Fire safety engineering -- Performance of structures in fire

ISO/TR 24679-4:2017 provides a fire engineering application relative to the fire resistance assessment of a fifteen-storey steel framed building following the methodology given in ISO 24679-1. This document describes the adopted process which follows the same step by step procedure as that provided in ISO 24679-1. The annexes of this document present the detailed assessment results obtained for the most severe fire scenarios on the basis of the outcome of this specific fire safety engineering procedure for the building. The fire safety engineering applied in this example to the office building with respect to its fire resistance considers specific design fire scenarios as well as the corresponding fire development. It takes into account fully-developed compartment fires. In realistic situations, activation of fire suppression systems and/or intervention of fire brigade are expected, but their beneficial effects are not taken into account. It should be noted that these severe fire scenarios have been selected for fire resistance purposes. Global structural behaviour is not explicitly considered, but implicitly included in the calculation formulae. Since the building of the example is located in a seismic region, principal structural elements are rigidly connected to each other. Load redistribution from heated elements to cold surrounding elements exists, but it's not taken into account in the design calculations. By this approach, design is conservative, while the process of safety checking is greatly simplified and clear. As a result, all the calculations were carried out by explicit algebraic formulae.

Ingénierie de la sécurité incendie -- Performance des structures en situation d'incendie

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Published
Publication Date
22-Aug-2017
Current Stage
6060 - International Standard published
Start Date
18-May-2017
Completion Date
23-Aug-2017
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TECHNICAL ISO/TR
REPORT 24679-4
First edition
2017-08
Fire safety engineering —
Performance of structures in fire —
Part 4:
Example of a fifteen-storey steel-
framed office building
Ingénierie de la sécurité incendie — Performance des structures en
situation d'incendie —
Partie 4: Exemple d'un immeuble de bureaux en structure acier de
quinze étages
Reference number
ISO/TR 24679-4:2017(E)
ISO 2017
---------------------- Page: 1 ----------------------
ISO/TR 24679-4:2017(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland

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ii © ISO 2017 – All rights reserved
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ISO/TR 24679-4:2017(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 2

4 Symbols .......................................................................................................................................................................................................................... 2

5 Design strategy for fire safety of structures ............................................................................................................................. 3

6 Quantification of the performance of structures in fire .............................................................................................. 3

6.1 General ........................................................................................................................................................................................................... 3

6.2 Step 1: Scope of the project for fire safety of structures ..................................................................................... 4

6.2.1 Built environment characteristics .................................................................................................................... 4

6.2.2 Fuel load .................................................................................................................................................................................. 5

6.2.3 Mechanical actions ......................................................................................................................................................... 6

6.3 Step 2: Identify objectives, functional requirements and performance criteria for

fire safety of structures .................................................................................................................................................................... 7

6.4 Step 3: Trial design plan for fire safety of structures ............................................................................................. 7

6.5 Step 4: Design fire scenarios and design fires .............................................................................................................. 8

6.5.1 Design fire scenarios .................................................................................................................................................... 8

6.5.2 Design fires (thermal actions) .............................................................................................................................. 9

6.6 Step 5: Thermal response of the structure ..................................................................................................................10

6.6.1 Steel columns and beams ......................................................................................................................................10

6.6.2 Other construction elements .................. ............................................................................................................12

6.7 Step 6: Mechanical response of the structure ...........................................................................................................12

6.7.1 Steel columns ...................................................................................................................................................................12

6.7.2 Steel beams ........................................................................................................................................................................13

6.8 Step 7: Assessment against the fire safety objectives .........................................................................................14

6.9 Step 8: Documentation of the design for fire safety of structures ...........................................................14

6.10 Factors and influences to be considered in the quantification process ..............................................15

6.10.1 Thermal properties.....................................................................................................................................................15

6.10.2 Mechanical strength of steel material ........................................................................................................15

6.10.3 Uncertainty of material properties ...............................................................................................................15

7 Guidance on use of engineering methods ...............................................................................................................................15

Annex A (informative) Building and framing design ........................................................................................................................16

Annex B (informative) Fuel and structural load ....................................................................................................................................26

Annex C (informative) Fire temperatures ....................................................................................................................................................31

Annex D (informative) Maximum temperature of insulated steel elements ...........................................................36

Annex E (informative) Critical temperature of steel columns, girders and beams ...........................................42

Bibliography .............................................................................................................................................................................................................................49

© ISO 2017 – All rights reserved iii
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ISO/TR 24679-4:2017(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

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described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

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URL: www.iso.org/iso/foreword.html.

