Technical Paper (Tsunami)

1

Estimation of Tsunami Forces on Three Different Types of Bridge

Superstructure

1Foo Siong KOON

*,

2Tze Liang LAU

1,2 School of Civil Engineering, Universiti Sains Malaysia (USM)

14300 Nibong Tebal, Penang, Malaysia *E-mail: [email protected]

Abstract Since the past two disastrous tsunami events (i.e. Indian Ocean and Tohoku Tsunami), a number of bridge superstructure were damaged by tsunami waves. In this study, laboratory experiments were conducted to

estimate the tsunami forces on three different types of bridge superstructure. The models which placed at various deck

clearances were downscaled at a ratio of 1:100 and subjected to tsunami bores of various runup heights in a 1 m x 1 m

with 40 m long wave flume. The experimental results revealed that the nature of wave attack on bridge model depends

on the relationship between deck clearances, nominal wave heights and bridge types. The time histories of forces in the

horizontal and vertical directions on bridge models due to incident waves were measured. The relationships among the

forces and pressures for the incident wave of nominal wave height around 60 mm were discussed in detailed. The

maximum horizontal force and front face pressure were attained when nominal height of the wave was achieved by the

flow. Stability of each bridge superstructure against sliding was evaluated. The experimental results provide vital

information for the dynamic analysis in the later stage of the study.

Keywords: Bridge, Tsunami, Force, Wave pressure, Deck clearance

1. Introduction

Tsunamis are destructive waves that propagate with

considerable speed from the sources toward the shore

with unique wave length and flow velocity [1]. As the

tsunamis approach the shoreline, the unremitting amount

of energy results in tremendous force that will acts on the

infrastructures nearby, including bridge superstructure.

The enormous destruction caused by the 2004 Indian

Ocean tsunami and the 2011 Tohoku tsunami had

demonstrated the annihilation power of tsunami that

caused serious damages on bridge superstructures . In the

wake of the both past tsunami events, about 81 bridges

out of 168 were washed away by the 2004 tsunami in

Sumatra [2] and at least 280 bridges were washed away

by the 2011 Tohoku tsunami [3].

In Malaysia, the primary mitigation measures of

tsunami countermeasure have been concerned and

evacuation strategies had been planned after the event of

2004 Indian Ocean Tsunami. However, the strategies did

not address the same mitigation tactic to the coastal

infrastructures especially the bridge superstructures

onshore nearby the coastline. Furthermore, there were no

evidence proving that the impact of tsunami on bridge

superstructures located onshore especially the coastal

area had been conducted in Malaysia.

Type of bridge superstructure plays an important role

for the tsunami-proof design. Up to date, there is no

proper way to estimate tsunami force acting on different

types of bridge in Malaysia. Due to the complexity and

substantial uncertainties of the tsunami phenomena,

theoretical approach for the determination of tsunami-

induced forces cannot be easily applied. On the other

hand, there is no study had been done in Malaysia with

the consideration of both wave height and wave velocity

based on the case of the 2004 Indian Ocean tsunami. The

stability of bridge subjected to tsunami force is still

unknown and thus there is no design guideline developed

in Malaysia for bridge to resist tsunami fluid force.

Hence, a comprehensive study of estimation of tsunami

force on different types of bridge superstructure should

be carried out and explored promptly.

The main objective of the study is to study the

characteristics of tsunami wave and its impact onto

onshore simplified deck, I-beam deck and box girder

with deck clearance of 3 m, each subjected to 6 m wave

heights at Penang Island, Malaysia.

2. Background

It is not viable to perform field investigation of

tsunami flow characteristic since the occurrence of

tsunami is rare and complex in nature. Furthermore, the

important flow characteristics such as flow depth, flow

velocity, forces and pressure distribution are further

complex especially in bridge superstructure [4]. There

were numbers of research on the development of design

guideline had also been conducted include Final Report

2

on Development of a Guideline for Estimating Bridge

Superstructures [5] and American Society of Civil

Engineers (ASCE 7) [6]. From the existing guidelines,

the loads considerations were: i) horizontal force, ii)

vertical force, iii) impact force, iv) hydrostatic force, v)

hydrodynamic force, vi) uplift force, vii) buoyant force

and viii) additional gravity force [5].

