Non-Linear Analysis On The Reinforced Concrete Structure Subjected To Fire

Fire resistance of structures is an important and difficult problem in structural engineering. Fire resistance of reinforced concrete structures is better than other ones, but it is also necessary to research the fire resistance properties. Because of the high cost of fire testing facilities and the fire tests itself, it is more and more important to use software of finite element analysis to model the nonlinear response of the reinforced concrete members and structures in fire. Based on the systematic analysis and conclusions of fire resistance research in this thesis, the nonlinear finite element response of reinforced concrete structures is analyzed. The main content is organized as follows:1. The research results of material performance of the steel and concrete at high temperature are summarized, including the thermal and mechanical performance parameters of steel and concrete in high temperature. These parameters are the important precondition to research the mechanical properties of reinforced concrete structures in high temperature.2. Based on the hybrid mathematic model of finite element-difference, the analysis program of the internal temperature field are developed, and the computing results show in good agreement with the existing test results. The results of test and theoretical analysis demonstrate that the method and the computer program given in the thesis are reliable.3. Considering the demand of modeling, the basic mechanical principles that the constitutive model should be satisfied is introduced in brief. Then the basic formulae of constitutive model of reinforced concrete are derived, and the basic steps of establishing the constitutive model of reinforcement and steel in high temperature are given. According to the constitutive model, the numerical solving method of elastic predictor and plastic corrector is established, and the efficiency of computing equivalent plastic strain and plastic deformation is greatly improved. Corresponding, the numerical algorithm of constitutive model of steel and reinforcement in high temperature is established based on the backward Euler method. According to the above constitutive model and numerical algorithm, the program of nonlinear analysis is developed, and the existing tests are simulated, which demonstrates the validity of the constitutive model and numerical algorithm. 4. A simplified calculation approach is provided, which can be used for calculating the ultimate bearing capability of reinforced concrete eccentric compression member. The effects of material property, mechanical performance and stability deterioration of structures under high temperature is considered. By this approach, ultimate bearing capability of the reinforced concrete eccentric compression member under different temperatures can be calculated accurately, and the results are matching well with the test data. Based on this, taking a four-face heated reinforced concrete column for example, each influence parameter on ultimate bearing capability is analyzed. The results indicate that concrete cover, concrete compressed strength and sectional dimension are the main influence parameters on ultimate bearing capacity, while the influences of steel ratio and steel yield strength can be ignored.5. Considering the constitutive model of materials at different temperatures, ultimate bearing capability of reinforced concrete slabs under membrane effect is analyzed. Based on the force equilibrium and geometric compatibility equation in the middle plane, two governing differential equations expressed by the deflection and Airy stress function are obtained. Based on the principle of energy, the deflection, membrane force, stress, strain and the ultimate bearing capability of slab are deduced by double Fourier series. The calculation results are in good agreement with the test results, therefore, the results can offer the reference for the fire resistance design of slab when the membrane effect is considered.6. The thermo-elasto-plastic material constitutive model in the condition of large deformation is deduced. The nonlinear analysis of reinforced concrete continuous beam in high temperature is made with beam-columniation element and load increment method. By calculating the ultimate bearing capability and mechanical properties of reinforced concrete continuous beam at high temperature, the change law of internal force redistribution and deformation were analyzed in different load positions, loads levels and different numbers of heating spans. The results show that, in heating process, the internal force redistribution and the forming of plastic hinges at high temperature in continuous beam are caused by the loading position and level, which can change the failure pattern of structure.