Study On Seismic Performance Of RC Bridge Column With ECC-CFST Composite Section In Plastic Hinge Region

Reinforced Concrete（RC）columns are severely damaged and have large residual deformation in the plastic hinge region under seismic loading,which will directly affect the safety and functionality of the bridge structure.Concrete filled steel tube（CFST）is widely used because of its good bearing capacity and durability.However,engineering practice found that the ductile properties of the inner and outer materials of the steel tube and concrete combination columns have large differences,and the synergistic effect of the two cannot be given full play,while the outer concrete materials have low tensile strength and are prone to cracking,resulting in lower seismic performance of the structure,and the application scope is limited.Engineered Cementitious Composite（ECC）has the characteristics of high ductility,strain hardening and multiple micro-cracking,etc.Its application in the field of seismic resistance can give full play to its material properties and improve the seismic performance of bridge columns in terms of ductility and energy dissipation capacity.In this context,this paper proposes the use of ECC-CFST composite section bridge pier structural form in the plastic hinge zone and investigates its seismic performance.The main research contents are as follows:（1）The current research status of seismic performance of existing ordinary reinforced concrete columns,steel tube concrete composite columns,new forms of structural columns and new material combined columns is summarized through literature analysis,and the structural form of ECC-CFST composite section columns is proposed to improve the seismic performance of reinforced concrete columns in the plastic hinge zone.Five models of columns with a scaling ratio of 1:5 were designed and fabricated with the variation of steel tube diameter and steel tube thickness as the main research parameters,and the proposed static tests were carried out.（2）The damage modes of all specimens were analyzed based on the test phenomena,and the seismic performance of the bridge pier was analyzed by the hysteresis curve,skeleton curve,stiffness degradation curve,ductility performance and energy dissipation capacity.The test results show that the structural form of ECC-CFST composite section design in the plastic hinge zone of the bridge pier has significantly improved the bearing capacity compared with the RC column,and the hysteresis curve of the bridge pier with specimen number ECS-102/EUS-102 is fuller and has better ductility and energy dissipation capacity.In addition,this structural form also enables the residual displacement of the members to be controlled within a reasonable range,which ensures the normal use of the structure and the rapid repair after the earthquake.（3）The finite element model was established by Open Sees finite element software combined with the test results using fiber sections,and the numerical analysis results were compared with the test results to verify the accuracy of the numerical model.After extracting the key point parameters of the finite element model,it was found that the ECC geometry size effect brought about by the change of tube size has an important influence on the structure,so the relationship between the ECC layer thickness t under the influence of tube diameter change and the length to slenderness ratio l_sr,buckling degradation parameter r _G,and fatigue degradation influence coefficient C_d was obtained by fitting the exponential function in combination with the parameters of the numerical model.（4）Based on the ECS-102 and EUS-102 finite element models,the geometric dimensions of the column cross-section in the range of 1/3～1/2 were selected with reference to the relevant specifications,and the parametric analysis was carried out for the pier models with different steel tube diameter sizes and different thicknesses under the same size.The results show that:the thickness of the corresponding ECC material layer due to the variation of the steel tube diameter has a large impact on the overall energy dissipation effect and the ductility performance of the specimen;the thickness of the steel tube also affects the effect of the steel tube on the filling material restraint in the core area of the specimen,which in turn affects the seismic performance of the specimen.Compared with ordinary reinforced concrete columns,the use of ECC-CFST composite section in the plastic hinge region can improve the overall seismic performance of the member,in which the ductility is significantly improved.However,this combined section form has an upper limit to improve the seismic performance of the bridge pier,so the reasonable setting of the steel tube diameter and thickness can make the specimen play its excellent performance and improve the seismic performance of the original reinforced concrete bridge column while reducing the cost.