ISO/TR 15656:2003
(Main)Fire resistance — Guidelines for evaluating the predictive capability of calculation models for structural fire behaviour
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
General Information
Standards Content (Sample)
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
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ISO/TR 15656:2003(E)
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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
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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
1)
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.
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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.
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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,
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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
n
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
2
between a natural fire in a laboratory and under real life service conditions ∆T . Analogically, there is the
3
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
n
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
1
exposure ∆Φ .
2
[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
n
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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.
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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
2
variable rate of surface reaction β
1
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;
1
H : the thermal exposure is that resulti
...
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