This document was prepared by Technical Committee ISO/TC 92, Fire safety, Subcommittee SC 4, Fire

safety engineering.
iv © ISO 2017 – All rights reserved
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ISO/TR 24679-4:2017(E)
Introduction

This document is an example of the application of ISO 24679-1, prepared in the format of ISO 24679-1. It

includes only those subclauses of ISO 24679-1 that describe the steps of the methodology for assessing

the performance of structures in fire. It preserves the numbering of subclauses in ISO 24679-1 and so

omits numbered subclauses for which there is no text or information relevant to this example.

This example is intended to illustrate the implementation of the steps of the fire resistance assessment,

as defined in ISO 24679-1. Only steps that are considered to be relevant to this example are well-detailed

in this document. The technical contents are based on the performance based verification methods for

fire resistance in the Building Standards Law of Japan, but were slightly modified for simplicity and

compatibility with ISO 24679-1.
© ISO 2017 – All rights reserved v
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TECHNICAL REPORT ISO/TR 24679-4:2017(E)
Fire safety engineering — Performance of structures in
fire —
Part 4:
Example of a fifteen-storey steel-framed office building
1 Scope

This document provides a fire engineering application relative to the fire resistance assessment of a

fifteen-storey steel framed building following the methodology given in ISO 24679-1. This document

describes the adopted process which follows the same step by step procedure as that provided in

ISO 24679-1. The annexes of this document present the detailed assessment results obtained for the

most severe fire scenarios on the basis of the outcome of this specific fire safety engineering procedure

for the building.

The fire safety engineering applied in this example to the office building with respect to its fire

resistance considers specific design fire scenarios as well as the corresponding fire development.

It takes into account fully-developed compartment fires. In realistic situations, activation of fire

suppression systems and/or intervention of fire brigade are expected, but their beneficial effects are

not taken into account. It should be noted that these severe fire scenarios have been selected for fire

resistance purposes.

Global structural behaviour is not explicitly considered, but implicitly included in the calculation

formulae. Since the building of the example is located in a seismic region, principal structural elements

are rigidly connected to each other. Load redistribution from heated elements to cold surrounding

elements exists, but it's not taken into account in the design calculations. By this approach, design is

conservative, while the process of safety checking is greatly simplified and clear. As a result, all the

calculations were carried out by explicit algebraic formulae.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 13943, Fire safety — Vocabulary
ISO 23932, Fire safety engineering — General principles
© ISO 2017 – All rights reserved 1
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ISO/TR 24679-4:2017(E)
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 13943 and ISO 23932 and the

following apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at http://www.iso.org/obp
3.1
design heat release of a room

amount of heat to be released in a room including movable fire load, fixed fire load and heat transferred

from adjacent rooms
Note 1 to entry: It is expressed in MJ.
3.2
equivalent fire duration time

duration of heating by a standard fire as specified in ISO 834-1 that gives equivalent thermal effect on

structural elements with a real fire
Note 1 to entry: It is expressed in min.
3.3
fixed fuel load density

heat of combustion of materials fixed to room, such as interior finish materials, equipment and so on,

per unit floor area of fire room
Note 1 to entry: It is expressed in MJ/m .
3.4
heat penetration factor
ratio of heat penetrated from adjacent rooms to the room in consideration
Note 1 to entry: It is dimensionless.
3.5
movable fuel load density

heat of combustion of movable room contents such as furniture, commodities and so on per unit floor

area of fire room
Note 1 to entry: It is expressed in MJ/m .
3.6
total heat release of a room

amount of heat possible to be released in a room including movable and fixed fire load

Note 1 to entry: It is expressed in MJ.
4 Symbols
For the purposes of this document, the following symbols are used.
A room floor area (m )
A surface area of interior lining material (m )
f heat penetration factor
2 © ISO 2017 – All rights reserved
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ISO/TR 24679-4:2017(E)
f nominal yield strength of steel at normal temperature (MPa)
G permanent load
K kinematic load
q movable fire load density (MJ/m )
Q heat of combustion of interior lining materials per unit area (MJ/m )
Q design heat release of a room (MJ)
Q live or variable load
t time (min)
t approved fire resistance time of a construction element (min)
t fire duration time (min)

t equivalent fire duration as replaced with a fire as specified in ISO 834-1 (min)

