Guideline on Structural Fire Engineering Part I- Fire Scenarios and Calculation of Temperature Under...

7/25/2019 Guideline on Structural Fire Engineering Part I- Fire Scenarios and Calculation of Temperature Under Fire Struct

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Structural Engineering Branch, ArchSD Page 1 of 87 File code : SEBGL-OTH6

Revision No. : 0 CTW/MKL/CHL/CHM/SCF

First Issue Date : December 2011 Current Issue Date : December 2011

SEB GUIDELINES

SEBGL OTH6

Guideline on Structural Fire Engineering

Part I: Fire Scenarios and Calculation of Temperature under Fire

Structural Engineering Branch

Architectural Services Department


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CONTENTS

Content Page

1. Introduction ..... 1

2. Fire Safety Codes in Hong Kong 8

3. Fire Safety Engineering and Structural Fire Engineering.. 10

4. Prescriptive and Alternative Approaches ..... 12

5. General Principles of Structural Fire Engineering Approach ....17

6. Applicability of Structural Fire Engineering Approach ......18

7.

Typical Fire Scenarios .... 23

8. Fire Modelling ..... 26

9. Design Fire ... 29

10. Temperature of Structural Elements .... 48

11. Thermal Actions for External Member ..... 49

12. Engaging Fire Engineering Consultants ... 61

13. Design Examples ...... 62

14. References .... 85

Annex A Sample Clauses in Engaging Structural Fire Engineering Consultant

Copyright and Disclaimer of Liability

This Guideline or any part of it shall not be reproduced, copied or transmitted in any

form or by any means, electronic or mechanical, including photocopying, recording, or

any information storage and retrieval system, without the written permission from

Architectural Services Department. Moreover, this Guideline is intended for the internal

use of the staff in Architectural Services Department only, and should not be relied on by

any third party. No liability is therefore undertaken to any third party. While every effort

has been made to ensure the accuracy and completeness of the information contained in

this Guideline at the time of publication, no guarantee is given nor responsibility taken by

Architectural Services Department for errors or omissions in it. The information is

provided solely on the basis that readers will be responsible for making their own

assessment or interpretation of the information. Readers are advised to verify all relevant

representation, statements and information with their own professional knowledge.

Architectural Services Department accepts no liability for any use of the said information

and data or reliance placed on it (including the formulae and data). Compliance with

this Guideline does not itself confer immunity from legal obligations.


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1. Introduction

1.1 There has been a large body of work written on the subject of performance

based structural fire engineering. Unfortunately, most of this information is

scattered throughout technical journals from different countries and

organizations, and not easily accessible to the practicing engineer. The

purposes of this Guideline are therefore to provide project officers in ourDepartment:

a)

background information on the behaviour of fire;

b)

the structural behaviour of structural steel, reinforced concrete,

composite structure and timber at elevated temperature;

c)

list of design references; and

d)

design examples,

when a structural fire engineering study is required for the design of

structural members under fire.

1.2 This set of Guideline is divided into two parts:

a) Part I will describe the fire scenarios development in a fire, the

techniques in fire modelling and the procedures to calculate the

maximum gas temperature and duration of a fire. Design examples

will be given to demonstrate the techniques. The gas temperature is an

important parameter in deciding whether a structural fire engineering

study is required. For example, if the computed gas temperature is

high enough such that the temperature of the structural steel exceeds

550oC, passive fire protection will likely be required, and hence a

structural fire engineering study may not be warranted in the detail

design stage.

b) Part IIwill first describe the heat transfer mechanisms from the fire to

the structural members, and the procedures to obtain the temperature

of the members during a fire. It will then focus on the structural

design of steel structure, reinforced concrete, composite structure and

timber exposed to fire, which will again be followed by design

examples.

1.3 Resources on Fire Safety Engineering

Project officers should note that this set of Guideline only provides an

overview on analysis and design of structural elementsexposed to fire, and

are therefore advised to conduct their own research on the details and

updated information. The following list the resources that may be helpful:

Hong Kong SAR Government Publications

For private buildings, approval of fire safety designs and inspection of the

buildings upon completion are held responsible by two Government

departments Buildings Department and Fire Services Department. The

building design shall be submitted to the Buildings Department to check


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against all fire aspects for approval. As government buildings are exempted

from the Buildings Ordinance, the design of these government buildings in

theory are not necessary submitted to Buildings Department; yet, our

Department is always required to submit to Fire Services Department. The

requirements and installation of fire protection systems are monitored by the

Fire Services Department. Buildings Department has issued the following

codes governing different aspects for fire safety:

1.

Buildings Department (1996), Code of Practice for the Provision of

Means of Escape 1996(Hong Kong: Building Authority).

2. Buildings Department (1996), Code of Practice for Fire Resisting

Construction 1996(Hong Kong: Building Authority).

3. Buildings Department (2004), Code of Practice for Means of Access for

Firefighting and Rescue 2004(Hong Kong: Buildings Department).

These three codes have just been replaced by the following unified code:

Buildings Department (2011), Code of Practice for Fire Safety in Buildings

2011(Hong Kong: Buildings Department).

This unified code consists of the following parts:

Part A - Introduction

Part B - Means of Escape

Part C - Fire Resisting Construction

Part D - Means of Access

Part E - Fire Properties of Building Elements and Components

Part F - Fire Safety Maintenance and Management

Part G - Fire Safety Guidelines

There is an annex Guidelines from Licensing Authorities to the unified

code.

Fire Services Department issued the following two codes on active fire

protection system or fire services installation:

1. Fire Services Department (2005), Code of Practice for Minimum Fire

Service Installations and Equipment (Hong Kong: Fire Services

Department).

2. Fire Services Department (2005),Code of Practice for Inspection and

Testing and Maintenance of Installations and Equipment (Hong Kong:

Fire Services Department).

Professional Associations

1. The Society of Fire Protection Engineers (SFPE) (www.sfpe.org/) is the

professional association of the US for fire protection engineering, and

published the following comprehensive text describing the fire science

that underpins fire protection engineering, and providing information in

the areas of the fundamental science and engineering concepts that are

applied in fire protection engineering, fire dynamics, fire hazard

calculations, design calculations, and fire risk analysis:


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DiNenno, P J et al (eds.) (2002), SFPE Handbook of Fire

Protection Engineering (Bethesda, Maryland: Society of Fire

Protection Engineers, 3rd

ed).

2.

British Standards Institution (www.bsi.org.uk) published the following

standards on the principles of structural fire engineering and the designof structural members for different materials:

BSI (2003), BS 5950-8: Structural Use of Steelwork in Building

Part 8: Code of Practice for Fire Resistant Design(London: British

Standards Institution).

BSI (2002),Eurocode 1: Basis of Design and Actions on Structures,

Part 1.2: Actions on Structures Actions on Structures Exposed to

Fire (BS EN 1991-1-2) (London: British Standards Institution).

BSI (2005), Eurocode 2: Design of Concrete Structures, Part 1.2:

General Rules, Structural Fire Design (BS EN 1992-1-2)(London:British Standards Institution).

BSI (2003), Eurocode 3: Design of Steel Structures, Part 1.2:

General Rules, Structural Fire Design (BS EN1993-1-2)(London:

British Standards Institution)

BSI (2005), Eurocode 4: Design of Composite Steel and Concrete

Structures, Part 1.2: Structural Fire Design (BS EN 1994-1-2)

(London: British Standards Institution).

BSI (2004), Eurocode 5: Design of Timber Structures, Part 1.2:

General Rules, Structural Fire Design (BS EN 1995-1-2)(London:

British Standards Institution).

