ISO/TR 24679-4

• Abstract:

  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.

• Status: Not Published
• ICS: 13.220.01 - Protection against fire in general
• Technical Committee: ISO/TC 92/SC 4 - Fire safety engineering
• Drafting Committee: ISO/TC 92/SC 4 - Fire safety engineering
• Current Stage: 6000 - International Standard under publication
• Start Date: 23-Sep-2026
• Completion Date: 26-Sep-2026

• Revises: ISO/TR 24679-4:2017 - Fire safety engineering — Performance of structures in fire — Part 4: Example of a fifteen-storey steel-framed office building
• Effective Date: 06-Jun-2022

Overview

ISO/TR 24679-4 is an ISO technical report in fire safety engineering that provides a practical example of performance of structures in fire for a fifteen-storey steel-framed office building. It shows how to apply the methodology from ISO 24679-1 through a step-by-step fire resistance assessment.

This document is especially useful for professionals involved in:

• Structural fire engineering
• Fire resistance assessment
• Steel building design
• Performance-based fire safety verification

The report focuses on a realistic office building case study and explains the process used to evaluate structural behavior under severe fire scenarios. It is designed to support clear, conservative, and simplified fire safety checking for steel-framed structures.

Key Topics

ISO/TR 24679-4 covers the main stages of a fire engineering study, including:

• Scope definition for the fire safety project
• Identification of objectives, functional requirements, and performance criteria
• Development of a trial design plan
• Selection of design fire scenarios and design fires
• Evaluation of the thermal response of the structure
• Assessment of the mechanical response of the structure
• Final comparison against fire safety objectives
• Documentation of the design and verification process

The example considers:

• A 15-storey office building
• Steel-framed construction with insulation
• Fully developed compartment fires
• Fire resistance based on explicit algebraic calculations
• Conservative assumptions, with suppression systems and fire brigade intervention not credited

The document also includes annexes with detailed results for:

• Building and framing design
• Fire load and structural load
• Fire temperatures
• Maximum steel temperatures
• Critical temperatures of steel columns, girders, and beams

Applications

ISO/TR 24679-4 is valuable in practical fire safety work where engineers need to understand how structural fire resistance is assessed in a real building example. It can support:

• Office building fire safety design
• Steel structure fire resistance verification
• Performance-based design workflows
• Comparative evaluation of fire scenarios
• Training and guidance for fire engineering teams

Because the example uses a structured methodology and conservative design assumptions, it is useful for organizations seeking a clearer path to fire resistance checking in steel-framed buildings, especially where regulatory or project documentation requires a transparent process.

Related Standards

ISO/TR 24679-4 is closely linked to several other standards used in fire safety engineering and fire resistance testing:

• ISO 24679-1 - Fire safety engineering - Performance of structures in fire - General methodology
• ISO 23932-1 - Fire safety engineering - General principles
• ISO 13943 - Fire safety - Vocabulary
• ISO 834-1 - Fire-resistance tests - General requirements
• ISO 834-5 - Fire-resistance tests for loadbearing horizontal separating elements
• ISO 834-8 - Fire-resistance tests for non-loadbearing vertical separating elements

For engineers, designers, and reviewers, ISO/TR 24679-4 offers a practical reference for applying structural fire engineering methods to a multi-storey steel office building.

FINAL DRAFT
Technical
Report
ISO/DTR 24679-4
ISO/TC 92/SC 4
Fire safety engineering —
Secretariat: AFNOR
Performance of structures in fire —
Voting begins on:
2026-07-28
Part 4:
Example of a fifteen storey steel-
Voting terminates on:
2026-09-22
framed office building
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Reference number
ISO/DTR 24679-4:2026(en) © ISO 2026

FINAL DRAFT
ISO/DTR 24679-4:2026(en)
Technical
Report
ISO/DTR 24679-4
ISO/TC 92/SC 4
Fire safety engineering —
Secretariat: AFNOR
Performance of structures in fire —
Voting begins on:
Part 4:
Example of a fifteen storey steel-
Voting terminates on:
framed office building
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
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AND TO PROVIDE SUPPOR TING DOCUMENTATION.
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BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
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ii
ISO/DTR 24679-4:2026(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 2
5 Design strategy for fire safety of structures . 3
6 Quantification of the performance of structures in fire . 4
6.1 General .4
6.2 Step 1: Scope of the project for fire safety of structures.5
6.2.1 Built environment characteristics.5
6.2.2 Fire load .8
6.2.3 Mechanical actions .9
6.3 Step 2: Identify objectives, functional requirements and performance criteria for fire
safety of structures .9
6.4 Step 3: Trial design plan for fire safety of structures .10
6.5 Step 4: Design fire scenarios and design fires .10
6.5.1 Design fire scenarios .10
6.5.2 Design fires (thermal actions) .10
6.6 Step 5: Thermal response of the structure . 12
6.6.1 Steel columns and beams . . 12
6.6.2 Other construction elements . .14
6.7 Step 6: Mechanical response of the structure .14
6.7.1 Steel columns .14
6.7.2 Steel beams . . 15
6.8 Step 7: Assessment against the fire safety objectives .16
6.9 Step 8: Documentation of the design for fire safety of structures .16
6.10 Factors and influences to be considered in the quantification process .17
6.10.1 Thermal properties .17
6.10.2 Mechanical strength of steel material .17
6.10.3 Uncertainty of material properties .17
7 Guidance on use of engineering methods . 17
7.1 Using calculation methods .17
7.2 Using experimental method .18
7.3 Using engineering judgement .18
Annex A (informative) Building and framing design . 19
Annex B (informative) Fire load and structural load .28
Annex C (informative) Fire temperatures .33
Annex D (informative) Maximum temperature of insulated steel elements .38
Annex E (informative) Critical temperatures of steel columns, girders and beams .44
Bibliography .51

iii
ISO/DTR 24679-4:2026(en)
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 through
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The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
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This document was prepared by Technical Committee ISO/TC 92, Fire safety, Subcommittee SC 4, Fire safety
engineering.
This second edition cancels and replaces the first edition (ISO/TR 24679-4:2017), which has been technically
revised.
The main changes are as follows:
— scope updated;
— Clause 3 terms and definitions updated;
— structure of document revised, with new figures included.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
ISO/DTR 24679-4:2026(en)
Introduction
This document is an example of the application 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.
The technical contents are based on the performance-based verification methods for fire resistance in
the Building Standards Law of Japan. This document is not intended to demonstrate full conformance to
a performance-based fire engineering regulation of Japan where extensive and detailed descriptions are
needed to obtain approval. Not all the details are presented in this document, but only a few examples of
verification are demonstrated following the procedure determined in ISO 24679-1.
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 considered. These severe fire
scenarios have been selected for fire resistance purposes.
Global structural behaviour is not explicitly considered, but implicitly assumed in the calculation formulae
by limiting excessive thermal deformation. In addition, since the building of this 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 does exist, but the beneficial effect was not taken into account
in the design calculations because of simplification. This approach is applicable to ductile structural frame
where restraint from surrounding cold structures would not cause excessive thermal stress to cause lateral
torsional buckling. By this approach, design is considered to be 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.
v
FINAL DRAFT Technical Report ISO/DTR 24679-4:2026(en)
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 for this building.
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 23932-1, Fire safety engineering — General principles — Part 1: General
ISO 24679-1, Fire safety engineering — Performance of structures in fire — Part 1: General
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13943, ISO 23932-1, ISO 24679-1
and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— IEC Electropedia: available at https:// www .electropedia .org/
— ISO Online browsing platform: available at https:// www .iso .org/ obp
3.1
critical temperature
temperature critical to cause any failure of an element by overall buckling, local buckling, excessive thermal
deformation or connection failure and so on
Note 1 to entry: It is expressed in °C .
3.2
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.