Number of experimental studies on wave pressures

and forces on bridge superstructures had been

investigated by Iemura et al. [7], Lau et al. [8], Nakao et

al. [9] and Kawasaki and Izuno [10]. Several formulas

had been proposed to evaluate the tsunami wave loadings

on bridge superstructures. Iemura et al. [7] proposed an

equation to determine drag force due to tsunami flow on

the bridge model. As for Lau et al. [8], the author

proposed empirical formulas for the slow-varying forces

on bridge decks by establishing pressure distribution on

bridge deck. In Japan, Nakao et al. [9] had investigated

the relationship of horizontal drag force for various

shapes of I-beam bridge models.

Tsunami waves on bridge superstructures had been

studied and scrudinised deeply by other countries such as

Japan and United States. However, the development of

Malaysia coastal bridge superstructures design in

accordance to relevant standards and codes still cease at

the infant stage due to the lack of knowledge on tsunami

impact topics. Besides, the ocean bathymetry,

topography, hydrology and geology of the coast have

great influence in determination of tsunami runup

mechanisms and impact forces of tsunami [11].

Therefore, this study had focused on one common coastal

profile in Penang Island in order to investigate the

tsunami characteristics on bridge superstructures.

3. Methodology

3.1 Data Collection

The beach profile of Penang Island was evaluated

from the General Bathymetric Chart of the Oceans

(GEBCO) software. Wave velocities and wave heights

were obtained from the past recorded laboratory data.

3.2 Experimental Setup

Physical modelling was downscaled to 1:100 based

on Froude Number Similitude Law. Fig. 1 illustrated the

setup of this experimental study. The hydraulic model

experiments were carried out in a wave flume of 40 m

long, 1 m wide and 1 m high wave flume. The flume's

platform represented the common beach profile of

Northwest Peninsular Malaysia, and was comprised of a

compound bed with continuous plane slope of 1:200 and

1:125 and a flat platform where the model was located as

shown in Fig. 1. The compound bed ended with a

horizontal flat plane where the downscaled building

model subjected to tsunami loading was located. The

bridge models were constructed from acrylic plates with

specific dimensions as shown in Fig. 2. In the experiment,

tsunami runup was simulated while the tsunami

drawdown was not considered in the study.

Figure 1. Schematic diagram of the experimental setup

I-Beam Deck Simplified Deck Box Girder

300 mm 300 mm 300 mm

135 135 mm 135 mm

28 mm

28 mm

28 mm 117 mm 65 mm

Figure 2. Constructed bridge models

3

Long period solitary wave was generated by sudden

releasing of mass water built up in a water tank. By

varying the released volume from the tank, different

wave forms and wave forces were produced. In this

experiment, water height of 0.72 m in the tank was used

to generate the wave with nominal wave height of 60 mm

at the location of bridge model. Upon released, water

flowing through the wave baffle was regulated and

eventually broke into bores and surges after travelling

past the slope of 1:4. The broken wave which consisted

of bores and surges then propagated across all remaining

wave flume sections. The travelled broken wave then

attacked the bridge model which was located at the

flumes horizontal bed.

3.3 Physical Modeling

Fig. 3 showed the schematic diagram of the

instrumentation and data acquisition system used in the

experiment. Capacitance type wave gauges were used to

measure the wave profiles at onshore (H2) and offshore

(H1) locations as illustrated in Fig. 2. The velocities of

the flow in the flume for various wave heights were

recorded by electromagnetic type current meter at V2

(Fig. 2). Both wave gauge and current meter were

installed at H2 and V2 with the absence of the model

during measurement. The wave height and velocity at H2

and V2 were then correlated with the wave height at H1.

