Fire Resistance of Prestressed Multispan Steel Truss Composite Slab

Abstract

Prestressed steel truss composite slab is composed of a precast prestressed bottom slab, a steel truss, and a postcast concrete layer. In order to investigate the fire resistance performance of prestressed multispan steel truss composite (STPC) slabs under the coupled effect of high temperature and load, four specimens of such composite slab containing two spans with various postcast concrete layer thicknesses were fabricated. Fire experiments were conducted following the ISO 834 standard temperature–time curve. Two of the specimens underwent two-span fire experiments, while the other two underwent single-span fire experiments. During the experiment, the bottom of the multispan STPC slabs exhibited serious bursting under fire and the precast concrete layer at the bursting locations fell off. The experiment result showed that the degree of bursting was much higher than that of nonprestressed composite slabs and single-span slabs. Although bond failure was observed at the composite interface where horizontal cracks developed, the STPC slabs still performed well in terms of load bearing, which could be attributed to the contribution of the steel truss. Under the coupled action of high temperature and load, concrete cracks were prone to occurring at the cut-off position of the top reinforcement near the supports; therefore, the anchorage length of these reinforcing bars should be appropriately extended. The damage to the STPC slab specimens after fire was found to be more serious for those with a less-think postcast concrete layer. In the single-span fire experiments, the structural performance of the unfired span was not prominently affected. Due to the bursting of the precast layer, the thickness of the STPC slabs was reduced, resulting in decreases in the thermal insulation and the fire resistance limit. The fire resistance limits of the STPC slabs with 60 mm and 80 mm postcast concrete layers were 90 min and 130 min, respectively. When a high fire resistance limit of the STPC slabs is required in a design, it is recommended that the thickness of the postcast concrete layer be no less than 60 mm.