ISO/TR 24679-5:2023
(Main)Fire safety engineering — Performance of structures in fire — Part 5: Example of a timber building in Canada
Fire safety engineering — Performance of structures in fire — Part 5: Example of a timber building in Canada
This document provides a fire engineering application relative to the fire resistance assessment of a multi-storey timber building according to the methodology given in ISO 24679-1. In an attempt to facilitate the understanding of the design process presented herein, this document follows the same step-by-step procedure as that given in ISO 24679-1. The fire safety engineering approach is applied to a multi-storey timber building with respect to fire resistance and considers specific design fire scenarios, which impact the fire resistance of structural members. A component-level (member analysis) approach to fire performance analysis is adopted in this worked example. Such an approach generally provides a more conservative design than a system-level (global structural) analysis or an analysis of parts of the structure where interaction between components can be assessed. An advantage of the component-level approach is that calculations can be done with the use of simple analytical models or spreadsheets. Advanced modelling using computational fluid dynamics is presented to replicate an actual office cubicle fire scenario and for assessing timber contribution to fire growth, intensity and duration, if any. The thermo-structural behaviour of the timber elements is assessed through advanced modelling using the finite element method. The fire design scenarios chosen in this document are only used for the evaluation of the structural fire resistance. They are not applicable for assessing, for example, smoke production, tenability conditions or other life safety conditions.
Ingénierie de la sécurité incendie — Performances des structures en situation d’incendie — Partie 5: Exemple d’un immeuble en bois au Canada
Le présent document développe une application de l’ingénierie de la sécurité incendie relative à l’évaluation de la résistance au feu d’un immeuble en bois de plusieurs étages conformément à la méthodologie donnée dans l’ISO 24679-1. Dans la volonté de faciliter la compréhension du processus de dimensionnement présenté ci-après, le présent document suit la même procédure par étapes que celle donnée dans l’ISO 24679-1. L’approche d’ingénierie de la sécurité incendie repose sur la résistance au feu d’un immeuble en bois de plusieurs étages et considère des scénarios d’incendie de dimensionnement spécifiques, lesquels impactent la résistance au feu des éléments structuraux. Dans l’exemple développé ci-après, il a été adopté, pour l’analyse de la tenue au feu, une approche au niveau du composant (analyse de l’élément). Une telle approche fournit généralement une conception plus prudente qu’une analyse (structurale globale) au niveau du système ou qu’une analyse des parties de la structure notamment lorsqu’une interaction entre les éléments peut être évaluée. L’approche au niveau du composant présente un avantage qui est que les calculs peuvent être effectués avec l’utilisation de modèles analytiques simples ou de tableurs. Une modélisation avancée utilisant la dynamique des fluides à l’aide d’un modèle de champs est présentée pour reproduire un scénario d’incendie réel d’une cabine de bureau et pour évaluer la contribution du bois dans la croissance, l’intensité et la durée du feu, le cas échéant. Le comportement thermo-structural des éléments en bois est évalué au moyen d’une modélisation avancée utilisant la méthode des éléments finis. Les scénarios d’incendie de dimensionnement choisis dans le présent document sont uniquement utilisés pour l’évaluation de la résistance au feu des éléments de la structure de l’immeuble. Ils ne s’appliquent pas à l’évaluation, par exemple, de la production de fumée, des conditions de tenabilité ou d’autres conditions relatives à la sécurité des personnes.
