Experimental Study On Fire Resistance Of Concrete Roof Of Underground Space Structure

At present,there are few research achievements on the fire resistance of the concrete roof of the underground space structure and the mechanical behavior of the concrete slab after fire.There is often a large amount of soil covering over the concrete roof of underground space structure,which will lead to the obvious difference between the thermal boundary conditions of the concrete roof surface and the concrete floor in the building structure under high temperature.Moreover,the moisture content of the concrete slab of the underground space structure is usually higher than that of the latter which is easier to crack under high temperature.With the rapid development and utilization of underground space structures such as underground commercial complex,underground garage and subway station,etc.,the research on fire resistance performance of the concrete roof of underground space structure is of great significance to the research on disaster prevention and reduction of underground structure.In this paper,the fire resistance of the concrete roof of underground space structure is studied by the method of the combination between experimental research and finite element simulation.Including the heating under constant load test and the static load test under fire were carried out on four concrete slabs under standard fire,and the compressive performance of the concrete slabs under fire was simulated and analyzed with finite element software.The main research contents are as follows:(1)We investigated whether there are influences on the fire resistance of the concrete slab for covering soil on the concrete slab surface,exposing to the fire and producing the vertical deformation on the side beam of slab according to the heating under constant load test under standard fire.The results show that covering wet soil on the concrete slab surface will reduce the internal temperature of the concrete and steel of the slab,but the concrete slab bottom is easier to crack for covering wet soil and its fire-resistant limit is decreased by 15 percent;When the side beam of the concrete slab is not subjected to fire and has no vertical displacement,its fireresistance performance is better than that of the side beam which is subjected to fire and has vertical deformation,and the fire-resistant limit is increased by 9.7 percent.(2)The stiffness and ultimate bearing capacity of the concrete slab after fire were analyzed by static load test which carried out on the concrete slab after fire.The results show that the maximum deformation of the concrete slab under fire is142.75 mm and that of the residual deflection after cooling is 97 mm,the ratio of the two deformation is 68.0 percent;the concrete slab surface covered with wet soil is cracked,which weakens the stiffness of the members and reduces the ultimate bearing capacity by 10.1 percent.(3)Using the finite element software ABAQUS to simulate the temperature field and the fire resistance of the concrete slab under fire and the results show that the simulated temperature field agree well with the measured temperature.The parameters of the finite element model were used to analyze the fire resistance of the concrete slabs,and the results show that the lower the load ratio,the lower the span-depth ratio,the higher grade of the concrete strength and the higher fire resistance of the concrete slabs restrained by the side beams.