Behaviour of composite floors with different sizes of edge beams in fire

Novel experimental results and extensive numerical studies on three one-quarter scale composite floor assemblies tested in fire are presented. The purpose is to investigate the effect of bending stiffness of protected edge beams on the fire behaviour of the assemblies. The focus of the paper is on the behaviour of interior panels with both rotational and inplane restraints along the four edges. Test results showed that tensile membrane action was mobilised at a deflection equal to about 1.0 of the slab thickness irrespective of the bending stiffness of the edge beams. An increase of the edge beam bending stiffness could help to reduce the slab deflection initially. However, as temperature increased cracks in the slab over the protected main or secondary edge beams developed, the composite slab-beam action was weakened and the benefit associated with a greater stiffness of the edge beams was lost. Subsequent part of the paper describes the distribution and development of membrane stresses in the slab and the steel beams in fire. It is noteworthy that due to composite action of protected edge beams and slab, the neutral axis across a section rises and falls within the steel web during the fire duration. However, at the slab centre above the unprotected intermediate secondary beams, no clear tensile region can be found.