Fire resistance -- Guidelines for evaluating the predictive capability of calculation models for structural fire behaviour

ISO/TR 15656:2003 provides guidance for evaluating the predictive capability of calculation models for structural fire behaviour. It is specific to models that are intended to predict the fire resistance or fire endurance of structural members or assemblies. Such models include models simulating the thermal behaviour and mechanical behaviour of fire-exposed load-bearing and/or separating structures and structural elements. In ISO/TR 15656:2003, the term model includes all calculation procedures that are based on physical models. These mechanistic-based or physical models encompass all the physical, mathematical and numerical assumptions and approximations that are employed to describe the behaviour of structural members and assemblies when subjected to a fire. In general, such physical models are implemented as a computer code on a digital computer. The application and extension of results from calculation methods are generally limited to performance resulting from standard tests. Aspects of ISO/TR 15656:2003 are applicable to calculation procedures not based on physical models. Mechanistic-based models can often be used to calculate the behaviour of structures in non-standard fire exposures. The process of model evaluation is critical in establishing both the acceptable uses and limitations of fire models. It is not possible to evaluate a model in total; instead, ISO/TR 15656:2003 is intended to provide methodologies for evaluating the predictive capabilities for specific uses. Documentation of suitability for certain applications or scenarios does not imply validation for other scenarios.

Résistance au feu -- Lignes directrices pour évaluer l'aptitude des modèles mathématiques à simuler le comportement des feux de structures

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Publication Date
30-Nov-2003
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6060 - International Standard published
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20-Oct-2003
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TECHNICAL ISO/TR
REPORT 15656
First edition
2003-12-01
Fire resistance — Guidelines for
evaluating the predictive capability of
calculation models for structural fire
behaviour
Résistance au feu — Lignes directrices pour évaluer l'aptitude des
modèles mathématiques à simuler le comportement des feux de
structures
Reference number
ISO/TR 15656:2003(E)
ISO 2003
---------------------- Page: 1 ----------------------
ISO/TR 15656:2003(E)
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ii © ISO 2003 — All rights reserved
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ISO/TR 15656:2003(E)
Contents Page

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

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

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

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

3 Terms and definitions........................................................................................................................... 1

4 Background information ...................................................................................................................... 2

4.1 General................................................................................................................................................... 2

4.2 Potential users and their needs........................................................................................................... 2

4.3 Predictive model capabilities, uncertainties of design component (from ISO/TR 12471).............. 2

5 Outline of methodology........................................................................................................................ 6

6 Definition and documentation of model and scenario...................................................................... 7

6.1 Types of models.................................................................................................................................... 7

6.2 Documentation ...................................................................................................................................... 9

6.3 Deterministic versus probabilistic .................................................................................................... 10

7 Evaluation ............................................................................................................................................ 10

7.1 Sources of errors in predictions ....................................................................................................... 10

7.2 Model application and use ................................................................................................................. 11

7.3 Model theoretical basis ...................................................................................................................... 12

7.4 Model solution..................................................................................................................................... 12

7.5 Comparison of model results ............................................................................................................ 14

7.6 Measurement uncertainty of data (from ISO/TR 13387-3)................................................................ 17

7.7 Model sensitivity ................................................................................................................................. 18

Bibliography ..................................................................................................................................................... 22

© ISO 2003 — All rights reserved iii
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ISO/TR 15656:2003(E)
Foreword

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

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

technical committees. Each member body interested in a subject for which a technical committee has been

established has the right to be represented on that committee. International organizations, governmental and

non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the

International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards

adopted by the technical committees are circulated to the member bodies for voting. Publication as an

International Standard requires approval by at least 75 % of the member bodies casting a vote.

In exceptional circumstances, when a technical committee has collected data of a different kind from that

which is normally published as an International Standard (“state of the art”, for example), it may decide by a

simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely

informative in nature and does not have to be reviewed until the data it provides are considered to be no

longer valid or useful.

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

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

ISO/TR 15656 was prepared by Technical Committee ISO/TC 92, Fire safety, Subcommittee SC 2, Fire

containment.