ISO/DTR 24679-4:2026(en)
3.3
equivalent fire duration
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.
3.4
heat penetration ratio
ratio of heat penetrated from adjacent rooms to the room in consideration
Note 1 to entry: It is dimensionless.
3.5
movable fire 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.
3.7
fire temperature rise coefficient
proportional coefficient to calculate fire room temperature as described by a function of 1/6-th power of
time
1/6
Note 1 to entry: It is expressed in K/min .
4 Symbols
For the purposes of this document, the following symbols are used.
A room floor area (m )
r
A surface area of interior lining material (m )
f
f heat penetration ratio
a
f nominal yield strength of steel at normal temperature (MPa)
y
G permanent load (kN/m )
K kinematic load (kN/m )
q movable fire load density (MJ/m )
l
q heat of combustion of interior lining materials per unit area (MJ/m )
f
Q design heat release of a room (MJ)
r
Q live structural load (kN/m )
t time (min)
t approved fire resistance time of a construction element (min)
A
ISO/DTR 24679-4:2026(en)
t fire duration (min)
f
t equivalent fire duration as replaced with ISO 834-1 standard fire (min)
eq
T limiting temperature for overall buckling of a column(°C)
B
T limiting temperature for bending failure of a beam (°C)
Bcr
T critical temperature of a steel element (°C)
cr
T limiting temperature for excessive deformation (°C)
DP
T limiting temperature for connection failure (°C)
CT
T limiting temperature for local buckling (°C)
LB
T maximum steel temperature under design fire action (°C)
s.max
T fire temperature in a room (K)
f
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 assumed to spread over a whole compartment. As a
result, a fully developed compartment fire is expected in each room of the building.
The structural elements are composed of insulated steel. To prevent failure of the structural elements and
connections, the thickness of insulation had been defined in order to limit their temperatures below their
critical temperatures.
In the prescriptive code applicable to this example, columns must be three hours fire resistance rated
construction on the first floor, two hours on the second to eleventh floors and one hour on the floors above.
By carrying out fire calculations, the thickness of fire insulation is rationalized. The method is based on
[1]
the performance-based verification methods for fire resistance in the Building Standards Law of Japan .
The standard procedure is shown in Figure 1. The fires are assumed at all the rooms respectively. The steel
temperatures are calculated for fully developed fires. Critical temperatures are determined by the minimum
of the limiting temperatures for overall buckling, local buckling, excessive deformation and connection
failure. The interactions between elements were not explicitly assessed but assumed as the building frame
is normally designed to be ductile to withstand the earthquake motion assumed the region. Finally, the
maximum steel temperature is compared with critical temperature element by element.

ISO/DTR 24679-4:2026(en)
Figure 1 — Verification method of fire resistance in Building Standards Law of Japan
6 Quantification of the performance of structures in fire
6.1 General
The steps of the design process considered in the conducted fire safety engineering study are detailed in 6.2
to 6.9.
ISO/DTR 24679-4:2026(en)
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 total area is 8 236 m and the building
height is 68,5 m. See Annex A for building drawings. According to the regulations, the building must be
[8]
constructed by fire-resistive constructions. In the prescriptive code, columns must be three hours fire
resistance 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, lifts and service shafts are enclosed by one-hour fire resistance rated walls.
The floor plan is shown in Figure 2. The office area is split into two rooms, XX01 and XX02, where the
th
symbol XX denotes floor number. For example, 1502 denotes room number 2 on the 15 floor. The two office
rooms are separated by an EI60 (60 minutes of integrity and insulation) wall as determined by ISO 834-8.
In addition, the office area is divided into two rooms by a non-fire resistance rated wall made of regular
gypsum board. In order to prevent fire spread between office area and corridor, the doors between office
rooms and corridor provide a fire resistance-rated E60 (60 minutes of integrity). The insulation criterion is
not required for fire doors in the Japanese regulation.
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 mineral wool cementitious mixture because this thickness is the minimum prescribed in
Japan for stickability and integrity. This thickness can be changed as long as stickability and integrity are
maintained.
NOTE This assumption is specific for this example.
— 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 resistance-rated for one hour in accordance to ISO 834-1 with exposure from the
outside (integrity and insulation). The windows are not fire-resistant. However, it was assumed that upward
fire spread was prevented by the use of appropriate materials conforming to the applicable regulations. The
floors are composite constructions of concrete slabs and steel deck with a 1 hour fire resistance rating (load
bearing, integrity and insulation). The beams are connected rigidly with deck floor slab of normal weight
concrete and profiled steel sheets with reinforcing bars.

ISO/DTR 24679-4:2026(en)
Key
1 office room (XX01)
2 office room (XX02)
3 corridor
4 vestibule
5 lift
6 stairs
7 machinery room
8 toilet
9 duct space
10 pipe space
fire resistance rated partition wall (EI60)
non-fire resistance rated partition wall
fire door (E60)
NOTE All the exterior walls are fire resistance rated (EI60).
Figure 2 — Typical floor plan
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
will be described later in 6.2.3.
The floor and beam plans are shown in Figure 3. The frame consists of simple 2 × 4 bays. The trial design of
principal construction elements is listed in Table 1. On the second floor, large cross-section steel elements
are used to withstand the large seismic actions. Under the fire condition, vertical loads are applied to second
floor columns as well as in cold condition. On the 15th floor, the elements are made of relatively thin and
small steel sections. The applied loads under the fire condition are small compared to the second-floor
elements. Further details are shown in Annex A.
[9]
Columns and girders are made of SN490 steel with a yield strength of 325 MPa. Secondary beams are
[10]
made of a SS400 steel with a yield strength of 235 MPa. Columns are made of box-sectioned tubes. Beams
and girders are made of H-sectioned elements. To prevent excessive temperature rise, the elements are

