Dynamic Bucking Analysis Of Pile Fountdations With Elevated Caps

Pile foundation especially those with elevated caps is widely applied as the foundation of industrial and civil constructions,airports and stations,road and bridge projects,offshore platforms,water conservancy and hydropower projects,ports and wharves and other fields because of its advantages like high bearing capacity,little settlement as well as its capability of utilization in various complex geological conditions.Under the combined action of dynamic loads such as structural self-weight and upper load(static load),seismic load and construction vibration etc.,the pile foundation with elevated caps may be destroyed because of losing stability,which is abrupt with quite serious consequences.So how to analyze the dynamic buckling of pile foundation with elevated caps is of great significance for the engineering theory as well as its practice.In this thesis,the analytical methods and analytical models on several pile-soil interaction are systematically discussed according to the existing research achievements relating to the buckling stability of pile foundation and the pile-soil interaction from home and abroad.On such basis,the dynamic nonlinear equation of pile foundation with elevated caps under the action of vertical transient striking load is established by introducing the pile-soil displacement theory with fluctuating characteristics and simplifying the motion differential equations of pile-soil interaction under dynamic action,considering the constitutive and geometrical nonlinearity;and the dynamical equations on time are gained by supposing the displacement shape functions and applying the Galerkin method to integrate;and then the B-R criterion to determine the stability of pile foundation with elevated caps is applied by solving the nonlinear equations on time and obtaining its displacement response,and a simplified method about applying finite element method to the buckling of pile foundation with elevated caps under dynamic action is poposed.Through proposing the simplified finite element method(FEM)which is different from the dynamic buckling analysis of pile foundation with elevated caps and applying the APDL language to compile different simplified finite element model program and obtain the displacement diagram of pile foundation with elevated caps of a certain node when simplified finite element model under the action of transient dynamic load and using the B-R criterion to judge the stability of pile foundation with elevated caps and comparing with the analysis results of abovementioned theoretical formula,the correctness and applicability of the simplified finite element method is verified and a new theoretical approach to the dynamic buckling analysis of pile foundation is provided as well.The main research work is as follows:(1)In his thesis,the origin and significance of the issue and the current research situation on the dynamic buckling of pile foundation at home and abroad is introduced and the limitations as well as advantages and disadvantages of the existing research achievements are discussed through summarizing a large number of related literature.(2)The basic theory of dynamic buckling of pile foundation with elevated caps is narrated and the analytical methods and analytical models on several pile-soil interaction are systematically discussed.And the basic formula of nonlinear dynamic buckling of pile foundation with elevated caps is established by simplifying mechanical model according to the mechanical characteristics.And the program design scheme is given and the basic theory of the Galerkin method is introduced through numerical computation by applying the Runge-Kutta method.(3)In order to overcome the common disadvantages of the huge number of model nodes,the quite large scale of problem and the low efficiency of calculating rate under pile-soil system dynamic interaction established by finite element method,in this thesis,through the abovededuced simplified calculation formula,a simplified finite element method is proposed which can be used to simplify the finite element method of pile-soil system dynamic analysis and to compile APDL language program by applying ANSYS software and establish a simplified finite element model of single pile among which the pile body is simulated by beam element and the soil body is simulated by elastic resistance element.(4)Through applying the simplified finite element model to analyze the dynamic buckling of the pile foundation with elevated caps to make sure the transient buckling characteristics of the pile foundation with elevated caps under the simplified finite element model is established,and he time displacement curve and the dynamic buckling value of each simplified finite element model are obtained,and the finite element model of the single pile is used to analyze the dynamic response under various transient loads,then the corresponding time displacement curve is obtained and the stability of the pile foundation is judged by the B-R criterion,and on such basis,compared with the analysis results of the basic equations of the nonlinear dynamic buckling problem of pile foundation with elevated caps established on the Galerkin integral method theory,the correctness and applicability of the simplified finite element method are verified.(5)The dynamic interaction of the pile-soil system is simulated by changing the height of the pile foundation,the depth of the pile,the pile diameter and the pile-soil stiffness of the simplified finite element model,and the relationship between the dynamic buckling of the pile foundation and the above factors is obtained.And through comparing the calculated results with the results obtained by the corresponding simplification method,it can provide the computation basis for the solution of the dynamic buckling problem of the pile foundation with elevated caps by using simplified finite element and it can also lay the theoretical foundation of solving the practical project problem by applying the simplified finite element method.(6)Based on the project case-the pile foundation design problem of the Maling River Bridge,the model when pile foundation with elevated caps is under complex geological conditions is established by using simplified finite element method and after analyzing the dynamic buckling,it proves that such simplified finite element method applying to the practical analysis of dynamic buckling projectof pile foundation with elevated caps is quite feasible and practicable.