ULTIMATE CAPACITY OF COMPOSITE FLOOR SLABS AT THE FIRE LIMIT STATE

摘要

Collapses of composite frame buildings in fire are an extremely rare event therefore the manner of structural failure is not well understood. In tests carried out on such structures and in many instances of real fires collapse has not occurred and therefore the exact failure mechanism is still a matter of debate. A number of investigations, including the authors own, have shown that the tensile membrane action that develops in the composite floor slab during the fire is extremely important in resisting collapse. This is because the large thermal strains that develop allow the floor to form a highly deflected shape whilst limiting the magnitude of damaging tensile mechanical strains. If the load capacity available through this tensile membrane action could be reliably quantified then it may not be necessary to apply fire protection to all beams leading to significant cost savings. A new method for determining the ultimate load capacity of composite floor systems in multi-storey building fires is presented. Initially the temperature distribution through the depth of the slab is calculated for the chosen design fire. For the temperature distribution through the slab the deflected shape and the resulting membrane stress-strain distribution are calculated using a rigorous analytical method. Finally an energy based method using the geometric form and stress-strain state calculated in the previous stage is used to determine the ultimate load capacity of the slab.