General Information
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Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 24679-5
First edition
2023-07
Fire safety engineering —
Performance of structures in fire —
Part 5:
Example of a timber building in
Canada
Reference number
© ISO 2023
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Design strategy for fire safety of structures . 2
4.1 General design process for fire safety of structures . 2
4.2 Practical design process for fire safety of structures . 2
5 Quantification of the performance of structures in fire . 2
5.1 Step 1: Scope of the project for fire safety of structures. 2
5.1.1 Built-environment characteristics . 2
5.1.2 Fuel loads . 6
5.1.3 Mechanical actions . 8
5.2 Step 2: Identifying objectives, functional requirements and performance criteria
for fire safety of structures . 8
5.2.1 Objectives and functional requirements for fire safety of structures . 8
5.2.2 Performance criteria for fire safety of structures . 9
5.3 Step 3: Trial design plan for fire safety of structures . 10
5.4 Step 4: Design fire scenarios and design fires (thermal actions) . 10
5.4.1 General . 10
5.4.2 Design fire scenarios . 11
5.4.3 Design fires (thermal actions) .12
5.5 Step 5: Thermal response of the structure .34
5.5.1 Charring of timber .34
5.5.2 Description of the thermal properties . 37
5.5.3 Scenario 3 .38
5.5.4 Temperature beyond the char layer .50
5.6 Step 6: Mechanical response of the structure . 51
5.6.1 Description of the mechanical properties . 52
5.6.2 Scenario 3 – Beam B1 . 52
5.6.3 Scenario 3 – Column C2 .56
5.7 Step 7: Assessment against the fire safety objectives .60
5.7.1 Beam B1 .60
5.7.2 Column C2 .60
5.8 Documentation of the design for fire safety of structures . 61
5.9 Factors and influences to be considered in the quantification process . 61
5.9.1 Material properties . 61
5.9.2 Effect of continuity and restraint (interaction between elements and
materials) . 62
5.9.3 Use of test results . 62
5.9.4 Fire spread routes . 62
6 Guidance on use of engineering methods .62
6.1 Using calculation methods . 62
6.2 Using experimental methods . 62
6.3 Using engineering judgment . 62
Bibliography .64
iii
Foreword
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This document was prepared by Technical Committee ISO/TC 92, Fire safety, Subcommittee SC 4, Fire
safety engineering.
A list of all parts in the ISO 24679 series can be found on the ISO website.
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
Introduction
This document provides an example of the application of ISO 24679-1. The procedure described in
this document is intended to follow the principles outlined in ISO 24679-1. It therefore preserves the
numbering of subclauses in ISO 24679-1, omitting numbered subclauses for which there is no text or
information relevant to this example.
The example provided in this document is intended to illustrate the implementation of the steps of
fire resistance assessment, as defined in ISO 24679-1, and to demonstrate how ISO 24679-1 can be
applied to different building regulatory systems. It is not intended to demonstrate full conformance of
a performance-based fire engineering design seeking approval. Therefore, only a limited number of fire
design scenarios and structural assessments are presented.
v
TECHNICAL REPORT ISO/TR 24679-5:2023(E)
Fire safety engineering — Performance of structures in
fire —
Part 5:
Example of a timber building in Canada
1 Scope
This document provides a fire engineering application relative to the fire resistance assessment of
a multi-storey timber building according to the methodology given in ISO 24679-1. In an attempt to
facilitate the understanding of the design process presented herein, this document follows the same
step-by-step procedure as that given in ISO 24679-1.
The fire safety engineering approach is applied to a multi-storey timber building with respect to fire
resistance and considers specific design fire scenarios, which impact the fire resistance of structural
members.
A component-level (member analysis) approach to fire performance analysis is adopted in this worked
example. Such an approach generally provides a more conservative design than a system-level (global
structural) analysis or an analysis of parts of the structure where interaction between components can
be assessed. An advantage of the component-level approach is that calculations can be done with the use
of simple analytical models or spreadsheets. Advanced modelling using computational fluid dynamics
is presented to replicate an actual office cubicle fire scenario and for assessing timber contribution to
fire growth, intensity and duration, if any. The thermo-structural behaviour of the timber elements is
assessed through advanced modelling using the finite element method.
The fire design scenarios chosen in this document are only used for the evaluation of the structural fire
resistance. They are not applicable for assessing, for example, smoke production, tenability conditions
or other life safety conditions.
