Study On Working Mechanism And Bearing Capacity Of The Composite Beam Exposed To Fire(High Temperature)

The concrete slab of the composite beam is usually connected to the steel beam by using the flexible shear studs, which forms an interface between the concrete and the steel beam to bear the external load. Many researchers have conducted a series of studies on the performance of the composite beams at ambient temperature systematically, and presented relative design methods and theories. Different to the performance of a composite beam at ambient temperature, the composite beam would failure in fire rather than the effect of the external loads, due to the different thermal expansion of the concrete and the steel, and the material degradation at elevated temperature. Because of these, the fire-induced slip in interface would cause the deterioration of the working capacities of the composite beams with the stiffness decreasing, which leads the composite beams to fracture eventually.Studies on the interface of composite beam at elevated temperature are not enough presently, researchers ignored the relative slip between the interface in order to simple the analysis in studying on the fire resistance analysis of the composite beam. However, in experiment and numerical analysis, the interface slip of composite beams will have disadvantages on their fire resistance. Therefore, it needs more parametric study to investigate the effects on the interface slips of the composite beams exposed to fire. Especially focusing on the thermal properties of the studs, the restraints at the beam end and the load ratio, this thesis completes the study on the mechanism and the bearing capacity of the interface of the composite beams at elevated temperature by using the numerical simulation and theoretical analysis. Finally, the thesis presents the design methods beneficial to improving the fire-resistant of the composite beams.The main study and the conclusions are listed in the below:(1) Firstly, the temperature distribution in the composite beam models, when the models exposed to the fire, were analyzed by using the thermal analysis of the numerical simulation. In the process, the simulated fire zone utilized the ISO834 standard temperature curve, considering the thermal properties of the material, such as the specific heat, density and thermal conductivity and conducting the convection coefficient and radiant coefficient as the real application. The results of the FEA and the experiment were compared and they are in good agreement, which validated the rationality and accuracy of the thermal analysis model. The temperature distribution gradient in steel webs is obvious in 15 min and the temperature gradient will lead to certain thermal expansion deformation in steel beam and affect the working performance of composite beam. Through the study of temperature distribution of the composite beam with shear studs,the temperature of the studs decreased from the roots along the thickness direction of the slab gradually. Considering the efficiency, in this thesis, the Spring elements were used to simulate the studs. Hence, the top flange temperature of steel beam is used to define the temperature of the Spring element.(2) The behaviors of the thermal-structural coupling of the composite beam were conducted at elevated temperature, in which the studs were still represented by the Spring element to reflect the slip properties in the interface. Based on the existing results of the push-out test about the studs at high temperature, the extracted force-displacement-temperature curve were used to define the nonlinear rigidity of the Spring element along the length direction of the steel beam. Using the technics, the analytical models of the composite beams were modeled and then were defined the results of the temperature distribution as the above Chapter. The results of the FEA and the experiment were compared and they are in good agreement, which verify the rationality and accuracy of thermal-structural coupling analysis model.(3) Based on the modeled methods and the validated properties of the models, this part implemented, 45 models, the parametric studies on the interface slip of the composite beam models at elevated temperature. These parameters are the restraints at the beam end(axial restraints and rotational restraints), the load ratio η and the stiffness of the shear connection R. By analyzing the mid-span deflections, the interface slip and the interface vertical uplift, this chapter studies the effects of the above parameters on the behavior of the composite beam models at elevated temperature. According to the results, the working mechanisms of the composite beams in fire were discussed for the theoretical study.(4) Based on the performance of the composite beam models in fire, the bearing capacity of the composite beams and the design methods were discussed. According to the fundamental hypothesis, the interface slip of the composite beams at elevated temperature and the formula of the bearing capacity were deduced by the theoretical method.