ISO/TC 92/SC 2/WG 2 - Calculation methods

This document identifies test methods already in existence and provides guidance on those that need to be developed to characterize the thermo-physical and mechanical properties of structural materials at elevated temperatures for use in fire safety engineering calculations. It is applicable to materials used in load-bearing construction in which structural and thermal calculations might be required to assess the performance of elements or systems exposed to either standard fire tests, real or design fire heating conditions.

ISO/TR 12470-2:2017 explains a methodology to determine the applicability of the results of fire resistance tests to actual applications. ISO/TR 12470-2:2017 is applicable to those non-loadbearing elements for which there is an ISO standard test procedure based upon the ISO 834 series for determining the fire resistance of a representative sample of the construction proposed for use in a specific building or just for general use. These elements are: - fire resisting door assemblies (excluding lift landing doorsets): - timber; - steel; - fire resisting vertical glazing ? metal framed: NOTE The rheology of glass is such that gravity has a disproportional influence on fire glass when it is heated to high temperatures and as a consequence, it is not possible to provide generic guidance on the extended application of horizontal glazed elements. - timber framed; - linear gap sealing; - service penetration sealing. Fire resistance testing furnaces have fairly restricted size limitations and as a consequence, there is little confidence that the result obtained on an element of construction tested in accordance with the standard methods will behave in a similar manner when installed in the final building. Direct and extended applications of test results are the two possible ways to ensure that an element that is not identical to the tested construction will have an acceptable probability of obtaining the same fire rating as that of the original tested specimen. In both cases, these applications generally refer only to the fire rating that the building element can expect to reach if it, or a representative sample of it, were to be tested in a furnace according to the standard fire test conditions used in the reference test. The criteria and methodology used in evaluating ductwork and dampers is significantly different from those used to evaluate conventional separating elements and for this reason, these forms of construction are not included in the scope of this document. It is planned that a subsequent part of this document may include guidance on these elements.

This document explains a methodology to determine the applicability of the results of fire resistance tests to actual applications. It is applicable to those loadbearing and simple vertical and horizontal separating elements for which there is an ISO standard test procedure based upon the ISO 834 series for determining the fire resistance of a representative sample of the construction proposed for use in a specific building or just for general use. These elements are: —   loadbearing elements; —   non-loadbearing elements: —   partitions: —   stud construction partitions; —   composite panel/SIPS partitions; —   ceiling membranes (horizontal partitions): —   jointed ceilings; —   composite panel ceilings. Direct and extended applications of test results are the two possible ways to ensure that a modified element has an acceptable probability of obtaining the same fire rating as that of the original tested specimen. In both cases, these applications generally refer only to the fire rating that the building element can expect to reach if it, or a representative sample of it, were to be tested in a furnace according to the standard fire test conditions used in the reference test. One of the most common variations is in respect of the size of the element in use. Fire resistance testing furnaces have size restrictions and as a consequence, there is little confidence that the result obtained on an element of construction tested in accordance with the standard methods will behave in a similar manner when installed in the final building. This document does not provide guidance on the application and extension of results arising from testing carried out on door and window assemblies, linear gaps or service penetration seals, which is covered in ISO/TR 12470-2. For some, but not all of the critical parameters, a summary of the possible influences is incorporated in the given examples.

The assessment detailed in ISO 834-11:2014 is designed to cover a range of thicknesses of the fire protection material, a range of steel sections characterized by their section factors, a range of design temperatures, and a range of valid fire resistance classification periods. ISO 834-11:2014 covers fire protection systems that include both passive (boards, mats, slabs, and spray materials) and reactive materials as defined in this document. The assessment procedure is used to establish: a) on the basis of the temperature data derived from testing loaded and unloaded specimens, a correction factor and practical constraints on the use of the fire protection system (the physical performance), and b) on the basis of the temperature data derived from testing unloaded short steel specimens, the thermal properties of the fire protection material (the thermal performance). The limits of applicability of the results of the assessment are defined together with permitted direct application of the results to different steel section sizes and strength grades (but not stainless steels) and to the fire protection system tested. The results of the tests obtained according to ISO 834‑10 and the assessment in this part of ISO 834 are directly applicable to steel sections of "I" and "H" cross-sectional shape and hollow sections. Results from analysis of I or H sections are directly applicable to angles, channels, and T-sections for the same section factor, whether used as individual elements or as part of a fabricated steel truss. The results of the assessment are applicable to fabricated sections. ISO 834-11:2014 does not apply to concrete-filled hollow sections, beams, or columns containing holes or openings of any type or solid bar.

ISO/TR 15657:2013 provides an overview of the advances that have been made in understanding how structures respond to fire. This is reviewed in terms of heat transfer to the structural elements from primarily nominal (furnace) fires changes in the elevated temperature, physical and mechanical characteristics of structural materials, and how the information is used in the analysis of structural elements for the fire limit state. In reviewing the fire scenarios the report concentrates primarily on standardized heating curves but includes the basis of characteristic curves, which may at some time in the future be adopted in a standardized way. Reference is made to time equivalent as a recognized methodology in relating a natural or characteristic fire to an equivalent period of heating in the ISO 834 furnace test.

ISO/TR 15658:2009 specifies procedures for the design, performance and reporting of fire tests which are not performed using standardized test equipment, such as furnaces or test chambers, and which are primarily duration- or time-based. It is applicable to all ‘natural' fire tests, which set out to evaluate the behaviour of structural frames, rooms (or a suite of rooms forming part of a building), with respect to fully developed fire conditions, regardless of whether or not the heat input is by means of natural sources, e.g. cribs or burners. It is not applicable to ‘reaction-to-fire' large-scale tests, which are primarily designed to evaluate materials and for which the heating rate is slower and the maximum rate of heat release is lower than that which would occur at full development.

ISO/TR 12471:2004 gives a review of the advances that have been made in measuring and understanding how structural materials respond to fire in terms of changes in their elevated temperature, and physical and mechanical characteristics, and to identify areas where further work is necessary to generate the data required. Analytical methods for heat transfer are combined with mechanical models to calculate structural behaviour from single elements up to complete frames under real fire and ISO Standard furnace heating conditions. ISO/TR 12471:2004 reviews advances in computational analysis and indicates how these can be used with probabilistic analysis to provide a risk-based approach to structural fire engineering design.

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.

Clause 3 contains explanations of some of the basic terms employed in the development and application of the concept of fire resistance. Clause 4 discusses the significance of fire resistance. Clause 7 covers the development and application of calculation methods which are associated with the results obtained from fire resistance tests. Clause 8 gives a survey of the methods presently available for structural fire design, followed by an enumeration of the precepts that appear to be applicable to their use. Clause 9 deals in more detail with the development and application of calculation methods which are directly related to an analytical determination of fire resistance.