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

SEB GUIDELINES

SEBGL – OTH6

Guideline on Structural Fire Engineering Part I: Fire Scenarios Sce narios and Calculation of Temperature under Fire

Structural Engineering Branch Architectural Services Department

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 1 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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 …………………...... 18

7.

Typical Fire Scenarios ………………………………………………….……... 23

8.

Fire Modelling ……………………………………………………………..…... 26

9.

Design Fire ………………………………………………………………….….. 29

10.

Temperature of Structural Elements …………………………………….…... 48

11.

……………………………………..... 49 Thermal Actions for External Member …………………………………….....

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 f or 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 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 reliance placed on it (including the formulae and data). Compliance with this Guideline does not itself confer immunity from legal obligations.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 2 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

1.

1.1

Introduction

There has been a large body of work work written on the the subject of performance 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 our Department: a)  b) c) d)

 background information on the behaviour of of fire; the structural behaviour of structural steel, reinforced concrete, composite structure and timber at elevated te mperature; list of design references; and design examples,

when a structural fire engineering study is required for the design of structural members under fire. 1.2

1.3

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 of a fire. Design examples will be given to demonstrate the techniques. techniques. The gas temperature temperature is an important parameter in deciding whether a structural fire engineering study is required. required. For example, if if the computed computed gas temperature temperature is high enough such that the temperature of the structural steel exceeds o 550 C, 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 II will first describe the heat transfer mechanisms from the fire to the structural members, and the procedures to obtain the temperature of the members members during a fire. It will then focus on the the structural design of steel structure, reinforced concrete, composite structure and timber exposed to fire, which will again be followed by design examples.

Resources on Fire Safety Engineering Project officers should note that this set of Guideline only provides an overview on analysis and design of structural elements exposed to fire, and are therefore advised to conduct their own research on the details and updated information. The following list the resources 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 Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 3 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

against all fire aspects for approval. As government buildings are exempted from the  Buildings Ordinance, 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  (Hong Kong: Building Authority). 2. Buildings Department (1996), Code of Practice for Fire Resisting Construction 1996  (Hong  (Hong Kong: Building Authority). 3. Buildings Department (2004), Code of Practice for Means of Access for  Firefighting and Rescue 2004 (Hong 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 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 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   Equipment   (Hong Kong: Fire Services Department).

Professional Associations

1. The Society of Fire Protection Engineers (SFPE) ( www.sfpe.org/ 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: Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 4 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

DiNenno, P J et al (eds.) (2002), SFPE Handbook of Fire  Protection Engineering   (Bethesda, Maryland: Society of Fire rd Protection Engineers, 3  ed). 2. British Standards Institution (www.bsi.org.uk  ( www.bsi.org.uk ) published the following standards on the principles of structural fire engineering and the design of 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: Design (London: British Standards Institution). BSI (2002), Eurocode (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) 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 7974  7974  only 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. The following parts are relevant to structural s tructural 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. Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 5 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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: 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 UK’s 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 rd (Aldershot: Association for Specialist Fire Protection, 3  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, 1.2 ,  EN1992 Part 1.2, 1.2 ,  EN1993 Part 1.2 and 1.2  and EN1994  EN1994 Part 1.2  1.2  on structural design design of concrete, steel, composite structures under fire. The book book also provides examples 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, 1.2 ,  EN1992 Part 1.2, 1.2 ,  EN1993 Part 1.2 and  EN1994 Part 1.2 on 1.2 on 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: 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 that are given in the  Eurocode 1 for designing steel structures subjected to fire by describing the design process together with worked examples. 5. Law, M and O’Brien, T (1989),  Fire Safety of Bare External Structural Steel   (Ascot: SCI) – Although this book is old, it is a classic in structural fire engineering. engineering. This book examines examines flame projection projection from openings in building facades and heat transfer calculation methods of fires to external unprotected steel columns. Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 6 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

6.  Newman, G M (1990),  Fire and Steel Construction: the Behaviour of Steel Portal Frames in Boundary Conditions  Conditions   (Ascot: The Steel nd Construction Institute, 2   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 sofwares (some being free for use) include: include: SAFIR (www.argenco.ulg.ac.be/logiciels/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 structures subjected to fire. Fire Dynamics Simulator (FDS) (www.fire.nist.gov/fds/index.html ( 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/ (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 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 (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 to Eurocode to  Eurocode 3. 3. Academic Institutions 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 f ire 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, Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 7 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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.

2.1

Fire Safety Codes in Hong Kong

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 performance in the event event of fire. Similar requirements requirements have been specified as Regulations 41(1), 41A, 41B, 41C and 41D in the  Building (Planning) Regulations  Regulations   and Regulation 90 of the  Building (Construction)  Regulations.  Regulations. Over the years, years, Buildings Buildings Department Department and Fire Services Services Department have issued the following codes on the performance requirements complying the statutory requirements: a) the Code of Practice for the Provision of Means of Escape 1996   1996   (the “MOE Code”);  b) the Code of Practice for Fire Resisting Construction 1996   1996   (the “FRC Code”); c) the Code of Practice for Means of Access for Firefighting and Rescue 2004 (the 2004 (the “MOA Code”); d) the Code of Practice for Minimum Fire Service Installations and  Equipment ; and e) the  the  Code of Practice for Inspection and Testing and Maintenance of  Installations and Equipment .

2.2

The MOE Code Code sets out the the requirements on on the provisions 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 the objective of assisting in 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, access staircases, fireman’s lifts as well as fire fighting and rescue stairways according to the area, use use and height of buildings. The FRC Code provides provides guidance on compliance with the requirements for fire resisting construction stipulated in Part XV of the  Building (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

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 8 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

with the type of use, the maximum compartmentation area or volume and requirements on protection of adjoining buildings and separation between different uses and occupancies ( Table 1). Table 1 Fire Resistance Rating Rating for Premises of Different Occupancies Occupancies Class

1 2 3 4

Use

Domestic Hotel bedroom Institution Institutional al Commercial

Fire Resistance Rating

Compartment Volume

 No limit

60 mins.

Not exceeding exceeding 2500 m 2 Not exceeding 10500 m

60 mins.

2

Place of public entertainment

 Not exceeding 2500 m

Educational establishments

Exceeding 2500 m 2  but not exceeding 10500 10500 m

60 mins.

5

6b 7

2.2

2

120 mins.

Bulk storage and  Not exceeding 28000 m and warehouse 10500 m 2 Car parking Not exceeding 10,500 m (Source: Source: modified from FS Code Part C)

120 mins 60 mins.

FS Code Buildings Department has just issued the Code of Practice for Fire Safety in  Buildings 2011  2011  (the “FS Code”), which consolidates and replaces the requirements of the MOE Code (now Part B of the FS Code), the FRC Code (now Part C of the FS Code) and the MOA Code (now Part D of the FS Code). As the FS Code has already already replaced these three three codes, reference in this set of Guideline will be based on the FS Code. Code. The objectives of the FS Code are: 1) 2) 3) 4) 5)

2.3

to allow occupants to escape during fire; to ensure that the fire and smoke would not spread beyond the room of origin; to ensure structural integrity of the structural elements for a specific  period of time; to prevent the outbreak of fire, to abate fire hazards, to suppress fire, to prevent loss of property; and to ensure structural integrity for fire fighters to perform their duties.

The Code of Practice for Minimum Fire Service Installations and the   Code of Practice for Inspection and Testing and  Equipment and the   Maintenance of Installations and Equipment   are enforced by the Fire Services Department. The first one provides the minimum fire protection systems required for different types of premises and the specifications for various fire service installations and equipment for meeting the statutory requirements. The second one is to indicate the type and nature of

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 9 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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 Institution of Fire Engineers of the UK ( www.ife.org.uk/ www.ife.org.uk/)) defines “fire engineering” (or sometimes termed as “fi re 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/ http://www.canterbury.ac.nz/)) defines “fire engineering” 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: engineering: fire prevention prevention (designed to reduce the chance of a fire occurring) and fire protection (designed to mitigate the the effects of a fire should it nevertheless occur). Fire  prevention includes eliminating or protecting possible ignition sources in order to prevent prevent a fire from from occurring. Fire protection measures measures may be  passive or active. Active measures include detection and alarm, fire extinction, and smoke control. Passive measures measures include structural fire 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 measures during a fire.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 10 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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

3.3

“Structural fire engineering” is a special branch branch within the fire 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 2 shows the interrelationship of fire engineering, fire protection engineering, and structural fire engineering.

