ofpotential evaporation rate, relative humidity and airtemperature, the correlation results imply that the impact ofsoil evaporation on gross recharge percentage becomes morenoticeable when the vadose zone thickness is larger than2.5m.

Rainfall-recharge relationship

The rainfall-recharge relationships for the 4 ranges of vadosezone thickness are shown in Figure 2.

Obviously, the gross recharge percentage increases as rainfallintensity increases. Its sensitivity to rainfall intensity isgenerally very high for rainfall intensity lower than 25mm/day, but decreases dramatically as rainfall intensity increases.These rainfall-recharge characteristics can be defined wellby a normal-log relationship, where the coefficients of

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Figure 2. Rainfall-recharge relationships for differentranges of vadose zone thickness

determination of the curves are generally high as shown inFigure 2.

These equations could be applied directly to estimate grossrecharge percentages for individual rainfall events, as theequations were developed from datasets that were generatedbased on individual rainfall events. To estimate gross rechargepercentages for rainfall clusters, the application of theseequations needs further adjustments as rainfall rechargecharacteristics at unconfined sandy aquifers were found tobe different between individual rainfall events and rainfallclusters [2].

Gross recharge percentage appears to depend strongly tooon the vadose zone thickness. Gross recharge percentagesare much higher for shallow water table conditions (1.0 to2.5m), compared to intermediate depth of water table (2.5 to4.0m) as depicted in Figure 2. However, its sensitivity to thevadose zone thickness is more significant under low rainfallintensity (<25mm/day), and decreases as rainfall intensityincreases.

Recharge Threshold

By integrating uncertainties associated with the regressedcoefficients, the probable range of recharge threshold foreach range of vadose zone thickness was estimated. Theestimated ranges of recharge threshold for unconfined sandyaquifers under an equatorial climate are shown in Table 1.

Table 1. Estimated ranges of recharge threshold values forunconfined

sandy aquifers under an equatorial climate

Vadose Zone Thickness (m) Range of Recharge Threshold (mm/day)

1.0 < VZ 2.0 0 – 0.12.0 < VZ 2.5 0.5 – 1.12.5 < VZ 3.0 0.8 – 4.13.0 < VZ 4.0 1.8 – 8.0

Sensitivity to Soil Hydraulic Parameters

In this study, the influence of soil hydraulic parameters ongross recharge percentage was investigated. This could beachieved through investigating the sensitivity of Uz (averagevertical velocity of fluid flow in the vadose zone) to the soilhydraulic parameters, as Uz has been found to have the mostsignificant impact on the residence time tR [3]. Among thesoil hydraulic parameters tested, Uz was found to be sensitiveto θs and Κs , and fairly sensitive to β, but not sensitive toθr and α (Figure 3).

Figure 3. Sensitivity of vertical flow velocity tosoil hydraulic parameters

In order words, the derived rainfall-recharge relationships inFigure 2 could still be applied for estimating gross rechargepercentages for sand textures with θs or Κs , whose valuesare not significantly different from the calibrated values inthe present study. Otherwise, some adjustments would needto be carried out before the derived GR-I relationships canbe used [3].

Conclusions

This study investigates the effects of various meteorologicaland hydrogeological factors on the gross rechargepercentages, the rainfall-recharge relationships and therecharge threshold values at an unconfined sandy aquiferunder an equatorial climate.

The correlation analyses show that rainfall intensity has themost significant impact on the gross recharge percentage fora wide range of vadose zone thickness, among all the testedmeteorological factors. The rainfall-recharge relationshipscould generally be well defined by a normal-log relationship.The rainfall-recharge relationships derived here are applicablefor yielding estimates of gross recharge percentages forunconfined sandy aquifers under an equatorial climate, usingrainfall intensity and vadose zone thickness as input variables.These equations could be applied directly to estimate grossrecharge percentages for individual rainfall events. Applyingthe rainfall-recharge relationships, recharge thresholds wereestimated for unconfined sandy aquifers under an equatorialclimate. The recharge threshold values were found to dependstrongly on the thickness of vadose zone.

For vadose zone thickness less than 2.0m, almost all rainfallintensities are capable of producing gross recharges atunconfined sandy aquifers under an equatorial climate, asthe recharge threshold is generally negligible. Nevertheless,the results show that recharge threshold values increasesignificantly as vadose zone thickness increases.

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From the sensitivity analyses, porosity and saturated hydraulicconductivity were found to have the most significant effectson the gross recharge percentage among all the tested soilhydraulic parameters.

References

[1] Sophocleous M. 1991. Combining the soil water balanceand water level fluctuation method to estimate naturalgroundwater recharge: practical aspects. Journal ofHydrology 124: 229-241.

[2] Tan BKS, Shuy EB, Chua HCL, Yee WK. Studies ongroundwater recharge characteristics at a reclaimed landsite with an equatorial climate using time series andspectral analyses. Hydrological Processes (Accepted).

[3] Tan BKS, Shuy EB, Chua HCL. Effects of meteorologicaland hydrogeological factors on gross recharge percentageat unconfined sandy aquifers with an equatorial climate.Hydrological Processes. (Submitted).

Field Column Study of Organic Removal Efficiencyin a Shallow Vadose Zone

Leong Chorng Min Melvin ([email protected]), Chua Hock Chye Lloyd ([email protected]),Lim Teik Thye ([email protected]), Shuy Eng Ban ([email protected]), Tan Soon Keat ([email protected]),

Lo Yat Man Edmond ([email protected]), Tay Tiong Lee Stephen and Tay Joo Hwa ([email protected])

Introduction

Studies using laboratory columns have generally been usedto investigate the Soil Aquifer Treatment (SAT) process.Laboratory columns, however, require the repacking of soilsand cause disturbance to soil particle orientation. Laboratoryconditions also do not reflect actual field conditions.Therefore, for this study, a field column under ‘undisturbed’conditions was used. The column was installed in-situ at theChangi East Reclamation site with minimal disturbance tothe soil inside the column and the test was conducted underactual conditions in the field.

The objectives of this study were to determine, under fieldconditions, organic removal efficiencies with varyinginfiltration rates under different operating conditions(saturated or unsaturated), using an undisturbed sand columninfiltrated with chlorinated secondary treated wastewater.

Field experiments

A diagram of the field experimental setup is shown inFigure 1. The field column is made of grade 304 stainlesssteel (0.5m diameter and 3.0m length). Installation of thecolumn was carried out using an excavator by the directpush method, ensuring “undisturbed soil conditions” withinthe sand column. The column was installed to a depth of2.5m below ground surface, with 0.5m of the columnremaining above ground to allow for ponding during saturatedruns. A multi-depth sampler was also installed in the middleof the column by the direct push method. A 15m3 fiberglassstorage tank was used for on-site storage of the secondaryeffluent used during the experiment. Chlorinated secondary

Figure 1 Schematic diagram of field installation

effluent was obtained fortnightly from a nearby wastewatertreatment plant with the following characteristics: pH = 6.9± 0.2, Conductivity = 876 ± 195 µS/cm, DOC = 7.5 ± 0.7mg/L and UVA(at 254nm) = 0.166 ± 0.012cm-1.

The operating cycles carried out during the experiment areshown in Table 1. An infiltration rate of 2 39m/d was initiatedin cycle 1 and care was taken to gradually step up flows toachieve saturation (Cycle 2). The same stepping up procedurewas carried out from Cycle 3 to 4. During extended periodsunder saturated conditions, biological clogging was observedto form at the soil surface inside the sand column. Theclogging layer is usually made up of algal and microbialgrowth, and suspended solids deposited on the sand surface.Clogging layers can impede infiltration (see end of Cycle 4,Table 1). After day 52, the experiment was stopped and thecolumn was dried. Preliminary results obtained during thefirst 52 days of testing are reported in this paper.

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Results

For the duration of the experiment, the groundwater levelremained within the column. Infiltration rates, therefore, hadto be monitored closely when stepping-up flows duringunsaturated cycles so as to prevent air entrapment inside thesand column. During Cycle 2, the infiltration capacity wasapproximately 5.6m/d for a ponding head of 0.3m. This cyclewas abruptly ended and was followed by an 8 day unsaturatedcycle (Cycle 3). During Cycle 4, a larger head of 0.42m wasmaintained as compared to Cycle 2. The average infiltrationcapacity, however, was smaller, at 5.0m/d. This diminishedinfiltration capacity was due to the rising groundwaterlevel coupled with the formation of a clogging layertowards the end of Cycle 4, which severely impairedinfiltration rates. During Cycle 5, infiltration rates decreasedto about 1.3m/d.

Figure 2 shows the DOC removed as a percentage of influentDOC concentration. Influent DOC concentrations varied inthe range of 6.34mg/L to 9.08mg/L during the experimentand the range of background DOC of the groundwater variedfrom 4.31mg/L to 5.70mg/L. From Figure 2, DOC removalwas clearly more pronounced under unsaturated conditionsas compared to measurements taken during saturatedconditions. Low infiltration rates below 3m/d duringunsaturated flows also produced greatest DOC removal of29 ± 4%. Average DOC removal for Cycles 1 and 3 were 21± 9% and 29 ± 4% respectively whereas Cycles 2 and 4 onlyachieved 10 ± 6% and 13 ± 5% removal respectively. Thecorresponding average infiltration rates were about 3.8 and3.0m/d for Cycles 1 and 3, and 5.7 and 5.0m/d for Cycles2 and 4 respectively. This implies that a reduction ofsaturation infiltration rates by about as little as 33% couldresult in the doubling of DOC removal. Greatest DOCremoval (30% of influent DOC) was observed during Cycle5 where the clogging layer limited infiltration rates to lessthan 1.5m/d. Based on additional samples obtained fromdifferent depths, for Days 9, 10 and 15, no removal of DOC

Table 1 Method of operation

Mode of Length Flow rate Infiltration Remarksoperation of cycle rate

(m3/day)(days) (m/day)

1 Unsaturated 15 0.47-0.97 2.39-4.94 Flow increased daily

2 Saturated 11 1.10±0.06 5.59±0.30 Constant head(ponding) (average) (average)

3 Unsaturated 8 0.48-0.86 2.48-4.38 Flow increased daily

4 Saturated 10 0.98±0.14 5.01±0.72 Biological clogging(ponding) (average) (average) layer formation towards

end of cycle

5 Unsaturated 8 0..46-0.23 2.32-1.18 Diminishing infiltrationrates due to surfaceclogging layer

was observed below 1.50m. From Days 38 to 43, 65% to100% of total DOC removed occurred in the first 0.75m ofsand even in saturated conditions with infiltration rates above4m/d. Greatest removals of 83% to 100% of DOC removedduring Cycle 5 took place in the first 1.00m of sand.

These sharp reductions in DOC removals coincided with asudden increase in influent DOC for Days 43 to 45. Theresponse of the column to sudden DOC loadings duringsaturated and unsaturated conditions can be compared inCycles 3 and 4. A similar increase in influent DOC wasobserved in Days 28 and 29 during Cycle 3, with little effecton total DOC removal. Cycle 3, which is under unsaturatedconditions, allows air to permeate into soil voids, unlikeCycle 4, which is probably under anoxic conditions in Days43 to 45. This suggests that aeration plays an important partfor microbial populations to withstand sudden DOC loadingsand DOC removal is sensitive to sudden changes in rechargewater quality. Lastly, comparing similar operating conditions,it can be observed that DOC removal increases withsuccessive cycles. An explanation for the increased removalof DOC with progressive cycles could be attributed togenerally lower infiltration rates (in Cycles 3 and Cycle 5)and the biological acclimation of the column.

Conclusions

Increasing infiltration rates during unsaturated cycles from2.4m/d to about 5m/d showed decreasing removals for DOC.Average removal rates of 29% for DOC and were the highestat infiltration rates of less than 3.0m/d under unsaturatedconditions. The lowest DOC removal rates (12%) occurredduring saturated conditions with average infiltrations ratesof about 5.01m/d to 5.59m/d. Decreasing average saturationflow by as little as 33% can double organic removals. Organicremovals were observed to be the greatest in the first 1.0mof the sand (83% to 100% removal).

Figure 2 Time series of DOC, % DOC removal and infiltration rate

conce

ntr

atio

n (

mg/L

)

Days

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Measuring Shear Stresses on a Hollow FibreMembrane with Particle Image Velocimetry (PIV)

Adrian Yeo ([email protected])Adrian Law Wing Keung ([email protected])

Anthony G. Fane ([email protected])

Introduction

Hollow fiber membrane modules are similar to tubularmembranes, but are self supporting and require no separatesupport or backing. Hollow fibers can be operated in lumento shell mode, where the feed is pumped through the hollowfibers, or in shell to lumen mode, where the feed is pumpedfrom the outside of the fibers and the permeate removedfrom the inside of the fibers. The use of hollow fibers havebeen found to be one of the more economical in terms ofenergy consumption, and have the highest surface area tovolume ratio of all the membrane modules.

Several mechanisms for the enhancement of the performanceof membranes due to gas sparging have been identified,including bubble induced secondary flow, physicaldisplacement of the mass transfer boundary layer, reductionin membrane fouling and pressure pulsing caused by slugs.Physical displacement and the reduction in fouling resistanceare thought to be the main reasons for the observedenhancement in gas sparged ultrafiltration with hollow fibermembrane systems. It has been found that bubbling can beexpected to be efficient for limiting particle deposition andconcentration polarization phenomena, but not internalfouling. Recent studies have also recognized that the use ofcoarse, rather than fine bubbling is much more effective inimproving fluxes. Also, loose, rather than tight fibers arerecommended to reduce fouling. A larger air flow ratedecreases the rate at which the pressure rises due to fouling.However, enhancement reaches a plateau as gas flow rateincreases.

For submerged systems, the benefits of bubbling appear tobe the most effective at low liquid velocities. At high liquidvelocities, the performance becomes shear dominated. Thestudies on aeration of submerged hollow fiber membraneshave so far focused on the effect of gas flow rates. However,the effect of bubble size and frequency on the performanceof hollow fibers in submerged systems is not welldocumented.

The objective of this study was to determine the effect ofbubble size, bubble frequency and cross flow on shear stresseson the surface of a hollow fiber membrane both held singlyand in a bundle. The fibers were both tight and loose. The

shear stresses would be related to the performance of themembrane fibre.

Particle Image Velocimetry

Particle Image Velocimetry is a whole field technique is anonintrusive measurement technique for steady and unsteadyflows. It allows the investigation of spatial flow structures.In the plane of the flow field, two dimensional velocityvectors are captured. This technique is uniquely suitable foruse in the study of shear stresses on the surface of themembrane. The shear stresses are sensitive to any disturbancein the boundary layer, which is usually less than a millimeterin thickness. Some techniques such as constant temperatureanemometry require that a probe be placed where the velocityis to be measured. The presence of the probe will inevitablyaffect the flow within this sensitive layer.

Experiments

PIV technique was used to measure the shear stress on thesurface of a single fiber under different bubbling conditions.Figure 1 shows the flow field around the fibre just after thepassage of a bubble. As can be seen from the figure, thereis turbulence induced by the wake of the bubble.

Figure 1 Velocity Vectors when no bubbling is present

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Various bubbling sizes and frequencies were studied. Foreach bubble size and frequency, 1000 sets of readings weretaken. The standard deviations of these readings were plottedtogether with the rate of increase of trans-membrane pressure.An example is shown in Figure 2 which indicates that therate of increase of pressure falls as the standard deviation ofthe shear stress increases.

Conclusions

The transient variations in shear stresses around a hollowmembrane fibre and the standard deviations of the shearstress variations were determined under different bubblingregimes. It was found that the standard deviation of theshear stress was important in controlling fouling when theshear rates were low.

This project will be extended to other aspects of fluid flowin membrane systems. Various configurations of membrane

Figure 2 Varitaion of rate of increase of TMP withthe standard deviation of shear stress

filtration system, such as the flat sheet system, will also bestudied.

This study is done in collaboration with the TemasekProfessor Program for Sustainable Water, supported byA*Star.

Propagation of Tsunami in Malacca Strait -A Solitary Wave Approach

Rosales-Sierra Victor ([email protected])Lim Teck Bin ([email protected])

Tan Soon Keat ([email protected])

The objective of this document is to report the results ofnumerical simulations of solitary wave propagation inSingapore waters from a tsunami initiated in the AndamanSea. A scenario of a tsunami generated northern Andaman issimulated. The results are compared with the Sumatra tsunamiof 26 December 2004. Based on the findings of the numericalsimulations results, the writers concluded that a northerntsunami would have a reduced amount of run-up in Singaporeand other locations along the Malacca Straits.

Introduction

The Maritime Research Centre recently studied differentanalytical initial conditions for tsunami numerical simulations,ROSALES-SIERRA, V., 2005. The model has been verifiedand validated with field measurements of the 26 December2004 Sumatra tsunami. In this study a new scenario with anorthern generated tsunami was analyzed. A model based on3D Navier Stokes finite elements code was used in this study.The topobathymetry needed to set up the numerical modelwas taken from the world data of NOAA, N. G. D. C. N.,2005. Local charts were used to supplement this information.A grid of unstructured elements was then designed to take

into account the details of the zones where the comparisonwas made: west coast of Banda Aceh and Singapore.