ISO/DTR 24679-4:2026(en)
insulated with a sprayed mineral wool cementitious material of a thickness over 25 mm. The bulk density is
at least 280 kg/m .
NOTE The mixing ratio of mineral wool and cement is 6:4.
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.
Figure 3 — Floor and beam plan
Table 1 — Summary of the 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
y
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
y
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
y 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.
ISO/DTR 24679-4:2026(en)
6.2.2 Fire load
Because the building is used as office space, significant amount of combustible is expected. See Annex B. The
design heat release of a room, Q , is calculated as the sum of the following components.
r
1) Movable fire load based on the use of the room, A q .
r l
The movable fire load density for an office area was selected to be 560 MJ/m following the applicable
[8]
building code. The results are shown in Table 2. For design purposes, the fire load density is taken as the
mean value plus 1,6 times of the standard deviation of existing surveys of fire load for office buildings. This
th
allows to estimate the upper 95 percentile value assuming log-normal distribution.
2) Fixed fire load based on the type of interior finish materials, ΣA q .
f f
The heat of combustion of interior finish materials is counted as fixed fire loads. The details of calculation
are shown in B.5.2. The results are summarized in Table 2.
3) Heat penetrated from adjacent rooms, Σf (A q + ΣA q ).
a r l f f
As the partition wall between the office areas XX01 and XX02 are not fire resistance rated, fire can spread
between the rooms after the failure of the partition wall. As a result, a part of combustion heat in an
adjacent room may affect the structural elements in the room of fire origin. The schematic process is shown
in Figure 4. In case of two rooms are separated by a non-fire rated wall, fire can grow to fully developed
stage in one of the rooms as shown in the left-side room in Figure 4(a). Then, the partition wall collapses
and fire spread to the right-side room as shown in Figure 4(b). Even after the combustibles in the right-side
room burnt out, fire heat from the left room penetrate into the right-side room as shown in Figure 4(c). As a
result, the structural elements in the right-side room is heated by the fire from both rooms. As a simplified
calculation rule, it was assumed that 40 % of the released heat is absorbed by the walls, ceiling and floor
of room of fire origin. The remaining 60 % of heat penetrates equally in all directions as determined in
reference [11]. To account for this mutual heating effect, it was estimated that 15 % of heat of combustion in
room XX01 penetrates to XX02. Similarly, 15 % of heat of combustion in room XX02 penetrates to XX01. The
calculated results are shown in Table 3.
(a) Fully developed fire in the left room(b) After the partition wall collapsed( c) After the left room is burnt out,
and fire spread to right room but right room is still burning
Figure 4 — Schematic process of heat penetration
Table 2 — Total fire load of rooms XX01 and XX02 (XX = 2nd and 15th floors)
Movable fire
Movable fire Fixed fire load Total fire load
load density Floor area
Floors Room No. Usage load A q ΣA q of room
2 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
2 to 15 202,
office 560 275 154 000 22 397 176 397
corridor pathway 32 125 4 000 13 722 17 722

ISO/DTR 24679-4:2026(en)
Table 3 — Design heat release of a room considering heat penetration from adjacent rooms
Heat penetration
Total fire load of adja- Penetrated heat Design fire
Floor Room Adjacent rooms ratio
cent rooms (MJ) (MJ) load (MJ)
f (-)
a
202,1502 176 397 0,15 26 460
83 108
Corridor 17 722 0,0 0,0
2 to 15
201,1501 56 649 0,15 8 497
184 894
Corridor 17 722 0,0 0,0
6.2.3 Mechanical actions
The mechanical actions in fire situation are determined in accordance with the applicable building code.
Permanent and movable vertical loads are considered, while no horizontal actions are considered such as
[8]
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 movable 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 to most buildings in Japan, the cross-sectional dimensions
are governed by seismic design. As a result, the load ratios of structural elements are fairly small during
normal use such as in the case of non-seismic and non-windy conditions.
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. As a result,
[8]
structural stability during the whole process of fire is necessary. To fulfil this objective, the functional
requirement is to have no failure of the principal building construction elements during the whole process
of fire, including the cooling phase. Consequently, the following performance criteria, in terms of structural
stability, are considered on an element-by-element basis.
— The temperature of the steel columns do not exceed the minimum of the critical temperatures for overall
buckling, local buckling, excessive deformation nor connections failures.
— The temperature of the steel beams and girders do not exceed the limit for bending failure nor connection
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. The fire
resistance rated constructions are selected so as to satisfy the followings:
— Fire compartment walls and floor constructions do not transmit excessive heat that can ignite
combustibles in opposite side (insulation criterion).
— Fire compartment walls and floor constructions and fire doors do not penetrate flame and/or hot gases
that causes fire spread beyond them (integrity criterion).

ISO/DTR 24679-4:2026(en)
6.4 Step 3: Trial design plan for fire safety of structures
The office building studied in this document has been designed for normal conditions, including seismic
and wind loads. The structural elements are composed of insulated steel. Achievement of the fire safety
objectives with regard to structural fire resistance relies on passive fire resistance coating only. The
beneficial effect of active measures such as sprinklers, fire brigade intervention, are not considered.
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 fully-developed compartment fire is assumed to grow
and decay until the burnout of the combustible materials in the room. 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 after non-fire resistant partition walls collapse.
All the unprotected openings are assumed to be broken and accounted for the ventilation calculations.
Protected openings, such as fire resistance 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 a
building room.
A nominal exterior fire is assumed to occur in the neighbour of the building. The severity and duration of
external fire may depend on urban conditions. Explicit prediction of exterior fire is out of the scope of this
design, but the standard fire as specified in ISO 834-1 for 60 min was assumed as a nominal fire exposure
based on the applicable building code. Localized fires were not taken into account.
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 5. The time-temperature curves
are calculated using Formula (3), an algebraic equation assuming a uniform temperature in a fire room:
16/
Tt Tt,(0t ) (3)
ff0
1/6
where the fire temperature rise coefficient α (K/min ) and fire duration t (min) are calculated in
f
accordance with the room geometry, window opening size and burning rate of the fuel. The fire duration, t ,
f
includes the effect of decay phase. Calculation details are provided in Annex C. The fire duration is converted
[5]
to equivalent ISO 834-1 fire duration based on the equivalency in time-heat flux area , i.e., total absorbed
energy. It is known that such a method has limitations, but in the absence of a harmonized approach in fire
testing and assessing structural behaviour using design fire curves, this method is deemed appropriate in
this document.
The results are shown in Figure 6. As the window areas are fairly large and fuel/air ratio is close to
stoichiometric, fire burns severely but the fire duration is fairly short. The fire temperatures are considerably
higher than the standard fire temperature as specified in ISO 834-1. The equivalent ISO 834-1 fire duration
was calculated to be 59,9 min in both fires for rooms XX01 and XX02 as shown in Table 4. Wall and floor
constructions were selected from 60 minutes fire resistance rated constructions in 6.8.