Figure 2 Relationship among among various branches branches in fire engineering engineering

3.4

Project officers should therefore therefore note that the term “fire engineering” 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.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 11 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

3.5

IStructE (2007: (2007: 1) states that “[t]raditionally, structural engineers did 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 complex buildings buildings to be constructed.” constructed.” Professor D. D. J. O’Connor 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] [fi re] 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 are two approaches for complying with the the statutory requirements for fire safety, namely: Prescriptive Provisions and 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, fireman’s 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 ( PD 7974-0). 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  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 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 in There is a danger that relation to technological advances. 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  BS PD 7974-0). 7974-0). However, project officers officers should should note note though though with such limitations, Prescriptive Provisions provide an acceptable solution Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 12 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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 of the FS Code) in a fire: 1) 2) 3)

stability, i.e. to avoid collapse of load-bearing elements ( Figure 3(a)), integrity, i.e. to resist fire penetration and inhibit spreading ( Figure 3(b) ); and 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 construction elements elements during fire (Source: Source: Wang 2002)

Similar provisions have been specified in the Code of Practice for Structural Use of Concrete 2004 (the 2004 (the “HK Concrete Code”) and Code of Practice for Structural Use of Steel 2005  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 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 from the room of origin. origin. For structural structural elements (including structural frame, beam and column), stability criterion must be satisfied, and the 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 Construction Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 13 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

Criteria to the satisfied

Elements of construction

Method of Exposure

Stability

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 2 function of the floor 

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: 2

Y = required and N = not required 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: 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 order to satisfy the statutory requirements. requirements. Compliance of of these  provisions is deemed to satisfy the statutory requirements laid down for fire resisting construction for buildings in Part XV of the  Building (Construction)  Regulations.  Regulations. The following paragraphs provides brief summary of these  provisions. 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 requirements for the insulation and stability. stability. For insulation insulation (e.g. for the floor slabs), it is specified that the mean and maximum o unexposed face temperatures should not be increased by more than 140 C o and 180 C respectively above above the initial value. 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 fire  protection 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 a nd 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. Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 14 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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 in Fire in  Fire Protection for Structural Steel in Buildings (ASFP 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 “Section Factor” (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 a nd 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 resistance ratings to suit his project. 4.2.5.5 In 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 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 where vibration or large large deflection is expected, more demanding sprayed material with higher dry density and cohesion properties should be used. used. Moreover if the 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 protection board is an expensive method, method, and may also susceptible susceptible to the the effect of of moisture. Hence, its application is also restricted to indoor steelwork with dry environment. environment. Intumescent fireproofing is a layer of paint which is applied along with the coating system on the structural 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 aesthetical appearance is required. required. Moreover, intumescent coating coating is the option option that can be applied to s teelwork 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 the HK Concrete Code Code states that the covers to steel reinforcement for fire protection shall follow the FRC Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 15 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

Code. The FS Code Part E specifies the minimum dimensions dimensions of structural members and covers to steel reinforcement for specified fire resistance rating in Tables E2, E4, E6 E6 and E7. The minimum covers covers and dimensions have been derived to ensure that the temperature of steel reinforcements does not exceed a s pecified critical temperature. For timber construction, the prescriptive protection is normally to protect the elements from fire by fire resistant cladding materials. 4.2.7 Prescriptive Provisions for roof structure 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. construction. Detailed discussion on fire protection to roof structure can be referred to SEBGL-OTH1 Guidelines on the Fire Resisting Construction for Roof Structures �available: http://asdiis/sebiis/2k/ resource_centre/). 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 is sued  APP-87: Guide to Fire  Engineering Approach  Approach  (available: http://www.bd.gov.hk/ http://www.bd.gov.hk/;; accessed: 4 September 2011) providing further guidance on fire engineering approach. Under APP-87  Under APP-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  Code  cannot be provided. 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 further stated that the Alternative Approach “provides a framework for engineers to demonstrate that the performance requirements of legislations are met, or in some cases bettered, to compensate for the deviation or shortfalls of the prescriptive 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 http://bsbiis/main/bsbiis/4.3.3.asp Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 16 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

different aspects. The focus of this this set of SEB Guideline Guideline will be, however, however, on application of Alternative Approach to assess the actual performance of the structural members members under fire. fire. This set of Guideline will particularly 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 fire resisting construction. 5.

General Principles of Structural Fire Engineering Approach

5.1

A ‘full’ performance-based approach to fire fire engineering 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 of carrying out a full performance-based performance-based study, study, it is usually to carry out a simplified performance-based approach, which is sufficient for structural engineer to understand and e xplain 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 members essential for maintaining stability of the structure that:  Rf ≥ E f where  Rf is the load carrying capacity of the structural member in a fire till the end of the required fire resistance rating; and and  E f f is    is required load carrying capacity by loads in the fire till the end of the required fire resistance rating.

5.2

The process process of such performance-based performance-based approach is therefore therefore similar to the 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 in  Figure 4.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 17 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

Figure 4 Steps in Structural Structural Fire Engineering Engineering Study (Source: Source: Modified from Kirby 2004) 6.

Applicability of Structural Fire Engineering Approach

6.1

As stated above, one one of the main reasons for the research and rapid 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 benefit of unprotected steel is to have more choices of architectural finishes/appearance of the steel thus thus enhancing the aesthetic effects. In a structural 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 officers should, however, note that in a small compartment with the the usual design fire load, the fire fire will likely to be fully developed. 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 Prescriptive Provisions for the structural

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 18 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

elements. Example 2 in Section 13  will 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 situa tions: a)

 b) c) 6.5

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. compartments/buildings. Examples of of such structures include: 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; external structural steelwork located outside t he facade of the building; and localised fire which is unlikely to flash over.

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, venues, the resulting gas gas temperature in a fire is low. The significant fire loads in sports stadium, 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 fire engineering study was employed in the project of Tin Shui Wai Public Library cum  cum  IRC, in which the structural steelwork above the swimming  pool in the IRC was left unprotected. 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) 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 in Hong 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

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 19 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

transfer due to internal radiation of the compartment compartment fire. 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 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 façade. Heat transfer transfer calculations are therefore required to check that the members remain below its critical temperature for the compartment fire and flame projection considered. considered. In our Department, such studies were employed in the projects of Dr Sun Yatsen 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 façade of the development were left unprotected.

Photo 1 External Stair in Dr Sun Yat-sen Museum

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

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 20 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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

Photo 2(c) External Stair in Internationa Internationall 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).

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 21 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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 cum IRC, cum IRC, 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 Combination of Prescriptive Provisions Provisions and Alternative Approaches in Tin Shui Wai Public Library cum cum IRC  IRC

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 22 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

7

Typical Fire Scenarios

7.1

Typically, a fire in a residential, residential, commercial, commercial, or institutional building 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 decays in accordance with the mass and 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 Quintiere 2000; Drysdale Drysdale 2000). Following ignition, fires in compartment typically have three distinct phases: the growth or preflashover, 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  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 Compartment Fire Time-temperature Time-temperature Curve

7.2

Growth or Pre-flashover Phase

7.2.1 Figure 7 shows 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. combustion. During this stage, the fire is localised and temperature distribution inside the enclosure is highly nonuniform. If this fire is promptly promptly discovered and/or and/or effective fire fighting is activated, it can be easily 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.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 23 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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, burn, the volume of smoke and hot gases in the upper layer increases, reducing the height of the interface between the two layers. layers. As this happens, happens, the temperature of the hot gas layer increases further. The rate of burning may also be significantly enhanced by radiant radiant feedback from this hot upper 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 leave the room through the openings, 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. In more dramatic situations, a sudden fresh air supply to an under-ventilated fire may lead to the so-called “back draught” ( 回  燃  燃 效 應 )  phenomenon,  posing serious hazards for fire fighting. fighting.

Figure 7 Typical compartment fire before pre-flashover phase (Source: 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 preflashover fire so as to develop an understanding of the production and spread of smoke smoke and toxic gases. Structural engineers 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 post-flashover fire. However, being able to predict the preflashover 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. Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 24 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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 quicker when it is aided by the partially burnt fuels in the smoke layer. The radiation from the 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” (閃燃效應) (Figure 8).

Figure 8 Flashover during fire (Source: 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, Daily, 8 September 2011) and was reported in the fire of 20 December 2011 in Po On Building on Mongkok Road ( Ming Pao, 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 Commerce Commerce has  has 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 engineering study. The conditions necessary necessary for flashover to occur depend on: Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 25 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

1) 2) 3)

sufficient fuel and ventilation for fire to develop to a significant size; sufficient hot gases trapped in the ceiling; and 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 heat release and the time to flashover. flashover. The usual consensus is that flashover occurs when the upper layer temperature reaches approximately 600°C and the radiant heat flux to the floor is about 20kW/m² (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 oxygen supplied. Large amount of 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. windows. This is a ventilation controlled fire and in sufficiently small compartments will result in fairly uniform temperatures at any level within the the compartment. For such ventilation ventilation controlled fire, 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. 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. 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 influencing the temperature, temperature, magnitude, magnitude, and distribution of a fire can be summarized as follows (Petterson 1973; Roytman 1975; Subramanian and Venugopal 1984):

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 26 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

1) 2) 3) 4) 5) 6) 7)

fire load type, density and distribution; combustion behaviour of fire load; compartment size and geometry; ventilation conditions of compartment (especially the window opening area); thermal properties of compartment boundary; thermal conductivity and diffusivity of the construction material; radiation levels from both within the compartment and through the windows.

8.2

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

Fire model Complexity

1. Nominal fires

Compartment fire 2. Time equivalence 3. parametric 4. localised

Simple

Intermediate

Fire behavior

Post-flashover fires

Pre-flashover

Temperature distribution

Uniform in whole compartment

 Non-uniform along plume

Input parameters

Design methods

Fire type, no physical  parameters

Fire load, ventilation conditions, thermal properties of  boundary, compartment size

Fire load and size, height of ceiling

Spreadsheet

Simple equations

Simple equations

5. Zone models 6. CFD Two-zone/ models multi-zone Advanced Complete Pre-flashover or timelocalised temperature relationship Time and Uniform in each space layer dependence (varying) Fire load, ventilation conditions, Detail input thermal for solving  properties of the  boundary, fundamental compartment equations of size, the fluid detailed input flow for heat & mass  balance of the system

Computer model

(Source: Source: Modified from IStructE 2007) Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 27 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

8.4

The simplified models models of fire are based on on fundamental physical parameters, which allow temperature prediction, the design density of fire load and the conditions of the ventilation. ventilation. Nominal fires fires are used used in the testing of construction members in in the standard fire resistance. 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 curve in 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 localised burning. These four simplified models will further be described in Section 9.