During the testing of the model, only the wave height H1

was measured in order to avoid the interference from the

instruments on the flow regime in the vicinity of the

model. Video and digital cameras were used to capture

the wave motion acting on the building model.

Tsunami forces and the wave pressures acted on the

bridge models were recorded by the load cell and

pressure gauges respectively. The positions of pressure

gauges were attached to the position of interest to study

with, which shown in Fig. 4. There were total five

pressure gauges used in the experiment and all five

pressure gauges were attached to the mid span distance

(whole span is 300 mm) of the bridge model. The front

face of the bridge model was defined as the face facing

the incident wave and hit directly by the wave. Before

the experiment was carried out, the bridge model with

deck clearance of 30 mm was mounted onto an I-section

with a load cell was installed on the flume. The bridge

model was then subjected to tsunami attacks (Fig. 5).

However, wave gauge and current meter at the location

of model were not installed to avoid the instruments

interference to the flow characteristics of tsunami wave

near the bridge model. Each case of studied was repeated

at least three times to ensure the repeatability of the

experiment.

4. Results and Discussion

4.1 Tsunami Wave Attack on Bridge Model

The nature of the tsunami wave attack on the bridge

models greatly depends on the relationship between the

deck clearance of the model and the shape of bridge

model with the nominal wave height of the approaching

tsunami. Fig. 6 shows the sequences of wave attack on

bridge model (Case SH30W60). The instant the wave

first reaches the point that directly parallel downward to

the front face of the bridge model is taken as t = 0 sec.

Pressure

Gauge

Load Cell

PG

1, P

G2

, PG

3, P

G5

PG

6, P

G7

, PG

8

Wave

Gauge

Current

Meter

Control Unit

Control Unit Data Logger

Data Acquisition

and Processing

Figure 3. Schematic diagram of instrumentation and data acquisition system

Fo

rce

4

When the leading edge of the wave reaches the front face

of bridge model, the wave flows beneath the model. The

wave height increases gradually and achieves its nominal

wave height of 60 mm. The model is then struck by the

incoming wave, creating a splash up impact force on the

model. The combination of downward water along the

front face of the bridge model and the incoming surge

result in a standing wave which continually strike the

model until the wave height reduces over time. The

model is then gush over by incoming wave and totally or

partially submerges under water, which depending on the

deck clearance. Such sequential wave attack is similarly

observed for all cases where the nominal wave is higher

than the bridge model.

Figure 4. Position of pressure gauges on each bridge model

Figure 5. Bridge model instrumented with load cell and pressure gauges

Load cell

Bridge model Pressure gauge

Figure 6. Sequence of wave attack on bridge model (SH30W60)

(a) t = 0.00 sec

(e) t = 6.00 sec (d) t = 4.80 sec

(c) t = 2.80 sec (b) t = 1.48 sec

5

4.2 Wave Force and Pressure on Bridge Model

The recorded force time histories for three bridge

models: simplified bridge model, I-beam deck and box

girder deck model with the incident wave and nominal

wave height around 60 mm at deck clearance of 30 mm

are displayed in Fig. 7 (a) to Fig. 9 (a). The maximum

horizontal force (Fx) of the three bridge models occurs

during a time frame when nominal wave height is

achieved by the flow. The force then decreases gradually

to zero when the wave heights at both front and back

faces of bridge models are equal. Based on the

experimental results, I-beam deck model has attains the

highest maximum horizontal resultant force of 9.5 N,

follow by simplified deck (8.4 N) and box girder deck

(7.6 N). The wave induced force is mainly contributed by

the incident surge up force that subtracted to the

drawback force that accumulates at the back face of the

bridge models. As for total vertical force (Fz). At time t =

3 sec, the bridge models is hit by the leading wave,

causing an initial uplift force. Then, the wave is followed

by the higher surge up wave, which has higher level than

the elevation of each bridge models, causing an

overtopping onto the deck surface. Therefore, additional

downward force has deduces the uplift force. Fig. 6 has

clearly shows the effect of overtopping onto the bridge

models. All three bridge models has experience the

overtopping phenomena until t = 15 sec. After that, the

total vertical force in all three bridge models has

increases. One possible reason might due to the uplift

force acting on the bridge models is greater than the

downward force.