BSI (2001), BS 7974: Application of Fire Safety Engineering

Principles to the Design of Buildings Code of Practice (London:

British Standards Institution).

BS 7974only gives a framework for the application of fire safety

engineering principles to the design of buildings. It is supported by

the PD 7974-0 to -7 series of Published Documents that contain

guidance and information on how to undertake detailed analysis of

specific aspects of fire safety engineering in buildings. Thefollowing parts are relevant to structural fire engineering:

a) Part 0: Guide to design framework and fire safety engineering

procedures

b) Part 1: Initiation and development of fire within the enclosure

of origin;

c) Part 3: Structural response and fire spread beyond theenclosure

of origin.


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3. The Institution of Structural Engineers (www.istructe.org.uk) published

the following two comprehensive texts providing guidance on the

behaviour and structural design of structural elements of all the principal

construction materials:

IStructE (2003), Introduction to the Fire Safety Engineering of

Structures(London: IStructE).IStructE (2007), Guide to the Advanced Fire Safety Engineering of

Structures (London: IStructE).

4. The Association for Specialist Fire Protection (www.asfp.org.uk) is a

trade association representing UKs manufacturers and installers of

passive fire protection products, and published the following book

(commonly known as the Yellow Book) on common proprietary

materials and systems as passive fire protection products:

ASFP (2004), Fire Protection for Structural Steel in Buildings

(Aldershot: Association for Specialist Fire Protection, 3rd

ed).

Publications and Reference Books

1.

Lennon, T (2011), Structural Fire Engineering (London : Thomas

Telford) - This updated book provides comprehensive but concise

summary of the principles of structural fire engineering and summarizes

EN1991 Part 1.2,EN1992 Part 1.2,EN1993 Part 1.2andEN1994 Part

1.2 on structural design of concrete, steel, composite structures under

fire. The book also provides examples on the structural design.

2. Lennon, T et al (2007), Designers Guide to EN 1991-1-2, 1992-1-2,

1993-1-2 and EN 1994-1-2 (London : Thomas Telford) - This book

provides guide to EN1991 Part 1.2,EN1992 Part 1.2,EN1993 Part 1.2

and EN1994 Part 1.2on structural design examples of concrete, steel,

composite structures under fire.

3. Wang, Y C (2002), Steel and Composite Structures, Behaviour and

Design for Fire Safety(London: Spon Press) This book explains the

fire behaviour, heat transfer in construction elements and structural

analysis, and describes the behaviour of steel and composite structures

in fire.

4.

Franssen, J M and Real, P V (2010), Fire Design of Steel Structures

(Berlin: ECCS) - This updated text explains and illustrates the rules thatare given in the Eurocode 1for designing steel structures subjected to

fire by describing the design process together with worked examples.

5.

Law, M and OBrien, T (1989),Fire Safety of Bare External Structural

Steel (Ascot: SCI) Although this book is old, it is a classic in

structural fire engineering. This book examines flame projection from

openings in building facades and heat transfer calculation methods of

fires to external unprotected steel columns.


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6.

Newman, G M (1990), Fire and Steel Construction: the Behaviour of

Steel Portal Frames in Boundary Conditions (Ascot: The Steel

Construction Institute, 2nd

ed) This book describes the behaviour of

structural steel portal frames in fire.

Design and Analysis Tools

There are computer softwares available that can be used to simulate fires in

buildings. Common available sofwares (some being free for use) include:

SAFIR (www.argenco.ulg.ac.be/logiciels/SAFIR/), a computer

software developed at the University of Liege for the simulation of the

behaviour of building structures subjected to fire.

Fire Dynamics Simulator (FDS) (www.fire.nist.gov/fds/index.html), a

computational fluid dynamics (CFD) model of fire-driven fluid flow

for heat transport from fires developed by National Institute of

Standards and Technology, the US Department of Commerce.

PyroSim (www.thunderheadeng.com/pyrosim/), a computer software

that can simulate temperature of a building during a fire.

Consolidated Model of Fire and Smoke Transport (CFAST)

(www.nist.gov/el/fire_research/cfast.cfm), a computer developed by

the National Institute of Standards and Technology (NIST) of the US

Department of Commerce, and is free software that use a two-zone

fire model used to calculate the evolving distribution of smoke, fire

gases and temperature throughout compartments of a building during a

fire.

OZONE (www.ulg.ac.be), a free computer software that combines a

two zone model and a one zone model to predict the temperature and

time relationship before and after flashover in a compartment. It can

also calculate the temperature of a steel section under that

compartment fire, and evaluate the fire resistance of simple steel

elements according toEurocode 3.

Academic Institutions

The University of Manchester holds the following site providing free

information on structural fire engineering (including the theories,prescriptive and alternative measures in fire protection, fire behaviour, fire

modeling, and structural design):

http://www.mace.manchester.ac.uk/project/research/structures/strucfire/

This site was developed under the direction of a Steering Group with

representatives from the Institution of Structural Engineers, Building

Control of the City of London, Arup Fire, the Concrete Centre, Corus,


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British Constructional Steelwork Association, Concrete Block Association,

BRE, etc.

Department of Civil and Natural Resources Engineering of the University of

Canterbury, NZ also develops the following site publishing their research

findings and containing links to the various sofwares for fire modeling:

http://www.civil.canterbury.ac.nz/fire/firehome.shtml

2. Fire Safety Codes in Hong Kong

2.1 A properly designed fire safety system of a building greatly reduces the loss

of life and property during a fire, or in the neighborhood of the building.

Nearly all building regulations and/or codes specify requirements for

buildings to be designed in such a way that they exhibit an acceptable level

of performance in the event of fire. Similar requirements have been

specified as Regulations 41(1), 41A, 41B, 41C and 41D in the Building

(Planning) Regulations and Regulation 90 of the Building (Construction)

Regulations. Over the years, Buildings Department and Fire Services

Department have issued the following codes on the performancerequirements complying the statutory requirements:

a)

the Code of Practice for the Provision of Means of Escape 1996 (the

MOE Code);

b)

the Code of Practice for Fire Resisting Construction 1996 (the FRC

Code);

c)

the Code of Practice for Means of Access for Firefighting and Rescue

2004(the MOA Code);

d)

the Code of Practice for Minimum Fire Service Installations and

Equipment; and

e)

the Code of Practice for Inspection and Testing and Maintenance of

Installations and Equipment.

2.2 The MOE Code sets out the requirements on the provisions for the

protection of buildings from the effect of fire by providing adequate means

of escape in the event of fire and other emergency. This is achieved by

recommending the assessment of population density of floor, the type of

usage, the minimum number of escape routes and their widths, the

maximum travel distance, the construction of escape routes and appropriate

signage etc. The MOA Code seeks to achieve the objective of assisting in

firefighting and in saving life of people in buildings by ensuring adequate

access for firefighting personnel in case of fire and other emergencies. This

is achieved by recommending adequate emergency vehiclur access, accessstaircases, firemans lifts as well as fire fighting and rescue stairways

according to the area, use and height of buildings. The FRC Code provides

guidance on compliance with the requirements for fire resisting construction

stipulated in Part XV of theBuilding (Construction) Regulations. It sets out

the provisions on protection of buildings from effects of fire by inhibiting

the spread of fire and by ensuring the integrity of structural elements and the

overall stability of buildings. This is achieved by specifying a minimum fire

resistance period (or fire resistance rating in the FS Code) in accordance


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inspections and tests which installations and equipment must normally pass,

and provides guidance on administrative procedures for application and for

inspection and testing and how these systems can be appropriately

maintained and inspected throughout the building life.