Initial conditions

An initial wave of 5m height was observed in the Sumatratsunami 2004. An over prediction of the tsunami height atvarious locations was intentionally produced by the use ofan analytical Cnoidal wave approximation. The reason ofthis over prediction is to get higher wave run-up at theselocations; otherwise the normal simulation heights wouldhave been small and difficult to compare.

Numerical simulations of wave propagation

Numerical simulations were carried out with TELEMAC-3D system developed by Janin, J. M., Lepeintre, F., andPechon, P., 1992. It is a three-dimensional computation codethat describes the 3D velocity field (u, v, w) and water depthh (or free surface S measured from the bed) at each timestep. The code solves the three-dimensional hydrodynamicequations under the following assumptions:

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• Navier-Stokes 3D equations with free surface changingin time,

• Negligible density variation in the mass conservationequation,

• Hydrostatic (general) and non hydrostatic pressureassumed,

• Boussinesq approximation for momentum.

The system comprises many modules which allow forexternal code-interfacing, making it possible for theintroduction of different initial boundary conditions in themodel and simulation.

Numerical domain

The domain is composed by an irregular grid of 4272 nodes.Refinement with smaller elements is localized in the regionaround Singapore and neighboring islands to simulate moreaccurately the run up to the tsunami. The bathymetry overthe domain is shown in Figure 1.

Figure 1. Domain bathymetry with detail around Singapore.The circles represent locations where wave height

comparisons were performed.

The bathymetry was obtained from the NOAA, N. G. D. C.N., 2005. Detailed information was added from local charts.Three locations selected for comparison are as shown inFigure 1.

Scenarios

Four scenarios were computed in the simulations. Table 1shows the characteristic of the different scenarios. The firstis the reproduction of the Sumatra tsunami in 2004 whichwas also used to validate the numerical model. In this scenariothe tsunami originated in the west coast of Sumatra, andreached a local height of 5 m. The initial direction ofpropagation was towards the NE, Titov, V., Rabinovich, A.B., Mofjeld, H. O., Thomson, R. E., and Gonzalez, F. I.,2005 . The second scenario describes the same tsunami height

but transported to a different location: northern AndamanSea. For this scenario the wave front was set quasiperpendicular to the Sumatra tsunami, and the wavepropagated in the direction SE initially. The epicenter of thisscenario followed the geological subduction line of thetectonic plates. The third and fourth scenarios concern anormalized smaller and bigger wave height with tsunamiinitial propagation towards the direction of SE as well. Theratio of normalization was . The last two scenarios servedto analyze the consequence of smaller and bigger northAndaman Sea tsunamis on the areas around Singapore.

Table 1. Numerical simulation scenarios.

Scenario Initial height (m) Initial direction

1 5.0 North-east

2 5.0 South-east

3 3.5 South-east

4 7.0 South-east

Results

Figure 2 shows the wave propagation for scenario 1 (left)and 2 (right). The arrows represent the initial direction ofthe tsunami. From Figure 2 it can be observed that the tsunamiwith direction NE (Sumatra earthquake) is more energeticthan the one with direction SE (northern Andaman Seaearthquake). Note that the diffracted tsunami entering theMalacca Straits with an initial direction of SE has lost moreenergy (wave height) than that of the Sumatra earthquake.The probable reason is that the tsunami was generated at theside of the Bengal Gulf. The Andaman Islands dissipate agreater amount of energy from the north Andaman Seatsunami than the energy from the Sumatra tsunami dissipatedby the Nicobar Islands. The shallow depths of the AndamanSea cause more refraction of the tsunami to the coast, thusreducing the wave energy that is directed into the MalaccaStraits. These two elements are more marked for the tsunamiin scenarios 2 - 4. Hence, the tsunami originated from thenorth with a SE wave direction would be expected toexperience greater dissipation of its energy before propagatinginto the Malacca Straits.

Figure 2. Domain model and tsunami propagation NE (left)and SE (right). Initial wave height H=5m.

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Free surface variations at three selected locations wereextracted from the numerical simulations. Figure 1 showsthe location of the points. The name of the nearest villagewas adopted for the points for easy reference: Port Dickinson,Malarko - Tanjung Piai and Kampong Saigon (from thefarthest to the nearest distance to Singapore).

From the comparisons in Figure 3 and Figure 4, it can beobserved that the tsunami coming from the northern location(scenario 2) is less energetic that the one generated inDecember 2004 near Banda Aceh (scenario 1). Figure 3shows the non-dimensional tsunami height arriving at eachlocation. Note a slight increment in height from Malarko -Tanjung Piai to Kampong Saigon. This is the expectedtransformation attributable to the shallower depth as thetsunamis approaches the coastal waters. Figure 4 shows thetrend of increase of the tsunami height arriving near toSingapore (at Kampong Saigon) for different initial waveheights of the tsunami waves in the SE direction (scenario2). The case simulating the 2004 Dec 26 incident of themore energetic tsunami is also included in Figure 4 for easycomparison. From this Figure, it is observed that even withan increase wave height of 7m, the anticipated increase insea-level is not likely to be worst off than that due to theDec-26 incident. This means that a higher risk is not expectedfrom a tsunami which propagates into the Malacca Straitfrom the north. More scenarios need to be analyzed to bettersupport this observation.

Conclusions

Based on the results of numerical simulations, a tsunamigenerated by a tsunamigenic earthquake at northern AndamanSea in the side of the Bengal Gulf close to the AndamanIslands would have a smaller effect in Singapore waterscompared to a tsunami generated in a location similar to thatof the 26 December 2004 Sumatra tsunami. The reasons arethe dissipation of energy attributable to the Andaman Islandsand the shallow depths in the Andaman Sea and the MalaccaStraits.

References

[1] EERI, 2005: Distribution of the Tsunami Heights of the2004 Sumatra Tsunami in Banda Aceh measured by theTsunami Survey Team.

[2] Janin, J. M., F. Lepeintre, and P. Pechon, 1992:TELEMAC 3D : A finite element code to solve 3D freesurface flow problems. Computer modelling of seas andcoastal regions, Southampton, (UK).

[3] NOAA, N. G. D. C. N., cited 2005: MultibeamBathymetry. [Available online from http://www.ngdc.noaa.gov/mgg/bathymetry/multibeam.html.]

[4] Rosales-Sierra, V., 2005: Initial Conditions on NumericalSimulations of Tsunami Propagation. InternationalConference on Geotechnical Engineering for DisasterMitigation & Rehabilitation, Singapore, World ScientificPublishing Company.

[5] Titov, V., A. B. Rabinovich, H. O. Mofjeld, R. E.Thomson, and F. I. Gonzalez, 2005: The Global Reachof the 26 December 2004 Sumatra Tsunami, 2045-2048pp.

[6] Tyvand, P. A. and K. B. Haugen, 2005: Initial free-surface elevation due to impulsive deflection of a two-dimensional bottom with small slope and small curvature.Fluid Dynamics Research, 36, 1-8.

Figure 3. Comparison of scenarios 1 and 2 for tsunami run-up(normalised) at three locations in the Malacca Straits. The initialTsunami Height H=5m

Figure 4. Comparison of 4 scenarios of tsunami run-up at KampongSaigon.

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ABSTRACTS OF RESEARCH REPORTSABSTRACTS OF RESEARCH REPORTSABSTRACTS OF RESEARCH REPORTSABSTRACTS OF RESEARCH REPORTSABSTRACTS OF RESEARCH REPORTSA Case Study for Structural Collapse AnalysisPrincipal Investigator: Pan Tso-ChienReport No: CEE/2005/144

This study investigates the effects of an external explosion on ahigh-rise commercial reinforced concrete (RC) building. The threatscenarios were defined in terms of a TNT equivalent of explosiveswith short and long standoff distances. The building is of cast-insitu RC structure, which comprises RC frames and shear cores.The study has investigated the structural response to explosion-induced overpressure which looks into the blast resistance of thebuilding, the post-blast fire resistance of the damaged building andthe adequacy of building regulations and codes against progressivecollapse. The results are summarized as follows:

1. Blast Resistance of the building: At a long standoff distance,both the transverse and longitudinal frames could still survive,post-blast, the vertical service loads when the heavily damagedbeams are removed.

2. Post-Blast Fire Resistance of the damaged building: Anapproximate duration of fire resistance of the structure mayrange from 10 min to 50 min depending on the heat curve andthe fire exposure condition assumed for the structural members.

3. Adequacy of Building Regulations and Codes againstProgressive Collapse: RC buildings in Singapore wouldexperience demands on strength and ductility, which are greaterthan their inherent strength capacities and ductility factors forthe blast event at a long standoff distance. More severeconsequences can thus be expected for the blast event at ashort standoff distance.

A systematic review reveals that advanced computationalmethodologies such as coupling Computational Fluid Dynamics(CFD) and Computational Solid Mechanics (CSM) with preciseexperiments should be sought. This will provide a balancedapproach to an integrated, defensive design to protect buildingsagainst multiple hazards. Therefore, more powerful computationalprograms with coupled CFD-CSM capabilities and precise testinghave to be developed in order to better predict the effects of blastload and the resulting structural responses.

Conversion of Food Waste into Biogas and BiofertilizerPrincipal Investigator: Tay Joo HwaReport No: CEE/2005/145

The objective of the project was to develop an integrated foodwaste bioconversion system to produce biogas and fertiliser. Thehybrid anaerobic solid-liquid (HASL) digestion system wasdeveloped to enhance food waste bioconversion in comparison withtraditional anaerobic digesters. The unique features of the innovationwere the enhancement of the conventional two-phase anaerobictreatment of food waste to increase the efficiency of methanogenesisprocess and the methane content in biogas, and to diminish thevolume of liquid effluent from traditional two-phase anaerobicdigestion (AD) system for solid organic waste treatment. Theexperiments were focused on the batch and semi-continuousoperation of the modified two-phase HASL system, which includeda semi-solid recycle reactor as the acidogenic reactor and an upflowanaerobic sludge blanket (UASB) reactor as the methanogenicreactor. Under the HASL system design, the effluent from themethanogenic reactor was used for the dilution of the effluent fromthe acidogenic reactor to maintain optimal pH for methanogenesis

in the methanogenic reactor, and part of the effluent stream wasrecycled into acidogenic reactor to reduce the volume of effluentto be discharged from the HASL system and hence required noadditional water for hydrolysis of food waste feedstock. The coupledsolid/liquid bioreactor was an enhanced two-phase AD system,which was more efficient in the conversion of food waste intomethane and carbon dioxide. 100% of the total methane generatedwas from the methanogenic phase with an average methane contentof 71%. In this modified AD system, the use of UASB reactorenhanced methanogenesis and increased the methane content inbiogas produced. After the success of the lab-scale (5 L) and alarger-lab-scale (100 L) HASL system development, the researchteam has been working on the development of a 3-tonne/day pilot-scale plant with a research grant from Singapore government andindustry.

Horticultural Waste: A New Material for ProducingQuality Soil SubstitutePrincipal Investigator: Wang Jing-YuanReport No: CEE/2005/146

The objective of the project was to develop a new artificial soilusing poor fertility subsoil and local organic materials, such ashorticultural compost and sewage sludge, for landscaping andgardening in Singapore. The addition of horticultural compostimproved the physical properties of subsoil by the reduction ofbulk density, but increase of total porosity, water holding capacityand hydraulic conductivity. The addition of sewage sludge providednutrients for plant growth. The addition of 2% of sewage sludge tosubsoil, amended with 4% of horticultural compost, ensured thebest performance of plant growth. The contents of heavy metals inplants grown in this artificial soil were significantly lower thantoxic levels. Further experiments showed that initial content ofsewage sludge in artificial soil should not exceed 4%, to prevent itstoxic influence, but portional addition of sewage sludge duringplants cultivation can enhance their development and growth.Increase of application rate of horticultural compost in addition tothe mixture of subsoil from 4 to 10% enhanced the plant growthand development. However, dispersibility (i.e., erodibility) ofartificial soil with sewage sludge was higher than in the control. Itwas found use of artificial soil produced from subsoil, 4% ofhorticultural compost and 2 or 4% of sewage sludge could supportplants growth and development without harmful impact of leachedheavy metals on environment. On the other hand, addition ofsewage sludge increased the content of enterobacteria in 4 – 10times, but during the 16 weeks of the field experiment, the contentof enterobacteria decreased by 15 – 40%. The soil substituteproduced from this project meets Class B sludge requirement. Thefindings from this project have been provided to local wasterecycling companies to study the possibility of producingbiofertilizer with addition of sludge and horticultural compost.

The Effect of External Flow on Interior Air QualityPrincipal Investigator: Adrian Law Wing KeungReport No: CEE/2005/147

We investigated the re-entrainment of pollutants around a low-riseindustrial building under opposing winds through experimentalmeans in a wind tunnel. Two scaled models of an industrial buildingfor electrowinning metal extraction were tested. The first modelwas a two-dimensional simplified segment of the building with ascale ratio of 1:40, while the second was a 1:100 three-dimensionalmodel of the full building. Particle Image Velocimetry (PIV) was

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adopted to provide the planar velocity measurements that illustratedthe flow distribution around the building. Flame Ionisation Detection(FID) with propane tracer gas was used to measure the concentrationdistribution.

The results of the 2D model with 90 degree opposing wind showthat the exhaust plume interacted with the opposing wind in twodifferent stages, namely ground attachment and bent-over. Theground attachment stage occurred under low wind speeds, wherebythe exhaust plume exhibited Coanda attachment with the groundsurface before being lifted off by the cross wind and circulated tothe leeward wake cavity. Upon further increase in the wind speed,the bent-over stage occurred whereby the exhaust plume detachedfrom the ground and was deflected upward over the roof, beforebeing entrained by the wake. The re-entrainment ratio decreasedwith the increase in wind speed within the range of wind speedstested, indicating that the range did not include the critical windspeed. Results from the 3-D model with 90 degree opposing windpainted a somewhat different picture and pointed to the significanceof the end conditions. The maximum re-entrainment alwaysoccurred near the two ends of the building, where the pollutantsmostly flowed around the ends rather than over the roof. The re-entrainment ratio was consistently higher at the two ends comparedto the central sections. A building re-entrainment index, BRI, isproposed to characterize the re-entrainment performance of a specificbuilding geometry.

With oblique wind angles, the re-entrainment distribution is complexand highly three-dimensional. The overall results pointed to evenhigher significance of the end conditions. The maximum re-entrainment always occurred at the downwind side of the intake,whereby the dominant pathway for the transport was for the pollutantto go around the downwind building end rather than over the roof.The deviations in re-entrainment ratio between the downwind endand central sections increased further with larger wind angles. Thequantitative dependence of re-entrainment with the wind angle wasdrastic. The average building re-entrainment index reduces almostlinearly from 110 in cross-wind to less than 10 at 45 degree andbeyond.

Theoretical and Experimental Study of the DamageCriteria for RC Structures subjected to Blast InducedGround ExcitationsPrincipal Investigator: Cheong Hee KiatReport No: CEE/2005/148

This study investigated the dynamic responses, buckling and post-buckling of steel columns and the dynamic failure of RC columnssubjected to strong vertical ground motions induced by undergroundexplosion. In the experimental tests, 51 steel columns and 38 _-scale RC columns were tested in a shake-table facility. The influenceof slenderness ratio, initial column imperfection, and the presenceof an added mass on the top of each column to simulate the weightof superstructure over the column were studied. To investigate theeffect of different ground motion frequencies, five differentacceleration waves were used as input waves. Also investigatedwas the effect of ground motion duration on the behaviour of thecolumns.

The results of a numerical treatment of the effects on these columnswere also reported. Several parametric effects, such as slendernessratio, added mass on the top of column, mass eccentricity, groundmotion frequency and duration, and the initial imperfection ofcolumn were taken into account.

It was found that the column axial acceleration response attenuatedas the stress wave propagated from the bottom to the top of the

column, and this attenuation depended on both the principalfrequency of input ground motion and the axial vibration frequencyof the column. The lateral response characteristics were stronglyrelated to both the frequency of the input wave and the naturalfrequency of the column. It was concluded that columns with smallimperfections may buckle if the vertical ground motion is strongenough. However, when the imperfection was large, the columnwill fail in the normal flexural dynamic response instead.

Three critical conditions for response were defined, viz., the dynamicbuckling critical condition, dynamic yielding critical condition andthe dynamic failure critical condition. The corresponding criticalpeak ground accelerations were also estimated for each column.

Wind Tunnel Study on the Aerodynamic Derivatives ofthe Ting Kau BridgePrincipal Investigator: Edmund ChoiReport No: CEE/2005/149

The Ting Kau bridge with its 1,177m length when completed, isone of the longest cable-stayed bridges in the world. It providesthe linkage between the new Chak Lap Kok Airport and other partsof Hong Kong. The bridge is situated within the typhoon belt.Thus it is important to establish the relationship between windactions and bridge response.

For cable-supported bridges, because of the high degree of flexibility,bridge movements are of primary concern in the design of thebridge. The vibration characteristics of a bridge are stronglydependent on the aerodynamic characteristics of its bridge deck.

The motion-induced wind load on a structure with a constant modeshape can be represented by the aerodynamic derivatives, (ADs),Hi* and Ai* (i=1, 2, 3 and 4) These derivatives are functions ofthe physical characteristics (shape and geometry) of the bridgedeck and wind speed. With these ADs known, the behaviour of thebridge under wind action can be predicted.