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 and
ISO 24679-1 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Design strategy for fire safety of structures
4.1 General design process for fire safety of structures
The built environment used in this example is a medium-rise office building. To accommodate tenant
office functions, the building is separated into multiple compartments by floors and walls. Given that an
office space typically consists of several office workstation or cubicles, it is likely that a fire will spread
to neighbouring elements and eventually across the entire floor surface. As such, a fully-developed
compartment fire is expected in each office suite of the building.
The structural elements are of glue-laminated timber beams and columns, where portions of the
primary structural timber elements are left exposed for aesthetic purposes. The secondary structural
elements are protected against fire using fire-resistance rated gypsum boards.
The fire development was studied using computational fluid dynamics (CFD) modelling, with specific
considerations for capturing the potential fuel contribution from the structural timber elements.
Time-temperature curves were produced, as well as relevant key events during the fire development
(growth, flashover conditions, consumed fuel load, etc.).
Simplified and advanced models have been used to define the thermal actions applied to the timber
elements. The thermomechanical behaviour of the main structure of the office building, based on
simplified and advanced methods, is carried out as a function of the actual thermal actions defined
previously.
4.2 Practical design process for fire safety of structures
Refer to ISO 24679-1 for more information about the various steps and parameters to be considered
when assessing the behaviour of structures subjected to fire exposure.
5 Quantification of the performance of structures in fire
5.1 Step 1: Scope of the project for fire safety of structures
5.1.1 Built-environment characteristics
The built environment consists of a 6-storey office building constructed with a timber structure. The
2 2
floor area of each storey is approximately 960 m for a total floor area of 5 760 m . Access to each floor
is provided by two reinforced concrete exit stairs located at each end of a public corridor. An elevator
shaft made of reinforced concrete is also provided and is located near the centre of the floor area.
Figure 1 illustrates the structural framing of the building. Every floor has a clear interior floor/ceiling
height of 3,0 m. These floor assemblies are required to form a fire separation with a fire-resistance
rating not less than 1 hour. Load-bearing walls and columns are required to provide a fire-resistance
rating not less than that required for the supported elements and assemblies.
[4]
According to the applicable national prescriptive provisions, a 6-storey office building using a timber
structural system is required to be fully protected by an automatic sprinkler system conforming to
[5]
NFPA 13. It is also required to have fire detection and fire alarm systems.
The primary and secondary structural elements consist of glued-laminated timber beams and columns
[6],[7]
of the 20f-E and 12c-E Spruce-Pine (SP) stress grades. The floor structure is made of traditional
visually-graded solid-sawn double tongue-and-groove plank decking, of the Spruce-Pine-Fir (SPF) No.2
[8]
visually-graded lumber grade. The plank decking is laid perpendicularly to the supporting secondary
beams, which are spaced every 2 m (centre to centre). All timber elements conform to the national
[9]
lumber grading rules. The structural engineering design, for ambient/normal conditions, conforms
[8]
to the relevant design standard.
Concealed connections between the primary and secondary structural elements are used, in which
metallic components such as self-tapping screws driven at 45° are fully embedded into the wood
members to limit potential thermo-mechanical degradation from fire exposure. Figure 2 illustrates the
floor structure and location of load-bearing elements. Figure 3 illustrates the connections and their
embedment into the load-bearing elements. The characteristics of the load-bearing elements assumed
in this example are given in Table 1. The dimensions of the main elements are greater than required
for structural purposes due to the embedment of the load-bearing elements; they need to be able to
provide sufficient bearing lengths to the embedded main and secondary beams. The chosen elements
considered for demonstrating the procedure of ISO 24679-1 are a main beam, B1, located above the fire
source and its supporting column, C2, towards the exterior wall.
a) Isometric view b) Front view
Figure 1 — Structural frame
Figure 2 — Typical floor structural configuration
a) "Before" frame assembly b) "After" frame assembly
Figure 3 — Detailing of the connections
Table 1 — Load-bearing elements characteristics — Preliminary design (ambient conditions)
Dimensions
Element Type Gypsum board
mm
a
B1 Glulam 20f-E 265 × 532 None
b
B2 Glulam 20f-E 175 × 456 None
C1 Glulam 12c-E 418 × 365 None
C2 Glulam 12c-E 342 × 365 None
Decking S-P-F No.2 89 × 133 1 × 16 mm Type X
c
Partitions Wood studs 38 × 89 2 × 13 mm Type X
a
At 8 000 mm centre-to-centre (c/c).
b
At 2 000 mm c/c.
c
At 600 mm c/c.