3. Fire Safety Engineering and Structural Fire Engineering

3.1 The Institution of Fire Engineers of the UK (www.ife.org.uk/) defines fire

engineering (or sometimes termed as fire safety engineering) as:

the application of scientific and engineering principles, rules (Codes),

and expert judgment, based on an understanding of the phenomena

and effects of fire and of the reaction and behaviour of people to fire,

to protect people, property and the environment from the destructive

effects of fire.

Similarly, the Department of Civil and Natural Resources Engineering of the

University of Canterbury, NZ (http://www.canterbury.ac.nz/) defines fireengineering as:

the art and science of designing buildings and facilities for life safety

and property protection in the event of an unwanted fire.

Fire engineering is, therefore, a broad term embracing a multi-disciplinary

approach (involving architects, building services engineers, structural

engineers, insurance companies, etc) to determine fire safety strategy for

buildings under fire conditions, including the control of fire spread and

addressing structural stability.

3.2 There are two broad aspects in the fire engineering: fire prevention

(designed to reduce the chance of a fire occurring) and fire protection

(designed to mitigate the effects of a fire should it nevertheless occur). Fire

prevention includes eliminating or protecting possible ignition sources in

order to prevent a fire from occurring. Fire protection measures may be

passive or active. Active measures include detection and alarm, fire

extinction, and smoke control. Passive measures include structural fire

protection, layout of escape routes, fire brigade access routes, and control of

combustible materials of construction. The term fire protection

engineering therefore comprises active and passive ways of providing

satisfactory protection level to buildings and/or its contents from fires.

Figure 1 shows the role of active and passive fire protection measuresduring a fire.


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Figure 1 Role of active and passive fire protection in a fire

3.3 Structural fire engineering is a special branch within the fire protection

engineering, and addresses the specific aspects of passive fire protection in

terms of analyzing the thermal effects of fires on buildings and designing

members for adequate load bearing resistance and to control the spread of

fire. Figure 2shows the interrelationship of fire engineering, fire protection

engineering, and structural fire engineering.

Figure 2 Relationship among various branches in fire engineering

3.4 Project officers should therefore note that the term fire engineering (or

fire safety engineering) embraces all aspects of fire prevention and fire

protection. Besides predicting the performance of structural elements under

fire, it also involves the study of the means of escape, smoke control, fire

spread control, design of sprinkler, alarm, fire-fighting systems, etc.

Structural engineering design mainly concerns passive fire protection. This

Guideline will focus on the structural fire engineering, rather than on the

architectural or BS aspects.


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3.5 IStructE (2007: 1) states that [t]raditionally, structural engineers did not

venture into fire design, due to their lack of knowledge of fire behavior .

Structural fire design brings together the disciplines of structural

engineering and fire engineering, to allow a performance-based design

approaches to be carried out which can allow more economic, robust,

innovative and complex buildings to be constructed. Professor D. J.OConnor of the Fire Engineering Research Centre of the University of

Ulster in the ordinary meeting of IStructE of 9 March 1995, once said: in

this developing field of [fire] engineering, structural engineers have a unique

opportunity to provide leadership to other building professionals.so that

structural engineers do not restrict their expertise simply to the provision of

safety based on passive fire protection, but understand the full complexities

of the life safety and the structural safety issues pertaining to total fire

engineering design.

4. Prescriptive and Alternative Approaches

4.1 There are two approaches for complying with the statutory requirements forfire safety, namely: Prescriptive Provisionsand Alternative Approach.

4.2 Prescriptive Provisions

4.2.1 The simplest approach to satisfy the statutory requirements is to follow

Prescriptive Provisions in the FS Code, which includes the provisions on

means of escape, emergency vehiclur access, firemans lifts, passive

protection, etc based on required fire resistance rating. These provisions

aim at providing adequate fire resisting construction to the elements of

construction of the buildings, providing adequate means of escape,

maximum travel distances, and specifying compartmentation within the

building and measures for protection of adjoining buildings. However, as

these provisions have to account for a wide range of buildings, they cannot

provide the optimum solution in terms of life safety, property protection,

cost-effective fire protection and operational requirements (PD 7974-0).

Perhaps, the main deficiency of Prescriptive Provisions is that they do not

meet the fire safety for complex buildings. BS PD 7974-0 quotes the

following conclusion of the Cullen report into the Pier Alpha offshore

disaster (in which 167 of the 229 people onboard on the oil platform in

North Sea were killed) for the Prescriptive Provisions:

Many regulations are unduly restrictive in that they are of a type that

impose solutions rather than objectives and are out of date inrelation to technological advances. There is a danger that

compliance takes precedence over wider safety considerations.

The conclusion highlights the main deficiency of Prescriptive Provisions.

Other disadvantages include: unable to anticipate all eventualities, unable to

provide an optimum solutions, and unable to meet with the current design

practice (BS PD 7974-0). However, project officers should note though

with such limitations, Prescriptive Provisions provide an acceptable solution


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for most buildings with straightforward construction, layout and use, and are

therefore adopted in the majority of cases.

4.2.2 The FRC Code (or the FS Code Part C), which applies to elements of

construction (including structural frame, fire barriers, fixed lights, fire doors,

fire shutters or other components, etc) specifies that one or more of the

following three criteria to be satisfied (details being specified in Table C2 ofthe FS Code) in a fire:

1)

stability, i.e. to avoid collapse of load-bearing elements (Figure 3(a)),

2) integrity, i.e. to resist fire penetration and inhibit spreading (Figure

3(b)); and

3) insulation, i.e. to prevent transfer of excessive heat such that the

unexposed surface of a fire resistant construction should not be heated

excessively and cause further ignition (Figure 3(c)).

Figure 3 Failure modes of construction elements during fire

(Source: Wang 2002)

Similar provisions have been specified in the Code of Practice for Structural

Use of Concrete 2004(the HK Concrete Code) and Code of Practice for

Structural Use of Steel 2005 (the HK Steel Code) issued by Buildings

Department.

4.2.3 To meet the stability criterion, a building element must perform its load

bearing function and carry the applied loads for the duration of the fire

without any structural collapse. The integrity and insulation criteria are the

ability of the building element to contain a fire in order to prevent fire

spreads from the room of origin. For structural elements (including

structural frame, beam and column), stability criterion must be satisfied, andthe other criteria may be required for specific structural element. For

example, for floor slab, integrity and insulation criteria must also be

satisfied in order to prevent fore spreads through floors. Table 2 lists the

criterion or criteria to be satisfied for main types of structural elements.

Table 2 Criteria for Different Elements of Construction


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Elements of

construction

Criteria to the satisfied

Method of

ExposureStability Integrity Insulation

Structural frame,

beam or column

Y N N Exposed faces

only

Floor including

compartment floor

Y Y Y Each side

separately

Roof forming part of

an exit route or

performing the

function of the floor2

Y Y Y From

underside

Loadbearing wall not

forming a separating

wall or fire

compartment wall

Y N N Each side

separately

External wall Y Y Y Each side

separately

Notes: Y = required and N = not required2 Project officers should also refer to Section 4.2.7 below, or SEBGL-OTH1

Guidelines on the Fire Resisting Construction for Roof Structures for roof not

forming part of an exit route and not performing the function of the floor.

(Source: FS Code Part C Table C2)

4.2.4 For structural elements, Prescriptive Provisions specify the material, shape

and size, thickness of fire protection materials and construction details to be

used in order to satisfy the statutory requirements. Compliance of these

provisions is deemed to satisfy the statutory requirements laid down for fire

resisting construction for buildings in Part XV of theBuilding (Construction)

Regulations. The following paragraphs provides brief summary of theseprovisions.

