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Performance Based Approach: Simple Calculation Methods

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 external columns.
  • 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 membrane action.
  • Re-distribution of moments with frame action.

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