Study On Seismic Behavior And Design Method Of PHC Pile-Cap Connections Subjected To Strong Earthquake

In recent years,prestressed high-strength concrete piles(PHC piles)have gained a rapid development in civil engineering in China.They have been widely used in the foundations of various types of engineering constructions.International survey data of structural damages under seismic loads show that it is possible that the PHC foundation could be severely damaged when it is subjected to strong earthquake.However,currently there are no design codes on the seismic design and seismic behavior of PHC pile foundation in China.Especially,there is a lack of understanding of the seismic design calculation and construction details of pipe pile and pile cap connections.Moreover,there are no real seismic damage data for PHC pile in China and the understanding of the seismic performance of PHC pile foundation is still very limited,which restricts the application of PHC pile foundation in high intensity seismic region.In order to promote the application of PHC pile foundation in high intensity seismic area,It is studied and summarized that the research results on the seismic performance of the connection between the pipe pile and the pile cap developed by domestic and international researchers.A new type of stiffening connection between the PHC pile and the pile cap for high intensity seismic region is suggested.In order to verify the differences of seismic performance fornovel stiffening connection,the common connection and strengthened connection suggested by the Chinese current national code,three full-scale connection specimens tests were conducted.The connection details are the common connection and strengthened connection which are suggested by code,and an internal T-shape steel connection which is suggested by author.All these specimens were subjected to constant vertical load and cycle lateral load.In the experimental study,the failure modes,hysteretic performance,stiffness degradation characteristics,plastic hinge mechanism,ductility energy dissipation and bearing capacity were recorded.The test results show that the three types of connections experienced different failure patterns.The common connection specimen show brittle failure,poor ductility performance and low flexural bearing capacity.Compared with the common connection specimen,the node flexural bearing capacity and the ductility for both strengthen ring connection specimen and stiffening connection were greatly improved.It shows that the stiffening connection has comprehensive advantages in the flexural bearing capacity,ductility,energy dissipation performance and construction.In order to further understand the stress state and failure mechanism of the connection specimens,to analyze the influence of different design parameters and to optimize the design parameters.three-dimensional numerical models were established with ABAQUS software to represent these three PHC pile-cap connection specimens.Elastic-plastic finite element analysis was carried out to simulate the behavior of the tested specimens and a good agreement wasachieved between the measured and predicted results.The study shows that for the common connection specimen,embedded depth at pile head significantly influences the flexural bearing capacity.With the increase of the embedded depth at pile head,the ratio between pile caps concrete constraint force and the anchoring reinforcement resistance is changed.When the embedded depth is0.5D,where D is the pile diameter,the common connection specimen can obtain the largest bending bearing capacity.For the strengthen ring connection specimen,the wall thickness of reinforcement ring obvious influences the flexural bearing capacity.It is fond that the reasonable value of wall thickness is0.5D,and it is suggested that the pile end anchorage reinforcement might be canceled.For stiffening connection specimen,it was suggested that the pile end anchorage reinforcement might be retained;the reasonable value of leg length(B)of T-shape steel and the depth(d T)of T-shape steel embedded the cap are 1.5D and 0.75 D,respectively.In order to further study the effect of the different connection details adopted by pile-cap connections on the seismic performance of the pile-cap-soil system,The six pseudo-static seismic tests of pile-cap connections scaled model with the different connection details were conducted in a 1.2m×1.2m×1.2m mini-geotechnical box.The connections details include three common connections(pile head is directly embedded in pile cap by 1.0D,0.5D and 0.2D,respectively),the strengthen ring connection,stiffening connection and pile partner connection.The research shows that the different connection detailsobviously influence the failure pattern and degree of pile head,the system energy dissipation ability,the level of ultimate bearing capacity and the pile bending moment distribution.However,the distribution and magnitude of soil pressures along cap sides and pile are not obviously influenced by the connections detail type.Finally,based on the experimental results,numerical simulation and theoretical analysis,the calculation formula of the pile head internal force is put forward by considering the action of soil resistance along the cap sides.The cracking moment and ultimate flexural bearing capacity calculation formula of no-core pipe pile,common core pipe pile,stiffening core pipe pile and reinforcement ring section are derived respectively,with the consideration of pile body damage model and node failure mode.The bending bearing capacity calculation formula and the relevant seismic details of the common connection,the strengthen ring connection and stiffening connection and pile partner connection are developed respectively.The concept of equivalent embedded depth is proposed to represent the bending bearing capacity of connections with different details.