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
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.