| On the basis of the urban traffic function,the modern subway constantly adds more and more commercial and living functions,and the surrounding ground and underground buildings(structures)are numerous,the construction is more and more complex,and the construction deformation control requirements are more and more strict.In many cities,construction of subway stations by means of cut and cover method is often the preferred technical solution.Subway station pit is mostly long and narrow,with small width and large depth.And these characteristics make it adopt the inner brace type envelope structure more.The steel tube bracing structure is widely used by its advantages of light weight,easy to hoist,prefabrication in factory,high quality control level,rapid and efficient construction,easy to remove,can be recycled and low cost.However,there are some local weakening issues in the steel tube bracing system,which affect its bearing capacity and safety to different degrees.It is easy to cause the deformation of the foundation pit to be too large,or even cause a chain effect,which eventually leads to the collapse of the foundation pit.The local weakening phenomena mainly include active joint,steel enclosing purlin,surface quality issues of diaphragm retaining wall and top beam,embedded steel plate,section mutation of axial force gauge and resist shear structure of corner bracing,etc.In this dissertation,by means of theoretical analysis,numerical simulation,laboratory test and field test,the issue of local weakening of steel tube bracing caused by active joint in the foundation pit engineering was studied in depth,and the research and development of bolt-fasten-wedge(BFW)active joint was carried out.The main research work and results are as follows:(1)In order to solve the problem of local weakening of steel tube bracing caused by active joint,the mechanical properties of the commonly used steel-wedge(SW)active joint were studied by theoretical analysis,numerical simulation and laboratory test,and the design principle,force transmission mode,mechanical properties and failure modes of SW active joint were revealed.The weakening degree of steel tube bracing caused by SW active joint was analyzed by establishing a fine numerical model of steel tube bracing with SW active joint.The numerical results show that SW active joint has obvious weakening effect on stiffness of steel tube bracing,and its bearing capacity,stiffness and safety reserve are insufficient.(2)The design ideas,working principles and mechanical properties of hoop-disconnectable active joint,splints active joint and anchor-seat active joint were expounded and the mechanical mechanism and failure modes of these types of active joints were revealed through theoretical analysis and summary of laboratory test results.The design conception of BFW active joint was put forward,the structure composition and working principle were expounded,and the calculation method of the design bearing capacity and design stiffness was established,they are all on the basis of the design experience of hoop-disconnectable active joint,splints active joint and anchor-seat active joint.(3)The plane-contact-surface BFW(P-BFW)active joint was developed,the force analysis model and theoretical calculation method were established,and the design parameters such as bearing capacity and stiffness were obtained.2 sets of P-BFW active joint full-size specimens were developed and manufactured,and the laboratory mechanical properties tests were carried out.The test results show that the P-BFW active joint has excellent bearing capacity,good stiffness and resistance to eccentricity.The test results are close to the theoretical calculation results,which verified the correctness of the theoretical calculation method.The nonlinear numerical model of P-BFW active joint was established,and the failure modes,load-displacement curves and load-strain curves were obtained through analysis.The numerical simulation results are in good agreement with the test results,so the mechanical properties of P-BFW active joint can be predicted effectively.(4)The arc-contact-surface BFW(A-BFW)single-legged and double-legged active joints were developed,and the design parameters such as bearing capacity and stiffness were obtained.4 sets of A-BFW single-legged and double-legged active joints full-size specimens were developed and manufactured,and the laboratory mechanical properties tests were carried out,then the failure modes,load-displacement curves and load-strain curves were obtained.The test results show that the A-BFW double-legged active joint has excellent bearing capacity,good stiffness and resistance to eccentricity.In addition,the structure of the arc-nested-contact surfaces avoids the separation between the wedge seat and the splints,which can improve the overall performance of the joint.The nonlinear numerical model of A-BFW double-legged active joint was established,and the failure modes,load-displacement curves and load-strain curves were obtained through analysis.The numerical simulation results are in good agreement with the test results,so the mechanical properties of A-BFW active joint can be predicted effectively.(5)The fine numerical models of steel tube bracings with P-BFW and A-BFW active joints were established respectively,and the corresponding laws of linear elastic deformation were revealed.The results of different numerical models show that the steel tube bracing with BFW active joint has a greater degree of strengthening and improvement in the axial deformation control compared with the steel tube bracing with SW active joint.BFW active joint provides better bearing capacity and stiffness,which can better improve the local weakening of steel tube bracing,improve the overall mechanical properties of steel tube bracing,and better control the deformation of foundation pit,compared with the SW active joint.(6)Through the field application tests of 3 sets of P-BFW active joint full-size specimens,we can get the results that the P-BFW active joints are able to perform convenient installation and demolition operations at the construction site,and have well realized the functions of adjusting the length of steel tube bracing,applying axial prestress,and bearing the axial force of steel tube bracing.The working performance and construction feasibility of P-BFW active joint were verified.The technical points of installation operation were summarized,and an improved design scheme was given for the problem of dislocation and mutual release of the contact surfaces between the splints and the wedge seat. |
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