ISO/DTR 24679-4:2026(en)
Key
X time (min)
Y heat release rate (MW)
Figure 5 — Heat release rates of office rooms, XX01 and XX02
Key
X time (min)
Y fire room temperature (°C)
1 equivalent fire as specified in ISO 834-1 (59,9 min)
Figure 6 — Fire room temperatures of office rooms, XX01 and XX02
Table 4 — Calculation results of fire room temperatures and equivalent fire duration
Fire temperature rise Equivalent ISO 834-1 fire
Fire duration
Room coefficient α duration
t (min)
1/6 f
(K/min ) t (min)
eq
XX01 715 30,9 59,9
XX02 658 35,0 59,9
NOTE The equivalent fire duration of both rooms happened to coincide in this specific example. In general,
the value is expected to vary by rooms.

ISO/DTR 24679-4:2026(en)
6.6 Step 5: Thermal response of the structure
6.6.1 Steel columns and beams
The above thermal actions are applied to the corresponding structural elements to calculate their
temperatures as a function of time. Heat transfer analyses were carried out by algebraic calculation formulae
over time to calculate the temperature rise in the steel beams and columns. The calculation did not consider
the temperature distribution of the steel section because the thermal conductivity of steel is fairly large and
because the critical temperatures for mechanical failures, such as buckling, bending and so on, are defined
[6]
for average steel temperatures over cross section based on experiments and theory . The formula takes
into account the section factors of the steel members and the applied insulation. The calculation details are
described in Annex D. The calculation results of column temperatures are shown in Figure 7 and Table 5.
The circles in Figure 7 denote the maximum steel temperature at the design condition. As the thickness of
cross-sections of the steel elements in 15th floor is smaller, the maximum temperature is higher on the 15th
floor compared with those on the 2nd floor. The maximum beam temperatures are shown in Figure 8 and
Table 6. The circles in Figure 8 denote the maximum steel temperature at the design condition.
Key
X fire duration (min)
Y maximum steel temperature (°C)
maximum temperature at design condition
1501/C1,C2
1502/C1,C2
201/C1
201/C2
202/C1
202/C2
Figure 7 — Maximum temperature of steel columns as functions of fire duration

ISO/DTR 24679-4:2026(en)
Table 5 — Maximum temperature of steel columns, T
s,max
Room No. Column Maximum temperature (°C)
201 C1 141
C2 130
202 C1 141
C2 129
1501 C1 218
C2 218
1502 C1 218
C2 218
Key
X fire duration(min)
Y maximum steel temperature (°C)
maximum temperature at design condition
1501/G1
1501/G3
1502/G1
1502/G3
201/G1
201/G3
202/G1
202/G3
201,1501/b1
202,1502/b1
Figure 8 — Maximum temperature of steel girders and beams as functions of fire duration

ISO/DTR 24679-4:2026(en)
Table 6 — Maximum temperature of steel beams, T
s,max
Room No. Beam Maximum temperature (°C)
G1 298
201 G3 304
b1 499
G1 297
202 G3 304
b1 498
G1 308
1501 G3 333
b1 499
G1 308
1502 G3 332
b1 498
6.6.2 Other construction elements
As for floors and walls, no explicit calculations were made, but the equivalent fire duration was compared
with the approved fire resistance time of the elements. The results are shown in 6.8.
6.7 Step 6: Mechanical response of the structure
6.7.1 Steel columns
The critical temperature of the steel columns is calculated by considering overall buckling T , local buckling
B
[1]
T , excessive thermal deformation T and connection failure T . The critical temperature of a steel
LB DP CT
column is then determined as the minimum of the four limiting temperatures as shown in Formula (4):
TTmin ,,TT ,T (4)

cr BLBDPCT
The limiting temperatures for overall buckling were calculated based on tangent modulus theory. The
limiting temperatures for local buckling were calculated by experimental correlation with non-dimensional
width thickness ratio. The limiting temperatures for excessive deformation correspond with a thermal
elongation of the elements that cause 1/50 of story drift. The limiting temperatures for connection failure
were assumed conservatively by 550 °C for full-strength strength rigid connections.
Details of calculation are shown in Annex E. Calculated critical temperatures are shown in Table 7. For most
cases, critical temperature is determined by limiting temperature for connection failure. In some cases,
critical temperature is determined by local buckling. The limiting temperature for excessive deformation,
i.e., storey drift due to large thermal elongation was not applied to this study because the building is not
very large.
ISO/DTR 24679-4:2026(en)
Table 7 — Critical temperature of steel columns
Limiting temperatures (°C) for
Critical
Room Position Symbol temperature, T
Overall buckling, Local buckling, Connection failure, cr
( C)
T T T
B LB CT
X5-Y1 C1 694 692 550
X4-Y1 C1 687 684 550
1501 550
X5-Y2 C2 690 687 550
X4-Y2 C2 679 673 550
X4-Y1 C1 687 684 550
X3-Y1 C1 687 684 550
X2-Y1 C1 687 684 550
X1-Y1 C1 694 692 550
1502 550
X4-Y2 C2 679 673 550
X3-Y2 C2 679 673 550
X2-Y2 C2 679 673 550
X1-Y2 C2 690 687 550
X5-Y1 C1 656 646 550
X4-Y1 C1 607 592 550
201 550
X5-Y2 C2 634 621 550
X4-Y2 C2 557 542 542
X4-Y1 C1 607 592 550
X3-Y1 C1 607 592 550
X2-Y1 C1 607 592 550
X1-Y1 C1 656 646 550
202 550
X4-Y2 C2 557 542 542
X3-Y2 C2 557 542 542
X2-Y2 C2 557 542 542
X1-Y2 C2 634 621 550
6.7.2 Steel beams
The critical temperature of steel beams is calculated by considering bending strength T , excessive thermal
Bcr
[1]
deformation T and connection failure T . The critical temperature of a steel beam is determined as the
DP CT
minimum of the three limiting temperatures, as shown in Formula (5):
TTmin ,,TT (5)

cr BcrDPCT
Since the beams are rigidly connected to the floor slab, it is assumed that lateral-torsional buckling will not
occur. In addition, web buckling will not occur as the beams are designed for seismic resistance. The critical
temperature for connection failure was taken as 550 °C, which is conservative for full moment connections.
Details of calculation are shown in Annex E. The calculated critical temperatures are shown in Table 8. For
most cases, critical temperature is governed by the limiting temperature for connection failure. The critical
temperature for excessive storey drift due to large thermal elongation was not applied to this study since
the structural beams of the building are not extremel
...