Wind tunnel section model testing technique is used to obtain theaerodynamic derivatives. The shape and geometry of the Ting Kaubridge deck is modeled. The vertical and torsional vibrationcharacteristics of the bridge are simulated by adjusting the mass ofthe model and the stiffness of the supporting springs.

Estimates of eight aerodynamic derivatives (ADs) or Scanlancoefficients have been found for a range of model wind speedscorresponding to full-scale wind speeds up to a level higher thanthe 60m/sec design wind speed. Clear trends were identified forcross derivatives H

3*, A

1*, and direct derivatives H

1*, A

2*, A

3*.

The methods for recovering the ADs using transient decay appearedto be reliable for a range of wind speeds.

Wind Characteristics of Thunderstorms and their Effectson Wind Loading Design in SingaporePrincipal Investigator: Edmund ChoiReport No: CEE/2005/150

Singapore is situated in the equatorial belt where there are no severetropical cyclones (typhoons). The monsoon winds over these regionsare also mild. Thus, strong winds due to these weather systems arerelatively less severe. On the other hand, equatorial regions havefrequent tropical thunderstorms. Wind speeds of the gust fronts ofthese thunderstorms can be relatively high. Thunderstorm windsplay an important role in defining the wind loading criteria in theequatorial region. Unfortunately, the existing knowledge onthunderstorm is very much inadequate.

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The objective of the project is to obtain the wind characteristics ofthunderstorm wind through on-site measurements of thunderstormwind. The project involves the installation of anemometers at fiveheights on a 150m tall tower. The anemometers are to be fastresponse, 3-directional anemometers capable of measuring the u, vand w velocity components. A rain gauge is also used to monitorthe rain condition during the thunderstorm. Data are recorded byhigh speed data-logging system and transmitted through telephoneline to NTU for processing and analysis.

Velocity profile, gust factor and energy spectrum of thunderstormwind are obtained from the analysis of the wind data. It is observedthat these wind characteristics are very different from those of thelarge scale wind. The normal design procedure for the dynamicdesign of structures due to wind action is found to be not satisfactory.A new dynamic gust factor method using running mean approachis proposed and produced satisfactory result. Findings of the studyare referred to in the new draft of the ISO Wind Loading code (ISOCD4354).

Characterization of Ground Motion in Tropical Soils dueto Transient Dynamic LoadingPrincipal Investigator: Leong Eng ChoonReport No: CEE/2005/151

The objectives of the research are:

(1) To study the free field wave propagation and attenuationcharacteristics from explosive sources and to quantify thecharacteristics of the ground motion particularly in tropicalsoils;

(2) To study the effect of degree of saturation in soils on wavepropagation characteristics;

(3) To determine dynamic soil properties of tropical soils inSingapore;

(4) To study the effect of blast wave loading on structures throughnumerical modeling, and

(5) To develop a triaxial dynamic measurement system for fieldmonitoring of wave propagation and attenuation through soils.

To serve these three objectives, field explosion tests, numericalsimulation of underground explosions, and data analyses werecarried out. The field explosion tests also included sitecharacterization works to determine the dynamic properties of thein-situ soil. The field tests were undertaken in various soil typesincluding sands, clays, and residual soils. The field explosion testsalso included a variety of buried targets to investigate theamplification effects on stress waves caused by these buried targets.The results from the site characterization works and the fieldexplosion tests were used to calibrate a numerical model. The 2-Dand 3-D numerical modelling works were undertaken using LS-DYNA, a commercial finite element program. The field test results,numerical results and published literature were used to developrelationships for predicting of free-field and reflected response ofSingapore soils. The TM5-855-1 (1986) equations for free-fieldstresses and motions were re-written using dimensionless parametersand Poisson’s ratio. It was also found from the analyses that thefree-field stresses and motions were affected by material damping.Charts were thus developed, as a function Poisson’s ratio andmaterial damping. The proposed equations based on the Poisson’sratio and damping ratio provided a more objective estimate of thefree-field stresses and motions.

Analysis of Dynamic Response and Characterisation ofIn-Structure Shock of Explosive Loaded StructuresPrincipal Investigator: Lu YongReport No: CEE/2005/152

This project was aimed to characterize the in structure shock inducedby subsurface explosions in soil. For this purpose, a numericalthree-phase model for soil under shock loading was developed. Afull coupled numerical model was established to simulate the entireprocess involving charge detonation in soil, stress wave propagation,interaction between soil and the concrete structure, and structuralresponse. The coupled model encompasses the SPH technique forthe close-in soil region where large deformation takes place, andthe Lagrangian FE technique for the remaining soil medium andthe concrete structure. Scaled physical models were tested as partof the ETSC test programme in 2004, and pertinent experimentaldata were processed and analyzed. The numerical model was verifiedagainst the ETSC and other benchmark data. Subsequently,parametric calculations were performed to examine the influencesof various parameters, and a simplified approach has been proposedfor the prediction of the in-structure shock.

Shear Strength of RC Columns Under Multi-DirectionalLoadingPrincipal Investigator: Paulus IrawanReport No: CEE/2005/153

Some reinforced concrete (RC) columns in high-rise buildings,bridges and elevated highways suffered severe damage during strongearthquakes due to brittle and catastrophic shear failure. Underseismic loads, depending on the direction of the ground motion andthe connection between the columns and other members, thesecolumns can experience any combination of shear force, axial force,bending moment and torsion. The disastrous collapse of RC columnsduring the earthquakes motivates a study on the behaviour of RCshort columns subjected to multi-directional forces, typical ofseismic or wind load.

Previous researches prove that low shear span ratio elements arecharacterized by high strength and stiffness in monotonic or lowcycle loading and fail at relatively low deformations, in brittlemanner. However, behaviour of relatively high strength concreterectangular short column under high axial load, especially whentorsion is present, is still needs to be further investigated.

Many studies have been conducted on prediction of shear strengthand finite element modelling of reinforced concrete columns.Nevertheless, the present shear design methods do not consistentlyprovide safe and efficient structures. The present finite elementmodels also do not predict accurately cyclic response of shortcolumn particularly under multi-axial loading. Therefore, additionalexperimental and analytical works need to be conducted.

This study consists of three parts. The first part investigates RCshort columns subjected to multi-directional loading. Nine identicalrectangular columns were tested in shear, torsion or combination ofthem under high axial load. Cyclic shear force was applied indouble curvature mode at different angles to the weak axis of columncross section.

The second part of this research simulates behaviour of RC shortcolumns under complex loading conditions by finite element method.The two-dimensional and three-dimensional cracked concrete modeldeveloped in University of Tokyo was modified. Though finiteelement analysis could provide accurate prediction of shear capacityof short columns, a more simple method is desired.

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In the third part of this study, the shear design method A proposedby Architectural Institute of Japan (AIJ) was modified based onrational evaluation of effective compressive strength of concrete,truss angle and stress of transverse reinforcement. The modifiedAIJ’s shear design method is proposed based on the newly derivedexpressions of effective compressive strength, angle of trussmechanism and stress in transverse reinforcement. Severalrestrictions in the original method become redundant and can beeliminated. The modified AIJ’s method provides better predictionof shear capacity of the 58 columns. The method is more rational,reliable and simpler. The method is not only applicable for shortcolumns but also likely applicable for more slender columns.

Ultrasound Treatment of Sludge for Anaerobic DigestionPrincipal Investigator: Show Kuan YeowReport No: CEE/2005/154

The objective of the project is to examine feasibility of lowfrequency ultrasound pretreatment of sludge for enhancement ofanaerobic digestion, and to establish optimum sonication conditionsfor a cost-effective pretreatment. The test results indicated that thesonication resulted in a reduction in the sludge particle size from43-65µm to 7.7-15µm, and an increase in the soluble COD to CODratio of up to 440%, indicating effective disruption of sludge solidsand transformation of organic substances into soluble form.Significant digester improvements in terms of COD removal, solidsremoval and biogas production were observed in all digesters fedwith sonicated sludge. Comparing to the control digester fed withuntreated sludge, the COD removal improved by 1-17%, 1-31%and 1-32% in the digesters fed with sludge sonicated at densitiesof 0.18W/mL, 0.33W/mL, and 0.52W/mL, respectively. The totalsolids (TS) removal improved by 1-21%■ 3-42% and 5-45% in therespective digesters. The improved COD and TS removal efficiencycorresponded with significant increase in biogas production by 45-175%, 140-220% and 86-220% in the respective digesters, as wellas an increase in methane composition by 2-19%.

The experimental results demonstrated that ultrasonication is anefficient solids destruction technique which results in significantreduction in sludge particle size and increase in soluble organics.Anaerobic digesters fed with sonicated sludge indicated enhancedbiochemical degradation of solids commensurate with increasedmethane production. Ultrasound waves applied to sludge couldgenerate cavitation bubbles, which grow and then violently collapsewhen they reach a critical size. The collapse of cavitation bubblesproduces powerful hydromechanical shearing forces along withlocalized high temperature and high pressure. These extremeconditions would lead to destruction of sludge flocs and conversionof partial complex organics into soluble forms, as well as bacterialcell wall rupture releasing ezo-enzymes to biocatalyse hydrolyticreactions. Moreover, breakage of strong chemical bonds ofrecalcitrant compounds is facilitated. Consequently, the sludge ismodified into amendable forms to favor subsequent anaerobicdigestion.

The enhancement could bring about several benefits in wastewatertreatment plant operation. These include capital cost saving out ofsmaller unit digester, cost savings in downstream sludge treatmentand disposal, and increase in energy yield derived from improvedbiogas production.

Application of Smart Materials in Health Monitoring ofCivil InfrastructuresPrincipal Investigator: Soh Chee KiongReport No: CEE/2005/155

The recent advent of ‘smart’ materials technologies has ushered in anew avenue for the development of structural health monitoring(SHM) systems. Smart piezoelectric-ceramic (PZT) materials, forexample, have emerged as high frequency mechatronic impedancetransducers (MITs) for SHM. As MIT, the PZT patches are not onlyrobust, cost-effective, and show high damage sensitivity, but are alsoideal for already constructed infrastructures and currently operationalmachinery because they only require non-intrusive externalinstallation. The piezo-impedance transducers, acting as collocatedactuators and sensors, employ ultrasonic vibrations (typically in 30-400 kHz range) to read the characteristic ‘signature’ of the structure,which contains vital information governing the phenomenologicalnature of the structure, and can be analysed to predict the onset ofstructural damages. High operational frequency ensures sensitivityhigh enough to capture any damage at the incipient stage itself,much before it acquires detectable macroscopic dimensions. Thisnew SHM technique is popularly called the electro-mechanicalimpedance (EMI) technique.

In spite of enormous potential due to its low-cost and high sensitivity,the EMI technique is still in the infancy stage as far as damageseverity assessment or access to the inherent damage mechanism isconcerned. Changes in the diagnostic signature and the nature, severityand type of damage are not well correlated. Till date, all the existingapproaches are non-parametric and statistical in nature, and are ableto utilize only the real part of signature. The information concerningdamage carried by the imaginary part is therefore lost. Besides, noattempt has been made to extract the mechanical impedance of theinterrogated structure from the electro-mechanical signatures, partlydue to the non-existence of suitable impedance models.

This research focused on utilizing the underlying PZT-structureelectro-mechanical interaction for impedance based structuralidentification and SHM using the EMI technique. A new concept ofactive signatures has been proposed to extract the more damage-sensitive information from the raw signatures and a new PZT-structureinteraction model has been developed based on the concept of‘effective impedance’. The proposed formulations can be convenientlyemployed to extract the hidden damage sensitive structural parametersfor any ‘unknown’ structure by means of surface-bonded PZT patches.A new experimental technique has been developed to ‘update’ themodel of the PZT patch, so as to enable it to extract the host structure’simpedance information much more accurately. A unified impedanceapproach has been developed to ‘identify’ the host structure from theextracted mechanical impedance spectra and to carry out quantitativeand parametric damage prediction. This has made possible greaterinformation about the nature of damage in terms of stiffness, dampingand mass changes, which was so far lacking. As proof-of-concept,the new diagnostic approach has been applied on representativeaerospace and civil structural components.

Further, in order to rigorously calibrate the piezo-impedancetransducers for damage assessment, comprehensive tests were carriedout on concrete specimens. An empirical fuzzy probabilistic damagemodel has been proposed for predicting damage level in concreteusing piezo-impedance transducers. In addition, a new experimentaltechnique has been developed to predict in situ concrete strengthnon-destructively using the EMI technique, thereby imparting it furtheredge over the contemporary non-destructive evaluation (NDE)techniques. Finally, the intermediate bond layer between the PZTpatch and the structure has been integrated into the impedance models,thereby enabling a rigorous analysis of the shear lag effect associatedwith the bond layer.

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In addition, the concepts of vibration-based methods have beenintegrated with the EMI technique to extend its applicability fordamage location identification and damage growth characterization.Natural frequency changes are extracted from the admittancesignatures, and the variations in the frequency changes are studiedfor damage location identification and characterization. Based onthe changes in natural frequencies, parametric methods for damagelocation identification and damage growth characterization havebeen developed. The damage location identification method usesthe natural frequency changes extracted from the e/m admittancesignatures and the mode shapes obtained from the FE model forundamaged structure. For damage growth characterization, statisticalmeasures for quantifying the pattern of frequency shifts have beenproposed instead of quantifying the admittance signature changes.The numerical and experimental verification for the two methodsare presented. Another major contribution of this research is thenovel integration of Bayesian network model to e/m impedance-based structural health monitoring. With limited data availabilityand in the presence of model errors this reasoning tool can be usedto enhance the structural damage identification. The errors obtainedin damage location identification due to limitations of the FE modelare eradicated using the integrated Bayesian networks.

Performance Based Design Guide for Fire Resistance ofBare Steel StructuresPrincipal Investigator: Tan Kang HaiReport No: CEE/2005/156

Current practice for fire resistance of steel structures is very muchbased on prescriptive-based design, which is derived from furnacetests conforming to ISO 834 fire curve. This is contrary to theinternational trend of building design development, which is towardsperformance-based design. Prescriptive-based design is often over-conservative as it assumes that steel frame members are isolatedpieces subjected to unrealistic ISO 834 curve that does not have anatural decay period, and once the limiting temperature is reached,frame members will collapse. It does not recognise the fact thatstructural members are interconnected and redistribution of actionscan take place. Consequently, fire protection of bare steel framesbecomes the accepted norm in Singapore. This is an expensiveoperation and is not always aesthetically pleasing especially fornovelty structures that emphasise architectural features such astransparency and sleekness.

The main purpose of this research is to promote steel constructionindustry in Singapore. This is done through

(1) quantification of realistic compartment fire load and the firecurve,

(2) heat transfer model for determining heat flux into steelmembers,

(3) experimental behaviour of restrained connections and restrainedcolumns,

(4) finite element (FE) model for fire resistance of steel members,and

(5) mechanical model which can be done manually or throughcomputer.

This research project seeks to bridge the gap between the disciplinesof structural engineering and fire protection engineering andproduces practical design guide for structural engineers. It isinterdisciplinary as it combines heat transfer model with FEMstructural model. The objectives are

(1) to develop a performance-based design by using more realisticfire curve,

(2) to ensure consistent level of fire safety in steel buildings,

(3) to promote steel construction by making it more economical,and

(4) to encourage more innovative design.

Two levels of design will be offered: viz. a more advanced levelusing computational mechanics programs developed in-house inNTU such as FEMFAN, and a basic level hinged on Eurocodecode member design. A design guide is also prepared for users.

Design of Reinforced Concrete Shear Wall with Opening-Strut-and-Tie ApproachPrincipal Investigator: Li BingReport No: CEE/2005/157

To date limited research on the seismic behaviourof walls withirregular openings has been conducted, and a general designrecommendation based on the strut-and-tie models and the capacitydesign procedures has been provided by the ACI 318 and NZS3101 codes. However, the behaviour of the structural walls withirregular openings subjected to seismic loadings and the details ofthe design procedures have not been adequately addressed.Therefore, experimental and analytical studies on the seismic designof walls with irregular openings were performed in this study. Sixflanged walls, consisting of one solid wall, three walls with irregularopenings, and two walls with regular openings were tested underreversed cyclic loadings. It was found that the flanged walls withirregular openings could perform similar to the solid flanged wall.All of the walls reached the flexural yield strength before failures.The modified strut-and-tie models developed based on the loadpaths of the walls with irregular openings could predict the behaviourof the walls satisfactorily; however, because of the presence offlanges, the walls often failed in sliding failure modes. A series ofparametric studies on the walls with openings were performed usinga reliable non-linear finite element programme. The selectedparameters included the flanges, the axial loadings, the differentloading schemes, the reinforcements in the different zones of thewalls with irregular openings, and the size and arrangement of theopenings etc. Both the quasi-static analysis and the dynamic analysiswere carried out. In addition, a seismic performance evaluation onwalls with openings was performed using the capacity spectrummethod. The seismic demands of these walls were investigated.Based on the experimental and analytical results, a general designapproach was proposed for the design of walls with irregularopenings. In this design method, flexural behaviour of the wallswas studied for the selections of strut-and-tie models and thedeterminations of overstrengths for the shear design, so that theundesirable failure modes could be avoided.