T limiting temperature for overall buckling (°C)
T limiting temperature for bending failure of a beam (°C)
Bcr
T critical temperature of steel element (°C)
T limiting temperature for excessive deformation (°C)
T limiting temperature for joint failure (°C)
T limiting temperature for local buckling (°C)
T maximum steel temperature under design fire action (°C)
s.max
T fire temperature in a room (K)
T initial temperature (K)
1/6
α fire temperature rise coefficient (K/min )
5 Design strategy for fire safety of structures

The built environment of this example is a medium-rise office building. Due to its use, the building is

separated into multiple compartments by floors and walls to accommodate tenant office functions. As

the combustible contents are distributed densely, fire is likely to spread over whole compartment. As a

result, a fully-developed compartment fire is expected in each room of the building.

The structural elements are composed of steel and are protected against fire. To prevent failure of the

structural elements and joints, the thickness of fire protection had been defined in order to limit their

temperatures below their critical temperatures during the fully-developed fire in each compartment.

6 Quantification of the performance of structures in fire
6.1 General

The various steps of the design process considered in the conducted fire safety engineering study are

detailed in 6.2 to 6.9.
© ISO 2017 – All rights reserved 3
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ISO/TR 24679-4:2017(E)
6.2 Step 1: Scope of the project for fire safety of structures
6.2.1 Built environment characteristics

The built environment is a steel framed 15-storey office building. The gross floor area is 8 236 m and

the building height is 68,5 m. See Annex A for building drawings. According to the regulations, the

[10]

building must be constructed by fire-resistive constructions. In the prescriptive code, columns must

be three hours fire-rated construction on the first floor, two hours on the second to eleventh floors

and one hour on the floors above. The building is separated horizontally by compartment floors at all

levels. Vertical shafts such as stairs, elevators and service shaft are surrounded by one hour-rated fire

resistance walls.

The floor plan is shown in Figure 1. The office area is split into two rooms, XX01 and XX02. Symbol XX

denotes floor number. For example, 1502 denotes room number 2 on floor 15. The two office rooms are

separated by an EI60 wall as determined by ISO 834-8. In addition, the office area is divided into two

rooms by a non-fire-rated wall made of regular gypsum board. The doors between office rooms and

corridor are fire-rated E60 to prevent fire spread between office area and corridor.

The building structure is a rigid moment-resistant steel frame. Specification of members such as

columns, beams and floor slabs are listed in Annex A. The columns, beams and girders are protected

against fire by a 25 mm sprayed rock wool cementitious mixture.
— Span of primary beams: 13,6 m;
— Span of secondary beams: 6,4 m;

— Spacing of columns: 6,4 m in direction of primary beams and 13,6 m in direction of secondary beams.

The applied load of design on the floors is taken as follows:
— live load: 2,9 kN/m (based on building code requirement);
— self-weight of floor: 3,65 kN/m .

In this document, only the results of construction elements in office rooms number: 201, 202, 1501

and 1502 are demonstrated.

The external walls are fire-rated for more than one hour. However, windows are not fire-resistant. The

floors are composite constructions of concrete slabs and steel beams. The composite slabs are made of

normal weight concrete and profiled steel sheets with reinforcing bars.
4 © ISO 2017 – All rights reserved
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ISO/TR 24679-4:2017(E)
Key
1 fire-rated partition wall (EI60)
2 non-fire-rated partition wall
3 fire door (E60)
NOTE All the exterior walls are fire-rated (EI60).
Figure 1 — Typical floor plan
6.2.2 Fuel load

As the building is used as office space, a significant amount of combustibles is expected. The design fuel

load of a room, Q , is calculated as the sum of the following components.
1) Movable fuel load based on the use of the room, A q .
r l
2 [10]

The movable fire load density for an office area is 560 MJ/m based on the building code. The results

are shown in Table 1.
2) Fixed fuel load based on the type of interior finish materials, ΣA Q .
f f
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ISO/TR 24679-4:2017(E)

The heat of combustion of interior finish materials are accounted as fixed fire loads. The details of

calculation are shown in B.5.2. The results are summarized in Table 1.
3) Heat penetrated from adjacent rooms, Σf (A q + ΣA Q ).
a r l f f