The dropped-ceiling assembly forms a cavity filled with non-combustible insulation for providing the
required sound transmission class (Figure 4). The exposed ceiling consists of a single layer of 16 mm
fire-rated gypsum board (e.g. Type X) fastened to the secondary beams in conformance with national
[10],[11]
specifications. With this specific configuration, a limited portion of the primary beams and
columns are left exposed and can thus contribute to fire growth and severity.
Partitions made from wood stud walls are used to separate the office suites and the public corridor
within the floor area. They are constructed using 38 mm × 89 mm wood studs spaced at 600 mm.
Two (2) layers of 13 mm Type X gypsum board (i.e. fire-resistance rated gypsum boards) are installed
on both sides of the studs, providing a 1 hour fire-resistance rating when tested by a standard fire-
[12]
resistance test. The inside cavities of the stud walls are filled with 89-mm thick non-combustible
insulation in order to provide both the prescribed fire-resistance rating and the sound transmission
class.
According to the applicable national prescriptive provisions, these partitions are not required to be
constructed as a fire separation and are not required to provide a fire-resistance rating because the
building is entirely protected by automatic sprinklers and the maximum travel distance from any part
of the floor area to an exit is not more than 45 m. Assessment of the fire performance of the partitions is
therefore beyond the scope of this document.
a) Isometric view b) View of the ceiling
Key
1 89 mm × 133 mm plank decking with double tongue-and-groove
2 concealed spaces filled with non-combustible insulation
3 16 mm type X gypsum board
Figure 4 — Floor assembly
For the purpose of this document, the office suite to be analysed is located on the second floor and
represents the compartment of fire origin. It is a 192 m open-space office suite in which cubicles with
computers, desks, chairs and filing cabinets are uniformly distributed across the floor area (Figure 5).
Key
1 window 1
2 window 2
3 window 3
4 window 4
Figure 5 — Isometric view of office suite (compartment of fire origin)
5.1.2 Fuel loads
Fire loads consist of the total energy content of combustible materials in a building, a space or an area
including furnishing and contents within a compartment (i.e. moveable fire load) and combustible
materials used as structural elements, interior finishes or installed in concealed spaces (i.e. fixed fire
load). The office suite where the fire is assumed to start consists of an open-space configuration with
28 cubicles and an engineered hardwood flooring of 13 mm in thickness, see Figure 5. Each cubicle
measures 1,8 m × 1,8 m (3,24 m ). The typical combustible materials found in cubicles are paper, wood,
plastic and textiles.
2 [13-15]
An average moveable fuel load density of 420 MJ/m is typically assigned for an office space.
However, it is typically recommended to use the 95th percentile value for fire design purposes. A 95th
2 [16]
percentile value of 760 MJ/m is suggested for offices in Reference [14]. Zalok found 95th percentile
fuel loads of 8 822 MJ and 15 666 MJ for small floor area cubicle offices (11 m ) and large floor area
enclosed offices (25 m ), respectively. It was also reported that offices with large floor area result
2 2
in lower fire load densities (626 MJ/m ), when compared to that of smaller floor areas (802 MJ/m ).
2 2
Given the large floor area of this example (192 m ), a value of 735 MJ/m is deemed appropriate, and
consistent with that provided in References [14-16].