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Studies on Inhibition and Feeding Strategies forDenitrification in Dupont Textiles and Interiors(Singapore) Pte Ltd (DTI)’s Anoxic BioreactorsPrincipal Investigator: Show Kuan YeowReport No: CEE/2005/158

Denitrifying bacteria are aerobic microorganisms that are able toutilize oxidized nitrogen compounds in place of oxygen. A numberof parameters may affect the denitrification process, such as pH,temperature, carbon sources (electron donors), nitrate and nitrite,dissolved oxygen (DO), and the presence of toxic compounds andso forth. Among these parameters, organic substrate or the electrondonor is one of the most important parameters that may influencethe process. This is especially true when these organic substancesare from industrial waste streams. It has been noted that variouswaste streams from different sources have different extent inhibitoryeffects on the denitrifying capability of the microbial community.This may result in malfunction of denitrifying bacteria in the anoxicbioreactors and in turn the process upset. It is therefore necessaryto study the inhibitory effects of these waste streams.

The objective of this work is to study the inhibitory effects of sixwaste streams from DTI on the denitrification capability of thedenitrifying bacterial consortium in DTI’s anoxic bioreactors. Inthis report, the mixed biomass from DTI’s anoxic bioreactors wasused as the inoculums in flasks to simulate the denitrificationprocess. Each waste stream was adjusted to have the same pH,TOC and concentration of nitrate prior to the experiments. Inaddition, pH change before and after denitrification was alsomonitored. The rate of nitrate removal was measured by monitoringthe concentrations of NO

3- and NO

2- periodically. At the end of the

experiments, the microbial community structure was characterizedby the diversity of the 16S rDNA sequences of the microbialconsortium using PCR (polymerase chain reaction) – DGGE(denaturing gradient gel electrophoresis) techniques. Theexperimental results show that the inhibitory effect of all the wastestreams tested is in the order of RCD > WWAQ > CTRL > PET> CSM > SWD > ERT. While RCD and WWAQ have the greatestinhibitory effects, ERT and SWD may enhance the denitrificationprocess. The microbial community diversity in DGGE patternsagrees with the kinetics results.

Integrated Structural Health Monitoring of HighwayBridges in Singapore using Distributed Fibre OpticSensorsPrincipal Investigator: Pan Tso-ChienReport No: CEE/2005/159

This joint research project between CEE and Building andConstruction Authority (BCA) was initiated to study the use ofdistributed fibre optic sensors to monitor structural health of highwaybridges in Singapore.

In this study, 3 bridge sections have been selected for investigation.The research team has collected data via wireless modem, emailedto users daily from a server until May 2004, when the logging ofdata was suspended due to the suspension of construction activitiesat the work site.

The data were also collected via surface mounted sensors after twoattempts to install FBG sensors inside the concrete segments failed.In the FBG method, the gauges failed to give out signal afterconcreting, which could have resulted from mishandling of thesensors and their protections during concreting.

The team has however continued to develop algorithms for datamining and anomaly detection. The algorithms developed havebeen applied to data obtained from an existing instrumentedstructure. Research in this area has been well received internationally.The development of methodologies for finite element modelupdating has also progressed well. Significant spin-off from theresearch has been the increased use of wireless data transmissionfor remote structural monitoring purposes.

Construction progress on the bridge site of PPSE has slowed downalmost to a standstill during 2004. The team has thus been waitingto capture the events of segment closure for the training of astructural health monitoring system. Due to the time frame of theresearch project, the major event of structural loading resultingfrom segment closure has not been captured when the final reportis produced.

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ABSTRACTS OF PhD THESESABSTRACTS OF PhD THESESABSTRACTS OF PhD THESESABSTRACTS OF PhD THESESABSTRACTS OF PhD THESESShear Strength Characteristics Under InfiltrationConditionsCandidate: Inge MeilaniReport No: CEE/PhD/2005/90

Singapore is located in the South East Asian region and has atropical climate. About two-thirds of Singapore’s land area is coveredwith residual soils. Many residual soil slopes have a deep groundwater table. The soils above the ground water table are unsaturatedwith negative pore-water pressures (i.e., matric suction). Matricsuction contributes significantly to the shear strength of soil and tothe safety of slopes. However, water infiltration during rainfallincreases the pore-water pressure in soil, resulting in a decrease inthe matric suction and the shear strength of the soil. Therefore,changes in the negative pore-water pressure associated with heavyrainfalls can result in slope failures. The purpose of this researchwas to study the shear strength characteristics of saturated andunsaturated soils under shearing-infiltration conditions. Thecharacteristics of pore-water pressure development and volumechange in soil under infiltration were investigated. Staticallycompacted kaolin specimens were used in this study to ensure thehomogeneity of soil specimens. In order to study the pore-waterpressures in soil during infiltration, a mini suction probe wasfabricated. Three mini suction probes were placed at 3/4, 1/2, and 1/4

heights of specimen, each at 120 degrees apart in the circumferentialdirection. This study contributed to the development of pore-waterpressure measuring devices for unsaturated soil testing. The minisuction probe can provide a fast response and can be used undera high matric suction for a long period of testing time. The shearing-infiltration tests results show that the failure envelope obtainedfrom the shearing-infiltration tests is unique regardless of theinfiltration rate. The failure envelope from the shearing-infiltrationtests is the same as the failure envelope from the consolidateddrained tests at mean net normal stress, p’ values from 0 to 150kPa. However, the characteristics of the failure envelope from theshearing-infiltration tests at p’ values from 150 kPa to 450 kPadepend on the matric suction. The failure envelope from theshearing-infiltration tests is higher than the failure envelope fromthe consolidated drained tests at matric suctions from 0 to 80 kPa.On the other hand, the failure envelope from the shearing-infiltrationtests is lower than the failure envelope from the consolidated drainedtests at matric suctions higher than 80 kPa. The effect of matricsuction and p’ values on the failure envelopes obtained from theconsolidated drained tests and the shearing-infiltration tests wasexplained using the characteristics of the volumetric water contentat failure, ■ wf. The total volumetric strain, water volumetric strainand pore-water pressures during the shearing-infiltration tests canbe computed using the volume change theory for unsaturated soils.The changes in the total volumetric strain can be related to matricsuction using the elastic moduli for the soil structure (i.e., H),while the water volumetric strain and matric suction changes arerelated using the water volumetric moduli (i.e., Hw). The computedwater volumetric strain and the computed pore-water pressure givea good agreement with the experimental water volumetric strainand the experimental pore-water pressure from the shearing-infiltration tests, respectively. The good agreement can be attributedto the use of Hw parameter from the appropriate scanning curve ofSWCC. The appropriate scanning curve should start from the matricsuction value at the beginning of the infiltration stage.

Instability Behaviour of a Granular Fill MaterialCandidate: Leong Wing KaiReport No: CEE/2005/PhD/91

This thesis presents an experimental study on the instabilitybehaviour of a granular fill material retrieved from a reclamationsite in Singapore. A preliminary study on the instability behaviourof the granular fill material under drained and undrained conditionswas first conducted. The conditions for the occurrence of instabilitywere then established. The failure mechanisms of dense granularsubmarine slopes and the conditions that govern the instability ofdilating granular soil were investigated. The instability behaviourof the granular fill along a drained stress path with decreasingconfining stress and constant deviator load was also studied. Theinstability behaviour of granular soils and its implications for slopestability was discussed. A framework that explains the instabilityoccurring in both loose and dense granular slopes was proposed.The relationship between instability and strain softening of granularsoils was discussed. Finally, the influence of fines content (FC) oncreep and instability behaviour of loose granular fill material wasinvestigated.

Shear Strength of Reinforced Concrete Slabs underSymmetric PunchingCandidate: Lee Sai ChengReport No: CEE/PhD/2005/92

This thesis presents the results of experimental and analytical studyon the punching resistance near the interior column regions ofreinforced concrete slabs. The purpose was to investigate thesuitability of current design guidelines for special situations of slabssupported on rectangular columns and provided with several typesof shear reinforcements, as well as the effect of high strengthconcrete on punching. A theoretical model based on the modifiedcompression field theory (MCFT) was developed to predict thepunching shear capacity, the inclination of cracked surface andstresses along the cracked surface. A simplified solution was furtherdeveloped as a practical design procedure for evaluating thepunching shear strength of slabs. The validity and accuracy of thetwo approaches had been verified by comparing the predicted shearresistance with the experimental results obtained from differentsources and the codes’ predictions. Both methods gave safe andconsistent predictions over a wide range of slab properties.

Evolutionary Programming for Civil EngineeringProblem SolvingCandidate: Xu JianfengReport No: CEE/PhD/2005/93

This thesis develops a general backcalculation algorithm based onevolutionary programming (EP) for structural parameteridentification and damage detection. The search efficiency oftraditional EP is improved by hybridizing with local searchstrategies. The elastic parameters of composite plates are determinedfrom the measured natural frequencies. The obtained results agreewell with those reported in the literatures. The backcalculationalgorithm is then integrated with the impedance-based method todevelop a new damage detection technique. A generic impedancemodel of PZT-structure interacting system is presented to predictthe electrical admittance. The effect of modeling errors issuccessfully minimized by a proposed fitness function. Experimentsare conducted on beams and plates to validate the impedance modeland the damage detection results. The results demonstrate that the

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proposed technique is able to simultaneously identify the locationand quantity of the damage, which overcomes the limitations of thetraditional impedance-based method.

Theoretical and Numerical Studies of Wave AttenuationAcross Parallel FracturesCandidate: Zhao XiaobaoReport No: CEE/PhD/2005/94

This thesis presents studies on wave attenuation across parallelfractures using displacement discontinuity theories. The parallelfractures represent rock joint sets in a rock mass and are assumedto be planar, dry, large in extent and small in thickness relative toincident wavelength. A theoretical approach is developed to derivetheoretical solutions of one-dimensional wave attenuation acrossparallel fractures by considering multiple reflections betweenfractures. In ultrasonic or seismic tests, the magnitude of stresswave is too small to mobilize nonlinear deformation of the fractures.Therefore, linearly deformational behaviour is adopted to study theeffects of parallel fractures on P-wave attenuation. However, at thevicinity of an explosion or impact, the magnitude of stress wave islarge enough to excite nonlinear deformation of the fractures. Insuch case, a static nonlinear model is applied to investigate theeffects of parallel fractures with nonlinear normal deformationalbehaviour on P-wave attenuation. When shear stress at fractureinterface reaches fracture shear strength, fracture slip occurs. Thenthe Coulomb slip model is used to study the effects of a singlefracture and parallel fractures on S-wave attenuation. In thetheoretical formulation, the Coulomb slip model is applied tosimulate the shear deformational behaviour. The dependence ofmagnitude of transmission coefficient on fracture spacing isgoverned by nondimensional fracture spacing (ξ) - ratio of fracturespacing to incident wavelength. Two important indices ofnondimensional fracture spacing, threshold value (ξthr) and criticalvalue (ξcri), are identified. They divide the area of nondimensionalfracture spacing into three parts: individual fracture area (ξ ≥ ξthr),transition area (ξthr > ξ > ξcri) and small spacing area (ξ ≤ ξcri). Indifferent areas, the magnitude of transmission coefficient hasdifferent tendencies with nondimensional fracture spacing andnumber of fractures. Combined with displacement discontinuousmodel, methods of plane wave analysis and propagator matrix areapplied to develop theoretical solution of in-plane wave attenuationacross parallel fractures with linearly deformational behaviour. Theeffects of parameters, such as incidence angle, normalized stiffness,nondimensional fracture spacing and number of fractures, areexamined. Further to the theoretical studies, numerical modellingon one-dimensional wave attenuation across parallel fractures isperformed with Universal Distinct Element Code (UDEC).Compared with theoretical solutions, UDEC is verified to be capableof modelling one-dimensional wave attenuation across parallelfractures with linearly and nonlinearly deformational behavioursand the Coulomb slip behaviour.

Mechanism of Sloping Capillary Barriers under HighRainfall ConditionsCandidate: Denny TamiReport No: CEE/PhD/2005/95

Rainfall-induced slope failure is one of natural disasters that occurfrequently in natural or engineered residual soil slopes. Previousresearch works indicate that rainfall has been found to significantlyaffect the pore-water pressures and the shear strengths of residualsoils. One of the possible preventive methods for rainfall-inducedslope failures is the utilisation of capillary barrier; a fine-grainedsoil layer placed over a coarse-grained soil layer. The constructionof a capillary barrier as a slope cover can significantly reduce the

infiltration of rainwater into the slope and keep the slopes in a safecondition. However, the effectiveness of capillary barriers underthe influence of high precipitation rates has never been fullyinvestigated and hence is the focus of this research. In this research,the mechanism of sloping capillary barriers under high rainfallconditions was assessed through both laboratory experiments andnumerical analyses. An infiltration box apparatus [2.45 m (L) x 2.0m (H) x 0.4 m (W)] was designed and constructed in laboratoryThis facility was equipped with comprehensive instrumentation anddata acquisition systems, such as pressure transducer-tensiometersystem; time domain reflectometry (TDR); magnetic flowmeter andelectronic weight balances. Rainfalls of different intensities anddurations can be simulated through a rainfall simulator located atthe top of the infiltration box. Physical models of sloping capillarybarriers, consisting of fine sand over gravelly sand and silty sandover gravelly sand, were constructed inside the infiltration box.The behaviour of these sloping capillary barrier models under highrainfall conditions was studied closely; Each series of experimentwas then analysed numerically using the drying and wettinghydraulic properties of the soils that were measured separately;The numerical model was verified by comparing the results of thenumerical analysis with those of the experimental data. Havingverified the numerical model, an extensive parametric study wasthen conducted by considering various parameters and conditionsthat could not be incorporated in the experiments using thelaboratory physical model. Capillary barriers were found to beeffective in minimising infiltration from rainwater into unsaturatedsoil slopes under high precipitation rates when appropriately selectedmaterials were used. It was found that the water storage capacityarid the lateral diversion of capillary barriers play an important rolein ensuring the effectiveness of the capillary barriers under thiscondition. Excessive breakthrough begins to occur once the potentialstorage of the capillary barrier is approached. The magnitude of thebreakthrough decreases when: (i) the thickness of the fine-grainedlayer of the capillary barrier increases; (ii) the contrast in thehydraulic properties of the fine-grained and coarse-grained layerincreases; (iii) the water-entry value of the coarse- grained layerdecreases; (iv) the inclination angle of the capillary barrier increases;(v) the intensity of the rainfall decreases; (vi) the duration of therainfall decreases; (vii) the total rainfall decreases; and (viii) theinitial condition is relatively dry. Apart from the effectiveness thecapillary barrier as mentioned above, it was also found that thehysteretic behaviour of the soils needs to be accounted for in themodelling of the unsaturated flow systems in order to providerealistic results. Furthermore, the appropriate soil-water characteristiccurve and permeability function (i.e., drying or wetting) should beused in accordance with the process that the soil actually experiences(i.e., the desorption process or the adsorption process), regardlessthe flux-boundary condition (i.e., infiltration or evaporation), inorder to capture the soil response correctly;

Predictive Instantaneous Optimal Control of Elastic andInelastic Structures based on Ground VelocityCandidate: Pang MiaoReport No: CEE/PhD/2005/96

A new predictive instantaneous optimal control (PIOC) algorithmthat is based on ground velocity is developed in this research. Thenew PIOC algorithm is proposed by introducing a new state spaceform. Since the earthquake ground velocity is not in high frequencyas compared with the ground acceleration, it can be predicted atcertain time steps beforehand in real-time domain with higheraccuracy. This ensures the synchronous execution of the currentlyproposed PIOC algorithm with the real-time application of thecontrol force. Furthermore, the negative effects of time delay canbe reduced to an acceptable level and the possibility that thecontrolled structure becoming unstable is reduced. Both elastic and

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inelastic structural models using active control are studied bynumerical simulation using both single degree of freedom (SDOF)and multi- degrees of freedom (MDOF) structural models todemonstrate the control efficiency using the PIOC algorithm. Morepractically, the PIOC algorithm is applied to two practicalapplications: (1) Generation of energy density spectra; and (2)Control of three-dimensional structural model. Results obtained bythese numerical studies show that the PIOC algorithm is effectiveto reduce both the maximum structural response and various energyforms, and it seems promising to deal with the problem of controlperformance degradation due to time delay.

Development of Aerobic Granules for Enhanced BiologicalWastewater TreatmentCandidate: Jiang HelongReport No: CEE/PhD/2005/97

This study demonstrates that aerobic granules can tolerate anddegrade high loads of toxic substrates. Aerobic granules werecultivated in column-type sequential batch reactors at a rate of 1.5kg phenol m-3 d-1. The compact granular structure offers severaladvantages, such as high biomass retention and protection of cellsagainst phenol toxicity. Aerobic granules can also make metabolicadaptations and remain stable at higher loadings, up to 2.5 kgphenol m-3 d-1. The Beta subclass of Proteobacteria and high G+CGram-positive bacteria had a significant presence in the granules.Of ten phenol-degrading bacterial strains isolated from the granules,three possessed 16S sequences belonging to dominant populationswithin the granules. Strain PG-O 1 had a high relative abundanceand may have contributed significantly to phenol degradation inthe granules. PG-O 1 was found to possess key physiological traitsthat allowed it to outcompete and dominate other microorganismswithin the granules.