ISO/TR 15656 is one of a series of documents developed by ISO/TC 92/SC 2 that provide guidance on

important aspects of calculation methods for fire resistance of structures:

 ISO/TR 15655, Fire resistance — Tests for thermo-physical and mechanical properties of structural

materials at elevated temperatures for fire engineering design
Others documents in this series are currently in preparation and include:

 ISO/TS 15657, Fire resistance — Guidelines on computational structural fire design

 ISO/TS 15658, Fire resistance — Guidelines for full scale structural fire tests

Other related documents developed by ISO/TC 92/SC 2 that also provide data and information for the

determination of fire resistance include:
 ISO 834 (all parts), Fire-resistance tests — Elements of building construction

 ISO/TR 10158, Principles and rationale underlying calculation methods in relation to fire resistance of

structural elements

 ISO/TR 12470, Fire-resistance tests — Guidance on the application and extension of results

 ISO/TR 12471 , Computational structural fire design — State of the art and the need for further

development of calculation models and for fire tests for determination of input material data required

1) In preparation.
iv © ISO 2003 — All rights reserved
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ISO/TR 15656:2003(E)
Introduction

Structural fire behaviour for a standard fire exposure has traditionally been experimentally determined by test

methods described by International Standards such as ISO 834 (all parts). For a variety of reasons,

calculation methods have been developed as alternative methodologies for determining the fire endurance or

fire resistance of structural members or assemblies. Since fire resistance is a critical component of fire safety

regulations, it is essential that objective assessments of the accuracy and applicability of such calculation

methods be conducted. In a review of the state of the art of computational structural fire design,

ISO/TR 12471, it was noted the “rapid progress in analytical and computer modelling of phenomena and

processes of importance for a fire engineering design stresses the need for internationally standardized

procedures for evaluating the predictive capabilities of the models and for documenting the computer

software.” The development of this Technical Report is toward that end.
© ISO 2003 — All rights reserved v
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TECHNICAL REPORT ISO/TR 15656:2003(E)
Fire resistance — Guidelines for evaluating the predictive
capability of calculation models for structural fire behaviour
1 Scope

This Technical Report provides guidance for evaluating the predictive capability of calculation models for

structural fire behaviour. It is specific to models that are intended to predict the fire resistance or fire

endurance of structural members or assemblies. Such models include models simulating the thermal

behaviour and mechanical behaviour of fire-exposed load-bearing and/or separating structures and structural

elements.

In this Technical Report, the term “model” includes all calculation procedures that are based on physical

models. These mechanistic-based or physical models encompass all the physical, mathematical and

numerical assumptions and approximations that are employed to describe the behaviour of structural

members and assemblies when subjected to a fire. In general, such physical models are implemented as a

computer code on a digital computer. The application and extension of results from calculation methods are

generally limited to performance resulting from standard tests. Aspects of this Technical Report are applicable

to calculation procedures not based on physical models. Mechanistic-based models can often be used to

calculate the behaviour of structures in non-standard fire exposures.

The process of model evaluation is critical in establishing both the acceptable uses and limitations of fire

models. It is not possible to evaluate a model in total; instead, this Technical Report is intended to provide

methodologies for evaluating the predictive capabilities for specific uses. Documentation of suitability for

certain applications or scenarios does not imply validation for other scenarios.
2 Normative references

The following referenced documents are indispensable for the application 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 13943:2000, Fire safety — Vocabulary
3 Terms and definitions

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

NOTE In discussions of models, the terms “evaluation”, “verification” and “validation” have taken on specific but

different meanings. There is no consensus on the requirements for an evaluation to be considered verification or validation.

The dictionary definition of “evaluate” is “to examine and judge.” “Verify” is defined as “to establish the truth, accuracy, or

reality of.” The definition of “validation” includes “the process of determining the degree of validity of a measuring device.”

“Valid” is considered to “imply being supported by objective truth or generally accepted authority.” For the purposes of this

Technical Report, no judgement is made as to what is required for a model to be “verified” or “validated.” The intent is to

review methodologies that are available to evaluate fire models for purposes of gaining verification or validation of such

fire models for their defined applications. The term “evaluation” is used in all cases. “For clarity it would be better for the

[1]

word (i.e. validation) not to be used at all but for people to say explicitly what they mean.”

