| Box cross-sectional bridges are widely used in urban cross-line bridges,large-span mountain bridges for their good mechanical properties;However,the shear lag effect of crosssectional bridges in the box has also changed in response to environmental climate,self deficiencies within the section and overload during service.In view of the above-mentioned characteristics,theoretical analysis,finite element numerical simulation as well as primary beam theory were used to analyze the shear lag effect of box girder in fracture free state and fracture damaged state under the effect of temperature.The main work of this paper is as follows:(1)Based on the expression of the axial displacement of the box girder under the temperature effect,the energy variational method is used to derive the governing differential equation of the shear lag effect of the box girder under the combined thermal-mechanical action.The corresponding normal stress expressions are obtained from the function of moment and shear force with the beam length.The equivalent load method is used to equalize the prestressed end of the prestressed tendon and the prestressed in the span under different beam distribution methods,and the corresponding normal stress expression is obtained.(2)The existing literature examples are introduced,and the analytical and numerical solutions in this paper are compared with the literature solutions to verify the reliability of the analytical method in this paper.The distribution of the temperature stress along the continuous beam under only the temperature gradient is studied,and the substantial reason for the change of the total temperature stress from the middle fulcrum to the edge fulcrum is revealed.Due to the large difference in temperature effect between the top plate and the bottom plate under the temperature gradient,the distribution of the normal stress with the beam length at the junction of the top plate and the web and the interface of the bottom plate and the web was analyzed respectively.It is found that the theoretical solution of the elementary beam at the top plate is much larger than The difference between the analytical solution and the numerical solution is EaT(z);while the theoretical results of elementary beams at the base plate and the analytical solution are only slightly different at the loads where the shear lag effect is obvious.(3)The influence of crack damage on the structural characteristics of box girder is analyzed,and the evaluation index expressions such as longitudinal prestress reduction coefficient,bending stiffness reduction coefficient and shear stiffness reduction coefficient are obtained.Based on this,the corresponding normal stress expression is obtained.The effect of the initial fracture height ratio on the tensile stress of the floor during the crack propagation process is studied.The results show that:in the loading process,the tensile stress and the load first have a linear relationship;after reaching a certain loading ratio,the tensile stress and the load are no longer linear.Instead,it increases with an "n"-shaped parabola that is higher than the original linear rate of change,and then decreases after reaching the main tensile stress level.In addition,the cracking load does not change with the joint height ratio all the time,but remains unchanged after reaching a certain value.(4)With the help of finite difference method,the governing equations of shear lag effect of box girder with variable cross-section are obtained,and a program for calculating shear lag effect of box girder with variable cross-section is compiled by using Matlab,and the rationality of the program result is verified by comparing the program result with the numerical solution.By introducing the influence coefficient of temperature effect,the results of considering temperature gradient and not considering temperature gradient are compared,and it is found that temperature gradient will weaken the shear lag effect of the roof of box girder with variable cross section by 34.9%～66.6%. |
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