Development of Aerobic Granules for SimultaneousOrganic Carbon and Nitrogen RemovalCandidate: Yang ShufangReport No: CEE/PhD/2005/98

This research work attempted to develop aerobic granules forsimultaneous organic carbon and nitrogen removal. For this purpose,four sequencing batch reactors (SBR) were operated at differentsubstrate N/COD ratios in the range of 5/100 to 30/100 by weight.Results showed that aerobic granules could form over the substrateN/COD ratios of 5/100 to 30/100, i.e. aerobic granulation would beindependent of the substrate N/COD ratios applied. However, thesubstrate N/COD ratio showed a significant effect on microbial andphysicochemical characteristics of aerobic granules. Nitrifying,denitrifying and heterotrophic populations would co-exist in aerobicgranules, and shifts in microbial populations in the granules wereclosely related .to the substrate N/COD ratio. Elementalcompositions of aerobic granules were highly related to the substrateN/COD ratios. Aerobic granules developed at high substrate N/COD ratios exhibited enhanced nitrification efficiency, while DOconcentration and mixing power significantly influenced theefficiency of denitrification by aerobic granules. It was demonstratedthat the stability of aerobic granules could be significantly improvedthrough selecting slow-growing nitrifying bacteria. The selectionof nitrifying bacteria in aerobic granules could be achieved bymanipulating the substrate N/COD ratio.

A Mechanical Impedance Approach for StructuralIdentification, Health Monitoring and Non-DestructiveEvaluation Using Piezo-Impedance TransducersCandidate: Suresh BhallaReport No: CEE/PhD/2005/99

The continuous ageing of civil-structures constructed during thelast few decades is creating maintenance problems for whichengineers are not logistically prepared. Similar problem isincreasingly felt for civil and military aircraft, which call for tedioushuman inspection routinely to detect damages, which, if unnoticed,could lead to great disasters. This concern has prompted researchfor the development of automated structural health monitoring(SHM) and non-destructive evaluation (NDE) systems. During thelast few years, electro-mechanical impedance (EMI) technique hasemerged as a new method for SHM/ NDE. In this technique, piezo-impedance transducers, acting as collocated actuators and sensors,employ ultrasonic vibrations (typically in 30-400 kHz range) toread characteristic ‘signature’ of the structure in question. Thisresearch proposed an impedance-based structural identification andSHM/ NDE using the EMI technique. A new PZT-structureinteraction model has been developed based on the concept of‘effective impedance’ and a new experimental technique devised to‘update’ the model of the PZT patches. The piezo-impedancetransducers have been rigorously calibrated for predicting damagelevel in concrete. In addition, a new experimental technique hasbeen developed to predict in situ concrete strength non-destructively.Finally, the intermediate bond layer between the PZT patch and thestructure has been integrated into impedance models, therebyenabling a rigorous analysis of the shear lag effect associated withthe bond layer.

Dynamic Instability Analysis of Elastic-PlasticCandidate: Liu YuminReport No: CEE/PhD/2005/100

This thesis investigates analytically and numerically dynamicinstability of elastic-plastic beams subjected to transverse impulsiveload. The beam dynamic instability is characterized into twocategories, i.e., symmetrical instability and asymmetrical instability.The symmetrical instability corresponds to the counter-intuitiveresponse investigated by many previous researchers. Theasymmetrical instability is defined as that a symmetrical beam loadedsymmetrically experiences asymmetrical response when the loadparameter is in a certain range. It is the first time to introduce theconcept of the asymmetrical instability of elastic-plastic beams inthe present study. A new three- degree-of-freedom (3-DoF) Shanley-type model is developed which is capable of capturing both thesymmetrical and the asymmetrical instability of elastic-plasticbeams. A general studying methodology, including sensitivityanalysis, uncertainty analysis, energy analysis, analytical solutionand numerical simulations, etc., is established to study the twotypes of beam dynamic instability. The effects of the impulsiveload duration, material parameters, slenderness ratio, boundaryconditions and load types on the beam dynamic instabilities arethoroughly explored.

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Flexural Behaviour of Confined High-Strength ConcreteColumnsCandidate: Nguyen Ngoc BaReport No: CEE/PhD/2005/101

This thesis is about a study on the flexural behaviour of plain andconfined HSC columns. An experiment has been carried out onforty plain and reinforced concrete columns having either square orcircular cross-sections. A new method for determining stress-straincurves of concrete from flexure tests was developed. A model forstress-strain curve of confined concrete subjected to flexure wasproposed. The existing formulas for estimating lateral deflection ofhinge-hinge RC columns under eccentric loading were refined inthis thesis. A computer program was written to analyse confinedconcrete columns under eccentric loading, which can be used toevaluate confinement models for strain gradient problems. Existingequivalent rectangular stress blocks for design of HSC columnswere evaluated and a new ERSB was proposed. The applicabilityof ERSB to design of members with noncircular cross-sections wasinvestigated and adjustment of ERSB factors for such a case wasproposed.

Effects of Lateral Soil Movements on PilesCandidate: Miao LingfengReport No: CEE/PhD/2005/102

A series of model tests in soft clay were carried out to determinethe ultimate soil pressure acting on the pile(s) undergoing lateralsoil movement. Using a specially designed apparatus, single pilesand pile groups with different pile spacings, numbers of piles andarrangements of piles were investigated. Numerical studies werealso carried out using three-dimensional finite element analyses.For the back analyses of the single pile model tests, the computedultimate soil pressures were in good agreement with those measuredfrom the model tests. Parametric studies of full- scale piles indicatedthat the behaviour of the piles was significantly influenced by thepile flexibility, the magnitude of soil movement, the pile headboundary conditions, the shape of soil movement profile and thethickness of the moving soil mass. Based on the experimental andnumerical results, a simplified method has been proposed to estimatethe maximum pile bending moment.

Design of Reinforced Concrete Walls with Openings forStrength and DuctilityCandidate: Wu HuiReport No: CEE/PhD/2005/103

In this study, experimental and analytical studies on the seismicdesign of structural walls with irregular openings were performed.Six flanged walls, consisting of one solid wall, three walls withirregular openings, and two walls with regular openings were testedunder reversed cyclic loadings. A series of parametric studies onthe walls with openings were performed using a reliable non-linearfinite element programme. Both the quasi-static analysis and the

dynamic analysis were carried out. In addition, a seismicperformance evaluation on walls with openings was performed. Basedon the experimental and analytical results, a general design approachwas proposed for the design of this kind of walls, in which flexuralbehaviours of the walls were studied for the selections of strut-and-tie models and the determinations of overstrengths for the sheardesigns, so that the undesirable failure modes could be avoided.

Flexural Behavior and Design of Irregular Flat plate FloorsCandidate: TavioReport No: CEE/PhD/2005/104

This thesis presents three relatively simple and effective proceduresfor computing bending moments and deflections of either regular orirregular reinforced concrete flat plate floors under gravity loading.The methods are applicable for predicting the deflections throughoutthe entire loading stages from the uncracked state to the fully crackedstate and even up to the ultimate limit state. The first method dealswith the grillage analysis based on the matrix or finite elementformulation to determine the distribution of moments in the slab. Toaccount for the nonlinear behavior of reinforced concrete section andmaterial, a combination of ACI effective moment of inertia, effectivetorsional rigidity, and reduced modulus of elasticity of concrete isincorporated in the analysis. Definitions of slab-beam elements,column dimensions and orientation are also generalized to take intoaccount the irregularity of slab geometry and column layout.Essentially, the second procedure follows the ACI Direct DesignMethod. The total static moment in a particular design strip will bedistributed to the midspan and support sections according to thedistribution coefficients of the ACI Direct Design Method. Somegeneralizations are included to account for the effects of theirregularity of slab geometry and column layout. The definitions ofeffective span, width of design strips, torsional stiffness of transversetorsional members, as well as column dimensions and orientationswill be generalized as far as practical. The latest method involves afinite element procedure incorporating the plate-bending elementformulation. The effect of cracking is considered through the use ofBranson’s effective moment of inertia approach. Prior to cracking,the concrete is treated as isotropic elastic material. After cracking,the concrete is considered to be an orthotropic material. The influenceof torsional moments is also taken into account in the formulation toobtain the total flexural moments in the slab. To accommodate theeffect of material nonlinearity, the modulus of elasticity of concreteis also reduced. The method also accounts for the effect of crackingin beams if the beams exist. A test of a large-scale irregular multipanelreinforced concrete flat plate floor is also reported. The flat platefloor was supported on fourteen columns located irregularly over thefloor layout. In the test, the flat plate floor was subjected to gravityloading until flexural failure occurred, which was then followed bypunching failures around the columns. Only the flexural behavior isreported here. Its behavior with respect to punching shear had beenreported earlier by Geng (2004). The accuracy of the proposedmethods has been confirmed with the experimental data from severaltest slabs and regular multipanel flat plate floors available in literature,as well as two irregular multipanel flat plate floors tested at NanyangTechnological University (NTU) – Singapore.

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PUBLICATIONSPUBLICATIONSPUBLICATIONSPUBLICATIONSPUBLICATIONSPublications of academic staff in journals and conference proceedings during the period from 1 July 2004 to30 June 2005. Authors who are not members of School are marked by *

Agus, S.S.*, Leong, E.C. and Rahardjo, H., 2005. Estimatingpermeability functions of Singapore residual soils. Journal ofEngineering Geology, April, Vol. 78, No. 1-2, pp. 119-133.

Ai, J.J.*, Yu, S.C.M.*, Law, A.W.K. and Chua, L.P.*, 2005. Vortexdynamics in starting square water jets. Physics of Fluids, Vol. 17,Article 014106 (published on-line January 2005).

Au, S.K., 2005. Reliability-based design sensitivity by efficientsimulation. International Journal on Computers and Structures. Vol.83, No. 14, pp.1048-1061.

Au, S.K., 2005. Using approximate response for unbiased reliabilityanalysis. Proceedings of the International Conference on StructuralSafety and Reliability (ICOSSAR’05), Rome, Italy, 19-22 June 2005,CD-ROM.

Au, S.K., Ching, J.* and Beck, J.L.*, 2005. Application of subsetsimulation methods to reliability benchmark problems. Proceedingsof the International Conference on Structural Safety and Reliability(ICOSSAR’05), Rome, Italy, 19-22 June 2005, CD-ROM.

Aw, T.G.*, Lim, T.H. and Gin, K.Y.H., 2005. Detection of coliphagesin environmental water samples by real-time PCR. Proceedings ofthe NTU-Stanford Symposium on the Environment: TechnologicalChallenges for Water Resources and the Environment, NanyangExecutive Centre, NTU, 31 May to 1 June 2005.

Bhalla, S.* and Soh, C.K., 2004. Electromechanical impedancemodeling for adhesively bonded piezo-transducers. Journal ofIntelligent Material Systems and Structures, Vol. 15, No. 12, pp. 955-972.

Bhalla, S.*, Soh, C.K. and Liu, Z.X.*, 2005. Wave propagationapproach for NDE using surface bonded piezo-ceramics. NDT&EInternational, Vol. 38, No. 2, pp. 143-150.

Brownjohn, J.M.W.*, Rizos, C.*, Tan, G.H., and Pan, T.-C., 2004.Real-time long-term monitoring of static and dynamic displacementsof an office tower, combining RTK GPS and accelerometer data.Proceedings of the 1st FIG International Symposium on EngineeringSurveys for Construction Works and Structural Engineering,Nottingham, UK, 28 June – 1 July 2004.

Brownjohn, J.M.W.*, Tor, Y.K., Lau, C.L. and Pan, T.-C., 2004.Dynamic structural response monitoring of tall buildings using GPS.Report, RG-11/99, Applied Research Project, School of Civil andEnvironmental Engineering, Nanyang Technological University.

Chang, M.F. and Teo, P.*, 2004.� Load transfer characteristics of boredpiles in Singapore’s Old Alluvium. Proceedings of the 15th SoutheastAsian Regional Conference, Bangkok, November, Vol. 1, pp. 217-222.

Chang, M.F., and Zhu, H.*, 2004. Pressuremeter test for evaluatingload transfer characteristics along bored piles in residual soils.Proceedings of the 2nd International Conference on SiteCharacteristics, Porto, Portugal, September, Vol. 2, pp. 1431-1437.

Cheah, C.Y.J., 2004. Public-private partnerships in infrastructuredevelopment: On value, risk and negotiation. Proceedings of the CIBW107 Globalization and Construction Symposium, Bangkok, Thailand,17-19 November 2004, pp. 735-746.

Cheah, C.Y.J., Garvin, M.J.* and Miller, J.B.*, 2004. Empirical studyof strategic performance of global construction firms. ASCE Journalof Construction Engineering and Management, Vol. 130, No. 6, pp.808-817.

Cheah, C.Y.J. and Wong, W.F., 2004. Management studies of theChinese construction industry: Which field of theories? Proceedingsof the 20th Annual Conference of the Association of Researchers inConstruction Management (ARCOM), Edinburgh, UK, 1-3 September2004, pp. 949-956.

Cheah, C.Y.J. and Chew, D.A.S., 2005. Dynamics of strategicmanagement in the Chinese construction industry. ManagementDecision, Vol. 43, No. 4, pp. 551-567.

Cheah, C.Y.J., Chen, P.H. and Ting, S.K., 2005. Globalizationchallenges, legacies, and civil engineering curriculum reform. ASCEJournal of Professional Issues in Engineering Education and Practice,Vol. 131, No. 2, pp. 105-110.

Cheah, C.Y.J. and Liu, J., 2005. Real option evaluation of complexinfrastructure projects: The case of Dabhol Power Project in India.Journal of Financial Management of Property and Construction, Vol.10, No. 1, pp. 55-68.

Cheah, C.Y.J. and Ting, S.K., 2005. Appraisal of value engineeringin construction in Southeast Asia. International Journal of ProjectManagement, UK, February, Vol.�23, No. 2, pp.�151-158.

Chen, P.H., Wan, C.Y.*, Tiong, R.L.K., Ting, S.K., and Yang, Q.Z.*,2004. Augmented IDEF1-based process-oriented informationmodeling. Automation in Construction, USA, November, Vol.�13, No.�6,pp.�735-750.

Chen, P.H., Cui, L.*, Wan, C.Y., Yang, Q.Z.*, Ting, S.K., and Tiong,R.L.K., 2005. Implementation of IFC-based web server forcollaborative building design between architects and structuralengineers. Automation in Construction, USA, January, Vol.�14, No.�1,pp.�115-128.

Chen, X.L., Zhao, Z.Y. and Liew, K.M.*, 2005. EFG based stabilityanalysis of piezoelectric FGM plates subjected to electricity, heat andnon-uniformly distributed loads. Boundary Elements XXVII, Orlando,USA, 15-17 March 2005, pp. 307-317.

Chen, X.W. and Chiew, Y.M., 2004. Closure to “Response of velocityand reynolds stress profiles to sudden change of bed roughness in open-channel flow”. Journal of Hydraulic Engineering, ASCE, USA, Vol.130, No. 6, pp. 586-589.

Cheng, L.Y. and Tiong, R.L.K., 2005. Minimum feasible tariff forBOT water supply projects in Malaysia. Journal of ConstructionManagement and Economics, UK, March, Vol. 23, No. 3, pp. 255-263.

Cheng, N.S., 2004. Analysis of bedload transport in laminar flows.Advances in Water Resources, Vol. 27, No. 9, pp. 937-942.

Cheng, N.S., 2004. Closure to “Exponential formula for bedloadtransport”. Journal of Hydraulic Engineering, ASCE, Vol. 130, No.5, pp. 474.

Cheng, N.S. and Chua, L.H.C., 2005. Comparisons of sidewallcorrection of bed shear stress. Journal of Hydraulic Engineering,ASCE, Vol. 131, No. 7, pp. 605-609.

Chew, D.A.S., Yan, S. and Cheah, C.Y.J., 2004. Creating andsustaining competitiveness of small and medium-sized constructionenterprises in China. Proceedings of the CIB W107 Globalization andConstruction Symposium, Bangkok, Thailand, 17-19 November 2004,pp. 25-34.


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Chiew, S.P., Lee, C.K., Lie, S.T. and Ji, H.L., 2005. Numericalmodelling of square tubular T-joints with a surface crack at corner.Proceedings of the 4th International Conference on Advances in SteelStructures (ICASS-2005), Shanghai, China, 13-15 June 2005, Vol. 2,pp. 1159-1164 (ISBN/ISSN: 0-080446-37-X).

Chiew, S.P., Lie, S.T., Lee, C.K. and Yu, Y., 2005. Debonding failuremodel for FRP retrofitted steel beams. Proceedings of the 4th

International Conference on Advances in Steel Structures (ICASS-2005), Shanghai, China, 13-15 June 2005, Vol. 1, pp. 579-586 (ISBN/ISSN: 0-080446-37-X).

Chiew, Y.M., Lim, S.Y. and Cheng, N.S., 2004. Scour and Erosion.Editor of Conference Proceedings of 2nd International Conference onScour and Erosion, 2 Volumes.

Ching, J.*, Au, S.K. and Beck, J.L.*, 2005. Reliability estimation ofdynamical systems subject to stochastic excitation using subsetsimulation with splitting. Computer Methods in Applied Mechanicsand Engineering, Vol. 194, No. 12-16, pp. 1557-1579.