Fig. 7 (b) to Fig. 9 (b) shows the recorded pressure

time histories for all three types of bridge models with

pressure gauges attached at different locations of study.

All bridge models are subjected to incident wave with

nominal wave height of 60 mm with deck clearance of 30

mm. The pressure shown in Fig. 7 (b) to Fig. 9 (b) are

normalised by the hydrostatic pressure (gh), where is

density of water, g is gravitational acceleration and h is

the nominal wave height. The front face pressure of all

bridge models exhibited similar trend as the horizontal

resultant force. This is because most of the front face

pressure was contributed to the horizontal resultant force.

For front face, after reaching the peak value, the

pressures then decrease gradually but remain at the

hydrostatic pressure for a much longer period

subsequently. Besides, it can be observed that the

maximum horizontal resultant force occurs almost the

same time instance with the occurrence of peak value of

the normalised front face pressure. The front face

pressure of all three bridge models are approximately 1.5

to 1.7 times of the hydrostatic pressure.

-25

-20

-15

-10

-5

0

5

10

15

0 10 20 30

Fo

rce

(N)

Time (sec)

Fx

Fz

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 10 20 30

No

rm

ali

sed

Press

ure

Time (sec)

PG2

PG5

PG1

PG6

PG7

PG8

(a) (b)

Figure 7. Time histories of (a) wave forces and (b) wave pressures on SH30W60 bridge model

-30

-25

-20

-15

-10

-5

0

5

10

15

0 10 20 30

Fo

rce

(N)

Time (sec)

Fx

Fz

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 10 20 30

No

rm

ali

sed

Press

ure

Time (sec)

PG8

PG2

PG7

PG5

(a) (b)

Figure 8. Time histories of (a) wave forces and (b) wave pressures on IH30W60 bridge model

6

On the other hand, the wave pressures at the back

face of all bridge models pick up slightly later than the

front face pressure. This is due to the fact that there is

time when no water can be observed at the back face of

all bridge models as the wave hit the bridge models. The

back pressures then decrease progressively to hydrostatic

pressure as time goes on. Based on the results, the front

face and back face pressures of the bridge models

achieve similar pressure which is about the hydrostatic

pressure when the wave heights at the front face and back

face of the bridge models are about the same. For the

bottom face pressure time histories of simplified deck

and box girder deck, the results show negative

normalised pressure at the initial stage (t = 2.5 sec). This

implies there is air entrapped at the bottom face of the

both bridge models. As time increases, the bottom face

pressures become positive, which imply the bridge

models have been thrusting upward gradually by the flow.

The pressure distribution of all three bridge models at

the front face are presented in Fig. 10 to Fig 12. The

pressure is normalised with hydrostatic pressure while

the elevation of the bridge point (z) is normalised with

nominal wave height (h). The best fit line which

represents the mean value of the measured pressure

recorded from the experiment. In the same graph, the

values of mean plus one standard deviation (mean + )

and mean plus two standard deviation (mean + 2) are

also plotted which correspond 68 % and 95 % percentiles

of the data, respectively. In this research, the proposed

prediction formulas (Eq. 1 to Eq. 3) for tsunami wave

pressure on the onshore bridge superstructures are adopts

the equation obtain from linear least squares regression.