4.2.5 Prescriptive Provisions for structural steel

4.2.5.1 For structural steelwork, Clause 12.2 of the HK Steel Code specifies the

quantitative requirements for the insulation and stability. For insulation

(e.g. for the floor slabs), it is specified that the mean and maximum

unexposed face temperatures should not be increased by more than 140oC

and 180oC respectively above the initial value. For stability, it is specified

that it should be able to carry the load without excessive deflection.

4.2.5.2 The FS Code contains Prescriptive Provisions for the required fireprotection to structural steel by encasing the members with concrete. The

main disadvantage of such method is that encasing increases the dead

weight of the structure resulting in enlarged member sizes and foundations.

Alternate materials in the form of sprayed mineral coating, intumescent

paint and proprietary fire protection board have therefore been used.

These alternate materials are permitted as prescriptive measures, provided

that appropriate test reports on their performance can be demonstrated.


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4.2.5.3 The required thickness of the alternate materials for fire protection of

structural steelwork can be determined from the performance data sheets,

published inFire Protection for Structural Steel in Buildings(ASFP 2002),

which is commonly referred to as the Yellow Book. The Yellow Book

provides a comprehensive guide of proprietary materials and systems of

fire protection to structural steelwork. For each type of fire protection

system, the thickness of fire protection is usually based on the SectionFactor (denoted by A/V (surface area divided by cross sectional area) or

Hp/A (heated perimeter divided by cross sectional area)) of the structural

member, since the rate at which the structural element will heat up is

proportional to the surface area of steel exposed to the fire and inversely

proportional to the mass or volume of the section. In a fire, a member with

low section factor will be heated up at a slower rate than one with high

section factor.

4.2.5.4 Detailed specification on the submittals, the alternate materials and the

workmanship has been included in the Clauses 15.66 15.72 of the

General Specification for Building 2007 of our Department. Project

officer are required to specify the type(s) of material and the fire resistanceratings to suit his project.

4.2.5.5 In the choice of the appropriate type of material, project officer should note

that sprayed mineral coating is the cheapest option, and can be rapidly

applied. Sprayed mineral coating is therefore a preferred option. However

due to its undulating finish and hence aesthetically unpleasant, it is usually

preferred in surfaces which are hidden from the view (e.g. concealed

behind false ceiling). The properties of the sprayed material shall also

cope with the use of the structure. For example, where vibration or large

deflection is expected, more demanding sprayed material with higher dry

density and cohesion properties should be used. Moreover if the

environment is moist (e.g. exterior steel stair or above a swimming pool),

then the sprayed mineral coating option is not advisable, as there is the

possibility of water seeping into it (because of the porous nature of sprayed

mineral). Proprietary fire protection board is an expensive method, and

may also susceptible to the effect of moisture. Hence, its application is

also restricted to indoor steelwork with dry environment. Intumescent

fireproofing is a layer of paint which is applied along with the coating

system on the structural steel members. Intumescent coating is applied as

an intermediate coat in a coating system (primer, intermediate, and

top/finish coat). Because of the relatively low thickness of this

intumescent coating (350-700 micrometers), nice finish, and anti-corrosive

nature, intumescent coating is a preferred option when aestheticalappearance is required. Moreover, intumescent coating is the option that

can be applied to steelwork in moist environment.

4.2.6 Prescriptive Provisions for reinforced concrete and timber

For reinforced concrete, fire protection is typically achieved by the

minimum dimensions and concrete covers to reinforcement for a given

standard fire resistance rating. Clause 4.3 of the HK Concrete Code states

that the covers to steel reinforcement for fire protection shall follow the FRC


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Code. The FS Code Part E specifies the minimum dimensions of structural

members and covers to steel reinforcement for specified fire resistance

rating in Tables E2, E4, E6 and E7. The minimum covers and dimensions

have been derived to ensure that the temperature of steel reinforcements

does not exceed a specified critical temperature.

For timber construction, the prescriptive protection is normally to protect theelements from fire by fire resistant cladding materials.

4.2.7 Prescriptive Provisions for roof structure

A particular Prescriptive Provision for roof structure is that it is not

classified as an element of construction under the definition in the FS

Code Part A, and hence there is no need to provide fire resisting

construction requirement for it, although there are special exceptional

circumstances (e.g. an exit route, performing the function of the floor, or

essential for the stability of an external wall) where roof elements require

fire resisting construction. Detailed discussion on fire protection to roof

structure can be referred to SEBGL-OTH1 Guidelines on the Fire ResistingConstruction for Roof Structures available: http://asdiis/sebiis/2k/

resource_centre/).

4.3 Alternative Approach

4.3.1 Alternative Approach (or more commonly called fire engineering

approach) is a performance based method. There has been a trend around

the world adopting of performance based method due to the well-publicized

benefits in fire safety, design flexibility, cost, and quality that can be

achieved. The use of performance-based approach should ensure an

equivalent level of safety of the building environment is not eroded.

4.3.2 In 1998, Buildings Department has issued APP-87: Guide to Fire

Engineering Approach (available: http://www.bd.gov.hk/; accessed: 4

September 2011) providing further guidance on fire engineering approach.

UnderAPP-87, the aim of fire engineering approach is stated to provide for

an overall level of safety that is equivalent to that which would result if fire

safety was achieved through full compliance with the prescriptive provisions

of the relevant codes of practices, even though the full prescriptive

provisions in the Code cannot be provided. The FS Code Part G now

replaces APP-87 and dedicates a full section providing guidance and

methods on using the fire engineering approach. Pang (2006) further stated

that the Alternative Approach provides a framework for engineers todemonstrate that the performance requirements of legislations are met, or in

some cases bettered, to compensate for the deviation or shortfalls of theprescriptive codes.

4.3.3 Similar to Prescriptive Provisions, Alternative Approach is available for

other aspects of fire protection engineering, e.g. in the provision of means of

escape and sprinkler system. BSB issued the Report on the Study on

Performance Based Fire Engineering Approach (available:

http://bsbiis/main/bsbiis/4.3.3.asp) in 2001 providing a summary these


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different aspects. The focus of this set of SEB Guideline will be, however,

on application of Alternative Approach to assess the actual performance of

the structural members under fire. This set of Guideline will particularly

stress on such the application of Alternative Approach on structural steel, as

one of the distinct advantages of Alternative Approach for structural steel is

that it may be unprotected, provided that the performance of the structural

steelwork can demonstrate to meet the statutory requirements of fireresisting construction.

5. General Principles of Structural Fire Engineering Approach

5.1 A full performance-based approach to fire engineering in buildings should

consider active and passive measures, movement of smoke and fire,

detection systems, fire safety management, structural response and risk

analysis. Instead of carrying out a full performance-based study, it is

usually to carry out a simplified performance-based approach, which is

sufficient for structural engineer to understand and explain how the structure

performs should it be subjected to severe fires. The main objective of a

structural fire engineering study is to verify for all structural membersessential for maintaining stability of the structure that:

Rf Efwhere Rf is the load carrying capacity of the structural member in a fire till

the end of the required fire resistance rating; and

and Efis required load carrying capacity by loads in the fire till the end of

the required fire resistance rating.

5.2 The process of such performance-based approach is therefore similar to the

process of designing structures to withstand wind (which requires an

estimate of the wind pressures over the building and an estimate of the

structural response). For a structural fire engineering performance-based

approach, the assessment involves three basic components namely: the

likely fire behaviour, heat transfer to structure, and the structural response.