8.5

Advanced models models take into account properties of of gas and the exchange of mass and energy. energy. Zone models models are simple computer models models that divide divide the considered fire compartment into separate zones, where the condition in each zone is assumed to be uniform. uniform. Two zone models 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 models are used for for pre-flashover 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 programmed and are available via  via  the internet. The most commonly used ones are CFAST (available: http://www.nist.gov/ http://www.nist.gov/)) and OZONE (www.ulg.ac.be ( www.ulg.ac.be). ).

8.6

The computational computational fluid fluid dynamics dynamics (CFD) (CFD) models forecast the the temperature 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 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. volume. CFD analysis is suitable suitable for very very large compartments. 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 gas temperature in fire compartment during fire from different models.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 28 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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

Design Fire

9.1

In order to carry carry out structural design under fire, the selection of a suitable 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 design fire fire for a compartment compartment fire: a) a

9.3

a nominal time-temperature curve uniform in space, and a “real fire” either specified in terms of parametric time exposure (the “parametric fire”), or obtained by computer modelling.

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, areas, building thermal properties, properties, etc. The standard standard timetemperature curves were originally derived from measurements of tests th taken early in the 20   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 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, fire, manufacturers can test their building building  product and find a fire resistance time that can be compared to other Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 29 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

 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) 834) contain defining equations for three distinct fire curves: standard, external and hydrocarbon ( Figure 10). The formula formula describing the standard standard time-temperature curve for the ISO the  ISO 834 fire 834 fire is: T  =  = 345 log (8t  (8 t +1) +1) + T o --- (1) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. 3.4.1) o where T   is the temperature (in C) at time t   (in minutes), and T o is the  o ambient temperature (taken as 20 C in Eurocode in Eurocode 1). 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 timetemperature 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, fires, as it has no decay phase, whereas in a real fire compartment temperature will reduce with the duration of 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. non-conservative. Such a realistic fire can be more 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. components. Most European countries have have standard fire curves similar to that in  ISO 834 standard 834 standard fire, and across the Atlantic, the US and Canada also use the standard fire curve in  ASTM E119  E119  which is similar to those in ISO in  ISO 834. 834. 9.3.3 External and Hydrocarbon Fire Curves Where the structure for which the fire resistance is being considered as external, Eurocode external, Eurocode 1 gives 1 gives a similar external fire curve. curve. This is the nominal nominal time-temperature curve to be used for structural members located in a façade outside the main structure but can be exposed to external plume of a fire coming either from the inside fire compartment, i.e. from a compartment situated below or adjacent adjacent to the external wall. The formula describing describing the external fire curve is: -0.32t  -0.32t  -3.8t  -3.8t  T  =  = 660(1 - 0.687e  – 0.313e )+20 --- (2) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. 3.6) In situations where petrochemicals or plastics form a significant part of the overall fire load,  Eurocode 1 gives 1  gives a hydrocarbon fire curve, representing a 2 fuel load of 200kW/m . The formula describing the hydrocarbon fire curve is: -0.617t  -0.617t  -2.5t  -2.5t   = 1080(1- 0.325e  –0.675e )+20 --- (3)( Eurocode T  =  Eurocode 1 Eqt. 1 Eqt. 3.7) Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 30 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

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 Curves

9.3.4 Time equivalent  Eurocode 1  provides for t-equivalent fire models. Law (1997) (1997) defines “tequivalent” 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 required for 11). Hence, it is applicable to calculate the fire resistance rating required the elements of construction within wi thin the building.

Figure 11 Graphical representation representation of time time equivalence Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 31 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

 Eurocode 1 gives 1 gives the following expression to calculate the time equivalent: t e,d  )×k c --- (4) e,d = (q f,d ×k b ×w f )×k  2 where q f,d = design fire load density (MJ/m ) (Table 5); w f = ventilation factor to take into account vertical and horizontal 0.3 4 openings=(6/ H   H ) [0.62+90(0.4-αv) ] in the absence of horizontal openings; k c = factor dependent on material=1.0 for protected steel and reinforced concrete;  H = the height height of the compartment compartment (m); αv =  Av/ A  A f ; the total total area area of the opening; opening;  Av  A f  the total floor area; thermal properties of of the and k b = factor to take into account the thermal 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 (= λρ c )) are known: Thermal inertia b (= λρ c ) (J/m²s K)

k b (min. m²/MJ)

≥2500 ≥720 to ≤2500 b2, a limit thickness  slim is calculated for the exposed material using:  slim =

3600t max λ 1 c1 ρ 1

--- (12)

If s If s1 > s  > slim  then b = b1. --- (13) Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 40 of 87

( Eurocode  Eurocode 1 Eqt. 1 Eqt. A.4) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. A.4a) File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

If s If s1  0.04m  and qt,d  <   < 75MJ/m  and b < 1160J/m s K, then Γ  then  Γ lim lim should  be further multiplied by k  as  as given by: q − 75  1160 − b   O − 0.04    k = 1 +   Eurocode 1 Eqt. 1  Eqt. A.10)  t  ,d   --- (20) ( Eurocode   0.04    75    1160  

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 41 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

9.4.4.9 Cooling Phase of Parametric Fire Curve During the cooling phase, the temperature of the parametric fire curve in cooling phase, θ  g   is assumed to decrease linearly at one of three rates o -1  between 625 and 250 C hr  , and is given by ( Eurocode ( Eurocode 1 Eqt. 1 Eqt. A.11a, 11b and 11c): *

*

θ  g  = θ max (t  –  – t  max . x) x) for t  max≤ 0.5 max – 625 (t  * * * * θ  g  = θ max )(t   – t  max . x) x) for 0.5 < t  max max – 250 (3 – t  max)(t  * * θ  g  = θ max (t  –  – t  max . x) x) for t  max  2 max – 250 (t  where

t*max=

0.2 × 10 −3 q t  ,d  O

t lim × Γ   and  x=   * * t lim × Γ / t max if  t max =  t lim 

 Eurocod e 1 Eqt. A.12) ( Eurocode

--- (21)

Examples 2 and 3 in Section 13 will give examples to derive parametric time-temperature curve using the methods detailed above.

9.4.5 Derivation of Parametric Fire using Law (1983)’s method Other approaches (e.g. Magnusson and Thelandersson 1970) to derive the  parametric time-temperature curve for a compartment fire have been developed, and they are similar to t o the procedure in  Eurocode 1 . For example, Law (1983) gave the following equation to calculate the 1 maximum temperature for the heating phase: -0.lη -0.5 θ max ) η --- (22) max = 6000 (1 - e  A −  Av where η= t  and heq is the window height.  Av / heq Law (1983) commented that this maximum temperature must be modified to take into account of the fuel available for combustion, and can be reduced  by the following factor: -0.05ψ  -0.05ψ  (1 - e ) --- (23)  L  = where ψ  = and L is the fire load (in kg wood equivalent).  Aw ( At  − Aw ) The time to reach m aximum temperature t max max in hour is simply given by: t max 3600 R (sec) max (hr) = L/ 3600 where  R(in kg/s)is the rate of burning, and Law (1983) gave the following equation to find  R:

(

)

 R= 0.18 1 − e −0.036η   Aw

heqW 

--- (24)  D where D and W are respectively the depth and width of the compartment.

1

 Ms Margaret Law MBE is the pioneer in structural fire engineering field and her method is a classic in the field and als o forms the basis for that in  Eurocodes.  Eurocodes.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 42 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

9.5

Fire in large compartments and travelling fires

9.5.1 In the previous discussion, it has been assumed that there would be uniform temperatures at any level within the compartment.  Eurocode 1 states that the design equations for the parametric time-temperature curve are only 2 valid for compartments with floor area up to 500m  and headroom up to 4m. In addition, the enclosure must have no openings through the ceiling and the thermal properties of the compartment linings must be within a limited range. As a result, common architectural layout in our Department, such as large enclosures, high ceilings, atria, large open spaces, multiple floors connected by voids, and glass façades, are excluded from its range of applicability. 6688-1-2:2 007: Background Backgr ound paper to the th e UK National N ational Annex to BS B S EN 9.5.2  PD 6688-1-2:2007: 1991-1-2  suggests that designers can ignore the  Eurocode 1  limitations on floor area and compartment height, and can expand the range of the compartment lining values. However, fires in such large compartments will tend to travel within the compartment as fuel is consumed at a rate governed  by the available ventilation. This causes variation in gas temperatures within such compartments. compartments. The compartment is too large for for a condition right for a flashover to develop and so the fire remains a localised fire which is moving throughout the entire compartment with different speeds and areas engulfed at the same time depending on how much fire load is available and how fast the the fire load is consumed. These fires have been labelled “travelling fires”. Such a fire could be a critical design case for the structure as the heating and cooling of the structures occurs at the same time relatively close to each other. Such travelling fires include the infamous ones in the World Trade Center Towers in New York in September 2001 and the Windsor Tower in Madrid, Spain in February 2005.

