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Prescriptive Design Methods in Fire

Regulations require structural members to have enough fire resistance. There are two broad standards for the design methods of fire resistance of buildings: prescriptive and performance-based.

A prescriptive method defines a structural fire design fairly precisely in terms of the materials used, shape and size of structural elements, thickness of fire protection materials and construction details etc. Traditionally, the design recommendations are mainly based on the experience with identical or similar standard fire tests. This concept works very well in a static situation but inhibits innovation and development of construction industry. It can become very restrictive in situations where designs need to evolve to meet architectural or aesthetic requirements. For these reasons, the prescriptive designs have been evolving for many years towards the performance-based designs.

A performance-based method for structural fire design is analogous to the process of designing structures to wind loads and seismic effects. The function of a structural element will be defined and a set of objective tests will be given that allows the evaluation of fire performance in relation to the key functional criteria: Is it strong enough to sustain the working load? Does it last long enough before it collapses or causes the collapse of structural components connected to it?

Unfortunately, a fire heated structural element in a building does not behave in an isolation manner. The continuity and interaction of the heated elements to the rest of the building inevitably cause additional thermal stresses. Their softening due to elevated temperatures will reduce their stiffness and lead to load redistribution. Ideally, the objective tests should be conducted by either full-scale fire tests or comprehensive numerical simulations. However, it is very difficult and expensive to conduct satisfactory tests for some performance aspects such as the steel beam-column connections, and this has meant that the structural fire designs have often evolved towards a hybrid between prescriptive and performance-based concepts.

This section discusses the background to the existing prescriptive approaches of structural fire design. The standard fire tests will be described first, followed by the typical prescriptive approaches adopted in the fire design of concrete, steel, masonry and timber structures.

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