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ISO/TR 15656:2003(E)
4 Background information
4.1 General

Structural fire behaviour for a standard fire exposure has traditionally been experimentally determined by test

methods described by standards such as ISO 834. For a variety of reasons, calculation methods have been

developed as alternative methodologies for determining the fire endurance or fire resistance of structural

members or assemblies. Since fire resistance is a critical component of fire safety regulations, it is essential

that objective assessments of the accuracy and applicability of such calculation methods be conducted. In a

review of the state of the art of computational structural fire design (ISO/TR 12471), it was noted that “rapid

progress in analytical and computer modelling of phenomena and processes of importance for a fire

engineering design stresses the need of internationally standardized procedures for evaluating the predictive

capabilities of the models and for documenting the computer software.” In an earlier review of fire-dedicated

thermal and structural computer programs, it was noted that programs are commonly only validated against

specific and limited test data. Little work had been presented by way of general validation of these methods.

ASTM has developed ASTM E 1355, Standard guide for evaluating the predictive capability of fire models.

This was used to develop the initial draft of this document. ISO/TC 92/SC 4 is developing guidelines,

ISO/TR 13389, Fire engineering — Assessment and verification of mathematical fire models. These

documents provide guidance that are applicable to any fire model but their primary intended applications are

to models that predict fire growth in compartments. A number of papers have been published on the

[2-10].

evaluation of a fire model . Some of these documents will be reviewed in ISO/TR 13389. A 1993 review of

[2]

seven thermal analysis programs and fourteen structural analysis was dedicated to fire endurance analysis .

[10]

An assessment of fire models based on a matrix of criteria and weighting factors has been presented .

Criteria include field of application (4 points), scientific verification (6 points), precision of method (2 points),

physical background (1 point), completeness (2 points), input existent (2 points), user friendliness (1 point)

and approval/standard or experience (2 points). The sum of the weighting factors is 20 points. The system

was applied to existing simplified methods for concrete, structural steel and timber.

4.2 Potential users and their needs

This Technical Report is intended to meet the needs of users of fire models. Users of models need to assure

themselves that they are using an appropriate model for an application and that it provides adequate accuracy.

Developers of performance-based code provisions and other approving officials need to ensure that the

results of calculations using mathematical models show clearly that the model is used within its applicable

limits and has an acceptable level of accuracy. The methodologies discussed in this Technical Report will

assist model developers and marketers in developing the documentation of predictive capabilities for specific

applications that should be available on their calculation methods. Part of model development includes the

identification and documentation of precision and limits of applicability, and independent testing. Educators

can use the methods to demonstrate the application and acceptability of calculation methods being taught.

This Technical Report should also be useful for educators of future model developers so future models of

greater complexity and availability are used within their limitations of application and precision.

4.3 Predictive model capabilities, uncertainties of design component (from ISO/TR 12471)

Few systematic studies of the predictive capabilities of models and related computer software, used for

describing the simulated fire exposure and the thermal and mechanical behaviour of fire exposed structures,

have appeared in the literature. Recent studies seem to indicate that the situation now is improved. Such

[1,11,12]

studies include compartment fire modelling and modelling of the thermal and mechanical behaviour of

[2,13]

structures . General categories have been identified regarding possible sources of error in using a

[1,11]

computer model to predict the value of a state-variable such as temperature or heat flux . The categories

specified are
a) unreality of the theoretical and numerical assumptions in the model,
b) errors in the numerical solution techniques,
c) software errors,
2 © ISO 2003 — All rights reserved
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ISO/TR 15656:2003(E)
d) hardware faults, and
e) application errors.