9.5.3 Cooke (1998) conducted a series of fire tests with uniform fire loads in a 4.5×8.75×2.75m high compartment in which ventilation was provided at one end. He found that there had been a progression of temperature temperature within the the compartment. Peak values occurred occurred near the source source of ventilation early in the fire and then progressed away from the opening as fuel was consumed.  Non-uniform heating across a compartment floor can cause a failure mechanism in the structure, which may not occur if uniform temperatures were applied to the structure. An example was quoted by Rein and SternGottfried (2011), when a cool, unheated bay in a multi-bay structure can  produce high axial restraint forces, which can result in failure of a heated element. However, there has has not yet been been any suggested suggested new approach to deal with the design fire for structural fire engineering in such fire scenarios. In such large compartment, probably a CFD model should be used. 9.6

Localised Fire

9.6.1 Localised fire is the burning of an individual fire load in a localised area and transfer the heat energy to the structural member by conduction, convection, or radiation. Scenarios where localised fires are most likely to occur include include (IStructE 2007): Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 43 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

a)  b) c)

large high spaces with with relatively limited fire load (e.g. in atria, shopping malls); areas where there are high levels levels of ventilation (e.g. hotel entrances); entrances); areas where fire load is relatively of low levels or spaced such that fire cannot readily spread from one area to another.

 Eurocode 1 Annex C gives the procedures to predict the gas temperature for a localised fire using fire plume models.

9.6.2 In a localised fire, it is assumed that the fire is well ventilated and fuelcontrolled similar to the fire that occurs in open space. The rate of burning burning is therefore characterized by the type, amount and configuration of the fuel.  Eurocode 1  gives two methods to calculate the temperature effects of a localised fire depending on the relative height of the flame and of the ceiling, i.e. whether the flame impinges the ceiling or not. 9.6.3 Flame not impacting the ceiling

Figure 16 Typical Localised Fire Fire impacting the ceiling (Source : Eurocode 1 )

For flame not impacting the ceiling ( Figure 16), the flame height  L f  (m) is calculated by:  L f = 0.0148 Q0.4 − 1.02 D --- (25)  Eurocod e 1 Eqt. C.1) ( Eurocode where Q (W) is the heat release rate of the localised fire source, and  D (m) is the characteristic length of the fire (usually taken as the “diameter of the flame”). The calculation of these two parameters ( Q  and  D) will be discussed in the latter paragraphs. When the flame is not impacting the ceiling, the temperature at height  z  along the symmetrical axis from the fire is given by: 2

θ = 20 + 0.25 Q ( z − z 0 ) 3 c

−

5 3

 ≤ 900 --- (26)

( Eurocode  Eurocode 1 Eqt. 1 Eqt. C.2)

where Qc is the convective part of the rate of heat release Q  and can be taken as 0. 0.8Q, and z  and z 0 is the virtual origin and is calculated by: Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 44 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

0 .4

 z 0 = 0. 0.00524Q 00524Q − 1. 1.02 D --- (27) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. C.3)  Note that the values of  z 0 are negative because the virtual origin is lower than the fire source. 9.6.4 Flame impacting the ceiling

Figure 17 Localised Fire Impacting Ceiling (Source: Source: Eurocode 1) 1)

In case of a localised fire with the flame tip impinging on the ceiling .

2

(Figure 17), the net heat flux h net  (W/m ) received by the structural elements at the level of the ceiling is given by the difference between the .

flux received by the member h and the heat energy lost by the member to the environment by convection and radiation, .

.

h net 

 Eurocode 1 Eqt. 1 Eqt. C.9) = h− α c (θ m,t  − 293) − ε mσ (θ m4 ,t  − 2934 ) --- (28) ( Eurocode

where θ m.t m.t is the temperature of the structural member at time t;

 100000 when  y ≤ 0.30  z '+ H  + r   ; h = 136300 − 121000 y when 0.30 ≤  y ≤ 1.0 and  y = + +  z  '  H   L h 1 . 7 −  150000 y when 1.0 ≤  y  .

--- (29) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. C.4) r = the horizontal distance distance between the vertical vertical axis of the fire and the point along the ceiling where the thermal flux is calculated;  H  =  = the distance between the fire source and the ceiling; 2 2  * 5 * 3 * − 2 . 4  D ( Q Q Q   D  D ) when Q D < 1.0  z ' =  and Q D* = ; 2 6 2.5 1 . 11 10  D × * *  2.4 D (1.0 − Q D 5 ) when Q D > 1.0

and

--- (30) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. C.7) * 0.33  Lh = the horizontal flame length = 2. 2 .9 H ( Q H  )  – H ; --- (31) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. C.5) Q * = Q H  --- (32) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. C.6) 1.11 × 10 6 H 2.5

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 45 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

The maximum temperature of the structural member is obtained by setting .

.

h net  =0, such that the flux received by the member h is the same as the heat energy lost by the member to the environment by convection and radiation, i.e. .

0 = h − α c (θ max

4 − 293) − ε mσ (θ max − 293 4 )

9.6.5 Simplified method in PD in PD 7974-1 The UK National Annex to Eurocode to  Eurocode 1 specifically 1 specifically states that the method in  Eurocode 1 is 1  is not applicable to localised fires, and directs designers instead to refer to the method in  PD 7974-1. 7974-1.  PD 7974-1 gives the following equation derived from experimental data by McCaffrey et al (1981) to calculate the rise in temperature above the localised fire: Q2

1

θ  = 6.85( ) 3 --- (33) 2  Aw he hk  At  o

where θ ( θ  ( C) is the rise in temperature above the ambient; Q (kW) is the rate of heat release; 2  Aw (m ) is the area of the ventilation opening; he (m) is the height of the ventilation opening; 2 and  At  (m ) is the total surface area of the compartment; 2

hk  (kW/m K) is the effective heat transfer coefficient, and depends on the thermal penetration time t  p (s) given by:  ρ c δ  t  p = ( )( ) 2 --- (34) λ  2 where δ  (m)  (m) is the thickness of the enclosure boundaries; -3  ρ (kgm ) is the density of enclosure boundaries; -1 -1 c (kJkg K  ) is the specific heat of enclosure boundaries; -1 -1 and  λ (kWm K  ) is the thermal conductivity of the enclosure boundaries. If t  p  is greater than the fire exposure time, i.e. heat transfer is transient or 2 non-steady, hk  (kW/m K) is given by: λρ c 0.5 hk  = ( ) --- (35) t  p If t  p  is less than the fire exposure time, i.e. heat transfer is steady hk  2 (kW/m K) is given by: λ  hk  = --- (36) δ  9.6.6 Heat Release Rate Q The total heat content of the combustible materials  E is given by ∑ M v ×  H v, and reference can be made to Table 6 for the calorific values  H v of common combustible materials. However, it is necessary necessary to convert convert E   E  into   into the rate of heat release Q.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 46 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

The heat release rate Q  in the pre-flashover stage of a fire has been empirically derived, and has been found to be proportional to the square of the time (the “t-squared fire”) as follows: t  Q = ( ) 2 --- (37) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. E.5) k  -0.5 where k  (sMW   (sMW ) is the fire growth constant and t   is the time (s). Hence, the relationship between E  between  E  and  and Q is given by: 2

3 E  Q = ( ) 3 --- (38) k    to reach Q=1050kW for typical t-squared fires Figure 18 shows the times t  to together with examples of combustible materials as given in  National Fire Codes NFPA 92B (NFPA, 2000), and Table 8  therefore gives the typical values of fire growth constant k . Once E  Once E  and  and k  are  are found, Q can be derived.

Figure 18 Relationship between Q and t  in  in t-squared fires (Source: Source: National Fire Codes NFPA 92B) 92B) Table 8 Values of fire growth rate k for typical fires Fire characteristics Slow Medium

Typical examples

Value of k 

Densely packed wood products 600 Solid wood furniture 300 Furniture with small account of  plastic Fast Some upholstered furniture 150 High stacked wood pallets Cartons on pallets Ultrafast Most upholstered furniture 75 High stacked plastic materials Thin wood furniture (Source: Source: Modified from National from  National Fire Codes NFPA 92B 92B ) Instead of deriving Q  from the total heat content of the combustible materials for localised burning, Table 9  gives the typical values of Q for common commodities given in National in  National Fire Codes NFPA 92B. 92B. Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 47 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

Table 9 Heat release rate rate Q of common commodities Commodity Heat Release Rate Q (kW) Waste paper baskets 4-18 Medium wastebasket with milk 106 cartons Large barrel with milk cartons 148 TV sets 120-290 Curtains, velvet, cotton 160-240 Arm chair 160 Upholstered chair with polyurethane 369 foam Christmas trees 500-650 (Source: Source: Modified from National from  National Fire Codes NFPA 92B 92B )

9.6.7 Characteristic length of the fire D fire  D Another parameter required is the characteristic length of the flame. By assuming that the flame to be circular in shape, the diameter of the flame  D is given by: 4Q  D = ( ) 0.5 --- (39) π  × RHR 2 where RHR where RHR (kW/m  (kW/m ) is the rate of heat release densities.  Eurocode 1 gives the different  RHR values for different occupancies. An 2  RHR value  RHR value of 250 kW/m  is recommended for t-squared fires in dwellings, hospital and hotel rooms, offices, classrooms, shopping centres and public 2 spaces in transport buildings, and an  RHR   RHR  value of 500 kW/m is recommended in libraries and theatres (cinema). Examples 4 and 5 in Section 13 will give examples respectively to derive time-temperature curves of localised fires not impacting and impacting the ceiling. 10.