Ching, J.*, Beck, J.L.* and Au, S.K., 2005. Hybrid subset simulationmethod for dynamic reliability problems. Proceedings of theInternational Conference on Structural Safety and Reliability(ICOSSAR’05), Rome, Italy, 19-22 June 2005. CD-ROM.

Chow, S.S., Gin, K.Y.H., Ke, J., Hu, C. and Reinhard, M.*, 2005. Theanalysis of endocrine disrupting chemicals in aqueous samples ofvarious matrices with liquid chromatography tandem massspectrometry. Proceedings of the NTU-Stanford Symposium on theEnvironment: Technological Challenges for Water Resources and theEnvironment, Nanyang Executive Centre, NTU, 31 May to 1 June 2005.

Chu, J. and Yan, S.W.*, 2004. Comparison of degree of consolidationestimated using settlement and pore water pressure for vacuumpreloading projects. Proceedings of the 15th Southeast GeotechnicalConference, Bangkok, 22-26 November 2004, Vol. 1, pp. 543-548.

Chu, J., Ho, M.G., Loke, W.L. and Leong, W.K., 2004. Effects ofscour and hydraulic gradient on the stability of granular soil slope.Proceedings of the International Conference on Scour and Erosion,Singapore, 15-17 November 2004, Vol. 2, pp. 439-452.

Chu, J., 2005. Unusual soil behaviour and its challenge to constitutivemodeling. Invited Paper, International Workshop on Modern Trendsin Geomechanics, Vienna, 27-29 June 2005.

Chu, J., Goi, M.H. and Lim, T.T., 2005. Consolidation of cementtreated sewage sludge using vertical drains. Canadian GeotechnicalJournal, Vol. 42, No. 2, pp. 528-540.

Chu, J., Klotz, C.* and Vermeer, P.A.*, 2005. The anisotropicbehaviour of a structured soil in Singapore. Proceedings of theInternational Conference on Problematic Soils, Cyprus, 25-27 May2005.

Chu, J. and Yan, S.W.*, 2005. Estimation of degree of consolidationfor vacuum preloading projects. International Journal ofGeomechanics, ASCE, Vol. 5, N., 2, pp. 158-165.

Chua, C.G.* and Goh, A.T.C., 2005. Estimating wall deflections indeep excavations using Bayesian neural networks. Tunnelling andUnderground Space Technology, Vol. 20, No. 4, pp. 400-409.

Chua, H.C.L. and Holz, K.P.*, 2005. Hybrid neural network-finiteelement river flow model. Journal of Hydraulic Engineering, Vol.131, No. 1, pp. 52-59.

Des Guo* and Yip, W.K., 2005. Behaviour of reinforced concretecolumn strengthened by prepacked concrete jacket. Proceedings ofthe 3rd International Conference on Construction Materials:Performance, Innovations and Structural Implications, Vancouver,Canada, 11 pp.

Dey, S. and Cheng, N.S., 2005. Reynolds stress in open channel flowwith upward seepage. Journal of Engineering Mechanics, ASCE, Vol.131, No. 4, pp. 451-457.

Dhakal, R.P.* and Pan, T.-C., 2004. Joint contribution to thedeformation of RC beam-column sub-assemblies. Proceedings of the2nd International Conference on Structural Engineering, Mechanicsand Computation (SEMC-2004), Cape Town, South Africa, 5-7 July2004, Chapter 267, pp. 1671-1676.

Dhakal, R.P.* and Pan, T.-C., 2005. Response of non-seismic framesto high-speed excitations. Proceedings of the International Conferenceon Structural and Road Transportation Engineering (START-2005),Indian Institute of Technology, Kharagpur, India, 3-5 January 2005.

Dhakal, R.P.*, Pan, T.-C., Irawan, P., Tsai, K.-C.*, Lin, K.-C. and Chen,C.-H., 2005. Experimental study on dynamic response of gravitydesigned RC connections. Engineering Structures, Vol. 27, No. 1, pp.75-87.

Dharma Ronny Budi and Tan, K.H., 2005. A numerical study ofrotational capacity of steel beams in fire. Proceedings of the 4th

International Conference on Advances in Steel Structures 2005(ICASS’05), China, 13-15 June 2005.

Dharma Ronny Budi and Tan, K.H., 2005. Alternative approach forlateral torsional buckling of unrestrained beams in fire. Proceedingsof the 4th International Conference on Advances in Steel Structures2005 (ICASS’05), China, 13-15 June 2005.

Faisal, A.* and Rahardjo, H., 2004. Unsaturated residual soils. �InvitedPaper. Proceedings of the Symposium on Tropical Residual Soil,Universiti Putra Malaysia (UPM), Serdang, Malaysia, 6-7 July 2004.

Feng, J., Lim, T.T. and Dong, Z.L., 2004.� Reductive transformationpathways and kinetics of carbon tetrachloride reduction by nano-scalezero-valent Fe and Fe/Ni.� Proceedings of the 3rd InternationalConference on Oxidation and Reduction Technologies for In-SituTreatment of Soil and Groundwater, San Diego, California, USA, 24-28 October 2004.

Feng, J. and Lim, T.T., 2005.� Pathways and kinetics of carbontetrachloride and chloroform reductions by nano-scale Fe and Fe/Niparticles: comparison with commercial micro-scale Fe and Zn.�Chemosphere, Vol. 59, No. 9, pp. 1267-1277.

Fung, T.C., 2005. Higher order accurate time step integrationalgorithms by equal order polynomial projection. Journal of Vibrationand Control, Vol. 11, No. 1, pp. 19-49.

Gho, W.M. and Gao, F., 2004. Parametric equations for stressconcentration factors in completely overlapped tubular K(N)-Joints.Journal of Constructional Steel Research, Vol. 60, pp. 1761-1782.

Gho, W.M., Gao, F. and Yang, Y., 2004. Stress concentration factorson completely overlapped tubular K(N)-joints under lap brace axialcompression. Proceedings of the 18th Australasian Conference on theMechanics of Structures & Materials, Perth, Western Australia, 1-3December 2004, pp. 1115-1120.

Gho, W.M., Yang, Y. and Gao, F., 2005. Cyclic performance ofcompletely overlapped tubular joints. Proceedings of the InternationalSociety of Offshore and Polar Engineers, ISOPE-2005, Seoul, Korea,19-24 June 2005, pp. 320-324.

Gin, K.Y.H. and Neo, S.Y.*, 2005. Analysis of microbial populationsin tropical reservoirs using flow cytometry. Journal of EnvironmentalEngineering, Vol. 131, No. 8, pp. 1187-1193.

Goh, A.T.C., 2005. Piles subjected to lateral ground movements.Invited paper, Special Seminar on Pile Foundation Practice, Institutionof Engineers Singapore, Singapore, 18 February 2005, pp. 1-38.

Goh, A.T.C. and Kulhawy, F.H.*, 2005. Reliability assessment ofserviceability performance of braced retaining walls using a neuralnetwork approach. International Journal for Numerical and AnalyticalMethods in Geomechanics, Vol. 29, No. 6, pp. 627-642.

Goh, A.T.C., Kulhawy, F.H.* and Chua, C.G.*, 2005. Bayesian neuralnetwork analysis of undrained side resistance of drilled shafts. Journalof Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 131,No. 1, pp. 84-93.

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Goh, K.H. and Lim, T.T., 2004.� Geochemistry of inorganic arsenicand selenium in a tropical soil: Effect of reaction time, pH, andcompetitive anions on arsenic and selenium adsorption.� Chemosphere,Vol. 55, No. 6, pp. 849-859.�

Goh, K.H. and Lim, T.T., 2005. Arsenic extractability in a fine soilfraction and influence of various anions on its mobility in subsurfaceenvironment.� Applied Geochemistry, Vol. 20, No. 2, pp. 229-239.

Goh, P.K.* and Wong, Y.D., 2004. Effects of distance and speed ondriver stop-versus-cross decision at traffic signals. Road and TransportResearch, Vol. 13, No. 3, pp. 36-44. �

Goh, P.K.* and Wong, Y.D., 2004. � A study of perception responsetime during the signal change interval. Applied Health Economicsand Health Policy, Vol. 3, No. 1, pp. 9-15. �

Goh, S.L., Wong, Y.D. and Sun, D.D., 2004. Reutilization of wasteconcrete into hot-mix asphalt pavement. EROU 2004, Singapore, 23September 2004.

Haijun Fang, Sun, D.D., Ming W., Phay, W. and Tay, J.H., 2005.Removal of humic acid foulant from ultrafiltration membrane surfaceusing photocatalytic oxidation process. Water Science and Technology,Vol. 51, Issue 6-7, p. 373.

Hay, C.T., Tan, H.L., Tang, T.M., Sun, D.D., Toh, A.C.*, Loh, C.K.and Lim, S., 2004. Reutilization of dredged marine clay into syntheticaggregate. Singapore Maritime and Port Journal (SMPJ).

Hay, C.T., Khor, S.L., Sun, D.D., and Leckie, J.O.*, 2005.Minimization of sludge production using membrane bioreactor.Singapore Stanford Partnership Symposium, 31 May to 1 June 2005.

He, G.H.*, Show, K.Y., Yan, Y.G.*, Liu, J.L.*, and Zheng, Q.G.*, 2004.Minimization of biosludge production on anaerobic-aerobic treatmentof industrial wastewater. Journal of Residuals Science and Technology,Vol. 1, No. 4, pp. 191-197.

Hou, L., Gin, K.Y.H. and Tay, J.H., 2005. Degradation study of volatiledisinfection by-products in a tropical soil aquifer system. Proceedingsof the NTU-Stanford Symposium on the Environment: TechnologicalChallenges for Water Resources and the Environment, NanyangExecutive Centre, NTU, 31 May to 1 June 2005.

Huang, Z.F. and Tan, K.H., 2004. Simple structural models to predictcritical temperature of a fire compartment within a semi-rigid steelframe. Proceedings of the 18th Australasian Conference on theMechanics of Structures and Materials (ACMSM 18), Perth, WesternAustralia, 1-3 December 2004.

Huang, Z.F. and Tan, K.H., 2005. 3-D Finite element simulation ofthe response of a steel frame subjected to fire. Proceedings of the 4th

International Conference on Advances in Steel Structures 2005(ICASS’05), China, 13-15 June 2005.

Ivanov, V., Tay, J.-H., Tay, S.T.-L., Jiang, R.H.-L., 2004. Removal ofmicro-particles by microbial granules used for aerobic wastewatertreatment. Water Science and Technology, Vol. 50, No. 12, pp. 147-154.

Ivanov, V., Stabnikov, V., Zhuang, W.-Q., Tay, S.T.- L. and Tay, J.-H.,2005. Phosphate removal from return liquor of municipal wastewatertreatment plant using iron-reducing bacteria. Journal of AppliedMicrobiology, Vol. 98, pp. 1152-1161.

Ivanov, V., Tay, J.-H., Liu, Q.-S., Wang, X.-H., Wang, Z-.W.,Maszenan, B.A.M., Yi, S., Zhuang, W.-Q., Liu, Y.-Q., Pan, S. and Tay.S.T.-L., 2005. Microstructural optimization of wastewater treatmentby aerobic granular sludge. In Stephan Bathe, Merle de Kreuk, BelindaMcSwain, and Norbert Schwarzenbeck (Eds.) Aerobic GranularSludge. Water and Environmental Management Series, London, IWAPublishing, pp. 43-52.

Jiang, H.-L., Tay, J.-H., Maszenan, A.M, and Tay, S.T.-L., 2004.Bacterial diversity and function of aerobic granules engineered in asequencing batch reactor for phenol degradation. Applied andEnvironmental Microbiology. Vol. 70, No. 11, pp. 6767-6775.

Jiang, J.S.*, Law, A.W.K., and Cheng, N.S., 2005. Two phase analysisof vertical sediment-laden jets. Journal of Engineering Mechanics,ASCE, Vol. 125, No. 3, pp. 308-318.

Krisdani, H., Rahardjo, H. and Leong, E.C., 2005. Behaviour ofcapillary barrier system constructed using residual soil. Proceedingsof the Geo-Frontiers Conference on Innovative Barriers and BarrierMaterials, Austin, Texas, USA, 24-26 January 2005.

Lam, J.S.L., 2005. Major shipping dynamics influencing containerport performance. Proceedings of the International Association ofMaritime Economists Annual Conference, Cyprus, 23-25 June 2005.

Lam, N.T.K.*, Pan, T.-C., Chandler, A.* and Megawati, K.* 2004.Cities without a seismic code II: Codification and risk assessment.Proceedings of the 13th World Conference on Earthquake Engineering,Vancouver, Canada, 1-6 August 2004, Paper No. 141.

Lan, S.R.*, Lok, T.S. and Heng, L.*, 2005. Composite structural panelssubjected to explosive loading. Journal of Construction and BuildingMaterials, Elsevier Science Publishing, UK, Vol. 19, Issue 5, pp. 387-395.

Law, A.W.K., Choi, E.C.C. and Britter, R.*, 2004. Re-entrainmentaround a low-rise industrial building: 2D versus 3D wind tunnel study.Journal of Atmospheric Environment, UK, Vol. 38, No. 23, pp. 3817-3825.

Law, A.W.K. and Ho, W.F*. 2004. Further investigations on jetspreading in grid turbulence. Proceedings of the 4th InternationalSymposium on Environmental Hydraulics, Hong Kong, China, 15-18December 2004, pp. 325-329.

Lee, C.K., Lie, S.T. and Shuai, Y.Y.*, 2004. On coupling ofreproducing kernel particle method and boundary element method.Computational Mechanics (USA), Vol. 34, No. 4, pp. 282-297.

Lee, C.K., Lie, S.T., Chiew, S.P. and Shao, Y.B., 2005. Numericalmodels verification of cracked tubular T, Y and K-joints undercombined loads. Engineering Fracture Mechanics, Vol. 72, No. 7,pp. 983-1009.

Lee, C.K. and Xu, Q.X.*, 2005. A new automatic adaptive 3D solidmesh generation scheme for thin-walled structures. InternationalJournal for Numerical Methods in Engineering (USA), Vol. 62, No.11, pp. 1519-1558.

Lee, S.*, Chang, L.M.*, and Chen, P.H., 2005. Performancecomparison of bridge coating defect recognition methods. Corrosion,January, Vol. 61, No. 1, pp. 12-20.

Leong, E.C., Yeo, S.H. and Rahardjo, H., 2004. Measurement of wavevelocities and attenuation using an ultrasonic test system. CanadianGeotechnical Journal (Canada), Vol. 41, No. 5, pp. 844-860.

Li, B. and Pan, T.-C., 2004. Seismic performances of reinforcedconcrete frames under low intensity earthquake effects. Proceedingsof the 13th World Conference on Earthquake Engineering, 1-6 August2004, Vancouver, Canada, Paper No. 127.

Li, X.B.*, Lok, T.S. and Zhao, J.*, 2005. Dynamic characteristics ofgranite subjected to intermediate loading rate. Rock Mechanics andRock Engineering, Springer-Verlag-Wien, Austria, February, Vol. 38,No. 1, pp. 21-39.

Li, Y.H.*, Zhang, H.Q., Tang, C.A.*, Yu, Q.L.* and Zhao, Z.Y., 2004.A numerical approach for studying self-organized criticality behaviourin rock failure progress process. Proceedings of the InternationalConference on computational methods, Singapore, 15-17 December2004.

Lie, S.T., Lee, C.K., Chiew, S.P. and Shao, Y.B., 2004. Validation ofstress intensity factors of a surface crack in tubular K-joints.Proceedings of the 18th Australasian Conference on the Mechanics ofStructures and Materials, Perth, Australia,1-3 December 2004, pp.1127-1133 (ISBN/ISSN: 9-058096-59-9).


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Lie, S.T., Lee, C.K., Chiew, S.P. and Yang, Z.M., 2004. Numericalmodels in linear and non-linear FE analysis of cracked rectangularhollow section joints. Proceedings of the International Conferenceon Computational Methods, Singapore, 15-17 December 2004.

Lie, S.T., Lee, C.K., Chiew, S.P. and Shao, Y.B., 2005. Validation ofa surface crack stress intensity factor of a tubular K-Joint. InternationalJournal of Pressure Vessels and Piping, (UK), Vol. 82, No. 8, pp. 610-617 (ISBN/ISSN: 0308-0161).

Lie, S.T., Lee, C.K., Chiew, S.P. and Shao, Y.B., 2005. Mesh modellingand analysis of cracked uni-planar tubular K-Joints. Journal ofConstructional Steel Research, (UK), Vol. 61, No. 2, pp. 235-264(ISBN/ISSN: 0143-974X).

Lie, S.T., Lee, C.K., Chiew, S.P. and Yang, Z.M., 2005. The ultimatebehaviour of cracked square hollow section T-joints. Proceedings ofthe 4th International Conference on Advances in Steel Structures(ICASS-2005), Shanghai, China,13-15 June 2005, Vol. 2, pp. 1099-1106 (ISBN/ISSN: 0-080446-37-X).

Lie, S.T. and Shao, Y.B., 2005. Parametric equation of stress intensityfactor for tubular K-joint under balanced axial loads. InternationalJournal of Fatigue, Vol. 26, No. 6, pp. 666-679.

Lie, S.T. and Shao, Y.B., 2005. Stress Intensity Factor (SIF) for tubularK-joints under balanced axial load – Part II: Parametric equation.Proceedings of the 4th International Conference on Advances in SteelStructures (ICASS 2005), Shanghai, P.R. China, 13-15 June 2005, pp.1127-1132.

