Study On The Compression Behavior Of Long Support Of Concrete Filled Double-Skin Steel Tubes For Subway Foundation Pit

At present,the conventional steel support is often used in the subway foundation pit support system.According to Technical specification for retaining and protection of building foundation excavations JGJ 120-2012,it is easy to cause the effective length of the component to be too long in the large-span foundation pit,which does not meet the length requirements of the steel support in the specification.And in the conventional subway foundation pit support scheme,it is often not set up rod on the support to meet the length requirements.If the traditional concrete filled double-skin Steel tube structure is adopted,the gravity of structure will increase,which will not only bring inconvenience to the transportation process,but also form a waste of concrete materials.To this end,long support of concrete filled double-skin steel tubes for subway foundation pit is proposed,This paper mainly studies its mechanical performance.The main work of this paper is as follows:(1)Use ABAQUS to carry out finite element simulation by selecting appropriate material constitutive model,contact mode and boundary conditions.Verified the feasibility and correctness of the simulation method by comparing the simulated result and experimental data of references;(2)Design long support of concrete filled double-skin steel tubes by considering the engineering project.The finite element models including different research variables are established by using the verified simulation method.The influence of hollow section ratio,diameter-to-thickness ratio,slenderness ratio,eccentricity and other factors on the compression behavior of the component is studied;(3)Considering the engineering project,two kinds of flange joints are designed to connect the multi-section supporting components,and the problem of disassembly and reuse of the component is to be solved.The influence of the number of bolts,flange thickness,flange type,bolt pre-tightening force and other factors on the compression behavior of components is studied.It is found that the failure mode of flange joint without stiffeners is basically the same as that of flange joint with stiffeners under eccentric compression.Under the eccentric compression,the upper and lower flange plates are subject to collaborative stress and deformation,and the stiffeners are beneficial to the collaborative stress of flange plates.The setting of stiffeners increases the bearing capacity of the components.In addition,Changing the number of bolts can make a influence on the ultimate capacity of both two types of flanged joint components.In the scope of design,increasing the flange thickness has a certain degree of influence on improving the ultimate capacity and ductility of the component,but there are some limitations.The bolt pre-tightening force has little effect on the mechanical properties of the two types of flange joints.(4)According to the existing formulas of bearing capacity of concrete filled double-skin steel tube,compare the calculation results with the finite element analysis results.It is found that the range of the ratio is 0.83-0.98 under the axial compression,and most of ratios are in the range of 0.90-1.02 under the eccentric compression.On the basis of parameter analysis,considering the existing formula in the specification and using the method of curve fitting,this paper summarizes the calculation formula which is suitable for a certain range to calculate the number of bolts of long support of concrete filled double-skin steel tubes for foundation pit which with flange joint with stiffeners.