Temperature of Structural Elements

The above paragraphs summarize the procedures to obtain the timetemperature curve curve for a compartment fire. This curve gives the gas temperature in the compartment. However, the structural structural elements will not have the same temperature as the gas, since heat has to be transmitted to the structural elements through conduction, convection and radiation ( Figure 6). In Part II  of this set of Guideline, the mechanism of heat transfer will be further discussed.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 48 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

Figure 19 Relationship of Gas Temperature Temperature and Member Temperature Temperature 11.

Thermal Actions for External Member

11.1 External structural members can be exposed to fire through the windows or openings of the compartment. The temperature of the external members depends on: a) the maximum compartment temperatures;  b) the size and temperature of flame fla me from openings; c) the heat transfer parameters of radiation and c onvection; The direction of the fire flame from the opening can also be deflected by wind.  Eurocode 1 Annex B provides procedures for the determination of the size and temperatures of the flames emerging from the openings to calculate the temperature of external members located outside a compartment. The method is based based on the original original derivation by Law and O’Brien (1989). (1989). The procedure procedure is first to determine the shape of the emerging or venting flame, then centre-line temperatures and finally heat transfer effects to the sstructural tructural members. 11.2 Ventilation Condition 11.2.1 As a plume vents from an opening, its shape is affected by the the enclosure's ventilation conditions as well as the window shape. The plume often surges out of the window, curling back to make contact with the external wall some distance above the opening, depending depending on its aspect aspect ratio. The overall height and width of the venting flame will depend on the window aspect ratio, as well as whether there are any horizontal or vertical  projections above or beside the window. Fire behaviour is al so influenced  by the amount of air that can reach and take part in the fire. In most common situations, the entry of air for combustion takes place through windows from which the flames are emerging. emerging. This is called a “no“nothrough draught” (or “no-forced “no-forced draught”) condition. condition. If there are windows on opposite sides of a compartment, such that a “through-draught” (or “forced draught”) condition is possible, or if additional air is being fed to Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 49 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

the fire from another source, the flames will tend to emerge from the whole area of window. In this throughthrough- or forced- draught draught condition, not only does the flame shape tend to be different, but also other relationships (e.g. the fire temperature and the temperature distribution along the flame axis) are found to change. 11.2.2 Law and O’Brien (1989) investigated the effect of no-forced and forceddraught conditions on flame shape and behaviour. Correlations of flame height and width were developed for both of these ventilation conditions as a function of compartment and window size, and burning rate. Generally, the flame for a forced-draught condition was found to emerge from the entire window area, its width being slightly wider than the window width and at an upward angle. Project officers should further note that the compartment may initially be in no-forced draught condition when some or all windows are close; but will change to through-draught condition when some of the windows break at elevated temperature. temperature. Such possibilities possibilities should be considered in the structural s tructural fire engineering study. 11.2.3 The following paragraphs will be divided into two parts: forced-draught, and no-forced draught conditions, in which the flame dimensions and the temperature of the flame for each condition will be calculated. 11.3 No-Forced Draught Condition 11.3.1 Temperature of the the Fire Compartment The rate of burning or the rate of heat release Q (MW) is given by:

 A f  q f,d  heq  Q = min  ; 3.15 ( 1 - e - 0 .036 /O )Av  τ   D/W    F   --- (40) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.4) where τF  is free burning fire duration (which may be taken as 1200s for  free burning of most types of furniture found in buildings). The temperature of the fire compartment c ompartment T  f  (K) is given by: -0.1/O -0.1/O 0.5 -0.00286Ω T  f  = 6000 (1 - e ) O (1-e ) + T  + T o (41) --( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.5)  A f  q f,d  where  is given by = and T o (K) is initial temperature (= 293 K).  Av At  11.3.2 External Flame Shape  Eurocode 1  gives the following expressions to calculate the flame dimensions ( Figure 20):

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 50 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

(a)

(b) wall above window

(c) no wall above or heq > 1.25w 1.25wt 

Figure 20 Flame Dimensions Dimensions under No-Forced Draught Draught (Source: Source: Eurocode 1) 1)

The flame height, L height,  L L (m) is given by: Q  L L = 1.9 (   ) 2 / 3  - heq --- (42) wt 

( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.7)

The flame depth is taken as 2/3 of the window height (=2/3 heq). The horizontal projection of flames  L H   depends on whether there is wall above the window and the width of the window as follows: The horizontal projection of flames  L H  (m)   (m) with a wall existing above the window (Figure 20(b)) is given by ( Eurocode 1 Eqt. B.8, B.9 and B.10) (43a):  L H  =  = heq /3 if heq ≤ 1.25w 1.25wt 0.54  L H  = 0.3 h 0.3 heq (h  (heq / w  / wt ) if heq > 1.25w 1.25 wt  and distance to any other window > 4w 4wt  0.54  L H  =  = 0.454 h 0.454  heq (h  (heq /2w  /2wt ) in other cases The horizontal projection of flames  L H  (m) with a wall not existing above the window ( Figure 20(c)) is given by: 1/3  L H  = 0.6 h 0.6 heq ( L  L L/ heq) --- (43b) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.11) The flame length along axis L axis  L f  (m) is given by ( Eurocode ( Eurocode 1 Eqt. 1 Eqt. B.11, B.12 and B.13) (44): when L when L L > 0,  L f  = L  = L L + heq /2 if wall exist above window or if heq ≤ 1.25w 1.25 wt 2 2 1/2  L f  = ( L L  + ( L H  - heq /3) )  + h  + heq /2 if no wall exist above window or if heq > 1.25w 1.25 wt

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 51 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

11.3.3 Centre-Line Temperature The flame axis is the centre-line of the venting plume, beginning at the widow opening and extending vertically up the external w all, in the middle of the plume as shown as the dashed line in Figure 20. The flame temperature at the window T w (K) is given by: T w =

520 1 − 0.4725( L f  wt  / Q)

+ T o --- (45)

( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.14)

The flame temperature along the flame a xis T  z  (K) is given by: . T  z  = (T  (T w - T o) (1-0.4725( L x wt  / Q)) + T  + T o --- (46) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.15) where  L x  is the axis length from the window to the point where the calculation is made. 11.3.4 Convective and Radiative Radiative Heat Transfer 2

The convective heat transfer coefficient α c (kW/m K) is given by: αc = 4.67 (1 / d eq ) 0.4 (Q / Av ) 0.6 --- (47)

( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.17)

where d eq eq is the geometrical characteristic of an external structure element (diameter or side). The emissivity of the flame with respect to face i of the t he column εz,i is given  by: − εz,i = 1 − e 0.3λ i --- (48) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.16) For column engulfed in flame, the value of the flame thickness λ i should be taken at the level of the top of the opening ( Figure 21(a)(1)).

Figure 21(a)(1) λ i for No-Forced draught condition for column c olumn

For beam engulfed in flame, the value of the flame thickness λ i should be taken at the level of the top of the opening ( Figure 21(a)(2)), and a distinction should be made between a beam that is parallel to the external wall of the fire compartment and a beam that is perpendicular to the external wall of the fi re compartment.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 52 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

Beam Perpendicular to Wall

Beam Parallel to Wall

Top of Flame below Top of Beam Beam Immediately Adjacent to Wall Figure 21(a)(2) λ i for No-Forced draught condition for beam

11.4 Forced Draught Condition 11.4.1 Temperature of the the Fire Compartment The rate of burning or the rate of heat release Q (MW) is given by:  A f  q f,d  Q= --- (49) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.18) τ  F  where τF  is free burning fire duration (which may be taken as 1200s for  free burning of most types of furniture found in buildings). The temperature of the fire compartment c ompartment T  f  (K) is given by: -0.00228Ω T  f  = 1200  = 1200 [ ( A f q f,d ] + T  + T o --- (50) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.19)  f,d)/17.5 - e  A f  q f,d  where  is given by = and T o (K) is initial temperature (= 293 K).  Av At  11.4.2 External Flame Shape  Eurocode 1  gives the following expressions to calculate the flame dimensions ( Figure 22(a)): Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 53 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

Figure 22(a) Flame Dimensions Dimensions under Forced Draught Draught (Source: Source: Eurocode 3) 3)

The flame height, L height,  L L (m) is given by: 1 0.43 Q  L L = [1366( ) ] − heq --- (51) 1/ 2 u  Av

( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.20)

where u is the wind speed and may be taken taken as 6 m/s. Therefore, Q  L L ≈ 0.628 1/ 2 - heq  Av The horizontal projection of flames  L H  (m)   (m) with a wall existing above the window is given by: 2 0.22  L H  =  = 0.605 (u ( u  / H   / H v)  ( L L + H   + H v ) --- (52) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. E.21) with u = 6 m/s, L m/s,  L H  is given by: 0.22  L H  = 1.33  = 1.33 ( L L + H   + H v ) / H  / H v The flame width w f  (m) is given by: w f  = w  = wt  + 0.4 L 0.4 L H --- (53)

( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.22)

The flame length along axis, L axis,  L f  (m) is given by: 2 2 1/2  L f  =  = ( L L  + L  + L H  ) --- (54)

( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.23)

11.4.3 Centre-Line Temperature The flame temperature at the window T w (K) is given by: T w =

520 1 − 0.3325 L f  ( Av )