-35

-30

-25

-20

-15

-10

-5

0

5

10

0 10 20 30

Fo

rce

(N)

Time (sec)

Fx

Fz

-1

-0.5

0

0.5

1

1.5

2

0 10 20 30No

rm

ali

sed

Press

ure

Time (sec)

PG2

PG5

PG7

PG8

PG6

Figure 9. Time histories of (a) wave forces and (b) wave pressures on BH30W60 bridge model

Mean

Mean +

Mean + 2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 1 2 3

z/h

P/pgh

Mean

Mean +

Mean + 2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 1 2 3 4

z/h

P/pgh

(a) (b)

Mean

Mean +

Mean + 2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 1 2 3

z/h

P/pgh

Figure 10. Pressure distribution of simplified deck at

front face

Figure 11. Pressure distribution of I-beam deck at

front face

Figure 12. Pressure distribution of box girder deck at

front face

7

At front face of simplified deck bridge, the mean:

P = gh (1.4856 z/h) /0.4093 (1)

At front face of I-beam deck bridge, the mean:

P = gh (1.3673 z/h) /0.3251 (2)

At front face of box girder bridge, the mean:

P = gh (1.4262 z/h) /0.395 (3)

where

P = Pressure (N/m2)

= Density of water (kg/m3) g = Gravitational acceleration (m/s2)

h = Nominal wave height for the incident wave (m)

z = Elevation from ground (m)

4.3 Stability against Sliding

All bridge models were constructed from acrylic

plate which used to represent the bridge models in the

laboratory test. The dimension of bridge models were

magnified from 13.5 2.8 30 cm to a bridge prototype

of 13.5 2.8 30 m under the scaling ratio of 1:100. All

bridge superstructures were examined under dry and wet

frictional conditions under tsunami loadings in this study.

Each acrylic plate had thickness of 3 cm. The coefficient

of dry and wet concrete used in this study are 0,6 and

0.45, respectively.

The self-weight of all bridge models were calculated

based on the dimensions shown in Fig. 4. Then, the

frictional resistant force of each bridge models under dry

and wet conditions can be computed from the calculated

self-weight of bridge superstructure. Lastly, the

calculated frictional resistant force was compared with

the horizontal resultant force of each bridge model in

each respective case, which had shown in Table 1 to

Table 3.

Table 1: Comparison of horizontal resultant force and

frictional resistant force for simplified deck


frictional resistant force for I-beam deck


frictional resistant force for box girder deck

5. Conclusions

The experiment has significantly achieved the main

objective of the study. Tsunami modeling and its impact

onto the onshore bridge models (simplified deck, I-beam

deck and box girder deck) were successfully carried out.

The forces and pressures acted on all the bridge models

are obtained from the experimental studies. The stability

against sliding of each bridge model is also evaluated.

These experimental results provide pragmatic

information for the dynamic analysis in the later stage of

the study that will contribute towards the design of

tsunami-resistant onshore bridge superstructures in

Penang Island, Malaysia.

ACKNOWLEDGEMENTS

The authors would like to express their deepest

gratitude to the School of Civil Engineering in Universiti

Sains Malaysia for opportunity to carry the research

without many obstacles. A whole hearted thanks to the

Case Measured

Horizontal

Force , Fx

(kN)

Deck Stability

Dry Condition Wet Condition

SH30W40 3829.2 Stable Unstable

SH30W60 8019.1 Unstable Unstable


SH40W40 2572.1 Stable Stable





Case Measured

Horizontal

Force , Fx

(kN)

Deck stability


IH30W40 5012.5 Unstable Unstable



IH40W40 1763.3 Stable Stable





Case Measured

Horizontal

Force , Fx

(kN)

Deck Stability


BH30W40 5066.8 Unstable Unstable








8

River Engineering and Urban Drainage Research Center

(REDAC) for providing a convenient environment to the

authors to carry out the laboratory test. Special

appreciations also goes to Mr. Liew Kok Kei and Mr.

Moon Wei Chek for their countless contributions in

conducting physical experiments.

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Technical Paper (Tsunami)

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