The simplest calculation methods are based on
the behaviour of individual members. These members could be in
the form of a column, beam, wall or floor slab. Guidance on the
design of structural members is presented in codes and design guides.
With member design, the effects of restraint to axial thermal expansion
are ignored. However, the effects of thermal gradients through
the cross section are generally considered.
The simple member calculation methods are typically based on
strength and provide no detail on the displacement history, or
maximum displacement, of the member during the fire.
If the design approaches are based on fundamental engineering
principles, with the strength of materials within the member
being reduced with increase in temperature, then they are valid
for any fire scenario. However, there are some cases where the
design procedures given in the codes, particularly relating to
concrete, composite construction and timber members, are only
valid for the standard time-temperature fire scenario, since
they have been derived from, and validated against, standard
fire test results. The designer should ensure that the calculation
approach adopted for estimating the structural response is valid
for the fire scenario considered.
It is generally accepted that the available calculation methods
for the design of individual members will provide acceptable
conservative answers. However, the design approach ignores the
true structural response of the building, which can be either
detrimental or beneficial to the survival of the building as
a whole. The important modes of behaviour that are generally
ignored in member design are described below:
- Spalling of concrete
The effects of thermal expansion of the beams laterally displacing
Any pulling in of external columns from catenary action of beams.
Any induced forces acting on a wall due to the movement of the
heated structure in the proximity of the wall.
Any beneficial effect of alternative load paths, catenary action
or membrane action.
The effect of induced compressive forces due to restrained thermal
expansion. These induced compressive forces could cause buckling
of vertical elements, local buckling of beams, increase the susceptibility
of concrete spalling, or increase the beneficial effect of compressive
Re-distribution of moments with frame action.