The steps in a typical structural fire engineering study are shown inFigure 4.


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Figure 4 Steps in Structural Fire Engineering Study

(Source: Modified from Kirby 2004)

6. Applicability of Structural Fire Engineering Approach

6.1 As stated above, one of the main reasons for the research and rapid advances

on structural fire engineering is to eliminate fire protection to steelwork.

That is, structural steel members can be unprotected, as fire protection to

steelwork can represent a significant part of the total steel structural cost and

the elimination of fire protection to steelwork therefore represents a

significant saving in construction cost to the client. Another benefit of

unprotected steel is to have more choices of architectural

finishes/appearance of the steel thus enhancing the aesthetic effects. In astructural fire engineering study, it is therefore required to predict the

structural performance of unprotected steel members under a real fire, so

that an equivalent level of fire safety can still be maintained.

6.2 Project officers should, however, note that in a small compartment with the

usual design fire load, the fire will likely to be fully developed. In such

circumstance, it may be safely assumed that the results from a structural fire

engineering study will not eliminate the fire protection to steelwork, and

project officers are advised to adopt Prescriptive Provisions for the structural


19/92




elements. Example 2 in Section 13will show that the room temperature

will rise up rapidly with time in a small compartment.

6.3

Project officers should further note that roof structure is not classified as an

element of construction as briefly discussed above, and hence no

structural fire engineering study is required to eliminate fire protection to

such steelwork.

6.4

In summary, under the current statutory requirements, structural fire

engineering is particularly applicable for the following situations:

a)

large compartments (especially with high headroom and limited fire

load) or open-sided buildings, as a fire is unlikely to fully develop in

these compartments/buildings. Examples of such structures include:

open-sided car park, sports stadium, indoor swimming pool, public

transport concourse in the projects of our Department, and casino or

cinemas in the private sector projects;

b)

external structural steelwork located outside the facade of the building;

andc)

localised fire which is unlikely to flash over.

6.5

Sports stadium, indoor swimming pool, transport concourse, casino and

cinemas

For sports stadium and indoor swimming pool, fire load is low and

headroom is high, whilst in transport concourse, casino and cinema, the

headroom is high. In these venues, the resulting gas temperature in a fire is

low. The significant fire loads in sports stadium, swimming pool and

transport concourse include the seating, the air ducts or the vehicles, which

will seldom lead to flashover of a localised fire. In our Department, fire

engineering study was employed in the project of Tin Shui Wai Public

Library cum IRC, in which the structural steelwork above the swimming

pool in the IRC was left unprotected.

6.6 Open-sided car parks

Similarly, for open-sided car parks, they have very high levels of ventilation

combined with a low fire load. Accordingly, UK Building Regulations 1991

(now Approved Document B Fire Safety Volume 2 issued under UK

Building Regulations 2000) allows that in open-sided car parks less than

30m high, 15 minutes fire resistance rating is normally sufficient, though no

similar provisions have been provided in the corresponding regulations inHong Kong. Structural fire engineering study can therefore be utilized to

find the temperature of the structural members under fire.

6.6 External steelwork

A structural fire engineering study is also warranted for external structural

steelwork or other load bearing members located outside the facade of the

building. There may be flames coming through windows and doors or heat


20/92




transfer due to internal radiation of the compartment fire. However, the

temperature of such steelwork will be less than the gas temperature within

the building due to the comparatively low net rate of heat transfer. However,

the location of these members relative to the windows is important, as

members placed directly opposite openings will receive more heat than

members shielded by a wall or faade. Heat transfer calculations are

therefore required to check that the members remain below its criticaltemperature for the compartment fire and flame projection considered. In

our Department, such studies were employed in the projects of Dr Sun Yat-

sen Museum (Photo 1), Improvement Works to Lei Yue Mun Park and

Holiday Village (Photo 2(a) and (b)), and International Wetland Park and

Visitor Centre at Tin Shui Wai (Photo 2(c)), in which structural steelwork

of the external staircases located just outside the faade of the development

were left unprotected.

Photo 1 External Stair in Dr Sun Yat-sen Museum

Photo 2(a) External Stair in Lei Yue Mun Park


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Photo 2(b) Steel Beams in External Corridor in Lei Yue Mun Park

Photo 2(c) External Stair in International Wetland Park

6.7 Localised fire

Localised fire may be caused by vandals or disposal of lit cigarette, resulting

the burning of an isolated item in an area with plentiful supply of oxygen,

where flashover is unlikely because of the limited fire load. Structural fire

engineering study will usually show that the structural integrity of the

building will not be affected by such localised fires. Effects of such

localised fires have been studied in the projects of Improvement Works to

Lei Yue Mun Park and Holiday Village (Photo 3) (for the burning of carton

exhibit and a/c unit).


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Photo 3 Items for Localised Fire on the Verandah Lei Yue Mun Park

6.8 Figure 5 illustrates the applicability and inapplicability of structural fire

engineering in the project Tin Shui Wai Public Library cumIRC, in which:

a) structural fire engineering was applied to study the effect of fire on the

unprotected steel trusses above the swimming pool (further details of the

study having been reported in Ho et al (2011));

b)

Prescriptive Provisions by providing a 2-hour fire-resistance rating

passive protection were followed for the steel trusses above the multi-

purpose rooms; and

c) the roof steel trusses were left unprotected as they were not classified as

elements of construction.

Figure 5 Combination of Prescriptive Provisions and Alternative

Approaches in Tin Shui Wai Public Library cumIRC


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7

Typical Fire Scenarios

7.1 Typically, a fire in a residential, commercial, or institutional building starts

in a single compartment, commonly known as compartment fire.

Compartments are typically rectangular in shape and not overly large with

small aspect ratios. The fire grows and decays in accordance with the massand energy balance within the compartment in which it occurs. The energy

released depends upon the quantity and type of fuel available and upon the

ventilation conditions. The different stages of fire development in a

compartment have been studied extensively (e.g. Cox 1995; Buchanan 2001;

Karlsson and Quintiere 2000; Drysdale 2000). Following ignition, fires in

compartment typically have three distinct phases: the growth or pre-

flashover, the fully developed or post-flashover, and the decay, which are

represented graphically in Figure 6. There is a rapid transition stage called

flashover between the pre-flashover and fully developed fire. NIST of the

US Department of Commerce uploads a video in the following URL

(accessed: 26 September 2011) showing the fire development in a

compartment:http://www.fire.nist.gov/tree_fire.htm

Figure 6 Typical Compartment Fire Time-temperature Curve

7.2 Growth or Pre-flashover Phase

7.2.1 Figure 7shows a typical compartment fire before flashover phase.During

this period the fire begins as either a smoldering or flaming fire depending

on availability of oxygen for combustion. During this stage, the fire islocalised and temperature distribution inside the enclosure is highly non-

uniform. If this fire is promptly discovered and/or effective fire fighting is

activated, it can be easily controlled. Even if there is no intervention, but

the first burning item is sufficiently far away from other combustible

materials, the fire may die out due to the difficulty of igniting other

combustible materials.


