Seismic Response And Fragility Analysis Of Long-Span Continuous Rigid-Frame Bridge With High-Rise Piers Considering Internal Force State

The long-span continuous rigid-framed bridge with high-rise piers is widely used in China,because it has outstanding advantages in crossing natural barriers like high mountains and deep rivers due to their excellent performance,such as large vertical stiffness,strong spanning capability and reasonable mechanical performance.Cantilever construction method are mostly adopted under construction of long-span continuous rigid-framed bridge with high-rise piers,and construction time is generally long.The internal force state of each construction stage changes constantly under the action of structural deadweight,box girder prestressing and construction loads.The loss of prestress and shrinkage and creep of concrete for the completed bridge are large.Therefore,the internal force state of a certain stage of the completed bridge have great differences depending on whether the construction process is considered or not and different shrinkage and creep years,which affects the accuracy of seismic response analysis results.In this study,a irregular long-span continuous rigid-framed bridge with high-rise piers is considered as the engineering background,in which the main bridge is a three-span continuous T-shaped rigid-frame bridge with a span layout of120+220+120m and the approach bridge is a four-span simply supported T-girder bridge with a span layout of 4×40m.Then effects of construction process,shrinkage and creep on the internal force state were considered,and seismic response and seismic fragility analysis based on the internal force state were carried out.The following conclusions can be drawn based on the numerical results:（1）To study the effects of actual construction process on the internal force state（named element initial strain state）of continuous rigid-frame bridge and the differences for seismic responses of main bridge under different internal force states,the analysis model of considering construction stage was established via MIDAS/Civil and the internal force state of completed bridge was obtained.Based on the equivalent load method,the calculation formulas of internal force equivalent load suitable for continuous rigid-frame bridge were put forward,and the internal force equivalent loads corresponding to different internal force states of the main bridge were calculated.The dynamic analysis model of the completed bridge considering the internal force state was established by Open Sees.Selecting 40 groups of typical near-fault ground motion records as input,the dynamic time history analysis of the bridge considering the construction process and different shrinkage and creep years was carried out.The results show that the construction process and prestress should be considered to ensure that its internal force state is consistent with the actual internal force state in the dynamic analysis of the continuous rigid-frame bridge.Internal force states of the main bridge can be accurately calculated and simulated by the formulas of internal force equivalent loads calculation.（2）To analyse the influence of internal force state of continuous rigid-frame bridge on its seismic fragility during construction and after completion,five typical T-shaped rigid-frames during construction and the initial stage of completed bridge were selected,the internal force state of each stage was obtained by construction stage analysis via MIDAS/Civil,the equivalent loads of T-shaped rigid-frames were calculated by equivalent load method,and the dynamic analysis model considering the equivalent internal force state was established based on Open Sees.Taking curvature ductility factor as the damage evaluation index of main girder root,main pier top and bottom in typical T-shaped rigid-frames,and displacement ductility factor as the damage index of the initial stage of completed bridge,then the threshold values of different damage stages were calculated.The influence of considering the equivalent internal force state on the seismic fragility of T-shaped rigid-frame and completed bridge in the initial stage,as well as the difference and variation law of seismic fragility between different construction stages were compared and analyzed.The results show whether in the typical construction stage or the initial stage of completed bridge,if the internal force state is not considered,its seismic fragility will be seriously underestimated.（3）To investigate correlations of duration indexes and nonlinear seismic responses under different duration ground motion,the definitions of duration with continuous-time intervals—namely,bracketed duration（D_b）and significant duration（D_s）,were selected.Based on the dynamic analysis model considering the internal force state established above,the correlation between duration indexes and seismic responses under the action of total duration ground motion was analyzed first.Then the effect of different duration indexes on nonlinear seismic responses was discussed under long-duration intercepted ground motion according to D_b0.1g,D_s5-75%and D_s5-95%.The results show that correlations of D_b and nonlinear seismic responses vary significantly depending on the absolute acceleration threshold.Correlations of D_s and nonlinear seismic responses are better than D_b,and D_s5-95%is preferred for calculating the total hysteretic energy dissipated.Also,the effect of the intercepted ground motion according to the indexes(D_b0.1g,D_s5-95%and D_s5-75%)on the maximum absolute seismic responses and the total hysteretic energy dissipated differs,which depends on the effective time interval of ground motion under different duration indexes.