As the walls between the office areas XX01 and the office areas XX02 are not fire-rated, there is a risk

for fire to spread between the rooms. A part of combustion heat in an adjacent room may affect the

structural elements in the other room. To account for this “mutual heating” effect, it was assumed that

15 % of heat of combustion (design rule in Japan) may penetrate to adjacent rooms separated by non-

fire-rated walls. The calculated results are shown in Table 2.
Table 1 — Fuel load of rooms XX01 and XX02 (XX = 2nd and 15th floors)
Movable fuel
Movable fuel Fixed fuel load Total fuel
load density Floor area
Floors Room No. Usage load A q ΣA Q load of room
r l f f
q A (m )
l r
(MJ) (MJ) (MJ)
(MJ/m )
201,
office 560 87,5 49 000 7 649 56 649
1501
2 to 15 202,
office 560 275 154 000 22 397 176 397
1502
corridor pathway 32 125 4 000 13 722 17 722

Table 2 — Design heat release of a room considering heat penetration from adjacent rooms

Heat penetration
Total fuel load of Penetrated heat Design fuel
Floor Room Adjacent rooms coefficient
adjacent rooms (MJ) (MJ) load (MJ)
f (-)
201 202,1502 176 397 0,15 26 460
83 108
Corridor 17 722 0,0 0,0
1501
2 to 15
202 201,1501 56 649 0,15 8 497
184 894
Corridor 17 722 0,0 0,0
1502
6.2.3 Mechanical actions

The mechanical actions in fire situation are determined in accordance with the building code.

Permanent and movable vertical loads are considered, while no horizontal actions are considered such

[10]
as seismic and wind actions. As a result, the load combination is :
1,0G + 1,0Q (1)

where G is sum of all the permanent loads, i.e. self-weight of the building and Q for the variable loads

representing the contents of the building. No snow load was considered in this document as the building

is located in non-snow region.

As the building is located in seismic region, the following load combination is applied for seismic

resistance design:
1,0G + 1,0Q + 1,0K (2)

where K is the (horizontal) kinetic action. Common for buildings in Japan, the cross-sectional dimensions

are governed by seismic design. As a result, the load ratios of structural elements are relatively small

during normal use such as in the case of non-seismic and non-windy conditions. Details are described

in Step 3.
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ISO/TR 24679-4:2017(E)

6.3 Step 2: Identify objectives, functional requirements and performance criteria for

fire safety of structures

As the building is used by multiple occupants, the building must not collapse during egress, firefighting

and rescue. In addition, as the building is located in an urban area, the building must not collapse

during the whole process of fire and subsequent cooling period to prevent fire spread to urban scale.

[10]

As a result, stability during whole process of fire is necessary. To fulfil this objective, the functional

requirement is to have no failure of the building construction elements during the whole process of fire,

including the cooling phase. Consequently, the following performance criteria, in terms of stability of

the structure, are considered on an element by element basis.

— The temperature of the steel columns does not exceed the minimum of the critical temperatures for

overall buckling, local buckling, excessive deformation and joint failures.

— The temperature of the steel beams and girders does not exceed the limit for bending failure and

joint failure. Shear failure does not precede the bending failure.

— Floor construction does not exceed the limit for mechanical failures, typically bending failure.

In addition, the following performance criteria are considered in terms of fire containment.

— Fire compartment walls and floor constructions do not transmit excessive heat that may ignite

combustibles in opposite side (insulation criterion).

— Fire compartment walls and floor constructions do not penetrate flame and/or hot gases that causes

fire spread beyond them (integrity criterion).
6.4 Step 3: Trial design plan for fire safety of structures

Preliminary designs, at room temperature, were carried out to determine the dimensions of the

structural members. As the building is located in a seismic region, the members are designed against

seismic actions as described by Formula (2).

The floor and beam plan is shown in Figure 2. The frame consists of simple 2 × 4 bays. The trial design

of principal construction elements is listed in Table 3. On the second floor, all the elements are made

of relatively thick large cross-sectioned steel sections in order to withstand the large seismic actions.

For the fire combination, large vertical loads are applied to second floor columns. On the 15th floor, the

elements are made of relatively thin and small sections but the applied loads in the fire combination are

also small compared with the second floor elements. Further details are shown in Annex A.