1/ 2

/ Q)

+ T o --- (55)

( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.22)

The flame temperature along the axis T  z  (K)  (K) is given by: 1/ 2

 L ( A ) )(T w − T 0 ) + T 0 --- (56) ( Eurocode T  z = (1 − 0.3325  x v  Eurocode 1 Eqt. 1 Eqt. B.25) Q where  L x  is the axis length from the window to the point where the calculation is made. Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 54 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

11.4.4 Convective and Radiative Radiative Heat Transfer 2

The convective heat transfer coefficient α c (kW/m K) is given by: αc= 9.8(1 / d eq ) 0.4 (Q /(17.5 Av ) + u / 1.6) 0.6 --(57) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. B.27) where d eq eq is the geometrical characteristic of an external structure element (diameter or side). Taking wind speed u = 6 m/s, α c is given by: αc = 9 .8(1 / d eq ) 0.4 (Q /(17.5 Av ) + 3 .75) 0.6 The emissivity of the flame with respect to face i of the t he column εz,i is given  by: − εz,i = 1 − e 0.3λ i For column engulfed in flame, the value of the flame thickness λ i should be determined as follows: a) if the level of the intersection of the flame axis and the column centerline is below the level of the top of the opening, the value of λ i shall be taken at the level of the intersection ( Figure 22(b)(1));  b) otherwise, the value of λ i  shall be taken at the level of the top of the opening ( Figure 22(b)(2)), except that if λ 4 < 0 at this level, the values at the level where λ 4 = 0 should be used.

(1) Flame axis intersects column axis below top of opening

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 55 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

(2) Flame axis intersects column axis above top of opening Figure 22(b) λ i for Forced draught condition (Source: Source: Eurocode 3 Figure B.6)

For beam engulfed in flame, the value of the flame thickness λ i should be determined as Figure 22(c) depending on whether the beam is located adjacent to wall.

(1) Beam nor adjacent to wall

(2) Beam immediately adjacent to wall

Figure 22(c) λ i for Forced draught condition (Source: Source: Eurocode 3 Figure B.7 )

11.5 Heat Transfer to External External Steelwork 11.5.1 Heat transfer to external steelwork can be classified to two different cases: namely where a member is not engulfed in flame, or where a member is engulfed in flame. A member that is not engulfed in flame receives radiative heat transfer from all the openings in that side of the fire compartment and from the flames projecting from all these openings, whilst a member that is engulfed in flame can receive convective heat transfer from the engulfing flame, plus radiative heat transfer from the engulfing flame and from the fire compartment opening from which it  projects. As member engulfed in flame is the conservative case in the assessment of heat transfer to external steelwork, only this case will be discussed as follows. Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 56 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

Two further cases need to be distinguished, namely: a vertical column member, and a horizontal horizontal beam member member being engulfed. engulfed. For a horizontal horizontal  beam member, if the beam is parallel to the external wall of the fire compartment, its average temperature T m should be determined for a point in the length of the beam beam directly above the centre centre of the opening. opening. For the case of the beam perpendicular to the external wall of the fire compartment, the value of the average temperature should be determined at a series of  points every 100mm along the length of the beam. The maximum of these values should then be adopted as the average temperature of the steel member  T   T m. 11.5.2 Radiative Heat Flux from from the Flames 11.5.2.1 Column Engulfed in Flame 2

The radiative heat flux from the flames  I z (kW/m ) is given by: ( I  z ,1 + I  z , 2 )d 1 + ( I  z , 3 + I  z , 4 )d 2  I z = --- (58) ( Eurocode  Eurocode 3 Eqt. 3 Eqt. B.18) 2(d 1 + d 2 ) 4 and  I z,1 z,1 = C 1εz,1σT z 4  I z,2 z,2 = C 2εz,2σT z 4  I z,3 z,3 = C 3εz,3σT w 4  I z,4 z,4 = C 4εz,4σT   z  where  I z,i z,i is the radiative heat flux from the flame to column face i; εz,i is the emissivity of the flames with wit h respect to face i of the column; i is the column face indicator (1), (2), (3) or (4) ( Figure 23); and C i is the protection coefficient of member face i. The emissivity of the flames εz,i for each of the faces (1), (2), (3) and (4) of the column should be determined from the expression for εz,i  given above. 11.5.2.2 Beam Engulfed in Flame 2

The radiative heat flux from the flames  I z (kW/m ) is given by:  I z =

( I  z ,1 + I  z , 2 )d 1

+ ( I  z ,3 + I  z ,4 )d 2 2(d 1 + d 2 )

--- (59)

( Eurocode  Eurocode 3 Eqt. 3 Eqt. B.21)

where  I z,i z,i is the radiative heat flux from the flame to beam face i; and i is the beam face indicator (1), (2), (3) or (4) ( Figure 23); Four cases are to be distinguished to calculate  I z,1  and I z,4 z,1, I z,2 z,2, I z,3 z,3 and I  z,4. Case1 (no forced draught with top of flame above the top of the beam):  Eurocode 3 Eqt. 3 Eqt. B.22 (60) gives: Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 57 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

4

 I z,1 z,1 = C 1εz,1σT w 4  I z,2 z,2 = C 2εz,2σT z,2 z,2 4 4  I z,3 σ(T z,1 z,3 = C 3εz,3σ(T  z,1 -T   z,2 )/2 4 4 I z,4 and σ(T  z,1 -T  z,2 )/2 z,4 = C 4εz,4σ(T  where εz,i is the emissivity of the flames with respect to face i of the  beam; T  z,1 is the flame temperature (in K) at level with the bottom of the beam; and T  z,2  is the flame temperature (in K) at level with the top of the  beam. C 4 may be taken as zero if the beam is immediately adjacent to the wall and the beam is parallel to the external wall of the fire compartment. Case 2 (no forced draught with top of flame below the top of the beam):  Eurocode 3 Eqt. 3 Eqt. B.23 (61) gives: 4  I z,1 z,1 = C 1εz,1σT w  I z,2 z,2 = 0 4 4  I z,3 (hz/d 2)C 3εz,3σ(T  σ(T z,1 z,3 = (h z,1 -T   x )/2 4 4 and I z,4 (hz/d 2)C 4εz,4σ(T  σ(T  z,1 -T  x )/2 z,4 = (h where

T  x is the flame temperature at the flame tip (can be taken as 813K); hz  is the height of the top of the flame above the bottom of the  beam.

Case 3 (forced draught with beam parallel to the wall, but not immediately adjacent to it, or with beam perpendicular to the wall):  Eurocode 3 Eqt. 3 Eqt. B.24 (62) gives: 4  I z,1 z,1 = C 1εz,1σT w 4  I z,2 z,2 = C 2εz,2σT z,2 z,2 4 4  I z,3 σ(T z,1 z,3 = C 3εz,3σ(T  z,1 -T   z,2 )/2 4 4 and I z,4 σ(T  z,1 -T  z,2 )/2 z,4 = C 4εz,4σ(T  Case 4 (forced draught with beam parallel to the wall and immediately adjacent to it): If the beam is parallel to the wall and immediately adjacent to it, only the  bottom face should be taken as engulfed in flame but one side and the top should be taken as exposed to radiative heat transfer from the upper 3 Eqt. B.25 (63) gives: surface of the flame. Thus, Eurocode Thus,  Eurocode 3 Eqt. 4  I z,1 z,1 = C 1εz,1σT w 4  I z,2 z,2 = Ф���C 2εz,2σT z,2 z,2 4 4  I z,3 σ(T z,1 z,3 = Ф���C 3εz,3σ(T  z,1 -T   z,2 )/2 I z,4 and z,4 = 0 where Ф  is the configuration factor relative to the upper surface of the flame for face i of the beam in Section 11.5.4.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 58 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

11.5.3 Radiative Heat Flux from from Opening 2

The radiative heat flux from an a n opening I  opening  I f f (kW/m    (kW/m ) is given by: 4  I f f =  Eurocode 3 Eqt. 3 Eqt. B.3)    = Ф f  εf (1- a (1- a z )σT  )σT f f  --- (64) ( Eurocode where Фf   is the overall configuration factor of the member for radiative heat transfer from that opening; εf  is the emissivity of the opening (may be taken as 1); az is the absorptivity of the flames. and 11.5.4 Overall Configuration Configuration Factor Фf  from  from the Window In calculating the configuration factor Ф f   for a given situation, a rectangular envelope should first be drawn around the cross-section of the member receiving the radiative radiative heat transfer. transfer. The configuration configuration factor should then be determined for the mid-point P of each face of this rectangle ( Figures 23 and 24). The configuration factor for each receiving surface should be determined as the sum of the contributions from each of the zones on the radiating surface (normally four) that are visible on the receiving surface. These zones should be defined relatively to the point X where a horizontal line  perpendicular to the receiving surface meets the plane containing the radiating surface. No contribution should be taken from zones zones that are not visible from the point P. If the point X lies outside the radiating surface, the effective configuration factor should be determined by adding the contributions of the two rectangles extending from X to the farther side of the radiating surface, then subtracting the contributions of the two rectangles extending from X to the nearer side of the radiating surface. 11.5.4.1 Receiving Surface Parallel to Radiating Surface The configuration factor Фi of zone i which is facing the radiating surface is given by: ai bi bi ai 1 −1 −1 + [ tan ( ) tan ( )] Фi = 2 0.5 2 0.5 2 0.5 2π  (1 + a i 2 ) 0.5 (1 + a i ) (1 + bi ) (1 + bi ) --- (65) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. G.2) where ai = hi /s;  bi = wi/s; s (m) is the distance from P to X; hi (m) is the height of the zone i on the radiating surface; and wi (m) is the width of that zone i. Figure 23 shows the demarcations of zone i of the receiving surface in a  plane parallel to that of the radiating surface.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 59 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

Figure 23 Receiving surface in a plane parallel to that of the radiating surface (Source: Source: Eurocode 3) 3)

11.5.4.2 Receiving Surface Perpendicular to Radiating Surface The configuration factor Фi of zone i which is perpendicular to the radiating surface surface is given given by: by: ai 1 1 −1 [tan −1 (a i ) − tan ( )] Фi = 2 2 2π  (1 + bi ) 0.5 (1 + bi ) 0.5 --- (66) ( Eurocode  Eurocode 1 Eqt. 1 Eqt. G.3) Figure 24 shows the demarcations of zone i of the receiving surface in a  plane perpendicular to that of of the radiating surface.

