Influence of laps on the behaviour of a composite slab under membrane action in fire

Membrane action increases the load-bearing capacity of a two-way slab due to increased deflection. The composite slab must transfer enough tensile forces at the laps of reinforcements to ensure adequate load-bearing capacity under membrane action. This study conducted load-bearing fire tests on a large-scale floor system consisting of steel beams and a composite slab with welded wire mesh of guaranteed welded point strength in order to investigate the transfer of tensile forces within the laps under the membrane action. The lap length of meshes was the wire spacing plus 50 mm (150 mm), shorter than the 250 mm specified by 1992-1-1. Two specimens were subjected to loads that were tested with either 2 or 2.67 times the predicted load based on yield line theory. The floor system of the test specimens resisted the load not only during the 216-min heating phase but also during the cooling phase, despite experiencing the integrity failure. The temperature of the welded wire mesh exceeded 600 °C, and the maximum vertical deflection was 1/11 of the short span. The wires outside the lap failed, and the laps transferred sufficient tensile forces until the wires yielded. Therefore, the lap length of the wire spacing plus 50 mm (150 mm) was adequate for transferring tensile forces. The test results indicated that the load-bearing capacity of the floor system based on ECCS was conservative, considering the strength degradation of the welded wire mesh at high temperatures.