Study On Temperature Action Of Steel-Concrete Composite Beam Bridges In Sichuan-Tibet Railway

The steel-concrete composite beam is a kind of structure that makes good use of the advantages of concrete and steel.Due to its excellent mechanical properties and economy,it has been widely used in the field of bridges in recent decades.The long-term exposure of bridges to the natural environment will induce uneven temperature fields,which will cause temperature effects such as temperature secondary stress,secondary internal forces and deformation.These temperature effects have adverse effects on the structure such as concrete cracking,steel plate buckling,bearing damage and so on.Moreover,the thermal conductivity of concrete and steel is quite different,which may aggravate the unevenness of temperature filed,and cause significant temperature effect.With the development of infrastructure,the climatic environment that bridges should confront is becoming more and more extreme,so the temperature effect should be underscored.Therefore,more in-depth studies of the temperature field and temperature effects of composite beams are necessary.The main research contents and conclusions of this thesis are as follows:1.Through numerical calculation of heat-fluid coupling analysis,the convective heat transfer coefficient on the surface of the concrete slab was calculated and compared with the measured value.The results are in good agreement,which verifies the reliability of the heat-fluid coupling numerical analysis.The two-dimensional heat-fluid coupling analysis model of the composite beam was established,and the area-weighted convective heat transfer coefficients of each surface of the composite beam are obtained.The results show that the temperature difference between the environment and the structure has little effect on the convective heat transfer coefficient.The convective heat transfer coefficient is related to the local wind field,wind vortices near the surface will lower it.Therefore,it cannot be determined only by the wind speed at a certain height above the surface.Calculate the convective heat transfer coefficient of each surface under different wind speeds,the formula of convective heat transfer coefficient of each surface are obtained by formula fitting.2.Calculate the convective heat transfer coefficient of the composite beam under different air pressures.The result shows that the pressure drop has a significant effect on the convective heat transfer coefficient.Through numerical analysis and formula fitting,the formula of convective heat transfer coefficient on the surface of composite beam in high altitude area is obtained.It can provide references for the analysis of temperature field of structures in plateau area.3.Through two-dimensional numerical analysis,the effect of the commonly used convective heat transfer coefficient and the convective heat transfer coefficient considering the heat-fluid coupling and the influence of air pressure on the temperature field of the composite beam is compared.The analysis shows that using the common convective heat transfer coefficients,which are generally too large and uniform on each surface,will cause the calculated composite beams to have a larger vertical temperature difference in concrete slab and a smaller lateral temperature difference in steel beams.4.The three-dimensional thermal-structural coupling analysis of railway composite beams is carried out,and the most unfavorable horizontal and vertical temperature gradients and corresponding temperature effects of composite beams are obtained.The results show that the under the action of temperature,the tensile stress on the bottom and sides of the concrete is large,which should be attached importance to in engineering practice.5.The most unfavorable temperature gradient of railway composite beams is extracted and the temperature distribution model is obtained.Long-term temperature gradient load data of railway composite beams in four different areas on the Sichuan-Tibet railway line is obtained by numerical analysis.Based on the reliability theory,the design representative temperature load is calculated.Through analyzing the representative temperature load and long-term temperatue field data,it is found that the unfavorable temperature load is not only related to daily climatic conditions,but also greatly related to changes of climatic conditions within a few days.Though comparing,it is found that the calculated temperature gradient modes in the four areas are different from the temperature gradient modes stipulated in the current specification of China.