[17]
The National Institute of Standards and Technology (NIST) evaluated the heat release rate
(HRR) of a single workstation covering a floor area of 1,93 m × 1,63 m (3,15 m ) as well as multiple
workstations (4 workstations assembled in similar manner as the single workstation). A total fuel load
mass of 273,2 kg was measured for the single workstation. Assuming an effective heat of combustion
of 18 MJ/kg, a total energy of 4 917 MJ is estimated for a single workstation. The HRR obtained in the
NIST study is illustrated in Figure 6 a). The workstation HRR development curve used for performing
the computational fluid dynamics (CFD) modelling in Fire Dynamics Simulator (FDS) version 6.7 is
shown in Figure 6 b). The workstation HRR development curve is further explained below.
a) HRR, as presented in Reference [17] b) HRR implemented in FDS
Key
X time (s)
Y heat release rate (kW)
Figure 6 — Heat release rate of a single workstation
For the purpose of this document, the moveable fuel load energy density, FLED, is comprised of a
total of 28 cubicles uniformly distributed along the 192 m floor area, FLED , and the hardwood
cubicles
flooring, FLED . In an attempt to replicate a uniform fuel load density of 735 MJ/m , each cubicle
flooring
has been set to 4 082 MJ. The cubicle individual HRR development growth has been kept similar to that
shown in Figure 6 b). The 13-mm thick hardwood flooring density is assumed to be 600 kg/m with an
effective heat of combustion of 18 MJ/kg. A resulting moveable FLED value of 735 MJ/m is obtained
using Formula (1).
FLED=+FLED FLED (1)
flooringcubicles
The total fuel mass, m , can be estimated from the fire load energy density (FLED), the total floor
Total
area, A , of 192 m , and an effective heat of combustion, H , of 18 MJ/kg (wood equivalent), using
f eff
Formula (2):
AF⋅ LED
f
m = (2)
Total
H
eff
The total fuel mass of 7 840 kg is required for determining the design fire curve, as presented in 5.4.2.
The moveable fuel load density calculated in Formula (2) does not include the potential contribution
from the timber structural elements. The latter will be explicitly considered when performing CFD
modelling, as described in 5.4.3.3.
5.1.3 Mechanical actions
[18]
According to the applicable national design requirements, the load combination, P, for a rare/
accidental event such as a fire is taken as shown in Formula (3):
P = 1,0 D+(αL + 0,25 S) (3)
where
D is the permanent load;
α is taken as 1,0 for storage areas, equipment areas and service rooms, or 0,5 for other occupancies;
L is the live load due to occupancy;
S is the snow load.
[18]
For an office space, a minimum live load of 2,4 kPa is prescribed in the national design provisions.
Given that the low probability that all floors of a multi-storey building would be structurally loaded
to its full live load simultaneously, a live load reduction factor can be used based on the tributary area
supported by columns. For a column supporting a tributary area greater than 20 m and for this type of
building occupancy, as with the case of column C2 (32 m ) in this office building, the applicable national
[18]
design requirements allow the applied live load to be multiplied by the value shown in Formula (4):
98,
03, + (4)
B
where B is the tributary area of the supporting column (m ).
While post-earthquake fires can occur, fires and earthquakes are both considered as rare events and
thus deemed not to occur at the same time. Therefore, horizontal actions due to wind and seismic
forces are typically not considered for structural fire-resistance, unless specifically stipulated in the
applicable building code.
5.2 Step 2: Identifying objectives, functional requirements and performance criteria for
fire safety of structures
5.2.1 Objectives and functional requirements for fire safety of structures
Conducting a rational fire safety design of structures requires the establishment of fire safety objectives
and functional requirements. With respect to fire resistance of structures, the qualitative objectives
typically relate to the fire safety of occupants as well as the fire protection of the building.
From the applicable national building code, the objective for fire safety is to limit the probability that, as
a result of the design, construction or demolition of the building, a person in or adjacent to the building
will be exposed to an unacceptable risk of injury due to fire caused by:
a) a fire impacting areas beyond its point of origin; and
b) collapse of physical elements due to a fire.
