Effect Of Fault Strike On Seismic Response Of Continuous Rigid Frame Bridges

The terrain in western China is dominated by mountainous areas,and seismic active faults are widely distributed.In order to accommodate the environment of multiple mountains and canyons,a great number of continuous rigid frame bridges have been constructed adjacent to seismic active faults.Near-fault ground motions with forward directivity effects contain velocity pulses and have caused severe bridge damage historically.Significant near-fault ground motion pulses do not only occur in the vertical fault direction,but also occur within a certain angle range.At present,the seismic active fault strike impact on bridges has not considered sufficiently in the domestic seismic analysis.Therefore,the influence of fault strike on the seismic response of continuous rigid-frame bridges under near-fault ground motions is studied based on engineering practice.The main work progress is as follows:(1)The research background is introduced,and the characteristics of near-fault ground motion are described briefly.The influence of related ground motion input angles on structural response at home and abroad is presented.Although there is a strong correlation between them,the current research is always focused on source direction uncertainty rather than the impact of fault strike.(2)Four large-span continuous rigid frame bridge models with pier heights of 126 m,63m,58 m,and 22 m are established by Midas Civil,and the vibration characteristics of the structures are analyzed.Based on the seismic wave database provided by the United States Pacific Earthquake Engineering Center for seismic research on transportation facilities,ten sets of near-fault ground motion records with similar site conditions are selected as inputs.Linear elastic time history analysis is performed on four bridges and elastoplastic time history analysis is performed on two bridges considered the pier plasticity.(3)The effect of the possible angles between the fault strike and the longitudinal bridge direction on the seismic response of continuous rigid frame bridges is analyzed under the action of near-fault bi-/ tri-directional ground motion.The results show that under the near-fault bi-/ tri-direction ground motion,no matter the structure is elastic or elastoplastic,the angles for the maximum seismic responses of main piers and main girders are in the range of 75 °-135 °in the longitudinal direction of the bridge,while the angles for maximum lateral seismic responses are in the range of 0 °-30 ° and 120 °-180°.On condition that the angle between the fault strike and the longitudinal bridge direction is not considered,there may cause an underestimation of the seismic response of the bridge.The underestimation range is 15%-40%.According to the above conclusions,no clear law of the influence of pier height and span can be deduced.The PGA(peak ground acceleration)has almost no effect under the plastic response condition of the piers.(4)The effect of near-fault vertical ground motion on the seismic response of continuous rigid-frame bridges is analyzed,and has compared to the seismic response of bridges only considering horizontal bidirectional ground motion.The results show that vertical ground motion has a great influence on the vertical bending moment response of the main beam under the condition of structural line elastic response,especially at the junction of the main piers and the side spans.The increase ratio can reach 2 times or more.Regardless of the elasticity or plasticity of the pier,the effect of the vertical ground motion on the seismic response of the piers is relatively small.