EXPERIMENTAL STUDY ON THE MECHANICAL BEHAVIOR OF COMPOSITE BEAMS WITH SUPERPOSED SLABS AFTER FIRE

In order to study the performance of composite beams with superposed slabs after fire, five composite beams with superposed slabs after exposure to fire and one composite beams with superposed slabs at room temperature were statically tested. Influence of factors such as the thickness of post cast slab, the stud spacing, the overlapping length of the precast slab on the upper flange of the steel beam, the joint form of precast slabs and exposure to fire or not on failure mode, bearing capacity and initial flexural stiffness of composite beams with superposed slabs were tested. The results indicate that the ultimate bearing capacity of integral composite beams with superposed plates are 9% higher than that of separated composite beams with laminated slabs after fire. The thickness of post-casting layer and the spacing of stud are the main factors influencing the initial flexural stiffness of integral composite beams with superposed plates after fire. Before and after the fire, the integral composite beams with superposed plates are subjected to bending failure under the action of load, and the failure modes are basically the same. However, after the action of fire with high temperature, the cracking of integral composite beams with superposed plates occurs earlier, and the flexural capacity and ductility decrease to different degrees, In this test, the flexural capacity and ductility of integral composite beams with superposed plates after fire under load are reduced by 23.3% and 55.4%, respectively, compared with those under normal temperature. The existence of the joint of precast base plate in the separated composite beams with laminated slabs damages the continuity of the precast base plate and causes the bond failure of the specimen. However, the cooperative performance of the superposed slabs is good at the place far away from the joint. The proposed calculation method for the residual strength and flexural rigidity of composite beams after fire has high accuracy.