Similarly, the objective for fire protection of the building is to limit the probability that, as a result of the
design, construction or demolition of the building, the building or adjacent buildings will be exposed to
an unacceptable risk of damage due to fire caused by:
a) a fire impacting areas beyond its point of origin; and
b) collapse of physical elements due to a fire.
In addition to these objectives, functional requirements are typically provided and linked to the
objectives to clarify the intent. With respect to structural fire resistance, the functional requirements
(also called functional statements in the applicable national building code) are to retard the effects of
a fire on areas beyond its point of origin and the retard failure or collapse due to effects of the fire. The
pairing of the functional statements and the objectives results in the following statements of intent:
a) to limit the probability that materials, assemblies or structural members will have insufficient
resistance to the spread of fire, which could lead to harm to persons;
b) to limit the probability that materials, assemblies or structural members will have insufficient
resistance to fire, which could lead to their failure or collapse, which could lead to harm to persons;
c) to limit the probability that materials, assemblies or structural members will have insufficient
resistance to the spread of fire, which could lead to damage to the building; and
d) to limit the probability that materials, assemblies or structural members will have insufficient
resistance to fire, which could lead to their failure or collapse, which could lead to damage to the
building.
In satisfying the functional requirements, it is essential to take into consideration the existence of
active and passive fire control systems and their effectiveness.
5.2.2 Performance criteria for fire safety of structures
Performance criteria are used to determine whether the objectives and functional requirements for the
fire safety of structures have been satisfied.
It is stipulated in a national design standard that structures are to be designed to prevent collapse of
the structure itself and to exhibit adequate load-bearing capacity and capability to maintain structural
[19]
integrity for a sufficient time. According to Reference [19], ensuring structural integrity for complete
burn out of the moveable fire load of any fire compartment is only required for tall (high) buildings. In
the context of this worked example, a 6-storey office building does not classify as a tall (high) building,
as defined in the applicable national prescriptive provisions, and is therefore not required to achieve
complete burnout of the moveable fuel content.
Therefore, the performance criterion used in this worked example is taken as the time at which the
impinging heat flux on the exposed surfaces of the timber elements reduces below 5 kW/m . Below this
threshold, it is assumed that the moveable fuel load will most likely be consumed and that smouldering
[20]
timber elements will stop charring and no longer contribute to the fire heat release rate. A reduction
in the net emitted energy to the exposed timber surfaces results in a reduction in the mass production
of volatiles, which will result in extinction if such reduction is sufficient (i.e. flaming will cease at the
timber surfaces). Moreover, auto-extinction of flaming combustion of Spruce with a density of 425 kg/
3 2
m has been found to occur if the timber mass loss rate reduces below 3,93 g/m ·s and the heat flux is
2 [21,22]
less than 43,6 kW/m . This threshold has been found to be dependent upon the timber species, but
[22]
not the density .
The objectives and functional requirements are deemed to be satisfied when the load-bearing function,
and separating function where appropriate, remain fulfilled until the criteria of heat flux and mass loss
rate are reached. The proposed performance criteria are considered as reasonable assumptions based
on scientific knowledge available at the time of writing this document. Other performance criteria
could be used as new evidence is made available, provided they are supported by technical test data.
5.2.2.1 Performance criteria to limit fire spread (compartmentation)
The compartmentation of a built environment in order to prevent or to limit the fire spread can be
achieved by load-bearing elements such as walls and floors, or by non-load-bearing elements, such as
partition walls, doors, windows, etc. These elements need to satisfy functional requirements related to
integrity, insulation and mechanical resistance or stability.
Separating elements used to compartment a building are required to achieve the following two
performance criteria, as found in ISO 834-1.
— Insulation criteria: the assembly is required to prevent the rise in temperature of the unexposed
side of separating (load-bearing and non-load-bearing) elements from being greater than 180 °C at
any location, or an average of 140 °C, above the initial temperature.