Lim, C.H.*, Ong, Y.H.*, Seah, C.C.*, Seah, Y.T.*, Lu, Y. and Wu,C.Q.*, 2004. Ground shock prediction for underground ammunitionstorage safety. Proceedings of the 31st DOD Explosives Safety Seminar,San Antonio, Texas, 24-26 August 2004 (in CD-ROM).

Lim, S.Y. and Nugroho, J.*, 2004. Observations on flow field aroundan abutment in a two-stage channel. Proceedings of the 2nd

International Conference on Scour and Erosion, Singapore, 14-17November 2004, Vol. 1, pp. 156-164.

Lim, S.Y. and Yang, S.Q., 2005. A simplified model of tractive-forcedistribution in closed conduits. Journal of Hydraulic Engineering,ASCE, USA, Vol. 131, No. 4, pp. 322-329.

Lim, T.H., Chen, Y.H., Gin, K.Y.H., Chow, S.S., Reinhard, M.* andTay, J.H., 2005. Kinetics of 17_-estradiol degradation in a rechargeaquifer system. Proceedings of the NTU-Stanford Symposium on theEnvironment: Technological Challenges for Water Resources and theEnvironment, Nanyang Executive Centre, NTU, 31 May to 1 June 2005.

Lim, T.H., Gin, K.Y.H., Chow, S.S., Chen, Y.H., Reinhard, M.* andTay, J.H., 2005. Microcosm evaluation of 17_-estradiol transformationduring groundwater recharge. Proceedings of the NTU-StanfordSymposium on the Environment: Technological Challenges for WaterResources and the Environment, Nanyang Executive Centre, NTU, 31May to 1 June 2005.

Lim, T.T., Chu, J., and Goi, M.H., 2004.� Assessment of heavy metalsleachability in clay-amended sewage sludge stabilized with cementfor use as fill material.� Journal of Residuals Science and Technology,Vol. 1, No. 3, pp. 157-164.

Lim, T.T., Chui, P.C. and Goh, K.H., 2005.� Process evaluation foroptimization of EDTA use and recovery for heavy metal removal froma contaminated soil.� Chemosphere, Vol. 58, No. 8, pp. 1031-1040.

Lim, T.T. and Goh, K.H., 2005.� Selenium extractability from acontaminated fine soil fraction: Implication on soil cleanup.�Chemosphere, Vol. 58, No. 1, pp. 91-101.

Liu, Q.S., Liu, Y., Tay, J.H. and Show, K.Y., 2005. Responses of sludgeflocs to shear strength. Process Biochemistry, Vol. 40, pp. 3213-3217.

Liu, Q.Y., Liu, Y. and Tay, J.H., 2004. The effects of extracellularpolymeric substances on the formation and stability of biogranules.Applied Microbiology and Biotechnology, Vol. 65, pp. 143-148.

Liu, Y. and Tay, J.H., 2004. State of the art of biogranulationtechnology for wastewater treatment. Biotechnology Advances, Vol.22, pp. 533-563.

Liu, Y., Liu, Q.S. and Tay, J.H., 2005. The initial conditions-dependentgrowth kinetics in microbial batch culture. Process Biochemistry, Vol.40, pp. 155-160.

Liu, Y.Q., Liu, Y. and Tay, J.H., 2005. Relationship between size andmass transfer resistance in aerobic granules. Letters in AppliedMicrobiology, Vol. 40, pp. 312-315.

Liu, Y., Liu, Y.Q., Wang, Z.W., Yang, S.F. and Tay, J.H., 2005.Influence of substrate surface loading on the kinetic behaviours ofaerobic granules. Applied Microbiology and Biotechnology, Vol. 67,pp. 484-488.

Liu, Y., Wang, Z.W., Liu, Y.Q., Qin, L. and Tay, J.H., 2005. Ageneralized model for settling velocity of aerobic granular sludge.Biotechnology Progress, Vol. 21, pp. 621-626.

Liu, Y., Wang, Z.W., Qin, L., Liu, Y.Q. and Tay, J.H., 2005. Selectionpressure-driven aerobic granulation in a sequencing batch reactor.Applied Microbiology and Biotechnology, Vol. 67, pp. 26-32.

Liu, Y., Wang, Z.W. and Tay, J.H., 2005. A unified theory for upscalingaerobic granular sludge sequencing batch reactor. BiotechnologyAdvances, Vol. 23, pp. 335-344.

Liu, Y.-Q., Tay, J.W., Ivanov, V., Moy, Y.-P. B., Yu, L., and Tay, T.-L.S., 2005. Influence of phenol on nitrification by microbial granules.Process Biochemistry, Vol. 40, pp. 3285-3289.

Lok, T.S. and Kulkarni, A.*, 2004. Woomera Trial: Response ofcantilevers to air-blast loading in 5-tonne trial in Woomera, Australia.Final (Post-test) Technical Report to DSTA, November 2004.

Lok, T.S. Lim, C.H. and Tan, J.S.Y.*, (Eds). December 2004.Protection of Structures against Hazards II -CI-Premier, Singapore.330 pages. (ISBN 981-05-0613-9).

Lok, T.S., Zhang, W.H. and Lim, C.H., 2004. Tensile failure ofconcrete subjected to shock loading. Proceedings of the 2nd

International Conference on Protection of Structures against Hazards,Singapore, 2-3 December 2004, CI-Premier, pp. 221-230. (ISBN 981-05-0613-9).

Lok, T.S., Zhao, P.J.*, Li, X.B.* and Lim, C.H., 2004. Characterizingdynamic properties of granite using SHPB. In Ohnishi, Y. and Aoki,K. (Eds.) Proceedings of the ISRM International Symposium – 3rd AsianRock Mechanics Symposium, Kyoto, Japan, 30 November – 2December 2004, Millpress Science Pub. Rotterdam, pp. 895-898.(ISBN 90-5966-022-6 Vol.2).

Low, B.K., 2005. Reliability-based design applied to retaining walls.Geotechnique, Vol. 55, No. 1, pp. 63-75.

Lu, Y. and Chiew Y.M., 2004. Using seepage as an auxiliary methodfor pier-scour countermeasure. Proceedings of the 2nd InternationalConference on Scour and Erosion, 14-17 November 2004, Singapore,Vol. 2, pp. 70-77.

Lu, Y., and Gao, F., 2004. Structural damage diagnosis using timedomain acceleration response data. Proceedings of the 18th

Australasian Conference on the Mechanics of Structures and Materials,Perth, Australia,1-3 December 2004, pp. 1225-1230.

Lu, Y., and Gu, X.M., 2004. Performance based seismic reliabilityevaluation of RC frames. Proceedings of the 8th InternationalSymposium on Structural Engineering for Young Experts, Xian, China,20-22 August 2004, pp. 530-536.

Lu, Y., Xu, K., and Tan, S.C., 2004. Characterization of effect ofreinforcing steel bars on concrete break-up under high-intensityexplosive loading. Proceedings of the 31st DOD Explosives SafetySeminar, San Antonio, Texas, 24-26 August 2004, (CD-ROM).

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Lu, Y., 2005. Inelastic behaviour of RC wall-frame system and itsanalysis incorporating three-dimensional effects. The StructuralDesign of Tall and Special Buildings. Vol. 14, No. 1, pp. 15-35.

Lu, Y., 2005. Underground blast induced ground shock and itsprediction using Artificial Neural Network. Computers andGeotechnics, Vol. 32, No. 3, pp. 164-178.

Lu, Y., and Gao, F., 2005. A novel time domain auto-regressive modelfor structural damage diagnosis. Journal of Sound and Vibration, Vol.283, No. 3-5, pp. 1031-1049.

Lu, Y., Gu, X.M., and Guan, J., 2005. Probabilistic drift limits andperformance evaluation of RC columns. Journal of StructuralEngineering, ASCE, Vol. 131, No. 6, pp. 966-978.

Ma, G.W., Liu, Y.M.*, Zhao, J.* and Li, Q.M.*, 2005. Dynamicinstability of elastic-plastic beam. International Journal of MechanicalScience, Vol. 47, pp. 43-62.

Ma, G.W. and Ye, Z.Q., 2005. Analysis of foam claddings for blastalleviation. Proceedings of International Conference on ImpactLoading of Lightweight Structures, Florianopolis, Brazil, 8-12 May2005, pp. 359-372.

Mao, T., Hong, S.Y.*, Show, K.Y., Tay, J.H. and Lee, D.J.*, 2004. Acomparison of ultrasound treatment on primary and secondary sludges.Journal of Water Science and Technology, Vol. 50, No. 9, pp. 91-97.

Megawati, K.*, Lam, N.T.K.*, Chandler, A.* and Pan, T.-C., 2004.Cities without a seismic code I: Hazard assessment. Proceedings ofthe 13th World Conference on Earthquake Engineering, Vancouver,Canada, 1-6 August 2004, Paper No. 129.

Megawati, K.*, Pan, T.-C., and Koketsu, K.*, 2005. Response spectralattenuation relationships for Sumatran-subduction earthquakes and theseismic hazard implications to Singapore and Kuala Lumpur. Journalof Soil Dynamics and Earthquake Engineering, Vol. 25, No. 1, pp. 11-25.

Melinda, F.*, Rahardjo, H., Han, K.K.* and Leong, E.C., 2004. Shearstrength of compacted soil under infiltration condition. ASCE Journalof Geotechnical and Geoenvironmental Engineering, August, Vol. 130,No. 8, pp.�807-817.

Na, Y.M.*, Choa, V., Teh, C.I. and Chang, M.F., 2005. Geotechncialparameters of reclaimed sandfill from cone penetration test. CanadianGeotechnical Journal, February, Vol. 42, No. 1, pp. 91-109.

Naidu, A.S.K.*, Bhalla, S.* and Soh, C.K., 2004. Recent developmentsin smart systems based structural health monitoring. NationalConference on Materials and Structures, NIT-Warangal, India, 23-24January 2004, pp. 273-278.

Nguyen, Trieu Thai Son and Pan, T.-C., 2004. Response of steelstructures to ground shocks. Proceedings of the 10th NationalUndergraduate Research Opportunities Programme Congress,Singapore, 25 September 2004, Agency for Science, Technology andResearch.

Ohno, T.*, Krauthammer, T.* and Pan, T.-C., 2004. Design andanalysis of protective structures against impact/impulsive/shock loads.Proceedings of the 1st International Conference on Design and Analysisof Protective Structures against Impact/Impulsive/Shock Loads(DAPSIL-2003), National Defence Academy of Japan, Tokyo, Japan,15-18 December.

Olszewski, P. and Xie, L.*,�2004. Traffic response to peak periodvariable road pricing. Proceedings of the Intelligent TransportationSociety (Singapore) Symposium 2004, 8-9 July 2004.

Olszewski, P. and Wibowo, S.S.*, 2005. Using equivalent walkingdistance to assess pedestrian accessibility to transit stations inSingapore. Proceedings of the 84th Annual Meeting of theTransportation Research Board, Washington, DC, USA, 9-13 January2005 (CD-ROM).

Olszewski, P. and Xie, L.*,�2005. Modelling the effects of road pricingon�traffic in Singapore. Transportation Research – Part A, Vol.39A,�pp. 755-772.

Ong, K.C.G., Tam, C.T. and Tan, T.H., 2004. Proceeding of the 29th

Conference on Our World in Concrete and Structures. Our World inConcrete and Structures, Singapore, 25-26 August 2004.

Ong, M.C., Lim, S.Y., Yu, G. and Tan, S.K., 2004. Abutment scourusing lightweight bed material. Proceedings of the 2nd InternationalConference on Scour and Erosion, Singapore, 14-17 November 2004,Vol. 1, pp. 187-198.

Pan, T.-C., 2004. From ground shocks to air blast – multiple-hazardprotection. Keynote Paper, Proceedings of the 3rd InternationalSymposium on New Technologies for Urban Safety of Mega Cities inAsia, The Department of Civil Engineering, Indian Institute ofTechnology, Kanpur, and the International Center for Urban SafetyEngineering (ICUS/INCEDE), Institute of Industrial Science, TheUniversity of Tokyo, 18-19 October 2004, Agra, India, pp. 1-13.

Pan, T.-C., 2004. Protecting buildings against multiple hazards. ICUS/INCEDE Newsletter, Vol. 3, No. 3, Oct–Dec, 2003, International Centerfor Urban Safety Engineering, Institute of Industrial Science, TheUniversity of Tokyo, Japan.

Pan, T.-C., Brownjohn, J.M.W.* and You, X.T., 2004. Correlatingmeasured and simulated dynamic responses of a tall building to long-distance earthquakes. Journal of Earthquake Engineering andStructural Dynamics, International Association for EarthquakeEngineering, Vol. 33, No. 5, pp. 611-632.

Pan, T.-C. and Lim, C.L., 2004. Approximate method for response ofa structure subjected to explosion-induced ground motions.Proceedings of the 2nd International Conference on StructuralEngineering, Mechanics and Computation (SEMC-2004), Cape Town,South Africa, 5-7 July 2004.

Pan, T.-C., You, X. T. and Cheng, K.W., 2004. Building response tolong-distance major earthquakes. Proceedings of the 13th WorldConference on Earthquake Engineering, Vancouver, Canada, 1-6August 2004, Paper No. 804.

Pan, T.-C., You, X.T. and Cheng, K.W., 2004. Response of Singaporebuildings to long-distance major Sumatra earthquakes. Invited Paper,Proceedings of the 3rd International Conference on ContinentalEarthquakes, Institute of Engineering Mechanics, China EarthquakeAdministration, Beijing, China, 12-14 July 2004.

Pan, T.-C. and Zhang, X., 2004. A case study for reinforced concretestructural collapse analysis – Literature review on blast phenomenaand modelling. Technical Report No. 1, PTRC-CEE/MHA/2003.07,Protective Technology Research Centre, Nanyang TechnologicalUniversity.

Pan, T.-C., 2005. Long-distance major Sumatra earthquakes – seismichazard and vulnerability, Singapore’s perspective. Keynote Paper,Proceedings of the, Strategic Planning Workshop 2005 – SeismicHazard Analysis of Peninsula Malaysia for Structural Design Purpose,Structural Earthquake Engineering Research, University of TechnologyMalaysia, Genting Highland, Malaysia, 21-24 April 2005.

Pan, T.-C., 2005. Protecting building structures against multiplehazards. Keynote Paper, Proceedings of the 1st InternationalConference of Urban Disaster Reduction, Kobe, Japan, 18-19 January2005, pp. II-2.1 – II-2.8.

Pan, T.-C., 2005. Protection of buildings against multiple hazards.Invited Paper, Proceedings of the Fire and Emergency Services Asia2005 - The New Threat Environment: Protecting the Community,National Fire Prevention Council, Singapore, 22-24 February 2005,pp. 202–211.


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Pan, T.-C., Lu, Y., Tan, K.H., Zhang, X., Wang, Z. and Hu, O., 2005.A case study for reinforced concrete structural collapse analysis. FinalReport No. NTU/PTRC/2005.01, Protective Technology ResearchCentre, Nanyang Technological University.

Pan, T.-C. and Megawati, K.*, 2005. Prediction of maximum probablestrong ground motion in Singapore due to Sumatra earthquakes.Report RGM-16/00, Applied Research Project, Protective TechnologyResearch Centre, School of Civil and Environmental Engineering,Nanyang Technological University.

Pan, T.-C., You, X.T. and Cheng, K.W., 2005. Assessment of groundmotion amplification and building damages. Report RGM-42/00,Applied Research Project, School of Civil and EnvironmentalEngineering, Nanyang Technological University.

Porter, K.A.*, Beck, J.L.*, Shaikhutdinov, R.V.*, Au, S.K., Mizukoshi,K.*, Miyamura, M.*, Ishida, H.*, Moroi, T.*, Tsukada, Y.* andMasuda, M.*, 2004. Effect of seismic risk on lifetime property value.Earthquake Spectra, Vol. 20, No. 4, pp. 1211-1237.

Qin, L., Liu, Y. and Tay, J.H., 2005. Denitrification on poly-_-hydroxybutyrate in microbial granular sludge sequencing batch reactor.Water Research, Vol. 39, pp. 1503-1510.

Qin, Q.W.*, Gin, K.Y.H., Lee, L.Y.*, Gedaria, A.I.* and Zhang, S.*,2005. Development of a flow cytometry based method for rapid andsensitive detection of a novel marine fish iridovirus in cell culture.Journal of Virological Methods, Vol. 125, No. 1, pp. 49-54.

Rahardjo, H., Lee, T.T., Leong, E.C. and Rezaur, R.B., 2005. Responseof a residual soil slope to rainfall. Canadian Geotechnical Journal,April, Vol. 42, No. 2, pp. 340-351.

Remesh Kuzhikkali and Tan, K.H., 2005. Inter-layer mixing inbuilding units during fire. Fire and Emergency Services Asia 2005,Singapore, 22-26 February 2005.

Remesh Kuzhikkali and Tan, K.H., 2005. Sensitivity of wall thermaldiffusivity on the fire dynamics in zone model. Fire and EmergencyServices Asia 2005, Singapore, 22-26 February 2005.

Rong, X.*, and Lo, E.Y., 2004. On the stability of shoaling Stokeswaves over a slowly varying bottom. Applied Ocean Research, Vol.26, No. 5, pp. 205-212.