Figure 24 Receiving surface in a plane perpendicular perpendicular to that of the radiating surface (Source: Source: Eurocode 1) 1)

11.5.5 Absorptivity of the Flames az For column engulfed in flame, the absorptivity az of the flame should be determined from:

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 60 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

a z  =  =

ε z,1 + ε z,2

+ ε z,3

 Eurocode 3 Eqt. 3 Eqt. B.20) --- (67) ( Eurocode 3 where εz,1, εz,2 and ε  and εz,3 are the emissivities of the flame for column faces (1), (2) and (3). For beam engulfed in flame, t he absorptivity az of the flame should be determined from a z  = 1 − e −0.3h --- (68) ( Eurocode  Eurocode 3 Eqt. 3 Eqt. B.26) where h is the height of the window. 11.5.6 Average Temperature of Steel Member Member Engulfed in Flame After obtaining T z,  I z,  I f f,  σ and α c, the average temperature of the engulfed steel column T m (K) is given by equating the flux received by the member with the heat energy lost by the member to the environment by convection and radiation, i.e. 4 σT m  + αcT m = I   = I z + I   + I f f + ( Eurocode  Eurocode 3 Eqt. 3 Eqt. B.2)   αcT z --- (69) where T z (K) is the flame temperature; 2  I z (kW/m ) is the radiative heat flux from the flame; 2 and I f f   (kW/m ) is the radiative heat flux from the corresponding opening. th

This is an equation involving the 4  order of T m, and in order to solve for it, iterative method may be used. Example 6 and Example 7 in Section 13 shows how to employ the above procedures to get the temperature of a steel member on the e xternal façade of a compartment. 12.

Engaging Fire Engineering Consultants

12.1 When it is determined to engage engage fire engineering consultant to carry out a fire engineering study, project officers should note that fire safety engineering is a multi-disciplinary field of study applying scientific and engineering principles to the effects of fire and of the reaction a nd behaviour of people to fire, and structural fire engineering is a special branch addressing 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. 12.2 Many fire engineering experts are mainly involved in the study of the means of escape, smoke control, fire spread control, design of sprinkler, alarm, firefighting systems, etc, and may not be able to carry out structural fire engineering study. Hence, if structural fire engineering is involved in the the fire engineering study, project officers need to specifically state in the brief of the consultancy agreement that the consultant shall have professional(s) with both fire engineering and structural engineering experiences so as to carry out computational studies, such as computational fluid dynamic analysis, fire scenarios, heat transfer modeling, structural analysis and design at elevated temperatures to determine their structural responses (including the adequacy of the members and the connections at elevated temperature). Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 61 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

12.3 SEB has engaged structural fire engineering experts experts to carry out structural fire engineering study for a number of projects, including Tin Shui Wai cum  IRC, Improvement Works to Lei Yue Mun Park and Public Library cum  Holiday Village, and A Permanent Planning and Infrastructure Exhibition Gallery at City Hall Annex. Annex A  contains a sample brief showing clauses that may be included in the invitation documents, and project officers may also approach the office of Chief Structural Engineer/1 for advice. 13.

Design Examples

13.1 Example 1 – Time Time Equivalence Equivalence

This example aims at calculating the time equivalence for a protected structural steel member to check whether the 1 hour protection is adequate for the member member in a standard fire fire test. Consider a compartment of of size 6m (B)× 6m(W) × 3.4m(H) within an office building with the following following data: 2

Fire load density: q f,k   = 511 MJ/m   (80% fractile value for offices) (Table 5) 2 Floor area: A area: A f  = 6m×6m = 36m 2 Ventilation area: A area:  Av = 3.6m×2m = 7.2m Window height: heq = 2m Height of compartment: H  compartment:  H  =  = 3.4m Fire resistance rating of the office building = 60 mins. Total area of enclosure: A enclosure:  At  = (2×6×6) + (4×3.4×6) = 153.6m Using (8), opening factor: O = Av

2

 At  = 7.2× 2 /153.6 = 0.0663m heq / A

1/2

Using (5), characteristic fire load density 2 q f,d  = q f,k × m× δq1×δq2×δn = 511×0.8×1.1×1×1 = 449.7 MJ/m Ventilation factor αv = Av / A f  =  = 7.2 / 36 = 0.2 αh = 0 0.3 4 w f  = (6 / H  / H ) [0.62 + 90(0.4 – αv)  / (1 + bv αh)] 0.3 4 = (6 / 3.4) [0.62 + 90(0.4 – 0.2) ] = 0.906 ≥ 0.5 2 Using (4)  with k b  = 0.07min m /MJ and k c  = 1.0 for protected steel, the equivalent time of standard fire exposure t e,d   ) × k c e,d  = (q f,d × k b × w f  = (449.7×0.906×0.07)×1.0 (449.7×0.906×0.07)×1.0 = 28.5 minutes Since te,d t lim lim = 826.2 – 625 (0.223t – 0.448) = 1106.2 – 139.4t Time when temperature drops to 20 °C Since

T = 1106.2 – 139.4t and when T=20, 20 = 1106.2 – 139.4t t = 7.79 hr

The parametric fire curve of the compartment in fire condition is shown in the following figure.

Part II of this set of Guidelines will show that the steel temperature follows more or less the gas temperature for unprotected steel, and hence there is no advantage advantage from the fire engineering study. 13.3 Example 3 – Compartment Fire (Low Fire Load) Load)

This example will determine the parametric fire curve for a fire compartment in an office which is of relatively low fire load level. The walls have two openings openings in total, each of 2m(B)×2m(D). 2m(B)×2m(D). A layer of plaster of 15mm thick has been been applied to all surfaces of the walls. The floor area 2 of the compartment is 20×20 m  and the height of the compartment is 2.5m. The thermal properties of the materials are: Reinforced concrete 3  ρ =  ρ = 2400 kg/m ; c p = 750 J/kgK; k  =  = 1.70 W/mK Plaster: 3  ρ =  ρ = 2700 kg/m ; c p = 900 J/kgK; k  =  = 0.48 W/mK Automatic water extinguishing system, automatic smoke detector and excellent safe access route are provided. For combustion factor, m = 1.0. Area of Compartment: 2

Area of opening A opening  Av= 2 × 2 × 2 = 8 m ; Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 65 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

2

Area of floor A floor  A f  =  = 20 × 20 = 400 m ; 2 Total surface area A area  At = 2 (20 × 2.5 + 20 × 2.5 + 20 × 20) = 1000 m Fire load: For the use of compartment as office, the characteristic fire load density q f,k  2 is 511 MJ/m  (Table 5). 2 For a total of 400 m  floor area, using Table 4(a), the factor for taking into account the fire activation risk due to size of compartment

δq1 = 1.5 +

x (400 - 250) = 1.527.

Using Table 4(b), the factor taking into account the fire activation risk due to the type of occupancy, δq2 is given as 1.00. Using Table 4(c), the factor taking into account the different active fire fighting measures, for the provision of aut omatic water extinguishing system, automatic smoke detector and excellent safe access route is: δn = δn1 × δn4 × δn8 = 0.61 × 0.73 × 0.9 = 0.4 Using (5), the design fire load is given by, q f,d  = q f,k  × m × δq1 × δq2 × δn = 511 × 1.0 × 1.527 × 1.0 × 0.4 2 = 312 MJ/m Using (7), the design fire load densities for total area is given by, 2 qt,d  = q f,d  × ( A f  / At ) = 312 × (400/1000) = 125 MJ/m Ventilation factor: Using (8), the ventilation factor O is given by, O

= Av

he

=8×

2

= 0.011 m

0.5

 At  1000 Time to reach maximum temperature

Using (9), the time to reach maximum temperature t max max for t min min = 25 min. is given by t max max

 0.2 × 10 −3 q t,d   0.2 × 10 −3 × 125  = max   , 0.416 , 0.416  = max  0.011 O     = max [2.27, 0.416]= 2.27 hr

Therefore the fire is ventilation control. Equivalent thermal inertia of material: Using (10), for floor, walls and ceiling, b floor  = bwall  = bceiling  = Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

 ρ conc

× c p × k  = 2400 × 750 × 1.7 = 1749 J/m 2s0.5K Page 66 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

For plaster layer of wall,  ρ  plaster  × c p

b plaster  =

× k  = 2700 × 900 × 0.48 = 1080 J/m 2s0.5K 2 0.5

Since b plaster  is  is less than bwall , bwall  =  = b plaster = 1080 J/m s K, using (15), b=

∑ (b  A ) = (1749 × 2 × 400) + [1080 × (1000 − 2 × 400 − 8)]  j

 j

1000 − 8

 At  −  Av

=

1399200  + 207360 992

2 0.5

= 1620 J/m s K

Heating phase of the parametric fire curve: *

The fictitious time t   is given by t control, using (17) and (18), Γ=

t

(O / 0.04)

2

(b / 1160 ) 2

* max =

=

*=

.