[11]

Columns and girders are made of SN 490 steel with a yield strength of 325 MPa. Secondary beams

[12]

are made of a SS 400 steel with a yield strength of 235 MPa. Columns are made of a box-sectioned

tube. Beams and girders are made of H-sectioned elements. To prevent excessive temperature rise, the

elements are insulated with a sprayed rock wool cementitious material of a thickness of 25 mm.

The floor slab is made of composite structure of profiled steel plate and concrete. The thickness of

concrete varies from 80 mm to 155 mm. The diameter of the reinforcing bars is 13 mm. The concrete

cover of reinforcing bars from the bottom side of the slab is 20 mm. Reinforcing wire mesh with 6 mm

diameter and spaced 100 mm, is located at 30 mm from the top side of the slab. The compressive

strength of concrete is 21 MPa.
© ISO 2017 – All rights reserved 7
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ISO/TR 24679-4:2017(E)
Key
Columns, girders and beams are not insulated.

All other columns, girders and beams are insulated by 25 mm thick sprayed rock wool.

Figure 2 — Floor and beam plan
Table 3 — Summary of structural members
Floor levels 2nd floor 15th floor
C1 box-600 × 40 box-600 × 22
Column (four sides exposed)
C2 box-600 × 45 box-600 × 22
SN 490 steel, f = 325 MPa
C3 box-500 × 36 box-500 × 19
G1 H-900 × 350 × 16 × 25 H-700 × 300 × 14 × 25
Primary beam (three sides exposed)
G2 H-900 × 300 × 16 × 25 H-700 × 250 × 14 × 22
SN 490, f = 325 MPa
G3 H-900 × 300 × 16 × 25 H-700 × 250 × 14 × 22
b1 H-350 × 175 × 7 × 11
Secondary beam (three sides exposed),
SS400, f = 235 MPa
b2 H-450 × 200 × 9 × 14
Steel decking 1,2 mm
Composite slab, concrete strength: 21 MPa
Concrete thickness minimum 80 mm, maximum 155 mm
NOTE See Annex A for details.
6.5 Step 4: Design fire scenarios and design fires
6.5.1 Design fire scenarios

It is assumed that each room can be an origin of fire. A compartment fire is assumed to grow and decay

until the burnout of the combustible materials in the rooms under investigation. No effect of active

suppression, such as sprinkler and/or manual intervention, is considered. Only one compartment fire is

considered at a time, but the fire may spread to adjacent rooms via non-fire-rated partition walls.

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ISO/TR 24679-4:2017(E)

All unprotected openings are assumed to be broken and accounted for the ventilation calculations.

Protected openings, such as fire-rated doors, are assumed to be closed and not accounted for in the

ventilation calculations.

A nominal localized fire with a constant heat release rate of 3 MW for 20 min, is considered in addition

to the fully-developed compartment fire. However, calculations for the localized fire are not included

in this document because the heat impact of a fully-developed fire is more severe than that of localized

fires in this case.

A nominal exterior fire is assumed to occur in the neighbour of the building. The standard fire

temperature as specified in ISO 834-1 for 60 min is assumed as the nominal fire for exterior heating.

6.5.2 Design fires (thermal actions)

A fully-developed compartment fire was considered. It is assumed that all the combustibles burn at

a constant rate. The heat release rates of office rooms are shown in Figure 3. The time-temperature

curves are calculated using Formula (3), an algebraic equation assuming a uniform temperature in a

fire room:
16/
Tt=+α Tt,(0≤≤t ) (3)
ff0
1/6

where the fire temperature rise coefficient α (K/min ) and fire duration t (min) are calculated in

accordance with the room geometry, window opening size and burning rate of the fuel. Calculation

details are provided in Annex C. The results are shown in Figure 4. As the window areas are fairly large

and fuel and air ratio is close to stoichiometric, fire burns severely but the duration is short. The fire

temperatures are considerably higher than the standard fire temperature as specified in ISO 834-1. The

equivalent fire duration time was calculated to be 59,9 min in both fires for rooms XX01 and XX02 as

shown in Table 4.
Key
Y heat release rate (MW)
X time (min)
Figure 3 — Heat release rates of office rooms, XX01 and XX02
© ISO 2017 – All rights reserved 9
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