Research On Seismic Behavior Of Steel Tube Reinforced Concrete Bridge Columns

The shear-fracture and flexural-crushing are two unrepairable earthquake-induced failure patterns of reinforced concrete bridge columns,which can often result in disastrous consequences such as collapse or overturning of total bridge system during earthquake events.The causes for the two failure patterns can be attributed to the insufficient shear resistance and deformability of bridge columns that can not accommodate their strength or deformation demand.The research object of the present study---steel tube reinforced concrete(STRC)bridge column,has been developed with the intention of eliminating the aforemotioned two categories of column failure modes.Benefiting from their excellent axial compressive capacity,flexural and shear resistance,and favourable post-earthquake repairability,STRC bridge columns exhibit a promising future of application in regions of seismicity.Currently,the research on the seismic behavior of STRC bridge columns is still in the infancy stage though some achievements of significance have been obtained.Against this background,this study was thus conducted to establish systematical understandings pertaining to the seismic behavior of STRC bridge column by means of cyclic loading test and numerical simulation,with the general purpose of providing both experimental and theoretical basis for engineering application of this type of bridge column.The main contents and conclusions of this study can be generalized as follows:1.Thirteen column specimens with shear span ratio of ?=3.0 were tested under constant axial load and cyclic-reversed lateral displacements to investigate the effects of axial load ratio,stirrup spacing and equivoluminal substitution,longitudinal steel diameter and number,steel tube specification and anchorage length on the hysteretic behavior of STRC columns subjected to flexural failure.Test results indicate that:the presence of a core steel tube would not alter the initial stiffness of the column;STRC columns subjected to flexural failure show plump and stable hysteretic loops with satisfactory deformability and energy dissipation capacity;STRC columns that having insufficient anchorage length for the core steel tube into the footing would experience severe damage at the column-footing joint and behaved extremely poor ductility;Based on the superposition principle,the formula for calculation of the flexural capacity of STRC columns are sugguested.On the basis of the fiber beam-column element available in the OPENSEES platform,a nonlinearity finite-element model(FEM)was developed to simulate the hysteretic responses of STRC bridge column specimens that have subjected to flexural failure.It is found that a satisfactory agreement was obtained between the FE model predicted and test measured hysteretic curves of the specimens.Then,the FE model was used to execute a parametric study with the parameters were not being involved in the testing program.2.To understand the effects of axial load ratio,stirrup spacing,longitudinal steel diameter and number,steel tube specification,and shear span ratio on the hysteretic behavior of STRC short bridge columns subjected to shear failure,a test matrix of thirteen specimens with the main shear span ratio of ?,=1.5 were tested under simulated seismic loading conditions.Test results show that:the presence of a core steel tube can prevent the short bridge columns from being subject to the typical failure pattern of shear-fracture;the failure pattern of STRC short bridge columns was ductile shear diagonal-compression failure with hysteretic loops showing no obvious pinching and slippage effects.Based on the research findings of CFST and RC shear components,expressions for shear strength prediction of STRC short bridge columns are suggested by introducing the parameters of shear span ratio,longitudinal steel ratio,and their modifying factors.By means of experimental data regression and theoretical derivation,the mothed for determining the load-displacement skeleton curves of STRC short bridge columns was suggested.Additionally,by considering the maximum point-directed hysteresis rule,the hysteretic model for STRC short bridge columns takes into account of some crucial parameters,i.e.axial load level,volumetric stirrup ratio,longitudinal steel ratio,and cross-sectional steel tube ratio,was finally etablished to provide a basis for nonlinear time-history analysis of such component.3.To improve the seismic behavior of bridge columns and to control the investment of earthquake fortification as while,a bridge column scheme that incorporating a core steel tube only into the lower-to-middle portion of the column body was proposed.The feasibility of the novel bridge column is experimentally evaluated by pseudo-static tests of eight bridge column specimens with shear span ratio of ?=3.0.Besides,the effects of axial load ratio,volumetric stirrup ratio,and core steel tube specification was also investigated.Test results demonstrate that the core steel tube embedment length is of an essential structural parameter in determining the failure pattern and mechanical property of this composite member;this novel bridge column scheme could achieve the balance of economy and seismic resistance through an reasonable design.The lateral strength of such bridge column can be obtained by introducing a intermediate variable of equivalent shear span ratio.By an example demonstration,the seismic design philosophy and corresponding calculation method are preliminarily discussed.Besides,some suggestions on structural measures are given.Based on ABAQUS software package,3D-FE models were established for analyzing the working mechanism of this type of bridge column suffered from different failure modes.The FM model of the design case was also developed to verify the reliability of the lateral strength calculation method,seismic design philosophy,and simplified mechanical model in shear checking of partially embedded STRC bridge columns.