Study On Elastoplastic Properties Of Reinforced Concrete Pier Column And Their Applications In Seismic Design Of Bridges

In recent years,many earthquakes occurred in the world caused numerous bridges to damage,which had resulted in significant effects on the safety of human life and property as well as the development of economy.Extensive post-earthquake reconnaissance has demonstrated that the economic consequences due to the loss of bridge function in small and medium earthquakes are worthy of attention,in addition to the collapse of bridges in severse earthquakes.The maximum displacement of structure under earthquakes reflects the damage and the nonlinear degree of structure.Thus,the displacement-based design of structures under multi-level earthquake motions can realize the bridge seismic targets better.Pier columns are the supporting members of reinforced concrete(RC)bridges,and also are the energy consuming components in earthquakes.Therefore,the non-linear performance of pier columns under repeated load has important influence on the seismic resistance of bridges.Understanding the deformation and energy dissipation capacity of pier columns and then developing the corresponding calculation theories are critical for implementing the displacement-based design of bridges.Although considerable research has been conducted on the seismic performance of RC columns,the research on the seismic performance of flexure-shear critical columns,which exhibit shear failure in the plastic hinge region,is not deep enough.Based on the support of National Natural Science Foundation of China,this study conducted experimental and analytical research on the seismic behavior of RC pier columns to improve the displacement-based design method of small and medium-sized reinforced concrete bridges.The main contents are as follows:1.24 scaled RC circular column specimens were constructed and tested statically.The effects of aspect ratio,axial load ratio,longitudinal steel ratio and transverse steel ratio on the three failure modes(flexure critical,flexure-shear critical and shear critical),hysteresis curve,hysteretic energy dissipation,equivalent damping ratio and damage zone length were investigated.The contributions of three deformation components(flexure,shear and slip)to total displacement were calculated according to experimental results.2.According to experimental results of flexure critical column specimens in this study and that selected from PEER-Column Database,the hysteretic curve of specimens in one complete cycle was investigated,and an expression for defining this hysteretic curve was proposed.Based on the proposed expression,a model for predicting the equivalent damping ratio of flexure critical pier columns was proposed using Jacobsen's approach.Then,taking the average maximum displacements predicted with time-history analysis of single-degree-of-freedom system under the action of 250 seismic waves as reference,the proposed model was modified.A modification factor of equivalent damping ratio,which incorporated the effects of relative yield force(the ratio of yield strength to weight of structures),period and peak ground acceleration,was developed.A procedure for associating the equivalent damping ratio of a structure as a whole with those of its element was established.3.By investigating the characteristics of hysteretic curves of extensive flexure-shear critical RC circular columns,an expression of hysteretic curve for flexure-shear critical RC columns under complete cyclic loading was developed.And then,an equivalent damping ratio model suitable for flexure-shear critical RC pier columns was proposed using Jacobsen' s approach and the proposed hysteretic curve expression.Considering that the complex form of the proposed model may seriously limit its application,a simple expression of the proposed model was developed.Then,the model was modified with the same method as that for flexure critical column specimens.A modification factor of equivalent damping ratio for flexure-shear critical RC columns,which incorporated the effects of relative yield force,period and peak ground acceleration,was developed.Differently from flexural critical columns,the equivalent damping ratio of flexure-shear critical columns was not only associated with ductility,but also with the ratio of second stiffness to yield stiffness,which affected the influence of pinching in hysteretic curves.4.Main factors influencing the equivalent plastic hinge length of piers were identified based on moment-curvature response analysis of cross-section.According to analysis results,the formula of equivalent plastic hinge length was proposed.The ultimate displacements of specimens measured in this study and selected from PEER-Column Database were used to modify the proposed formula,and then a practical formula of equivalent plastic hinge length was developed.Considering the tension shift effect caused by diagonal shear cracks angle in plastic hinge region,a formula of equivalent plastic hinge length suitable for flexure-shear critical RC pier columns was developed by theoretical deduction.5.Using the proposed equivalent damping ratio models that have been modified with time history analysis,and equivalent plastic hinge length formulas and for flexure and flexure-shear critical columns,as well as the method for determining the equivalent damping ratio formula of bridge structures from that of its pier columns,a displacement-based design of multi-span continuous beam bridge was conducted.