For 10 zone models and 3 field models for the compartment fire, the Loss Prevention Council provides the

following information: degree of validation, limitations, and restrictions on compartment size, number of vents

[12]

and number of fuels that can be accommodated, and number of organizations using the model . Useful

conclusions are drawn with respect to input/output data, experience of using the models, model validation,

[2]

and potential limitations. A survey discusses the theoretical background to 7 thermal and 14 structural

behaviour, fire-dedicated, computer programs, together with their strengths and weaknesses. The differences

between the programs were found to lie mainly in the material models adopted, the material data input, the

user-friendliness and documentation of the software. The majority of the available fire-dedicated structural

programs still require significant development and, as most of them are not user-friendly or properly

documented, using them effectively and universally would be very difficult.
[1]

Applied to fire exposed steel columns, comparative calculations are reported of the structural behaviour by

five computer programs. In terms of the ultimate resistance of the columns, the calculated results are very

similar, with a maximum difference between two programs of 6 %. Greater differences are observed for the

displacements of the columns, probably mainly due to different ways of considering the residual stresses at

increasing temperature in the program. When evaluating the results, it is important to note that the same

mechanical behaviour model for steel at transient elevated temperatures (the one in ENV 1993-1-2,

Eurocode 3 — Design of steel structures — Part 1-2: General rules —– Structural fire design) was used in all

computer programs.

For sensitivity and uncertainty studies of relevance for structural fire design, there are very few reported in the

[14-16]

literature. The most comprehensive studies are probably still those presented by 20 years ago . The

methodology developed for these studies is quite general and applicable to a wide class of structures and

structural elements. To obtain applicable and efficient final safety measures, the probabilistic analysis is

numerically exemplified for an insulated, simply supported steel beam of I-cross section as a part of a floor or

roof assembly. The chosen statistics of dead and live load and fire load are representative for office buildings.

With the basic data variable selected, the different uncertainty sources in the design procedure were identified

and dissembled in such a way that available information from laboratory tests could be utilized in a manner as

profitable as possible. The derivation of the total or system variance var(R) in the load bearing capacity R was

divided into two main stages: variability var(T ) in maximal steel temperature T for a given type of

max max

structure and a given design fire compartment, and variability in strength theory and material properties for

known value of T .
max

The results obtained are the decomposition of the total variance in maximum steel temperature T into the

max

component variances as a function of the insulation parameter κ = A k /(V d) (see Figure 1), where A is the

n i i s i i

interior surface area of the insulation per unit length, d the thickness of the insulation, k the thermal

i i

conductivity of the insulating material corresponding to an average value for the whole process to fire

exposure, and V the volume of the steel structure per unit length. Increasing κ expresses a decreased

i n
insulation capacity.
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ISO/TR 15656:2003(E)

Figure 1 — Separation of total variance in maximum steel temperature T into component variance

max
as a function of insulation parameter κ

The component variances refer to the stochastic character of the fire load density q, the uncertainty in the

insulation properties κ, the uncertainty reflecting the prediction error in the theory of compartment fires and

heat transfer from the fire process to the structural member ∆T , and a correction term reflecting the difference

between a natural fire in a laboratory and under real life service conditions ∆T . Analogically, there is the

decomposition of the total variance in the load bearing capacity R into component variances as a function of

the insulation parameter κ (see Figure 2). The component variances refer to the variability in the maximum

steel temperature T variability in material strength M, the uncertainty reflecting the prediction error in the

max

strength theory ∆Φ , and the uncertainty due to the difference between laboratory tests and in situ fire

exposure ∆Φ .
[17]

Uncertainty studies of fire-exposed concrete structures are scarce. A report breaks the total variance in fire

resistance or load-bearing capacity into component variances as a function of the slenderness ratio λ for an

eccentrically compressed, reinforced concrete column (see Figure 3). The component variances are related to

the following stochastic variables: f is the compressive strength of concrete at ordinary room temperature, f

c s

is the strength of reinforcement at ordinary room temperature, b is the width of the cross section, h is the

height of the cross section, x is the position of tensile reinforcement, x is the position of compressive

t c

reinforcement, f is the yield stress of steel as a function of temperature T, and k is the thermal conductivity

S,T c
of concrete.

Figure 2 — Separation of total variance in load bearing capacity R into component variances as a

function of insulation parameter κ
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ISO/TR 15656:2003(E)

NOTE Concrete B25, percentage of reinforcement µ = 0,2 %, b = h = 30 cm, eccentricity e = 0,2 h.

Figure 3 — Separation of total variance in resistance or load-bearing capacity R into component

variances as a function of slenderness ratio λ for an eccentrically compressed,
reinforced concrete column
[18]

Results of sensitivity studies regarding a fire engineering design of timber structures have been reported .

