Experimental and numerical evaluation of composite floor systems under fire conditions

• Tested 3 composite floor assemblies in fire to study stiffness of secondary beams.
• Abaqus/explicit was used to simulate results with good correlation.
• Stiffness of the secondary edge beams increased, the slab deflection decreased.
• Composite action between the edge beams and the concrete slab plays a key role.
• The commencement of tensile membrane stage was marked by one of three indicators.

This paper presents novel experimental results and observations from three one-quarter scale tests on two-way concrete slabs supported by protected steel edge beams under fire conditions. The sizes of the protected secondary edge beams were varied to study the effect of beam stiffness on the fire behaviour of the assemblies. Test results showed that as the stiffness of the protected secondary edge beams increased, the slab central deflection decreased and failure of the slab occurred later. However, composite action between the edge beams and the concrete slab plays a key role in mobilising this beneficial effect. Once the composite slab-beam action is weakened by cracks in the slab over the main or secondary edge beams, the benefit associated with a greater stiffness of the edge beams is lost. Tensile membrane action was mobilised at a deflection equal to 0.9 to 1.0 of the slab thickness irrespective of the bending stiffness of the edge beams. The commencement of tensile membrane stage was marked by one of three indicators: (a) concrete cracks which formed a peripheral compressive ring in the slab; (b) horizontal in-plane displacements along the slab edges; and (c) horizontal and vertical displacements of four corner protected steel columns. The test results were used to validate a finite element model developed using Abaqus/Explicit. Good correlation between the predicted and experimental results was obtained.