TC /SC
Date:  2026
TC /SC /WG ISO/DTR 24679-4
ISO/TC 92/SC 4
Secretariat: AFNOR
Date: 2026-07-14
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 cas d'incendie — Partie 4: Exemple
d'un immeuble de bureaux en structure acier de quinze étages

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Published in Switzerland
Contents Page
Foreword . 5
Introduction . 6
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 2
5 Design strategy for fire safety of structures . 3
6 Quantification of the performance of structures in fire . 4
6.1 General . 4
6.2 Step 1: Scope of the project for fire safety of structures . 5
6.3 Step 2: Identify objectives, functional requirements and performance criteria for fire
safety of structures . 10
6.4 Step 3: Trial design plan for fire safety of structures . 11
6.5 Step 4: Design fire scenarios and design fires . 11
6.6 Step 5: Thermal response of the structure . 13
6.7 Step 6: Mechanical response of the structure . 16
6.8 Step 7: Assessment against the fire safety objectives . 19
6.9 Step 8: Documentation of the design for fire safety of structures . 19
6.10 Factors and influences to be considered in the quantification process . 20
7 Guidance on use of engineering methods . 20
7.1 Using calculation methods . 20
7.2 Using experimental method . 20
7.3 Using engineering judgement . 21
Annex A (informative) Building and framing design. 22
Annex B (informative) Fire load and structural load . 32
Annex C (informative) Fire temperatures . 38
Annex D (informative) Maximum temperature of insulated steel elements . 43
Annex E (informative) Critical temperatures of steel columns, girders and beams . 50
Bibliography . 58

Foreword
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bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documentsdocument 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 drawnISO draws attention to the possibility that some of the elementsimplementation of this
document may beinvolve the subjectuse of (a) patent(s). ISO takes no position concerning the evidence,
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Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT)), see
www.iso.org/iso/foreword.htmlthe following URL: .
This document was prepared by Technical Committee ISO/TC 92, Fire safety, Subcommittee SC 4, Fire
safety engineering.
This second edition cancels and replaces the first edition (ISO/TR 24679-4:2017), which has been
technically revised.
The main changes are as follows:
— scope updated;
— Clause 3 terms and definitions updated;
— structure of document revised, with new figures included.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

Introduction
This document is an example of the application 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.
The technical contents are based on the performance-based verification methods for fire resistance in the
Building Standards Law of Japan. This document is not intended to demonstrate full conformance to a
performance-based fire engineering regulation of Japan where extensive and detailed descriptions are
needed to obtain approval. Not all the details are presented in this document, but only a few examples of
verification are demonstrated following the procedure determined in ISO 24679-1.

ISO/DTR 24679-4:(en)
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 for this 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 considered. It should be noted
that theseThese severe fire scenarios have been selected for fire resistance purposes.
Global structural behaviour is not explicitly considered, but implicitly assumed in the calculation formulae by
limiting excessive thermal deformation. In addition, since the building of this 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 does exist, but the beneficial effect was not taken into account in the
design calculations because of simplification. This approach is applicable to ductile structural frame where
restraint from surrounding cold structures would not cause excessive thermal stress to cause lateral torsional
buckling. By this approach, design is considered to be 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.

vii
ISO/DTR 24679-4:(en)
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 for this building.
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 834-1:1999/Amd.1:2012, Fire-resistance tests — Elements of building construction — Part 1: General
requirements
ISO 834-5, Fire-resistance tests —Elements of building construction — Part 5: Specific requirements for
loadbearing horizontal separating elements
ISO 834-8, Fire-resistance tests — Elements of building construction — Part 8: Specific requirements for non-
loadbearing vertical separating elements
ISO 13943, Fire safety — Vocabulary
ISO 23932-1, Fire safety engineering — General principles — Part 1: General
ISO 24679-1:2019, Fire safety engineering — Performance of structures in fire — Part 1: General
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13943, ISO 23932 and-1, ISO 24679-
1 and the following apply.
ISO and IEC maintain terminologicalterminology databases for use in standardization at the following
addresses:
— — IEC Electropedia: available at https://www.electropedia.org/
— — ISO Online browsing platform: available at https://www.iso.org/obp
3.1 3.1
critical temperature
temperature critical to cause any failure of an element by overall buckling, local buckling, excessive thermal
deformation or connection failure and so on
Note 1 to entry: It is expressed in °C .
ISO/DTR 24679-4:(en)
3.2 3.2
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.
3.3 3.3
equivalent fire duration
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.
3.4
fixed fire 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 3.5
heat penetration ratio
ratio of heat penetrated from adjacent rooms to the room in consideration
Note 1 to entry: It is dimensionless.
3.5 3.6
movable fire 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 3.7
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.
3.7 3.8
fire temperature rise coefficient
a proportional coefficient to calculate fire room temperature as described by a function of 1/6-th. power of
time.
1/6
Note 1 to entry: It is expressed in K/min .
4 Symbols
For the purposes of this document, the following symbols are used.
A room floor area (m )
r
A surface area of interior lining material (m )
f
ISO/DTR 24679-4:(en)
f heat penetration ratio
a
f nominal yield strength of steel at normal temperature (MPa)
y
G permanent load (kN/m )
K kinematic load (kN/m )
q movable fire load density (MJ/m )
l
q heat of combustion of interior lining materials per unit area (MJ/m )
f
Q design heat release of a room (MJ)
r
Q live structural load (kN/m )
t time (min)
t approved fire resistance time of a construction element (min)
A
t fire duration (min)
f
t equivalent fire duration as replaced with ISO 834-1 standard fire (min)
eq
T limiting temperature for overall buckling of a column(°C)
B
T limiting temperature for bending failure of a beam (°C)
Bcr
T critical temperature of a steel element (°C)
cr
T limiting temperature for excessive deformation (°C)
DP
T limiting temperature for connection failure (°C)
CT
T limiting temperature for local buckling (°C)
LB
T maximum steel temperature under design fire action (°C)
s.max
T fire temperature in a room (K)
f
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 assumed to spread over a whole compartment. As a result, a fully
developed compartment fire is expected in each room of the building.
The structural elements are composed of insulated steel. To prevent failure of the structural elements and
connections, the thickness of insulation had been defined in order to limit their temperatures below their
critical temperatures.
In the prescriptive code applicable to this example, columns must be three hours fire resistance rated
construction on the first floor, two hours on the second to eleventh floors and one hour on the floors above.
By carrying out fire calculations, the thickness of fire insulation is rationalized. The method is based on the
[1] [1]
performance-based verification methods for fire resistance in the Building Standards Law of Japan . The
standard procedure is shown in Figure 1Figure . The fires are assumed at all the rooms respectively. The steel
temperatures are calculated for fully developed fires. Critical temperatures are determined by the minimum
of the limiting temperatures for overall buckling, local buckling, excessive deformation and connection failure.
The interactions between elements were not explicitly assessed but assumed as the building frame is normally
designed to be ductile to withstand the earthquake motion assumed the region. Finally, the maximum steel
temperature is compared with critical temperature element by element.
ISO/DTR 24679-4:(en)
Figure — 1 — Verification method of fire resistance in Building Standards Law of Japan
6 Quantification of the performance of structures in fire
6.1 General
The steps of the design process considered in the conducted fire safety engineering study are detailed in 6.26.2
to 6.96.9.
ISO/DTR 24679-4:(en)
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 total area is 8 236 m and the building
height is 68,5 m. See Annex AAnnex A for building drawings. According to the regulations, the building must
[8] [8]
be constructed by fire-resistive constructions. In the prescriptive code, , columns must be three hours fire
resistance 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, lifts and service shafts are enclosed by one-hour fire resistance rated walls.
The floor plan is shown in Figure 2Figure . The office area is split into two rooms, XX01 and XX02, where the
th
symbol XX denotes floor number. For example, 1502 denotes room number 2 on the 15 floor. The two office
rooms are separated by an EI60 (60 minutes of integrity and insulation) wall as determined by ISO 834-8. In
addition, the office area is divided into two rooms by a non-fire resistance rated wall made of regular gypsum
board. In order to prevent fire spread between office area and corridor, the doors between office rooms and
corridor provide a fire resistance-rated E60 (60 minutes of integrity). The insulation criterion is not required
for fire doors in the Japanese regulation.
The building structure is a rigid moment-resistant steel frame. Specification of members such as columns,
beams and floor slabs are listed in Annex AAnnex A. The columns, beams and girders are protected against
fire by a 25 mm sprayed mineral wool cementitious mixture because this thickness is the minimum prescribed
in Japan for stickability and integrity. This thickness can be changed as long as stickability and integrity are
maintained.
Note: NOTE This assumption is specific for this example.
— — 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 resistance-rated for one hour in accordance to ISO 834-1 with exposure from the
outside (integrity and insulation). The windows are not fire-resistant. However, it was assumed that upward
fire spread was prevented by the use of appropriate materials conforming to the applicable regulations. The
floors are composite constructions of concrete slabs and steel deck with a 1 hour fire resistance rating (load
bearing, integrity and insulation). The beams are connected rigidly with deck floor slab of normal weight
concrete and profiled steel sheets with reinforcing bars.
ISO/DTR 24679-4:(en)
ISO/DTR 24679-4:(en)
Key
1 office room (XX01)
2 office room (XX02)
3 corridor
4 vestibule
5 lift
6 stairs
7 machinery room
8 toilet
9 duct space
10 pipe space
fire resistance rated partition wall (EI60)