Γ t, and since the fire is ventilation

(0.011 / 0.04) 2 (1620 / 1160 ) 2

= 0.0388

.

Γ tmax = 0.0388 × 2.27 = 0.0881 hr

Using (16), the temperature of the parametric fire curve in heating phase T is given by: -0.2t* -1.7t* -19t* T = 20 + 1325 (1 – 0.324e  – 0.204e  – 0.472e ) * where t  is 0.0388t (in hour). The maximum temperature of the parametric fire curve in heating phase, Tmax is then given by: -0.2t*max -1.7t*max -19t*max Tmax = 20 + 1325 (1 – 0.324e  – 0.204e  – 0.472e ) -0.2x0.0881 -1.7x0.0881 -19x0.0881 = 20 + 1325 (1 – 0.324e  – 0.204e  – 0.472e ) = 573 °C Cooling phase of the parametric fire curve: *

For t max  < 0.5, using (21), the temperature of the parametric fire curve in cooling phase, T is given by: *  x), where x T = Tmax – 625 (t* – t max. x), where x =  = 1 for t max max > t lim lim = 573 – 625 (0.0388t – 0.0881) = 628 – 24.25t Time when temperature drops to 20 °C Since

T = 628 – 24.25t, and when T=20, 20 = 628 – 24.25t t = 25.07 hr The parametric fire curve of the compartment in fire condition is shown in the following figure:

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 67 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

13.4 Example 4 – Localised Fire

This example is to calculate the temperature of the flame at a distance of 0.43 meters above the fire source for the case developed above of a localised fire of: a) a wastepaper basket in an office;  b) a Christmas tree in a library. This example shows the case where the fire cannot reach the ceiling of the floor. Wastepaper basket in an office Using Table 9, the rate of heat release of the fire of a wastepaper basket is: Q = 0.018 MW 2 Using Table 7, a heat release rate density of 250kW/m  is used for office. The area of the fire surface required to release a power of 0.018MW is thus given by: 2  A fire = 0.018/0.25 = 0.072 m Assuming a circular shape of the fire yields a diameter, using (39), 0.5  D = (4 × 0.072 / π)  = 0.303 m Using (25), the length of the flame 0.4 0.4  L f  = 0.0148 Q  – 1.02 D =  D = 0.0148 (0.018 × 106)  – 1.02(0.303) = 0.436 m Using (27), virtual origin of the fire source 0.4 0.4  z 0 = 0.00524 Q  – 1.02 D =  D = 0.00524 (0.018 × 106)  – 1.02(0.303) = -0.0452 m Using (26), temperature on the flame axis 2/3 -5/3 θ(z) = θ(z) = 20 + 0.25 Qc  ( z-z   z-z 0) 2/3 -5/3 = 20 + 0.25 (0.8 × 0.018 × 106)  (0.43 + 0.0452) = 531°C Christmas tree in a library Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 68 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

Using Table 9, the rate of heat release of the fire of a Christmas tree is: Q = 0.65 MW 2 Using Table 7, a heat release rate density density of 500kW/m  is used for library. The fire surface required to release a power of 0.65MW is thus given by: 2  A fire = 0.65/0.5 = 1.3 m Assuming a circular shape of the fire yields a diameter, using (39), 0.5  D =  D = (4 × 1.3 / π)  = 1.29 m Using (25), the length of the flame 0.4 0.4  L f  =  D = 0.0148 (0.65 × 106)  – 1.02(1.29) = 1.81 m  = 0.0148 Q  – 1.02 D = Using (27), virtual origin of the fire source 0.4 6 0.4  z 0 = 0.00524 Q  – 1.02 D =  D = 0.00524 (0.65 × 10 )  – 1.02(1.29) = -0.208 m Using (26), temperature on the flame axis 2/3 -5/3 θ (z)  z-z 0) ( z-z  (z) = 20 + 0.25 Qc 6 2/3 -5/3 = 20 + 0.25 (0.8 × 0.65 × 10 ) (1.8 + 0.208) = 526°C In Section 9.6.5, it has been noted that the UK National Annex to  Eurocode 1  specifically states that the method in  Eurocode 1  is not applicable to localised fires, and directs designers instead to refer to the method in  PD 7974-1. 7974-1. Hence, the example of a Christmas tree in a library will be calculated using PD using  PD 7974-1as 7974-1as follows: Using Table 9, rate of heat release of fire of a Christmas tree is: Q = 0.65 MW The geometric data of the library are as follows:  H  =  = 3.4 m, W  =  = 2.9 m, L m,  L =  = 3.75 m, δ = 0.15 m with opening size of he = 1.3 m and b = 1.2 m The thermal properties of reinforced concrete as wall materials are as follows: 3  ρ =  ρ = 2450 kg/m , c = 0.75 kJ/kgK, λ kJ/kgK,  λ =  = 0.0017 kW/mK Using (34), the thermal penetration time 2

2

  ρ c  δ   =  2450 × 0.75  0.15  = 6080 s t  p =          λ    2    0.0017    2   As t  p is greater than the fire exposure time of a library, using (35), the effective heat transfer coefficient 0.5

0.5  λρ c  0.0017 × 2450 × 0.75      hk  =    t  p  =   6080      

2

= 0.02267 kW/m K Using (33), the rise in temperature above the localized fire 1

  Q 2   3  θ = 6.85   A h 2 h  A    w e k  t   Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 69 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

1

    3 650   2 ( ) ( ) × × × × × 1 . 3 1 . 2 1 . 3 0 . 02267 2 . 9 3 . 75     2

= 6.85

= 593 ˚C The temperature of the flame T  =  = 593 + 20 = 613 ˚C The results are then checked using the computer software CFAST, which is free for downloaded. The compartment is simulated using the data from the above case. CFAST is a two-zone fire model that can be used to calculate the evolving distribution of temperature throughout compartments of a  building during a fire. These can range from very small containment vessels, 3 3 on the order of 1 m   to large spaces on the order of 1000 m . A compartment is divided into two control volumes, a relatively hot upper layer and a relatively cool lower layer, each of which is internally uniform in temperature and composition. composition. The modelling equations used in CFAST take the mathematical form of an initial value problem for a system of ordinary differential equations (ODEs). These equations are derived using the conservation of mass, the conservation of energy (equivalently the first law of thermodynamics), the ideal gas law and relations for density and internal energy. These equations predict as functions of time quantities such as  pressure, layer height and temperatures given the accumulation of mas s and enthalpy in the two layers. The user need to input data about the building geometry (compartment sizes, materials of construction, and material  properties), connections between compartments (horizontal flow openings such as doors, windows, vertical flow openings in floors and ceilings, and mechanical ventilation connections), fire properties (fire size and species  production rates as a function of time), and specifications for detectors, sprinklers, and targets (position, size, heat transfer characteristics). The model of the compartment used in this example is shown as follows:

The time-temperature graph of the fire of the Christmas tree at the ceiling of the compartment from the software is plotted as follows, and the peak temperature is at 623˚C.

Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 70 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

���

13.5 Example 5 – Localised Fire

This example shows the case where the fire can reach the ceiling of the floor. Consider the maximum flux received at the level of the ceiling by a localised fire in a stack of paper. The paper weighs 10kg and the vertical distance from the fire source to the ceiling is 2.6 m. From Table 6 and using (6) with the net calorific value of paper is 20MJ/kg, the total heat content  E=10×20  E=10×20 = 200 MJ From Table 8, take a fire growth rate between fast and medium, i.e. k  = -0.5 200sMW . Using (38), the heat release rate of the fire 2

2

3 E  3 × 200 3 Q = ( )3 = ( ) = 2.08 MW k  200 2 Using Table 7, a heat release rate density of 500kW/m  is used. The fire surface required to release a power of 2.08MW is thus given by: 2  A fire = 2.08/0.5 = 4.16 m Assuming a circular shape of the fire yields a diameter, using (39),  0.5  D =  D = (4×4.16 /π)  = 2.30 m Using (25), the length of the flame 6 0.4  L f  = 0.0148 (2.17×10 )  – 1.02(2.30) = 2.67 2.67 m Using (32), * 6 2.5 6 6 2.5 Q H   = Q / (1.11×10  H  ) = 2.17×10 /(1.11×10 ×2.6 ) = 0.179 Using (30), * 6 6 2.5 Q D  = 2.17 × 10 / (1.11 × 10 × 2.30 )= 0.231 Using (31), horizontal flame length *  0.33  0.33  Lh = (2.9 H (2.9 H (Q H  ) ) - H  - H  =  = (2.9 × 2.6 (0.179) ) – 2.6 = 1.67 m Structural Engineering Branch, ArchSD Revision No. : 0 First Issue Date : December 2011

Page 71 of 87

File code : SEBGL-OTH6 CTW/MKL/CHL/CHM/SCF Current Issue Date : December 2011

*

For Q D