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7.2.2 The connective plume of hot gases above the burning object will rise to the

ceiling and spread horizontally to form an upper hot layer, called the

ceiling jet. At this stage, the enclosure may be approximately divided into

two zones: an upper zone of hot smoke, and a lower zone of cold air. The

division between the upper and lower zones is the neutral plane, above

which smoke flows out of the enclosure and below which fresh air is

supplied into the enclosure. As the fire continues to burn, the volume ofsmoke and hot gases in the upper layer increases, reducing the height of the

interface between the two layers. As this happens, the temperature of the

hot gas layer increases further. The rate of burning may also be significantly

enhanced by radiant feedback from this hot upper layer. Over time the

combustion products will start to flow out the door opening when the

interface drops below the door soffit or open window of the compartment

(Figure 7). Hot gases will then leave the room through the openings, and

fresh air from the surrounding spaces will rush into the compartment to

make up for the air leaving the hot gas layer and continue to feed the fire. If

there are insufficient openings in a typical compartment, the rate of burning

will decrease, and it may self-extinguish even the fuel is not fully consumed.

However, it may grow again if fresh air is supplied into the enclosure. Inmore dramatic situations, a sudden fresh air supply to an under-ventilated

fire may lead to the so-called back draught () phenomenon,posing serious hazards for fire fighting.

Figure 7 Typical compartment fire before pre-flashover phase(Source: Parkinson and Kodur 2006)

7.2.3 Pre-flashover fire does have very significant influence of life safety since

toxic products of combustion can quickly give rise to untenable conditions.

This period is therefore critical for evacuation and fire-fighting. As such,

the majority of studies in fire dynamics have concentrated on the pre-

flashover fire so as to develop an understanding of the production andspread of smoke and toxic gases. Structural engineers may consider that

pre-flashover fire does not have a significant impact on the strength and

stiffness of structural members because of the low temperature when

compared with post-flashover fire. However, being able to predict the pre-

flashover fire behaviour enables structural engineer to investigate structural

behaviour under localised fires in such buildings as car parks, stadia and

airports, where due to large spaces, flashover is not possible.


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7.3 Flashover

As the fire grows in size and the layer of gases develops, owing to a lack of

oxygen in the smoke layer, a large quantity of partially burnt fuel will also

accumulate in the smoke layer. Meanwhile, the burning flame will become

larger and penetrate the smoke layer. Flame spread becomes quicker when

it is aided by the partially burnt fuels in the smoke layer. The radiation fromthe burning flames and the high temperature smoke layer will increase the

burning rate of the existing fire. All this will accelerate a positive burning

loop. A point will be reached when the incident radiation on the unburned

combustible materials in the enclosure becomes so high that objects distant

from the seat of the fire become ignited at almost the same time. If there is a

sufficient supply of air, this will result in full involvement of all combustible

materials in the fire. The transition from localised to fully developed fire

tends to be rapid and is known as flashover

() (Figure 8).

Figure 8 Flashover during fire(Source: Wang 2002)

Flashover lasts an extremely short duration, often seconds, and was held to

result in the death of a 27-year-old fireman on a fire on an industrial

building in Tsuen Wan in 2007 (China Daily, 8 September 2011) and was

reported in the fire of 20 December 2011 in Po On Building on Mongkok

Road (Ming Pao, 21 December 2011). TVB has recorded the latter flashover

in his news, and the video can be found in the following URL (accessed: 21

December 2011):

http://www.youtube.com/watch?v=jtDsaGgAZIc

NIST of the US Department of Commercehas also uploaded a video in the

following URL (accessed: 26 September 2011) showing the flashover in

compartment fire:

http://www.nist.gov/fire/upload/NS_multi.wmv

Another video showing flashover at a real fire is in the following URL

(accessed: 26 September 2011):

http://www.youtube.com/watch?v=_8btCZmrJzI&feature=related

Whether flashover will occurs and the time to flashover are both very

important for evacuation and fire-fighting, though is usually ignored in

structural fire engineering study. The conditions necessary for flashover to

occur depend on:


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1) sufficient fuel and ventilation for fire to develop to a significant size;

2)

sufficient hot gases trapped in the ceiling; and

3)

geometry of the room that must allow the radiant heat flux from the

hot layer to reach critical ignition levels at the level of the fuel items

Various analytical and experimental methods have been derived to estimate

the critical value of the heat release and the time to flashover. The usualconsensus is that flashover occurs when the upper layer temperature reaches

approximately 600C and the radiant heat flux to the floor is about 20kW/m

(Peacock et al 1999).

7.4 Fully developed or Post-flashover Phase

During the post-flashover phase, the very high temperature and radiant heat

flux in the compartment would cause all combustible fuel to burn when

there is sufficient oxygen supplied. Large amount of combustible gases are

produced at this stage, which burns when mixed with oxygen. The fire

severity will be controlled by the rate of supply of air through openings such

as doors and windows. This is a ventilation controlled fire and insufficiently small compartments will result in fairly uniform temperatures at

any level within the compartment. For such ventilation controlled fire, it is

normal to witness flames burning out through the openings, as any unburnt

gases, which leave through the opening will be able to burn due to the new

supply of outside oxygen. It is only during post-flashover phase, the highest

temperature, the largest flame and the highest rate of heating occur, leading

to fire spread and direct impact upon the structural integrity of the

compartment. The structural design of member in a post-flashover phase is

therefore critical, and is the focus of structural fire engineering.

7.5 Decay phase

The production rate of volatile gases is decreased as the fuel content in the

compartment is depleted (typically occurs when 70% of the fuel has been

consumed), and the decay phase of the fire will then begin. During this

period the temperature in the room decreases as the fire intensity decreases.

With burning thermoplastics and liquid hydrocarbon fuels, the decay phase

can be extremely short. However, with cellulosic materials, such as wood,

which chars, the decay stage is much longer and is of primary interest when

examining the fire resistance of structural elements of a building.

Ultimately, the decay rate will be a function of the quantity and physical

arrangement of combustible contents (such as the size and shape of openings)

within the compartment, and the thermal properties of the room boundaries.Typically, as a fire enters the decay period it begins to change from a

ventilation-controlled fire to a fuel-controlled fire.

8 Fire Modelling

8.1 The above paragraph describes the various phases of a fully developed

compartment fire. The factors influencing the temperature, magnitude, and

distribution of a fire can be summarized as follows (Petterson 1973;

Roytman 1975; Subramanian and Venugopal 1984):


27/92




1)

fire load type, density and distribution;

2)

combustion behaviour of fire load;

3)

compartment size and geometry;

4)

ventilation conditions of compartment (especially the window opening

area);

5)

thermal properties of compartment boundary;6)

thermal conductivity and diffusivity of the construction material;

7)

radiation levels from both within the compartment and through the

windows.

8.2 When dealing with post-flashover fire, the ignition phase is generally

neglected, because although this stage is generally the most critical for

human life, ignition phase is assumed to be dealt with active fire fighting

measures (e.g. sprinklers), which, if effective, will suppress the fire before it

becomes a fully-developed fire (Figure 1). Upon entering the post-

flashover phase, structural fire engineering will be useful to check the

stability of the structures. The temperature distribution insider the structure

must therefore be calculated.

8.3 The temperature distribution inside the structure is usually calculated based

on the gas temperature from many alternative methods, e.g. nominal fire

curves, parametric fire curves, the zone or fluid dynamics models, using heat

transfer analysis. Table 3 lists various options for fire modelling.

Simplified and advanced models of fire may be distinguished. The first four

fire models can be considered as simple models, whereas the zone and CFD

models are advanced models.