non-fire resistance rated partition wall

fire door (E60)
NOTE All the exterior walls are fire resistance rated (EI60).
Figure 2 — Typical floor plan
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
will be described later in 6.2.3Step 6.2.3.
The floor and beam plans are shown in Figure 3Figure . The frame consists of simple 2 × 4 bays. The trial
design of principal construction elements is listed in Table 1Table 1. On the second floor, large cross-section
ISO/DTR 24679-4:(en)
steel elements are used to withstand the large seismic actions. Under the fire condition, vertical loads are
applied to second floor columns as well as in cold condition. On the 15th floor, the elements are made of
relatively thin and small steel sections. The applied loads under the fire condition are small compared to the
second-floor elements. Further details are shown in Annex AAnnex A.
[9][9]
Columns and girders are made of SN490 steel with a yield strength of 325 MPa. Secondary beams are made
[10][10]
of a SS400 steel with a yield strength of 235 MPa. Columns are made of box-sectioned tubes. Beams and
girders are made of H-sectioned elements. To prevent excessive temperature rise, the elements are insulated
with a sprayed mineral wool cementitious material of a thickness over 25 mm. The bulk density is at least
280 kg/m .
NoteNOTE The mixing ratio of mineral wool and cement is 6:4.
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.
Figure 3 — Floor and beam plan
Table 1 — Summary of the 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
y
C3 box-500 × 36 box-500 × 19
G1 H-900 × 350 × 16 × 25 H-700 × 300 × 14 × 25
ISO/DTR 24679-4:(en)
Floor levels 2nd floor 15th floor
G2 H-900 × 300 × 16 × 25 H-700 × 250 × 14 × 22
Primary beam (three sides exposed)
SN 490, fy = 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, fy = 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.
6.2.2 Fire load
Because the building is used as office space, significant amount of combustible is expected. See Annex B. The
design heat release of a room, Q , is calculated as the sum of the following components.
r
1) 1) Movable fire load based on the use of the room, A q A q .
r l r l
The movable fire load density for an office area was selected to be 560 MJ/m following the applicable building
[8] [8]
code. . The results are shown in Table 2Table 2. For design purposes, the fire load density is taken as the
mean value plus 1.,6 times of the standard deviation of existing surveys of fire load for office buildings. This
th
allows to estimate the upper 95 percentile value assuming log-normal distribution.
2) 2) Fixed fire load based on the type of interior finish materials, ΣA q ΣA q .
f f f f
The heat of combustion of interior finish materials is counted as fixed fire loads. The details of calculation are
shown in B.5.2B.5.2. The results are summarized in Table 2Table 2.
3) 3) Heat penetrated from adjacent rooms, Σf (A q + ΣA q A q + ΣA q ).
a r l f f r l f f
As the partition wall between the office areas XX01 and XX02 are not fire resistance rated, fire maycan spread
between the rooms after the failure of the partition wall. As a result, a part of combustion heat in an adjacent
room may affect the structural elements in the room of fire origin. The schematic process is shown in
Figure 4Figure . In case of two rooms are separated by a non-fire rated wall, fire maycan grow to fully
developed stage in one of the rooms as shown in the left-side room in Figure 4(a)Figure (a). Then, the partition
wall collapses and fire spread to the right-side room as shown in Figure 4(b)Figure (b). Even after the
combustibles in the right-side room burnt out, fire heat from the left room penetrate into the right-side room
as shown in Figure 4(c)Figure (c). As a result, the structural elements in the right-side room is heated by the
fire from both rooms. As a simplified calculation rule, it was assumed that 40 % of the released heat is
absorbed by the walls, ceiling and floor of room of fire origin. The remaining 60 % of heat penetrates equally
[11]
in all directions as determined in reference[11] . To account for this mutual heating effect, it was estimated
that 15 % of heat of combustion in room XX01 penetrates to XX02. Similarly, 15 % of heat of combustion in
room XX02 penetrates to XX01. The calculated results are shown in Table 3Table 3.