— Integrity criteria: the assembly is required to prevent the passage of flame or gases hot enough to
ignite a cotton pad or through gaps formed through separating (load-bearing and non-load-bearing)
elements.
The performance criterion is that no spread of fire occurs beyond the compartment of fire origin, until
both the impinging heat flux on the exposed surfaces of the timber elements and mass loss rate reduce
below the thresholds cited in 5.2.2. The insulation and integrity criteria are to be fulfilled for this
entire period of time. The performance criteria provided herein are assumed to be acceptable, based on
scientific data and engineering principles. Other performance criteria can potentially be used, provided
they are supported by technical test data.
5.2.2.2 Performance criteria to limit structural damage (structural stability)
Load-bearing elements used in a building are required to achieve the following performance criterion,
as found in ISO 834-1.
— Load-bearing criterion: the load-bearing elements used to provide the structural stability are
required to maintain the applied loads during the complete duration of the fire including the decay
phase, or a specified period of time.
Even in the absence of collapse, deformation can still affect exit paths, endangering life safety, and can
cause considerable property damage. As such, prevention of collapse and/or limitation of deformation
are essential for load-bearing structural members and for load-bearing barriers, which also provide
fire containment.
The performance criterion is that no excessive deflection and no structural collapse of any load-bearing
element will occur until both the impinging heat flux on the exposed surfaces of the timber elements
and mass loss rate reduce below the thresholds cited 5.2.2. The performance criteria provided herein
are assumed to be acceptable, based on scientific data and engineering principles. Other performance
criteria can potenially be used, provided they are supported by technical test data.
5.3 Step 3: Trial design plan for fire safety of structures
The trial design plan for fire safety of structures is an elaboration of the strategy for fire safety of
structures and consists of a set of design elements for the fire safety of structures, such as stability
and compartmentation. For the purpose of this document, a preliminary design was carried out in
normal (room) conditions to determine the dimensions of the various structural timber components, as
presented in Table 1.
Should the fire safety objectives stated in 5.2 be inconclusive using these preliminary dimensions, an
iterative design process is to be undertaken to determine suitable dimensions of the timber elements
that will fulfil the load-bearing function at both ambient and fire conditions.
5.4 Step 4: Design fire scenarios and design fires (thermal actions)
5.4.1 General
Design fire scenarios and design fires are an important step in the assessment of the performance
of structures in fire. It is noted that a design fire scenario is a specific qualitative description of the
development of a fire whereas a design fire (thermal actions) is a quantitative description of assumed
fire characteristics within a design fire scenario.
5.4.2 Design fire scenarios
[23]
Figure 7 shows the event tree of 4 fire scenarios for office buildings. It can be observed that the
scenario where sprinklers do not activate and where fire department response is 20 min has the lowest
probability of occurrence (P = 0,011). However, this design fire scenario would become more challenging
for a structural frame within a relatively small compartment if the fire were to become fully-developed.
The most likely scenario to occur is when sprinklers activate and the fire department responds rapidly
(P = 0,801). These scenarios are selected for further analysis.
where:
P is the scenario probability;
P is the fire initiation probability;
P is the sprinkler activation probability;
P is the response time of the fire probability.
Figure 7 — Event tree of an office building, as presented in Reference [23]
The following four scenarios are chosen for analysis in the present example. Some rationale for the
selection is also provided for each scenario.
1) Scenario 1: HRR development of medium and fast t -fires assuming the maximum ventilation
factors (glass from all windows is assumed to be open/broken) in which fuel contribution from
structural timber elements and the effects of sprinkler activation on fire growth and the response of
the fire department are ignored. This scenario would represent the comparison basis for verifying
that the assumptions and parameters used in the CFD modelling are sufficiently conservative.
2) Scenario 2: fully-developed fire in which fuel contribution from structural timber elements and
the effects from sprinkler activation on fire growth and the response of the fire department are
ignored. This scenario represents the comparison basis for verifying and quantifying timber
contribution to fire growth, intensity and duration using a CFD modelling.
