Theoretical Analysis And Experimental Studies On Stability Of Super Wide Hollow Pier With Round End And Thin Wall

The existing railway bridges are mostly single-track bridges and double-track bridges in China, sometimes the ordinary single-track or double-track railway station has to be set up on the bridge because of the terrain limited, the railway station is generally composed by a number of tracks, so the railway bridges should be built into multi-track form inevitably. It is neither economical nor irrational to build solid pier when the height of bridge is higher, the hollow pier not only saves masonry, but also been loaded reasonable, so the construction of multi-track super wide hollow pier will be often encountered, especially the height of 30~60m multi-track super wide concrete hollow high pier have been used and developed more and more widely in the railway bridge engineering in China. Super wide hollow pier with round end and thin wall have the characteristic of columns structure, which also belongs to spatial plate and shell structure, so the mechanical characteristics is very complex. In the 1970 s, the stability problem of cylindrical, conical and rectangular hollow pier have been studied by some domestic research institutes, but the study is limited to single or double track concrete hollow pier, such super wide hollow pier with round end and thin wall is not involved. This paper seeks to make an accurate evaluation of overall stability and local stability of such multi-track super wide hollow pier with round end and thin wall, then summarize reasonable countermeasures to reduce the designing blindly, make the design rational to follow. And provide reference for similar project structures designing in the future to safety and reliability of super wide hollow pier with round end and thin wall design.In this paper, the four-track hollow pier with round end and thin wall of Lanzhou-Chongqing railway Lanzhou hub Dashaping super large bridge is taken as an example to analyze of the stability performance from theoretical calculations and experimental studies, and gets the instability characteristics and buckling mode of the hollow pier, reveals its instability failure mechanism, analyzes the impact of vertical clapboard to the stability of hollow pier, and then provide a reference for the future stability studies and engineering design of these super wide hollow pier with round end and thin wall.The main contents and conclusions are as follows:(1) Making finite element numerical analysis of the engineering instance pier. The original model with clapboard and the other model pier without clapboard are established to make linear elastic stability analysis. Calculation shows that the first order buckling mode of the two models show local buckling, the overall instability do not appear. The height, wall thickness, concrete strength, and vertical clapboard of the pier have a certain influence on linear elastic stability of super wide pier, the vertical clapboard and wall thickness impact more obviously of all.(2) Making nonlinear buckling analysis of the pier with clapboard and the one without clapboard respectively based on finite element method conclude that the buckling loads of the pier with clapboard is 4.48 times bigger than the pier without clapboard when nonlinear is not considered. The buckling loads of the pier with clapboard is 4.39 times bigger than the pier without clapboard by making geometric nonlinearity analysis when the structural initial imperfection is considered. The buckling loads of the pier with clapboard is 1.215 times bigger than the pier without clapboard by making geometric nonlinearity and material nonlinearity analysis when the structural initial imperfection is considered. Therefore, the vertical clapboard can be set to improve the elastic stability capacity of hollow pier, but the contribution to ultimate bearing capacity is limited when geometric nonlinearity and material nonlinearity are taken into account. It recommended that the vertical clapboard could be canceled in the future design of the same type of pier from the view of convenient to the construction.(3) The results of nonlinear buckling analysis shows that the stability carrying capacity of the pier is reduced significantly after the initial defect of the structure and nonlinear are considered. In which the e elastic-plasticbuckling load is 1/4.4 of the elastic buckling load of the pier without clapboard, while the elastic-plastic buckling load is only 1/16 of the elastic buckling of the pier with clapboard. It shows that the instability of the super wide hollow pier does not occur in the material linear elastic phase, but occurs in the elastic-plastic phase. The instability damage of pier is due to the material reach to its ultimate strength, so it is more accurate to make nonlinear elastic-plastic analysis.(4) For the pier without clapboard, the ultimate stability bearing load by nonlinear buckling analysis is 487028 kN, the stability safety factor K is 12.45; while the ultimate stability bearing load of pier with clapboard is 591628 kN, the stability safety factor K reaches to 15.02. It can clearly be seen that they both have sufficient safety margin by nonlinear elastic-plastic buckling analysis, the stability of such super wide hollow pier with thin wall can be with high security even though the initial defect and nonlinear are both considered.(5) Based on the Euler’s formula and railway bridges norms to calculate the overall stability bearing capacity of the hollow pier, and calculate the local stability bearing capacity of hollow pier according to the buckling of plates and shells theory. By comparing with the results of finite element analysis, we can obtain that the calculated stability safety factor based on the railway bridges norms is close to the stability safety factor that calculated by FEM which structural geometric defects and double nonlinear are taken into account, it shows that the overall buckling load formulas of pier based on the railway bridges norms is reasonable.(6) The Q345 steel is choosed as the experimental material based on the model similarity theory, producing two reduced-scale model respectively according to size of the pier with clapboard and the pier without clapboard, the experimental results and the finite element elastic-plastic buckling numerical results are in good agreement, it illustrates that the model experiment can simulate the instability force characteristics of the structure and the instability mode well, and it also verified that the analysis of the stability of super wide hollow pier is correct in this paper.