(a) Fully developed fire in the left room (b) After the partition wall collapsed and fire( c) After the left room is burnt out, but right
spread to right room room is still burning
Figure 4 — Schematic process of heat penetration
ISO/DTR 24679-4:(en)
Table 2 — Total fire load of rooms XX01 and XX02 (XX = 2nd and 15th floors)
Movable Total fire
Movable fire Fixed fire
Room fire load Floor area load of
Floors Usage load A q load ΣA q
r l f f
No. density q A (m ) room
l r
(MJ) (MJ)
(MJ/m ) (MJ)
201,
office 560 87,5 49 000 7 649 56 649
2 to 15 202,
office 560 275 154 000 22 397 176 397
corridor pathway 32 125 4 000 13 722 17 722
Table 3 — Design heat release of a room considering heat penetration from adjacent rooms
Heat
Total fire load of
Adjacent penetration Penetrated Design fire
Floor Room adjacent rooms
rooms ratio heat (MJ) load (MJ)
(MJ)
f (-)
a
202,1502 176 397 0,15 26 460
83 108
Corridor 17 722 0,0 0,0
2 to 15
201,1501 56 649 0,15 8 497
184 894
Corridor 17 722 0,0 0,0
6.2.3 Mechanical actions
The mechanical actions in fire situation are determined in accordance with the applicable building code.
Permanent and movable vertical loads are considered, while no horizontal actions are considered such as
8[8]
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 movable 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 to most buildings in Japan, the cross-sectional dimensions
are governed by seismic design. As a result, the load ratios of structural elements are fairly small during
normal use such as in the case of non-seismic and non-windy conditions.
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. As a result, structural
[8] [8]
stability during the whole process of fire is necessary. . To fulfil this objective, the functional requirement
is to have no failure of the principal building construction elements during the whole process of fire, including
ISO/DTR 24679-4:(en)
the cooling phase. Consequently, the following performance criteria, in terms of structural stability, are
considered on an element-by-element basis.
— — The temperature of the steel columns shalldo not exceed the minimum of the critical temperatures for
overall buckling, local buckling, excessive deformation nor connections failures.
— — The temperature of the steel beams and girders shalldo not exceed the limit for bending failure nor
connection failure. Shear failure does not precede the bending failure.
— — Floor construction shalldoes not exceed the limit for mechanical failures, typically bending failure.
In addition, the following performance criteria are considered in terms of fire containment. The fire resistance
rated constructions are selected so as to satisfy the followings:
— — Fire compartment walls and floor constructions shalldo not transmit excessive heat that maycan ignite
combustibles in opposite side (insulation criterion).
— — Fire compartment walls and floor constructions and fire doors shalldo 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
The office building studied in this document has been designed for normal conditions, including seismic and
wind loads. The structural elements are composed of insulated steel. Achievement of the fire safety objectives
with regard to structural fire resistance relies on passive fire resistance coating only. The beneficial effect of
active measures such as sprinklers, fire brigade intervention, are not considered.
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 fully-developed compartment fire is assumed to grow
and decay until the burnout of the combustible materials in the room. 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 after non-fire resistant partition walls collapse.
All the unprotected openings are assumed to be broken and accounted for the ventilation calculations.
Protected openings, such as fire resistance 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 a
building room.
A nominal exterior fire is assumed to occur in the neighbour of the building. The severity and duration of
external fire may depend on urban conditions. Explicit prediction of exterior fire is out of the scope of this
design, but the standard fire as specified in ISO 834-1 for 60 min was assumed as a nominal fire exposure
based on the applicable building code. Localized fires were not taken into account.
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 5Figure . The time-temperature curves are
calculated using Formula (3)Formula (3),, an algebraic equation assuming a uniform temperature in a fire
room:
ISO/DTR 24679-4:(en)
(3)
1⁄6
𝑇𝑇 =𝛼𝛼𝑡𝑡 +𝑇𝑇 ,   (0≤𝑡𝑡≤𝑡𝑡 )
𝑓𝑓 0 𝑓𝑓
(3)
1/6
where the fire temperature rise coefficient α (K/min ) and fire duration t (min) are calculated in accordance
f
with the room geometry, window opening size and burning rate of the fuel. The fire duration, t , includes the
f
effect of decay phase. Calculation details are provided in Annex CAnnex C. The fire duration is converted to
[5][5]
equivalent ISO 834-1 fire duration based on the equivalency in time-heat flux area ,, i.e., total absorbed
energy. It is known that such a method has limitations, but in the absence of a harmonized approach in fire
testing and assessing structural behaviour using design fire curves, this method is deemed appropriate in this
document.
The results are shown in Figure 6Figure . As the window areas are fairly large and fuel/air ratio is close to
stoichiometric, fire burns severely but the fire duration is fairly short. The fire temperatures are considerably
higher than the standard fire temperature as specified in ISO 834-1. The equivalent ISO 834-1 fire duration
was calculated to be 59,9 min in both fires for rooms XX01 and XX02 as shown in Table 4Table 4. Wall and
floor constructions were selected from 60 minutes fire resistance rated constructions in 6.86.8.

Key
X time (min)
Y heat release rate (MW)
Figure 5 — Heat release rates of office rooms, XX01 and XX02
ISO/DTR 24679-4:(en)
Key
X time (min)
Y fire room temperature (°C)
1 equivalent fire as specified in ISO 834-1 (59,9 min)
Figure 6 — Fire room temperatures of office rooms, XX01 and XX02
Table 4 — Calculation results of fire room temperatures and equivalent fire duration
Fire temperature rise Equivalent ISO 834-1
Fire duration
Room coefficient α fire duration
t (min)
f
1/6
(K/min ) t (min)
eq
XX01 715 30,9 59,9
XX02 658 35,0 59,9
NOTE The equivalent fire duration of both rooms happened to coincide in this specific example. In general,
the value is expected to vary by rooms.
6.6 Step 5: Thermal response of the structure
6.6.1 Steel columns and beams
The above thermal actions are applied to the corresponding structural elements to calculate their
temperatures as a function of time. Heat transfer analyses were carried out by algebraic calculation formulae
over time to calculate the temperature rise in the steel beams and columns. The calculation did not consider
the temperature distribution of the steel section because the thermal conductivity of steel is fairly large and
because the critical temperatures for mechanical failures, such as buckling, bending and so on, are defined for
[6] [6]
average steel temperatures over cross section based on experiments and theory . The formula takes into
account the section factors of the steel members and the applied insulation. The calculation details are
described in Annex DAnnex D. The calculation results of column temperatures are shown in
Figure 7Figure and Table 5Table 5. The circles in Figure 7Figure denote the maximum steel temperature at
the design condition. As the thickness of cross-sections of the steel elements in 15th floor is smaller, the
maximum temperature is higher on the 15th floor compared with those on the 2nd floor. The maximum beam
temperatures are shown in Figure 8Figure and Table 6Table 6. The circles in Figure 8Figure denote the
maximum steel temperature at the design condition.
ISO/DTR 24679-4:(en)
Key
X fire duration (min)
Y maximum steel temperature (°C)
maximum temperature at design condition

1501/C1,C2
1502/C1,C2
201/C1
201/C2
202/C1
202/C2
Figure 7 — Maximum temperature of steel columns as functions of fire duration
Table 5 — Maximum temperature of steel columns, T
s,max
Room No. Column Maximum temperature (°C)
201 C1 141
C2 130
202 C1 141
C2 129
1501 C1 218
ISO/DTR 24679-4:(en)
Room No. Column Maximum temperature (°C)
C2 218
1502 C1 218
C2 218
Key
X fire duration(min)
Y maximum steel temperature (°C)
maximum temperature at design condition

