Reasearch On Innovative Prestressed Support Earth Retention System

In the foundation pit project,the Innovative Prestressed Earth Retention Support System(IPS)can not only actively control the deformation of the foundation pit,but also provide a large space for construction work.It is a deep foundation pit internal support technology with good comprehensive benefits.In the using process of IPS,it is necessary to dynamically tension the strand at both ends to realize the active control of the deformation of the foundation pit.Thus,The slippage of the steel strands occurs frequently during the entire life cycle of the IPS.In fact,structural systems containing slidable rope elements represented by IPS are common in Civil Engineering.Changes in the external load in the structure will cause the slip of the continuous cable at the stays,and the slip of the continuous cable will Causes redistribution of structural internal forces.For a structural system containing slidable rope elements,if the influence of rope slippage is not considered,it will inevitably cause serious distortion of the calculation result,so a reasonable simulation of the slippage process of the cable can accurately solve such structural equilibrium conditions.However,the existing methods can not solve the problem of cable slip in a structural system that includes a slidable cable unit.Simulations using a contact element in a finite element software are complex and computationally expensive;The analysis of the real process of cable slip often requires complicated mathematics and physics methods,and it is difficult to apply to engineering practice;the use of discontinuous cable equivalent simulation of the sliding process usually requires iteration,and the calculation process is complex and the convergence cannot be guaranteed.In addition,most methods can not consider the impact of frictional forces that may exist in cable support node on the calculation results.It is the key and difficult point to analyze the cable length change accurately according to the slip process.The change of the cable length is mainly caused by the slip of the cable at both ends.On the basis of related researches on cable slip problem,this paper innovatively proposes the sliding element method: the sliding element method takes the slip s at the kNuckle joint as the basic variable,and firstly extracts the slip stiffness matrix of the cable.Then,according to the cable force balance conditions on both sides of the bracing node,a sliding element force balance equation is established to determine the slippage of the cable during the slip process at each bracing node.Finally,the change of original length of cable is calculated according to the displacement coordination conditions,and then the balance cable force is calculated.The sliding element method has clear thinking and high computational efficiency.It can also consider the friction force during the cable sliding process.It can simulate the cable slip problem well and quickly solve the static equilibrium of the cable.Around the sliding element method,this paper develops theoretical derivation,numerical simulation,program development,and mainly completes the following tasks:(1)In this paper,the basic concepts,structural features and stress mechanism of IPS are introduced firstly.According to the type of kinked braces,the sliding of strands is classified into frictionless slip and frictional slip.The basic framework of this article is thus determined.(2)An equivalent simulation method for frictionless slip of continuous cables is proposed,namely the sliding element method.At the same time,the frictionless slippage of continuous cable in the joint of IPS is taken as engineering background.The sliding element method is introduced in detail.The concept,basic assumptions,unkNown variables,and the calculation principle and steps of the sliding element method are derived.On this basis,the single element cable model and the multi-node cable model were solved according to the sliding element method respectively,and the Calculation results of the sliding element method was compared with the theoretical solution to verify the accuracy of the sliding element method.(3)The influence of the friction force at the stay bracing on the cable slip is analyzed.The judging method of the frictional slip process and the balance relationship of the cable force are deduced.The equivalent simulation method of the friction slip of the continuous cable is proposed—sliding element method Considering the friction force.At the same time,the friction and slippage of the continuous fishbone girder braces at the sliding joint is taken as the engineering background.The concept,basic assumptions and unkNown variables of the sliding element method considering friction are introduced in detail,and the friction is also considered,the calculation principle and steps of the sliding element method considering friction are derived Strictly.On this basis,the single element cable model and the multi-node cable model were solved according to the sliding element method respectively.The calculation results of the sliding element method considering the friction force were compared with the theoretical solution to verify the accuracy of the sliding element method.(4)The sliding element method and the sliding element method considering friction force are unified theoretically,both methods can be collectively called the sliding element method.The program macro file SVM is written according to the basic idea of the sliding element method.The typical working conditions of the IPS in the foundation pit construction process are selected and the structural performance of the prestressed fish web accurately analyzed by the macro file SVM.In addition,the macro file SVM is used to analyze two typical problems of single-joint cable-stayed girder slip and friction-slip of prestressed concrete strands,which further validates the accuracy and practicality of the sliding element method and the macro file SVM.