3) Scenario 3: fully-developed fire in which fuel contribution from structural timber elements
is considered, but the effects of sprinkler activation on fire growth and the response of the fire
department are ignored. The results from the CFD modelling of this scenario are compared to that
of the CFD modelling of scenario 2.
4) Scenario 4: same as scenario 3, but with consideration of the effects of sprinkler activation on fire
growth but without the fire department response.
The floor configuration overlooked in this document results in having the cubicle of fire origin located
directly beneath the mid-span of a timber beam to facilitate ignition of that beam. However, as previously
indicated, this document is not intended to demonstrate full conformance of a performance-based fire
engineering design seeking approval. Therefore, only a limited number of fire design scenarios are
being presented and evaluated to demonstrate how the design process given in ISO 24679-1 can be
applied in this situation. However, in a design that will be submitted to the authorities for approval,
multiple scenarios are to be considered by the designers.
5.4.3 Design fires (thermal actions)
As mentioned in ISO 24679-1, actions for consideration when assessing the behaviour of a structure in
fire include thermal actions or design fires from realistic fire scenarios. Typically, thermal actions or
design fires are given either as time-temperature relationships or as time-heat flux relationships. When
estimating the temperature or heat flux effects on separating and structural elements, both convective
and radiative heat effects are to be considered.
In this example, design fires are determined using simple analytical formulae and numerical
calculations from computational fluid dynamics (CFD) modelling. Scenario 1 serves in determining
a simple representation of an office fire using t -fires. Scenario 2 is produced using CFD modelling
without consideration of timber elements contributing to fire growth and intensity and without any
intervention from fire fighters or automatic sprinklers. The results are compared to those of scenario
1, namely with respect to growth rate, flashover conditions, ventilation conditions and heat release
rate. Scenario 3 is the same as scenario 2, but with consideration of timber elements when they reached
critical conditions for ignition and combustion. Scenario 4 is only used to demonstrate the effectiveness
of automatic sprinklers on fire growth and heat release rate.
5.4.3.1 Design fire for scenario 1 (t -fire)
A design fire typically includes an incipient phase characterized by a number of sources, a growth
phase ranging from fire propagation to flashover conditions, a fully-developed phase characterized by
a steady burning rate based on ventilation conditions, a decay phase where fire severity declines and
lastly the extinction where no more energy is produced. Events such as automatic sprinkler activation
and window glass breakage will influence the development and growth of a given design fire scenario.
As detailed in ISO/TS 16733-2, the maximum heat release rate following flashover of a design scenario
can be taken as the lesser of the ventilation-controlled and fuel-controlled heat release rates, Q and
v
Q respectively. The maximum ventilation-controlled heat release rate can be estimated using
fuel
Formula (5).
QA=1 500 H (5)
vV V
where
A is the sum of the area of all openings, in m ;
V
H is the average height of openings, in m.
V
The compartment has 4 openings, 6,7 m in width by 1,8 m in height, for a total area, A , of 48,24 m . All
V
windows are the same dimensions. The average height, H , is taken as 1,8 m, resulting in a maximum
V
ventilation-controlled heat release rate of 97 MW.
As presented in ISO/TS 16733-2, an office building can be modelled using a medium growth t -fire, α
taken as 0,012 kW/s . The growth period, τ , is then calculated as 2 844 s (47,4 min), according to
growth
Formula (6):
Q
max
τ = (6)
growth
α
During the growth phase, a mass of burned fuel, m , of 5 114 kg is determined according to
growth
Formula (7):
τ
growth
Qdτ 3
∫ ατ
τ
growth
m == (7)
growth
HH3
effeff
It is typically assumed that 80 % of the remaining fuel at the start of the steady-state period is pyrolized
during this period. As such, the duration of the steady-state period, τ , is taken as 404 s (6,7 min),
steady
calculated according to Formula (8):
08, mm−
()
totalgrowth
τ = (8)
steady
QH/
maxeff
A cons
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