Analysis Of Mechanical Properties Of Pile-Column-Bearing System In Nonlinear Soils

Pillar pier with pile foundation is the common substructure of standard span girder bridge,and it is often located in nonlinear soils.In engineering design calculation,the bearing is simulated as spring or nonlinear contact element.The influence of eccentric bending moment caused by large shear deformation of bearing under vertical force can not be considered.However,the shear deformation has great influence on mechanical properties of pile-column-bearing system under accidental loads.The pile-column-bearing system in nonlinear soils is taken as the research object in the dissertation.The proposed element considering large shear deformation and P-Δeffect is used to research the mechanical properties of pile-column-bearing system.The dissertation solves the numerical analysis problem of structures with large shear deformation under the action of compression,shear and bending,which has important theoretical significance and practical value.The main work and innovative achievements are as follows.（1）A new column element considering large shear deformation and P-Δeffect is proposed,and it can be used to analyze the mechanical properties of pile-column-bearing affected by large shear deformation and geometric nonlinearity.The analysis results of the pile-column-bearing system show that the eccentric bending moment caused by the large shear deformation of the bearing has a great influence on the mechanical properties of the pile-column-bearing system.（2）According to the hyperbolic p-y curve of clay proposed by Wang Huichu and the hyperbolic p-y curve of sand proposed by Wang Guocui,another unified form of hyperbolic p-y curve is developed with two new key parameters m and y _Ldefined,and it is the p-y curve under m method.Parameter m is the ratio of elastic foundation coefficient of soils and parameter y _Lis the displacement eigenvalue of sois.Parameter m can be determined by pile test under horizontal force or selected by experience.The parameter y _Lcan be calculated according to the formula of the p-y curve under m method,or determined empirically,or calculated inversely according to pile test under horizontal force.Three Cases analysis of test piles shows that the horizontal displacement of the pile at ground calculated by the p-y curve under m method is basically consistent with the test value.In addition,the p-y curve under the（mz+C）method is developed.And it contains three key parameters,namely,the ratio of elastic foundation coefficient m,the foundation coefficient C and the displacement characteristic value y _L.The values of parameters can be determined as following steps:（1）m₁named as the ratio of elastic foundation coefficient of the upper layer soil is selected empirically or determined by pile test under horizontal force,and C₁named as the foundation coefficient of the upper layer soil can be calculated according to the formula of the p-y curve under the（mz+C）method.Then the ratio of elastic foundation coefficient and the foundation coefficient of subsoils can be determined according to formula of the p-y curve under the（mz+C）method;（2）y _Lcan be determined by pile test under horizontal force.If there is no pile test,it shall be selected empirically or determined according to the the formula of the p-y curve under the（mz+C）method.The analysis of model piles shows that calculated values of horizontal displacements of model piles both at ground and at top by the p-y curve under（mz+C）method are basically consistent with the measured values.（3）A new method named asλ_i-V_i-F_cricorrelation curve method,is proposed for calculation of the buckling critical load of the pile-column-bearing system or pile-column system in nonlinear soils.Case analysis is done,and the results show that the buckling critical load of the pile-column-bearing system will decrease as the eccentric bending moment and horizontal force at the column top increases.The critical buckling load of pile-column-bearing system calculated by m method is larger than byλ_i-V_i-F_cricorrelation curve method.So it is suggested that the adverse effect of nolinear soils should be considered in the calculation of critical buckling load of pile-column-bearing system in engineering practice.（4）The eccentric bending moment of bearing and the P-Δeffect of pier pile foundation have a certain influence on the first-order natural circular frequency of the pile-column-bearing system,but have little influence on the high-order circular frequency.