Table 3 Various Fire Models

Fire model1. Nominal

fires

2. Time

equivalence

Compartment fire 5. Zone models6. CFD

models3. parametric 4. localisedTwo-zone/

multi-zone

Complexity Simple Intermediate Advanced

Fire behavior Post-flashover fires Pre-flashoverPre-flashover or

localised

Complete

time-

temperature

relationship

Temperature

distributionUniform in whole compartment

Non-uniform

along plume

Uniform in each

layer

Time and

space

dependence

(varying)

Input

parameters

Fire type,

no physicalparameters

Fire load,ventilation conditions,

thermal properties ofboundary,

compartment size

Fire loadand size,

height of

ceiling

Fire load,

ventilation

conditions,

thermalproperties of

boundary,

compartment

size,detailed input

for heat & mass

balance of thesystem

Detail inputfor solvingthe

fundamentalequations of

the fluidflow

Design

methodsSimple equations

Spread-

sheet

Simple

equationsComputer model

(Source: Modified from IStructE 2007)


28/92




8.4 The simplified models of fire are based on fundamental physical parameters,

which allow temperature prediction, the design density of fire load and the

conditions of the ventilation. Nominal fires are used in the testing of

construction members in the standard fire resistance. Time equivalent

method is also used to relate the exposure of a structural element in a real

fire to an equivalent period of heating in the nominal time-temperature curvein the standard fire resistance test. In a parametric model, it is assumed that

the whole compartment is burning at the same time and attains the same

temperature throughout a single zone model. Eurocode 1 provides

simplified expressions for calculating the single zone post-flashover fires

using parametric expressions that describe the entire heating and cooling

cycle by including the fire load, ventilation characteristics, compartment

geometry, and the thermal properties of the surrounding walls floor and

ceiling. Localised fires are important in structural fire engineering, when

flashover is unlikely and the structure is subject to localised burning. These

four simplified models will further be described in Section 9.

8.5 Advanced models take into account properties of gas and the exchange ofmass and energy. Zone models are simple computer models that divide the

considered fire compartment into separate zones, where the condition in

each zone is assumed to be uniform. Two zone models exist in which the

height of the compartment is separated into two gaseous layers each with

their own temperature cycle. Three zone models exist in which there is a

mixed gas layer separating the upper and lower gas levels. Two-zone or

multi-zone models are used for pre-flashover fires. When a pre-flashover

fire develops into a post-flashover fire, and the two-zone model will become

a one-zone model. A number of zone models have been programmed and

are available via the internet. The most commonly used ones are CFAST

(available: http://www.nist.gov/) and OZONE (www.ulg.ac.be).

8.6 The computational fluid dynamics (CFD) models forecast the temperature

and pressure growth in the finite elements of space in time. CFD has been

shown to be successful in the modelling of smoke movement in large spaces

and atria, and has therefore been applied to the modelling of fires. CFD

modelling is a numerical approach to representing fluids that divides a fluid

domain into small volumes and considers conservation of mass, energy etc.

within each volume. CFD analysis is suitable for very large compartments.

Software exists that can represent the very wide range of physical

phenomena known to affect fire behaviour including compartment geometry,

heat release rates of burning fuel, complex ventilation conditions, turbulent

gas flow, soot production and many others. Figure 9 shows the gastemperature in fire compartment during fire from different models.


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Figure 9 Time-Temperature Curves from Different Fire Models(Source: Modified from Ghoreishi et al 2009)

9. Design Fire

9.1 In order to carry out structural design under fire, the selection of a suitable

fire of assumed characteristics, which is referred to as the design fire, is

one of the most important steps in this process. A design fire is generally

considered to be a quantitative description of temperature of a fire with time

based on reasonable assumptions about the type and quantity of

combustibles, ignition method, growth of the fire and its spread from the

first item ignited to subsequent items, and the decay and extinction of the

fire.

9.2 There are two types of design fire for a compartment fire:

a) a nominal time-temperature curve uniform in space, and

a a real fire either specified in terms of parametric time exposure (the

parametric fire), or obtained by computer modelling.

9.3 Nominal time-temperature Curves

9.3.1 The nominal time-temperature curves are a set of curves with no physical

parameters taken into account. That is, these curves are independent of

various parameters known to affect fire intensity including fire load,ventilation areas, building thermal properties, etc. The standard time-

temperature curves were originally derived from measurements of tests

taken early in the 20th

century, and involves an ever-increasing air

temperature inside the compartment, even when all combustible fuel is used

up. The standard fire is primarily used in experimental fire tests, as

although it does not resemble a real fire, it can be replicated in a controlled

environment. By using a standard fire, manufacturers can test their building

product and find a fire resistance time that can be compared to other


30/92




building products. Since all products are tested and exposed to the same fire

they can be compared due to the consistency in the tests.

9.3.2 Most internationally recognized codes (including the Eurocode 1 and ISO

834) contain defining equations for three distinct fire curves: standard,

external and hydrocarbon (Figure 10). The formula describing the standard

time-temperature curve for theISO 834fire is:T= 345 log (8t+1) + To---(1) (Eurocode 1Eqt. 3.4.1)

where T is the temperature (inoC) at time t (in minutes), and To is the

ambient temperature (taken as 20o

C inEurocode 1).

The standard fire curve represents a typical fire based upon a cellulosic fire

in which the fuel source is wood, paper, fabric, etc. This form of time-

temperature relationship has, however, a limited similarity to the

temperatures in real compartment fires, and was indeed not intended to be

representative of a real fire scenario, but instead it is an envelope that

represents maximal values of temperature during fire that may occur in

buildings. It is conservative for long duration fires, as it has no decay phase,

whereas in a real fire compartment temperature will reduce with the durationof the decay phase. However, for shorter duration fires, particularly where

upholstered furniture and thermoplastics may be involved in a real fire, the

standard curve may be non-conservative. Such a realistic fire can be more

severe than the standard fire in the early stages of fire development, when

evacuation and rescue activities are required to be undertaken. This point

should be considered together with the trend that the wood furnishing used

in the old days have been replaced by high fuel loads from polyurethane

furniture, plastics and other synthetic materials nowadays resulting in large

and fast growing fires.

Nevertheless, although this curve does not really represent the temperature

build-up in a real fire, this has become the standard design curve used in the

furnace test of components. Most European countries have standard fire

curves similar to that in ISO 834standard fire, and across the Atlantic, the

US and Canada also use the standard fire curve in ASTM E119 which is

similar to those inISO 834.

9.3.3 External and Hydrocarbon Fire Curves

Where the structure for which the fire resistance is being considered as

external,Eurocode 1gives a similar external fire curve. This is the nominal

time-temperature curve to be used for structural members located in a faade

outside the main structure but can be exposed to external plume of a firecoming either from the inside fire compartment, i.e. from a compartment

situated below or adjacent to the external wall. The formula describing the

external fire curve is:

T= 660(1 - 0.687e-0.32t

0.313e-3.8t

)+20 ---(2) (Eurocode 1Eqt. 3.6)

In situations where petrochemicals or plastics form a significant part of the

overall fire load, Eurocode 1gives a hydrocarbon fire curve, representing a

fuel load of 200kW/m2. The formula describing the hydrocarbon fire curve

is:

T= 1080(1- 0.325e-0.617t

0.675e-2.5t

)+20 ---(3)(Eurocode 1Eqt. 3.7)


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The external and hydrocarbon fires are similar in shape but the hydrocarbon

fire curve has temperatures 75% higher temperature due to the higher

calorific values of petrochemicals or plastics.

Figure 10 Nominal Fire Curves9.3.4 Time equivalent

Eurocode 1 provides for t-equivalent fire models. Law (1997) defines t-

equivalent as the exposure time in the standard fire resistance test which

gives the same heating effect on a structure as a given compartment fire.

Time equivalent is to relate the exposure of a structural element in a real fire

to an equivalent period of heating in the standard fire resistance test (Figure

11). Hence, it is applicable to calculate the fire resistance rating required forthe elements of construction within the building.

