Study On The Mechanical Characteristics And Failure Mechanism Of Pile Foundation Under The Interaction Of Tension And Bending Moment

In the performance-based seismic design method of Bridges,the pile foundation is usually used as a capability protection member to resist the bending moment,shear force and axial force under normal service state and ultimate bearing capacity state,as well as the maximum bending moment,shear force and axial force from the piers under the earthquake action.When subjected to the maximum bending moment transmitted by the pier,the outermost pile in the pile group foundation may be subjected to great tension.Based on this,this paper takes the bridge pile foundation of Tianjin rail transit Z2 line as the engineering background,and studies the mechanical characteristics and failure mechanism of pile foundation under the interaction of tension and bending moment.The main research contents and results of this paper are as follows:(1)The failure mechanism of single pile under the interaction of tension and bending moment is analyzed,and the failure state of large and small eccentric single pile under the interaction of tension and bending moment is determined.Based on the failure mode of single pile section under the interaction of tension and bending moment,the theoretical calculation formula of bending capacity of single pile with large and small eccentricity is derived.The simplified calculation formula of bending capacity of the single pile under the interaction of tension and bending moment is put forward,and compared with the numerical analysis results,which shows that the simplified calculation formula has better accuracy.(2)Based on the failure mechanism of single pile under the interaction of tension and bending moment,the mechanical characteristics of the single pile under the interaction of tension and bending moment are analyzed,and different laws of deformation and internal force of pile with external load were studied.The influence of tension,bending moment,bending rigidity and boundary conditions on the mechanical characteristics of the single pile under the interaction of tension and bending moment is discussed.The analysis shows that when the single pile is subjected to different axial tension,the distance between the reverse bending point and the top of the pile is almost unchanged.When the pile foundation is subjected to the same axial tension,the shear strength is increased with the bending moment.Under a certain pile length,there is an optimal value between pile diameter and bearing capacity.When the loading is the same,the response of fixed pile is greater than that of free pile.(3)The failure mechanism and mechanical characteristics of pile group under horizontal earthquake action are analyzed,the failure process of pile group and the failure state under various levels of horizontal earthquake action are discussed,and the deformation and internal force distribution of pile group under different levels of earthquake action are studied.Based on the failure mechanism of pile group,the seismic design method of pile group considering the bending stiffness of pile group is proposed,including shear design method of pile group and bending design method of pile group,which is validated by an example.