Seismic Demand Mechanism And Quantitative Model For Near-fault Bridge Piers Under The Coupling Effect Of Pulse-like And Vertical Ground Motions

With the development of resilient city,traditional seismic design method based on structural safety is gradually changing to the resilience seismic design method which focuses more on the performance recovery of structures.Near-fault ground motions have the characteristics of pulse effect and large vertical component,which leads to the increase the ductility demand and residual displacement of bridge structures,and thus causing more severe challenges to the rapid recovery of bridge structure from earthquakes.Previous studies have analyzed the influence of near-field pulse effect and vertical ground motion effect on the ductility demand of bridge piers,and proposed composite structures to reduce the residual displacement,such as the unbonded prestressed concrete(UBPRC)column.However,there is a lack of quantitative research on the coupling effect of pulse effect and vertical ground motion,and insufficient research on the residual displacement of the pier,which is also the crucial problem restricting the performance recovery of nearfault bridge structures.In this paper,the Open Sees model of reinforced concrete and UBPRC piers are set and a series of measured near-field ground motions are selected to comprehensively study the ductility demand and residual displacement of piers under near-field excitation.1.A qualified model of pier ductility under the coupling effect of near-field horizontal pulse and vertical ground motion is proposed.In this paper,taking curvature ductility as the damage index,the three-dimensional probabilistic seismic demand model based on horizontal ground motion intensity and vertical to horizontal strength ratio is established through nonlinear time history analysis;Through the effect of vertical ground motion on the axial compression ratio,the three-dimensional capacity limit state model is also established.Finally,the seismic fragility surface for bridge piers considering pulse intensity and vertical ground motion intensity is obtained by the seismic demand and capacity model.2.The residual displacement of RC and UBPRC piers under near-field earthquake are studied.In this paper,the numerical model of RC pier is compared with the experimental results to verify the effectiveness of the numerical model in calculating residual displacement,and the influence of vertical ground motion effect on residual displacement of bridge pier is analyzed by the method of data statistics.;In order to solve the problem of excessive residual displacement of RC pier,a new type of UBPRC pier is evaluated.By comparing the ductility demand and residual displacement of the two piers under quasi-static analysis and near-field earthquake,the effectiveness of UBPRC pier as a method to reduce the residual displacement of RC pier is verified.3.The residual displacement spectra of UBPRC pier based on the ratio of structural period to pulse period is established.To qualify the influence of pulse period on residual displacement,the residual displacement spectra of UBPRC pier is creatively put forward and fitting formula is also given;The influence of key structural parameters on the residual displacement spectra under the combined action of horizontal and vertical ground motions is focused.The research results of this paper can provide an empirical formula for quantifying the coupling effect of near-field pulse effect and vertical ground motion effect,and provide a new structural scheme for reducing the residual displacement of RC pier,and provide the reference for the seismic safety of bridges in near fault area.