Figure 11 Graphical representation of time equivalence


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Eurocode 1gives the following expression to calculate the time equivalent:

te,d= (qf,d kb wf)kc ---(4)where qf,d = design fire load density (MJ/m

2) (Table 5);

wf = ventilation factor to take into account vertical and horizontal

openings=(6/H)0.3

[0.62+90(0.4-v)4

] in the absence of horizontalopenings;

kc = factor dependent on material=1.0 for protected steel and

reinforced concrete;

H = the height of the compartment (m);

v = Av/Af ;Av the total area of the opening;

Af the total floor area;

and kb = factor to take into account the thermal properties of the

enclosure

= 0.7 when there are no horizontal openings and bounding

surfaces are unknown, or when the bounding surfaces (and

hence the thermal inertia b(= c )) are known:

Thermal inertia b(= c ) (J/ms

K) kb(min. m/MJ)

2500 0.04 (0.055)

720 to 2500 0.055 (0.07)


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Figure 12 Typical Time-Temperature Behaviour

in a Compartment Fire

Eurocode 1divides the fire development of a parametric fire into two phases:the heating phase, and the decay phase, and Figure 13 shows the typical

parametric fire curve in Eurocode 1. As only the post-flashover and decay

phases of a fire will be taken into account in the parametric fire model, the

parametric time-temperature curve obtained using in the Eurocode 1 only

describes the fully-developed phase of the fire without considering the

growth phase of the fire, although results of fire tests with ordinary

furnishings reveal that even in small fire compartments it can take some

minutes to reach the fully developed fire from the initial fire.

9.4.3 Eurocode 1further states that the design equations for the parametric time-

temperature curve derived using the formulae in Eurocode 1are only valid

for compartments with the following conditions:

1) with floor areas up to 500m2and heights up to 4m;

2)

no openings through the ceiling;

3) with mainly cellulosic-type fire loads;

4) with thermal inertia 400 b 2,000 J/m2s

0.5K; and

5) with opening factor 0.02 O 0.2.


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Figure 13 Typical Parametric Curve inEurocode 1

9.4.4 Derivation of Parametric Fire usingEurocode 1

9.4.4.1 InEurocode 1, the design fire load for floor area, qf,dis given by:

qf,d = qf,k m q1 q2 n ---(5)(Eurocode 1Annex E)

where qf,k is characteristic fire load density per unit floor area (MJ/m2)

m is the combustion factor

q1 is the factor taking into account the fire activation risk due

to the size of the compartment (Table 4(a))

q2 is the factor taking into account the fire activation risk due

to the type of occupancy (Table 4(b))

and n =

=10

1i

ni is a factor taking into account the different active

fire fighting measures i (Table 4(c)).

The combustion factor mis a function of the spatial properties of the fuel

and location of the fuel relative to the fires ignition source and is a

measure of the influenced of the compartment on the burnability of the

fuel source. Usually, it is assumed to be 1. However, value of 0.8 has

been suggested, although Babrauskas and Williamson (1978) suggests that

the value can actually be lower than 0.7.


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Table 4(a) Fire activation risk factor due to compartment size

Compartment floor

areaAf(m2)

Danger of fire

activation q1

25 1.10

250 1.15

2,500 1.905,000 2.00

10,000 2.13

(Source:Eurocode 1Annex E)

Table 4(b) Fire activation risk factor due to occupancy use

Danger of fire activation q2 Examples of Occupancies

0.78Art gallery, museum, swimming

pool

1.00Offices, residence hotel, paper

industry

1.22Manufactory for machinery &

engines

1.44Chemical laboratory, painting

workshop

1.66Manufactory of fireworks or

paints

(Source:Eurocode 1Annex E)

Table 4(c) Fire activation risk factor due to fire fighting measures

ni Function of Active Fire Fighting Measures

Automatic Fire Suppression Automatic Fire Detection Manual Fire Suppression

Automatic

water

extinguishing

system

Independentwater supplies

Automatic firedetection alarm

Automaticalarm

transmission

to fire

brigade

Work

fire

brigade

Off site

fire

brigade

Safe

access

routes

Fire

fighting

devices

Smoke

exhaust

system

0 1 2By

heat

By

smoke

n1 n2 n3 n4 n5 n6 n7 n8 n9 n10

0.611.

00.87

0.

70.87 or 0.73 0.87 0.61 or 0.78

0.9 or

1.0 or

1.5

1.0 or

1.5

1.0 or

1.5

(Source:Eurocode 1 Annex E.1)

9.4.4.2 Characteristic Fire Load Density qf,k

In the above equation, it is necessary to obtain the characteristic fire load

density per unit floor area qf,k. The term fire load refers to the quantity

of combustibles within an enclosure and not the loads (forces) applied to

the structure during a fire. Fire load density refers to the quantity of fuel

per unit area, and is normally expressed in terms of MJ/m2. Sometimes, it

is expressed in terms of kg/m2of wood equivalent (1 kg wood = 18MJ). It

is an indication of the quantity of heat energy (in joules), which can be


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liberated by the complete combustion of all combustible materials in a

room. Surveys of combustibles for various occupancies (including offices,

retail, hospitals, warehouses, etc) have been undertaken and the fire load

densities have been available. However, as the fire load density is highly

variable, fire load data are usually given in terms of the mean and 80th

percentile, i.e. the value that is not exceeded in 80% of the rooms or

occupancy of the survey data. The latter level of fire load density is usuallytaken as the characteristic fire load density and is sometimes taken as being

distributed according to a Gumbel distribution. Typical fire load densities

for different occupancy uses are shown in Table 5.

Table 5 Fire Load Densities for Different Occupancy Uses

Occupancy

Average Fire

Load Density

(MJ/m2)

80t

Percentile Fire

Load Density(MJ/m

2)

Dwelling 780 948 (870)

Hospital (room) 230 280 (350)

Hotel (room) 310 377 (400)

Library 1500 1824 (2250)

Office 420 511 (570)

Classroom of a school 285 347 (360)

Shopping centre

Manufacturing

600

(300)

730 (900)

(470)

Manufacturing and storage

Hospital (storage)

(1180)

(2000)

(1800)

(3000)Notes:

1 Fire load densities for other occupancies can be found in Table 3.4.1a of

International Fire Engineering Guidelines(ABCB 2005). Project officers should

note that average values are given in International Fire Engineering Guidelines,

and a multiplier should be applied to get the 80thpercentile fire load values.

2 Values in bracket are given inPD 7974-1.3 Gumbel distribution is assumed for the 80% percentile values.

(Source:Eurocode 1 Annex E.4)

Alternatively, the fire load density may be calculated by:

qf,k= MvHv/Af ---(6) (Eurocode 1Eqt. E.2)

where qf,k is the fire load density (MJ/m2)

Af is the floor area (m2)

Mv is the total mass of the vth

combustible material

Hv is the calorific value of the vth

combustible material

(MJ/kg).


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Table 6 Net calorific valueHv(MJ/kg) of common combustible materials

Combustible Materials Value ofHv(MJ/kg)

Solids

Wood 17.5

Other cellulosic materials

Clothes

Cork

CottonPaper, cardboard

Silk

Straw

Wool

20

Carbon

Anthracit

Charcoal

Coal

30

Chemicals

Paraffin series

Methane

Ethane

Propane

Butane

50

Olefin seriesEthylene

Propylen

Butane

45

Aromatic series

Benzene

Toluene

40

Alcohols

Methanol

Ethanol

Ethyl alcohol

30

Fuels

Gasoline, petroleum

Diesel

45

Pure hydrocarbons plasticsPolyethylene

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