The study reports deals with the sensitivity of the charcoal layer penetration for a fire-exposed timber structure

as a function of certain material input in a defined simulation model, including the influence of varying the

[19]

thermal conductivity of the charcoal and the rate of surface reaction (see Figure 4). Another study

presented a first-order reliability analysis (FORM) of fire-exposed wood joist assemblies. By using non-linear

least-square regression analysis on 42 full-scale tests, a time-to-failure model is developed, predicting the

deterministic value of the resistance of the assembly. The exposure parameter is defined as the duration of

the ventilation controlled compartment fire predicted by the fire load, and the window area and height,

assuming constant rate of burning. Expressions describing the total system and component variances are

developed which, when quantified, lead to a determination of the safety index β.
© ISO 2003 — All rights reserved 5
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ISO/TR 15656:2003(E)
Key
X time, in minutes
Y depth, in millimetres

Figure 4 — Depth of charring as a function of time for variable thermal conductivity k of charcoal and

variable rate of surface reaction β
5 Outline of methodology

In this Technical Report, the evaluation of fire models is broken into seven primary components:

a) identification or definition of the model and scenario being evaluated;

b) evaluation of the application and use of the model when applied to a specific use;

c) identification of sources of errors in the predictions;

d) evaluation of the appropriateness of the theoretical basis and assumptions used in the model when

applied to the entire class of problems addressed by the model;

e) evaluation of the mathematical and numerical robustness of the model and the accuracy of the computer

code;

f) evaluation of the uncertainty and accuracy of the model results in predicting of the course of events;

g) evaluation of the model sensitivity to parameters.

Sufficient documentation of calculation models, including computer software, is absolutely necessary to

assess the adequacy of the scientific and technical basis of the models, and the accuracy of computational

procedures. Also, adequate documentation will help prevent the unintentional misuse of fire models. Scenario

documentation provides a complete description of the scenarios or phenomena of interest in the evaluation to

facilitate appropriate application of the model, to aid in developing realistic inputs for the model, and criteria for

judging the results of the evaluation.

A model should be assessed for a specific use in terms of its quantitative ability to predict outcomes. Even

deterministic models rely on inputs often based on experimental measurements, empirical correlations, or

estimates made by engineering judgements. Uncertainties in the model inputs can lead to corresponding

uncertainties in the model outputs. Sensitivity analysis is used to quantify these uncertainties in the model

outputs based upon known or estimated uncertainties in model inputs.
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ISO/TR 15656:2003(E)

In general, the results of measurement are only the result of an approximation or estimate of the specific

quantity subject to measurement, and thus the result is complete only when accompanied by a quantitative

statement of uncertainty. Guidance for determining the uncertainty in measurement is provided in the Guide to

the Expression of Uncertainty in Measurement.

The computer implementation of the model should be checked to ensure such implementation matches the

stated documentation. An independent review of the underlying physics and chemistry inherent in a model

ensures appropriate application of sub-models that have been combined to produce the overall model.

Information on methodologies discussed in this Technical Report can also be found in ISO/TR 13387-3:1999,

Fire safety engineering — Part 3: Assessment and verification of mathatical fire models, and ASTM E 1355.

These two documents are the primary documents used to prepare this Technical Report. ASTM E 1895,

Standard guide for determining uses and limitations of deterministic fire models, provides an overall

methodology for the systematic evaluation of fire models by model users, model developers and authorities

having jurisdiction. While the scopes of these documents were all deterministic fire models, they tend to reflect

an emphasis on models for the compartment fire itself. Emphasis in this Technical Report is on models for

predicting structural fire behaviour.
6 Definition and documentation of model and scenario
6.1 Types of models

Fire models for structures normally consist of a heat-transfer model that provides the thermal profile input

needed for the mechanical model and the mechanical model itself. Models available at present for structural

fire engineering design have been systematically characterized with reference to a matrix of models for

[20,21]

structure versus models for thermal exposure . In the matrix (shown in Figure 5), there are two types of

thermal models:

 H : the thermal exposure is the standard fire resistance test with the nominal temperature-time curve;

 H : the thermal exposure is that resulti
...

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