Sachidanandham, R.*, Gin, K.Y.H. and Poh, C.L.*, 2005. Monitoringof active but non-culturable bacterial cells by flow cytometry.Biotechnology and Bioengineering, Vol. 89, No. 1, pp. 24-31.

Show, K.Y. and Tay, J.H., 2004. Conversion of sludge into novelmaterials for construction applications. Transactions of the MaterialsResearch Society of Japan, Vol. 29, No. 5, pp. 1957-1960.

Sachs, T.*, Tiong, R. and Koo, J., 2005. Establishment of foreignconstruction companies in Korea and the Korean construction industryin a foreign perspective. Proceedings of the International Symposiumon Globalization and Construction 2004, CIB and AIT Bangkok, 17-19 November 2004, pp. 421-432.

Singh, D.*, Goh, N.H. and Tiong, R.L.K., 2005. Procurement andcontract award practices – A review of the Singapore constructionindustry. Proceedings of the 9th Pacific Association of QuantitySurveyors Congress, 27-28 June 2005, China, pp. 140-151.

Singh, D.* and Tiong, R.L.K., 2005. A fuzzy decision framework forcontractor selection. Construction Engineering and Management,ASCE, Vol. 131, No. 1, pp. 62-70.

Singh, D.* and Tiong, R.L.K., 2005. Development of life cycle costingframework for highway bridges in Myanmar. International Journalof Project Management, Vol. 23, pp. 37-44.

Soh, C.K., Liu, Y. and Yang, Y.W., 2005. Closure to “A displacementequivalence-based damage model for brittle materials”, Part I: Theory;Part II: Verification 2005. ASME Journal of Applied Mechanics, Vol.72, pp. 306-307. Journal of Applied Mechanics, ASME, Vol. 72, No.1, pp. 308.

Stabnikova, O., Ang, S.S., Liu, X.-Y., Ivanov, V., Tay, J.-H., Wang, J.-Y., 2005. The use of Hybrid Anaerobic Solid-Liquid (HASL) systemfor the treatment of lipid-containing food waste. Journal of ChemicalTechnology and Biotechnology, Vol. 80, No. 4, pp. 455-461.

Sun, D.D., Leckie, J.O.* and Lee, P.F., 2005. Removal of membranefoulant using nano-structured material. Proceedings of the NTU-Stanford Symposium on the Environment: Technological Challengesfor Water Resources and the Environment, Nanyang Executive Centre,NTU, 31 May to 1 June 2005.

Sun, D.D., Lee, P.F. and Leckie, J.O.*, 2005. Development ofnanostructured material for membrane fouling control in watertreatment system. Proceedings of the 1st NTU Nano Symposium,Nanyang Executive Centre, NTU, 29 January 2005.

Tami, D., Rahardjo, H. and Leong, E.C., 2004. Effects of hysteresison steady-state infiltration in unsaturated slopes. ASCE Journal ofGeotechnical and Geoenvironmental Engineering, September, Vol. 130,No. 9, pp. 956-967.

Tami, D., Rahardjo, H., Leong, E.C. and Fredlund, D.G.*, 2004.Design and laboratory verification of a physical model of slopingcapillary barrier. Canadian Geotechnical Journal, October, Vol. 41,No. 5, pp. 814-830.

Tan, K.H. and Tong Kebo*, 2004. Strut and tie analysis for deep beamswith different web reinforcement and openings. Proceedings of the18th Australasian Conference on the Mechanics of Structures andMaterials (ACMSM 18), Perth, Western Australia, 1-3 December 2004.

Tan, K.H., Phng, G.H.* and Toh, W.S.*, 2004. Fire resistance ofcomposite steel-concrete columns: Experimental evaluation andnumerical analysis. Proceedings of the 2nd International Conferenceon Steel and Composite Structures, Seoul, South Korea, 2-4 September2004.

Tan, K.H., Tang, C.Y. and Tong, K.*, 2004. Shear strength predictionsof pierced deep beams with inclined web reinforcement. Magazineof Concrete Research (UK), Vol. 56, No. 8, pp. 443-452.

Tan, K.H., Toh W.S.* and Phng, G.H.*, 2004. Fire resistance of steelcolumns subjected to different restraint ratios. Proceedings of the 2nd

International Conference on Steel and Composite Structures, Seoul,South Korea, 2-4 September 2004.

Tan, K.H. and Yao Yao, 2004. Fire resistance of RC columns subjectedto 1-, 2-, and 3-face heating. ASCE Journal of Structural Engineering(US), Vol. 130, No. 11, pp. 1820-1828.

Tan, K.H. and Huang, Z.F., 2005. Structural responses of axiallyrestrained steel beams with semi-rigid moment connection in fire.ASCE Journal of Structural Engineering (US), Vol. 131, No. 4, pp.541-551.

Tan, L.H., Gin, K.Y.H., Hu, C., Ke, J., Chow, S.S. and Reinhard, M.*,2005. The fate of endocrine disrupting chemicals during soil aquifertreatment. Proceedings of the NTU-Stanford Symposium on theEnvironment: Technological Challenges for Water Resources and theEnvironment, Nanyang Executive Centre, NTU, 31 May to 1 June 2005.

Tan, S.C.*, Lu, Y., Xu, K. and Weerheijm, J.*, 2004. Developmentof debris break-up model and its initial verification against DLVclamped slab test. Proceedings of the 31st DOD Explosives SafetySeminar, San Antonio, Texas, 24-26 August 2004, (CD-ROM).

Tan, T.H. and Nguyen Ngoc Ba, 2004. Applicability of AS 3600:2001and NZS 3101:1995 to circular high strength concrete columns.Proceedings of the 18th Australasian Conference on the Mechanics ofStructures and Materials (ACMSM 18), Perth, Western Australia, 1-3 December 2004, pp. 887-892.

Tan, T.H. and Nguyen Ngoc Ba, 2005. Flexural behaviour of confinedhigh-strength concrete columns. American Concrete InstituteStructural Journal, Vol. 102, No. 2, pp. 198-205.

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Tang, C.Y. and Tan, K.H., 2004. An interactive mechanical modelfor shear strength of deep beams. ASCE Journal of StructuralEngineering (US), Vol. 130, No. 10, pp. 1534-1544.

Tay, C.M., Tan, K.M., Tjin, S.C., Chan, C.C. and Rahardjo, H., 2004.Humidity sensing using plastic optical fibers. Microwave and OpticalTechnology Letters, 5 December 2004, Vol. 43, No.5, pp. 387-390.

Tay, C.M., Tan, K.M., Tjin, S.C., Chan, C.C. and Rahardjo, H., 2004.Humidity sensing using plastic optical fibers. In Anbo Wang (Ed.)Proceedings of SPIE, The International Society for OpticalEngineering, Sensors for Harsh Environments, Philadelphia, PA, USA,27 October 2004, Vol. 5590, pp. 77-83.

Tay, J.H., Liu, Q., Liu, Y., Show, K.Y., Ivanov, V. and Tay, S. T.L.,2004. Aerobic granulation in a pilot-scale SBR. Invited Paper,Workshop on Aerobic Granular Sludge, Institute of Water Quality andWaste Management, Munich, Germany, 27-28 September 2004.

Tay, J.H. and Show, K.Y., 2004. Issue Editors, Resources from Sludge:Forging New Frontiers. Journal of Water Science and Technology,the International Water Association, Vol. 50, Issue 9.

Tay, J.H., Show, K.Y. and Hung, Y.T.*, 2004. Seafood processingwastewater treatment. In Lawrence K Wang, Yung-Tse Hung, HowardH. Lo, and Constantine Yapijakis (Editors) Handbook of Industrialand Hazardous Waste Treatment, 2nd ed., Marcel Dekker, Inc., NewYork, pp. 647-684.

Tay, J.H., Show, K.Y., Lee, D.J.* and Hong, S.Y.*, 2004. Reuse ofwastewater sludge with marine clay as a new resource of constructionaggregates. Journal of Water Science and Technology, Vol. 50, No. 9,pp. 189-196.

Tay, J.H., Show, K.Y. and Lua, C.H., 2004. Water pollution in theAsia Pacific-basics, status, challenges, responses and outlook. InvitedPaper, Proceedings of the UNEP Asia-Pacific Leadership Programmeon Environmental and Sustainable Development, Shanghai, China, 26-31 July 2004.

Tay, J.-H., Tay, S.T.-L., Ivanov, V. and Hung, Y.-T.*, 2004. Applicationof biotechnology for industrial waste treatment. In Lawrence K Wang,Yung-Tse Hung, Howard H. Lo, Constantine Yapijakis, (Eds.)Handbook of Industrial Wastes Treatment. 2nd Edition, revised andexpanded, Marcel Dekker, N.Y., pp. 585-618.

Tay, J.H., Liu, Q., Liu, Y., Show, K.Y., Ivanov, V. and Tay, S.T.L.,2005. A comparative study of aerobic granulation in pilot- andlaboratory-scale SBRs.� In Stephan Bathe, Merle de Kreuk, BelindaMcSwain, and Norbert Schwarzenbeck, (Eds.) Aerobic GranularSludge. Water and Environmental Management Series. London, IWAPublishing, pp. 125-134.

Tay, S.T.-L., Moy B.Y.-P., Maszenan, A.M. and Tay, J.-H., 2005.Comparing activated sludge and aerobic granules as microbialinoculum for phenol biodegradation. Applied Microbiology andBiotechnology. On line publication January 2005 1007/s00253-004-1858-1.

Ting, S.K., 2004. Use of recycled aggregates in structural concrete.Invited lecture, National Construction Environment Seminar, NEA &SCAL, Singapore, 2 September 2004, 12 p.

Tiwari, M., Chu, J. and Teh, C.I., 2004. Effect of loading mode andrate on the drained behavior of marine clay. Proceedings of the 15th

Southeast Geotechnical Conference, Bangkok, 22-26 November 2004,pp. 67-72.

Tran, Cao Thanh Ngoc and Pan, T.-C., 2004. Effects of earthquakeground motion to various kind of buildings. Proceedings of the 10th

National Undergraduate Research Opportunities ProgrammeCongress, 25 September 2004, Agency for Science, Technology andResearch, Singapore.

Tu, Z.G., and Lu, Y., 2004. A robust stochastic genetic algorithm(StGA) for global numerical optimization. IEEE Transactions onEvolutionary Computation. Vol. 8, No. 5, pp. 456-470.

Wanatowski, D. and Chu, J., 2004. Drained behaviour of sand underplane-strain conditions. Proceedings of the 15th Southeast GeotechnicalConference, Bangkok, 22-26 November 2004, pp. 61-66.

Wang, Y.*, Show, K.Y., Tay, J.H. and Sim, K.H.*, 2004. Effects ofcationic polymer on start-up and granulation in UASB reactors.Chemical Technology and Biotechnology, Vol. 79, No. 3, pp. 219-228.

Wang, Z.H., Au, S.K. and Tang, K.H., 2005. Heat transfer analysisusing a Green’s function approach for uniformly insulated steelmembers subjected to fire. Engineering Structures, Vol. 27, pp. 1551-1562.

Wang, Z.Q.*, Lu, Y., Hao, H.* and Chong, K.*, 2004. A full couplednumerical analysis approach for buried structures subjected tosubsurface blast. Computers and Structures. Vol. 83, No. 4-5, pp. 339-356.

Wang, Z.Q.* and Cheng, N.S., 2005. Secondary flows over artificialbed strips. Advances in Water Resources, Vol. 28, No. 5, pp. 441-450.

Wong, T.S.W., 2005. Discussion of “Kinematic wave model of urbanpavement rainfall-runoff subject to traffic loadings” by Chad M.Cristina and John J. Sansalone. Journal of Environmental Engineering,ASCE, Vol. 131, No. 1, p. 170.

Wong, T.S.W., 2005. Influence of loss model on design discharge ofhomogeneous plane. Journal of Irrigation and Drainage Engineering,ASCE, Vol. 131, No. 2, pp. 210-217.

Wong, T.S.W., 2005. Kinematic wave method for storm drainagedesign. In J.H. Lehr and J. Keeley (Eds.) Water Encyclopedia: Surfaceand Agricultural Water. John Wiley, Ostrander, Ohio, USA, pp. 242-245.

Wong, W.F. and Cheah, C.Y.J., 2004. Issues of contractual chain andsub-contracting in the construction industry. Proceedings of the 20th

Annual Conference of the Association of Researchers in ConstructionManagement (ARCOM), Edinburgh, UK, 1-3 September 2004, pp. 671-680.

Wong, Y.D., Ho, J.S.* and Foo, H.Y.T.*, 2004. Left-Turn-On-Redtraffic scheme in Singapore. ITE Journal, Vol. 74, No. 11, pp. 24-29.�

Wu, C.Q.*, Hao, H.* and Lu, Y., 2004. Numerical simulation ofdamage of masonry and masonry infilled RC frame structures to blastground motions. Proceedings of the 8th International Symposium onStructural Engineering for Young Experts, Xian, China, 20-22 August2004, pp. 523-529.

Wu, C.Q.*, Hao, H.* and Lu, Y., 2005. Dynamic response and damageanalysis of masonry structures and masonry infilled RC frames to blastground motion. Engineering Structures, Vol. 27, No. 3, pp. 323-333.

Wu, M., Sun, D.D., Chung, M. and Tay, J.H., 2004. Nanofiltration ofsemiconductor indium phosphide wastewater for direct reuse. IWACongress at Marrakech, 19-24 September 2004.

Wu, M., Sun, D.D. and Tay, J.H., 2004. Effect of operating variableson rejection of indium using nanofiltration membranes. Journal ofMembrane Science, Vol. 240, Issue 1-2, pp. 105-111.

Wu, M., Sun, D.D. and Tay, J.H., 2004. Separation of indium usingnanofiltration membranes: a non-stead-state study. Journal ofMembrane Science, 1 November 2004, Vol. 243, Issues 1-2, pp. 215-222.

Wu, M., Sun, D.D., Tay, J.H. and Jiang, Y., 2004. Membrane surfacestudy using atomic force microscopy and its impact on wastewaterrecycling wastewater cost. Journal of Ultrapure Water, 1-5 September2004, UP210620.

Wu, M. and Sun, D.D., 2005. Characterization and reduction ofmembrane fouling during nanofiltration of semiconductor indiumphosphide (InP) wastewater. Journal of Membrane Science, Vol. 259,No. 1-2, p. 135.


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Wu, M., Sun, D.D. and Tay, J.H., 2005. Characterization andultrafiltration of semiconductor indium phosphide (InP) wastewaterfor recycling. Journal of Environmental Technology, Vol. 26, pp. 111-120.

Xie, S., Lim, T.H., Gin, K.Y.H. and Tay, J.H., 2005. Rapid detectionof active bacterial populations in diverse environmental waters usingflow cytometry. Proceedings of the NTU-Stanford Symposium on theEnvironment: Technological Challenges for Water Resources and theEnvironment, Nanyang Executive Centre, NTU, 31 May to 1 June 2005.

Xu, H., Tay, J.H., Foo, S.K., Yang, S.F. and Liu Y., 2004. Removalof dissolved copper and zinc by aerobic granular sludge. Water Scienceand Technology, Vol. 50, pp. 155-160.

Xu, K., Lu, Y. and Lie, S.T., 2004. Analysis of non-loading carryingfillet weld joints using SGBEM. International Journal of Fracture,Vol. 129, No. 2, pp. 149-159.

Xu, K., Lu, Y. and Lie, S.T., 2005. Load-carrying welded jointsanalysis using boundary element method. International Journal ofSolids and Structures, Vol. 42, No. 9-10, pp. 2965-2975.

Yan, S.W.* and Chu, J. 2004. Use of explosive method in the soilimprovement works of a highway project. Geotechnical Engineering,Proceeding of ICE, London, Vol. 157, GE4, pp. 259-265.

Yang, H., Rahardjo, H., Leong, E.C. and Fredlund, D.G.*, 2004.Factors affecting drying and wetting soil-water characteristic curvesof sandy soils. Canadian Geotechnical Journal, October, Vol. 41, No.5, pp. 908-920.

Yang, H., Rahardjo, H., Leong, E.C. and Fredlund, D.G.*, 2004. Astudy of infiltration on three sand capillary barriers. CanadianGeotechnical Journal, August, Vol. 41, No. 4, pp. 629-643.

Yang, H., Rahardjo, H., Wibawa, B. and Leong, E.C., 2004. A soilcolumn apparatus for laboratory infiltration study. GeotechnicalTesting Journal, ASTM International, July, Vol. 27, No. 4, pp. 347-355.

Yang, R., Wong, K.K.F.* and Pan, T.-C., 2004. Predictive inelasticstate control of modern structures during earthquakes. Journal ofStructural Control and Health Monitoring, Vol. 11, No. 4, pp. 291–309.

Yang, S.F., Tay, J.H. and Liu, Y., 2005. Effect of substrate N/CODratio on the formation of aerobic granules. Journal of EnvironmentalEngineering, Vol. 131, pp. 86-92.

Yang, S.Q.*, Tan, S.K. and Lim, S.Y., 2004. Velocity and sedimentconcentration profiles in sediment- laden flows. Journal of ChinaOcean Engineering, Vol. 18, No. 2, pp: 229-244.

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