Research On Bearing Capacity Of Concrete Slab At Ambient (Elevated) Temperatures Based On Steel Strain Difference Method

At present,lots of tests and theoretical studies are conducted on the bearing capacities of the concrete slabs at ambient(elevated)temperatures.For the ambient condition,the yield line theory is often used to predict the carrying capacity of the concrete slab,and the tensile membrane action is also considered.However,the load-deflection relationship during the membrane action stage is linear,and thus this relationship is not conformed to the actual nonlinear structural stiffness.Meanwhile,the theoretical method for the simply supported slabs is usually extent to that at the elevated temperatures,and the effect of the restraint on the carrying capacities of the concrete slab is not considered.Hence,according to the above research,the following studies are conducted in this paper,and they are:(1)By means of the deflection control method,the load carrying capacities of seven concrete two-way slabs are conducted.In addition,the effect of the aspect ratio,the thickness,the reinforcement ratio and the corners’ restraint on the load carrying capacities of the concrete slabs is investigated.Finally,the failure criterion during the descending stage of the load-deflection curve is established.According to the experimental results,the whole failure criterion is that the failure load is 0.8 time of the limit load,and the ultimate deflection is 1.2 time of the peak deflection.(2)Based on the concept of steel strain difference,a five-stage load-displacement curve calculation method is proposed.For the membrane effect stage,the linear relationship between the steel strain difference and slab rotation angle and the ultimate load failure criterion are proposed,and the calculation method of two-way slab load-deflection curve is established to consider the influence of vertical shear force.Finally,the rationality of the method is verified by experiment and numerical simulation,and compared with other theories.On this basis,the value of steel strain difference and slab angle is analyzed,and the results show that the two parameters have important influence on the development trend of slab bearing capacity.(3)Fire tests are carried out on two restrained two-way concrete slabs,and the data of crack,temperature,in-plane(out-plane)and binding force are obtained.The test results show that the large area of crack and perforation hole in the bottom of the slab are caused by in-plane restraint,which greatly reduces the fire resistance of the concrete slab.In addition,the ultimate bearing capacity of concrete slab at high temperature is calculated and analyzed by using the theory of steel strain difference.The results show that the limit bearing capacity of concrete slab under high temperature is reduced by in-plane restrain.(4)Vulcan software is used to carry out numerical analysis on the fire behavior of the steady constraint slab in this research group,and the influence of concrete expansion strain on the membrane mechanism of the restraint concrete slab is studied.In addition,the fire behavior of unsteady restrained concrete slab is numerically analyzed by using spring element.And the influence of support type,restraint degree,thickness and reinforcement ratio on the deformation of concrete slab,film mechanism and fire resistance limit are studied.The results show that it is reasonable to use geometric nonlinear model to analyze the fire behavior of steady constraint slab.Larger concrete expansion strain leads to larger deformation.For unsteady restrained slab,the type and degree of support have a decisive influence on its deflection trend and fire resistance limit under fire,and the increase of slab thickness and reinforcement ratio is beneficial to improve its fire resistance limit.