1501/G1
1501/G3
1502/G1
1502/G3
201/G1
ISO/DTR 24679-4:(en)
201/G3
202/G1
202/G3
201,1501/b1
202,1502/b1
Figure 8 — Maximum temperature of steel girders and beams as functions of fire duration
Table 6 — Maximum temperature of steel beams, T max
s,
Room No. Beam Maximum temperature (°C)
G1 298
201 G3 304
b1 499
G1 297
202 G3 304
b1 498
G1 308
1501 G3 333
b1 499
G1 308
1502 G3 332
b1 498
6.6.2 Other construction elements
As for floors and walls, no explicit calculations were made, but the equivalent fire duration was compared with
the approved fire resistance time of the elements. The results are shown in 6.86.8.
6.7 Step 6: Mechanical response of the structure
6.7.1 Steel columns
The critical temperature of the steel columns is calculated by considering overall buckling T , local buckling
B
[1] 1
T , excessive thermal deformation T and connection failure T . . The critical temperature of a steel
LB DP CT
column is then determined as the minimum of the four limiting temperatures as shown in
Formula (4)Formula (4)::
ISO/DTR 24679-4:(en)
{ }
𝑇𝑇 =𝑚𝑚𝑚𝑚𝑚𝑚𝑇𝑇 ,𝑇𝑇 ,𝑇𝑇 ,𝑇𝑇 𝑚𝑚𝑚𝑚𝑚𝑚{𝑇𝑇 ,𝑇𝑇 ,𝑇𝑇 ,𝑇𝑇 }
𝑐𝑐𝑐𝑐 𝐵𝐵 𝐿𝐿𝐵𝐵 𝐷𝐷𝐷𝐷 𝐶𝐶𝑇𝑇 𝐵𝐵 𝐿𝐿𝐵𝐵 𝐷𝐷𝐷𝐷 𝐶𝐶𝑇𝑇
(4)
The limiting temperatures for overall buckling were calculated based on tangent modulus theory. The limiting
temperatures for local buckling were calculated by experimental correlation with non-dimensional width
thickness ratio. The limiting temperatures for excessive deformation correspond with a thermal elongation of
the elements that cause 1/50 of story drift. The limiting temperatures for connection failure were assumed
conservatively by 550 °C for full-strength strength rigid connections.
Details of calculation are shown in Annex EAnnex E. Calculated critical temperatures are shown in
Table 7Table 7. For most cases, critical temperature is determined by limiting temperature for connection
failure. In some cases, critical temperature is determined by local buckling. The limiting temperature for
excessive deformation, i.e., storey drift due to large thermal elongation was not applied to this study because
the building is not very large.
Table 7 — Critical temperature of steel columns
Limiting temperatures (°C) for
Critical
Connection
Room Position Symbol temperature,
Overall buckling, Local buckling,
failure,
T ( C)
cr
TB TLB
TCT
X5-Y1 C1 694 692 550
X4-Y1 C1 687 684 550
1501 550
X5-Y2 C2 690 687 550
X4-Y2 C2 679 673 550
X4-Y1 C1 687 684 550
X3-Y1 C1 687 684 550
X2-Y1 C1 687 684 550
X1-Y1 C1 694 692 550
1502 550
X4-Y2 C2 679 673 550
X3-Y2 C2 679 673 550
X2-Y2 C2 679 673 550
X1-Y2 C2 690 687 550
X5-Y1 C1 656 646 550
X4-Y1 C1 607 592 550
201 550
X5-Y2 C2 634 621 550
X4-Y2 C2 557 542 542
X4-Y1 C1 607 592 550
X3-Y1 C1 607 592 550
X2-Y1 C1 607 592 550
202 X1-Y1 C1 656 646 550 550
X4-Y2 C2 557 542 542
X3-Y2 C2 557 542 542
X2-Y2 C2 557 542 542
ISO/DTR 24679-4:(en)
Limiting temperatures (°C) for
Critical
Connection
Room Position Symbol temperature,
Overall buckling, Local buckling,
failure,
T ( C)
cr
T T
B LB
T
CT
X1-Y2 C2 634 621 550
6.7.2 Steel beams
The critical temperature of steel beams is calculated by considering bending strength T , excessive thermal
Bcr
[1] 1
deformation T and connection failure T . . The critical temperature of a steel beam is determined as the
DP CT
minimum of the three limiting temperatures, as shown in Formula (5)Formula (5)::
(5)
𝑇𝑇 =𝑚𝑚𝑚𝑚𝑚𝑚{𝑇𝑇 ,𝑇𝑇 ,𝑇𝑇 }
𝑐𝑐𝑐𝑐 𝐵𝐵𝑐𝑐𝑐𝑐 𝐷𝐷𝐷𝐷 𝐶𝐶𝑇𝑇
(5)
Since the beams are rigidly connected to the floor slab, it is assumed that lateral-torsional buckling will not
occur. In addition, web buckling will not occur as the beams are designed for seismic resistance. The critical
o
temperature for connection failure was taken as 550 C550 °C, which is conservative for full moment
connections.
Details of calculation are shown in Annex EAnnex E. The calculated critical temperatures are shown
in Table 8Table 8. For most cases, critical temperature is governed by the limiting temperature for connection
failure. The critical temperature for excessive storey drift due to large thermal elongation was not applied to
this study since the structural beams of the building are not extremely long.
Table 8 — Critical temperature of steel beam
Limiting temperatures (°C) for
Critical
Room Position Symbol temperature,
Connection failure,
Bending failure, TBcr (°C)
(°C)
Tcr
TCT (°C)
X4 G1 623 550
X5 G1 633 550
1501 Y1 G3 690 550 550
Y2 G3 693 550
— b1 587 550
X1 G1 612 550
X2 G1 623 550
X3 G1 623 550
1502 550
Y1 G3 690 550
Y2 G3 693 550
b1 b1 587 550
X4 G1 663 550
201 X5 G1 662 550 550
Y1 G3 694 550
ISO/DTR 24679-4:(en)
Limiting temperatures (°C) for
Critical
Room Position Symbol temperature,
Connection failure,
Bending failure, T (°C)
Bcr
T (°C)
cr
T (°C)
CT
Y2 G3 697 550
— b1 616 550
X1 G1 654 550
X2 G1 663 550
X3 G1 663 550
202 550
Y1 G3 694 550
Y2 G3 697 550
b1 b1 616 550
6.8 Step 7: Assessment against the fire safety objectives
For the steel elements, the maximum temperatures T in Tables 5Tables 5 and 66 are checked if they are
s,max
lower than the critical temperatures T in Tables 7Tables 7 and 88. In this document, all the elements meet
cr
the criteria. See Formula (6)Formula (6)::
T ≤ T (6)
s,max cr
In case of floors and walls, approved fire resistance time, t , is compared with equivalent fire duration, t , as
A eq
shown in Formula (7)Formula (7)::
t ≤ t (7)
eq A
In this example, the equivalent fire duration is 60 min both in XX01 and XX02 rooms. Thus, the wall and floors
are one-hour fire resistance -rated as per ISO 834-5 or ISO 834-8, or longer.
6.9 Step 8: Documentation of the design for fire safety of structures
This document is prepared following th
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