Fire Resistance Analysis Of Reinforced Concrete Slab By Finite Element Method

Reinforced concrete is a kind of very important construction material in civil engineering, whose fire resistance is significant for structural safety and stability, so it is necessary to study the fire resistance of reinforced concrete slabs. Up to now, there are two ways of studying the fire resistance of reinforced concrete slabs: experimental method and finite element method (FEM). Experimentation can offer the deformation and the destroy course in fire, which provided precious data for the numerical analysis. But the experimentation is expensive in time and money and limited by experimental conditions, so as not to carry through the research of the effect of full-scale parameters on fire resistance. On the base of the accurate result and correct model validated by experimentation, the FEM can simulate diverse parameters, present the effects of the parameters on fire resistance of structure, and provide the theory gist for optimum design. So it has great significance for structural fire resistance design. In this paper the finite element models of reinforced concrete slabs were developed by ANSYS, temperature field of structure in fire were simulated by thermal analysis, and structural mechanics capability and destroy character in fire were analyzed.The behavior of multi-span continuous reinforced concrete slab in the case of local fire and entire fire was investigated by FEM. The results showed that : 1) When the location of the fire is different, the location of plastic hinge and scope of the plastic area are different, but both of them have relationship with the end of the negative moment reinforcement; 2) When the whole slab was fired, the middle span rose because of the structural holistic action, and at last took on promontory; but when the slab was fired in middle span, the deflection of the middle span was increased in negative y direction always.The effects on the structural fire resistance of cover thickness, the concrete quota, sectional height, the length of negative moment reinforcement were investigaed. It was discovered:1) The effective height reduced with the increasing of cover thickness, which induced to the decreasing of the stiffness and ultimate bending moment of reinforced concrete slab, but at the same time the temperature increasing ratio of reinforced steel was slowly in fire, it resulted in the lower strength and stiffness reduced ratio and better fire resistance. The optimal cover thickness(20～25mm) was recommended on the basis of comprehensive understanding, 2) Properly increasing sectional height could effectively improve the fire resistance of structure, 3) The fire resistance of structure was not optimal whatever the concrete quota percentage of reinforcement was too much or too small, and research results showed that the fire resistance of structure was optimal when the concrete quota wasρ=0.598%, 4) The length of negative moment reinforcement has significant effect on structural performance, which affect directly the location of destroy hinge and length of plastic period, and the finite element analysis results showed that: the length of the negative moment reinforcement should adopt 1/3 times of the calculational length of slab, whether the length was too long or too short can reduce the fire resistance.Then the finite element models of the reinforced concrete column-steel beam-reinforced concrete slab composite structure was developed to investicate mechanics behavior of the structure. The results showed that the deformation of structure both in normal and high temperature was the dish tilt trend, but the developing process of cracks were different. Because of the membrane effect, the fire resistance time of structure is much longer than which is limited by the rule. Then the effect of load ratio on fire resistance were investigaed, the result showed that the load had a great effect on the fire resistance.