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Structural Fire Engineering

Objective

The objective of this project is to produce a performance based design guide for the structural fire engineering of real buildings subject to real fires. This guide will pull together both the skills of the fire scientist and the experience of the structural engineer to provide a framework within which designers can be free to develop site solutions based on real performance criteria.

Background

The Building Regulations Approved Document B provides a performance based regulatory framework for the fire safety of buildings which allow for alternative fire engineering methods as a means of satisfying the mandatory requirements. The development of Fire Safety Engineering Design codes, DD240 and BS9999 provide an overview of fire safety engineering but contain no detailed guidance on fire engineering applied to structural systems.

The most common and traditional design method for ensuring strength and stability to steel, timber and some concrete structures during a fire is to cover all exposed surfaces with a protective material. Although this philosophy has proved adequate the prescriptive nature of this approach can impose unnecessary costs and stifle innovation. This is a result of the assumption that the members are uniformly heated, together with a disregard of both the actual load levels (both fire and static) during a fire and the true material behaviour at elevated temperatures. The development of structural fire design codes such as BS5268: Part 4(timber), BS5950: Part 8(steel), BS5628: Part 3(masonry), Eurocode2: part 1.2(concrete), Eurocode 3: part 1.2 (steel) and Eurocode 4: Part 1.2(composite steel and concrete) together with the fire part of Eurocode 1(actions) provides a more solid scientific foundation for the provision of fire resistance. However, these design codes were developed from standard fire tests in isolated beams and columns and there is general agreement that such tests ignore the significant structural contribution available through the interaction between members. This was shown in 1990 when a fire developed in a partly completed 14-story office block on the Broadgate development in London. Large sections of steel frame were totally exposed at the time of the fire. Although the structure was unprotected it did not collapse. A back analysis of the structure showed that the codes were unnecessarily conservative and did not take account of the true behaviour of the building

During the last few years there have been significant developments in the area of fire engineering. Complementary analytical and experimental research projects have investigated the behaviour of natural fires and helped to improve our knowledge of material, component and system interactions at elevated temperatures. For the industry to benefit from these developments there is a need to combine the expertise of the fire scientist with that of the structural engineer so that a holistic approach can be made to the behaviour of real buildings subject to real fires. Such an interdisciplinary approach is essential to the efficient implementation of the current generation of National and European fire standards for structures.

The development of this design guide will reduce the burden of prescriptive fire resistance requirements on both the regulatory authorities and the construction profession and allow more innovative, cost effective and safer solutions to be adopted. This project will also encourage innovation in the fire design of structures by bring together fire science and structural engineering in to a single clear and concise document.

Programme of work

This project can be conveniently split into two closely related but distinct tasks: -

  • More realistic fire models
  • Existing fire modelling is restricted to a fire defined according to the standard time-temperature response used in fire resistance tests. This project will consider methods for design fire based on the parameters, which influence fire growth and development. The available data, including the data from the Cardington tests, will be used to produce a more realistic fire model.

  • More realistic structural models
  • Observations from real fire, experimental evidence and numerical studies have suggested that whole buildings have a fire resistance over and above that predicted from standard tests on individual structural elements. This project will use this data to develop structural models that reflect the enhanced performance from interactions between structural and non-structural members.

To make the final design guidance capable of being used and understood by both fire and structural engineers the output from this project will include simple calculation procedures (checklists), tabulated data and/or design charts will be supported by a detailed description of the background theory used and a number of worked examples.

Dissemination

A number of mechanisms will be used to disseminate the findings from this project to practitioners. During the development of the design guide the Institution of Structural Engineers' Fire Engineering Group will advise and give feedback on the practicalities of the approach adopted. A number of universities will be targeted and encouraged through presentations given by members of the project team to develop courses based on this approach. Peer reviewed journal papers will also be prepared during the life of the project. This web site will also be used to disseminate the findings to a wider audience.

Finally, a workshop will be organised in October 2002 to present the design method to an audience of structural and fire engineers, fire protection manufactures, regulators and material suppliers. The feedback from this workshop will be used to amend the design guide.

For further details contact:

Dr David Moore

Director

Centre of Structural Engineering

BRE

+44 1923 664578

Email: mooredb@bre.co.uk

 

 


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