Broadband Strong Ground Motion Simulation And Seismic Disaster Analysis Of Bridges In Active-fault Regions

Strong ground motions in active-fault region exhibit strong velocity pulse and larger permanent ground displacement,affected by the rupture directivity effect,the fling-step effect,and the hanging wall effect.These ground motions may have significant influences on near-fault or fault-crossing bridges.The lack of near-fault records has been restricting the systematic investigations on the seismic performance of near-fault or fault-crossing bridges.Using geophysics-based ground motion simulation for earthquake engineering application is an effective approach to solve the shortage of near-fault ground motion records.It is also a research hotspot for bridging the gap between seismology and earthquake engineering.In this regard,this study focus on broadband ground motion simulation and seismic response analyses of near-fault and fault-crossing bridges.Firstly,a fault-rupture-based broadband ground motion simulation approach was proposed.Secondly,the spatial distribution characteristics of broadband ground motion and bridge seismic response in active-fault regions were studied.Then the seismic response of fault-crossing bridge was conducted.Finally,the theory and approach of seismic risk assessment for regional-scaled bridge system were investigated.The main works and conclusions are summarized as follows:(1)An approach of fault-rupture-based broadband groung motion simulationBased on the comparison of the pros and cons for the syhthetic ground motions of the Wenchuan earthquake using four ground motion simulation methods,an approach of broadband ground motion simulation was proposed by incorporating the strong velocity pulse,larger permanent ground displacement,and the nonlinear soil response.Comparisons between the synthetic ground motions with recorded ones and the ground motion models indicate that the propose approach is an accurate and effetive tool for simulating broadband strong ground motions for earthquake engineering applications.(2)The spatial distribution characteristics of broadband ground motion in active-fault regionsA parametric investigation of spatial distribution characteristics of broadband ground motion using proposed ground motion simulation approach was conducted.Twelve simulation parameters were considered in the parametric investigation.The simulation results show that fault type is the main factor affecting the spatial distribution of the direction of rupture directivity effect.Rupture velocity and shear-wave velocity of crustal medium affect the intensity of rupture directivity effect.Fault dip mainly affects the hanging/foot wall effect,the hanging/foot wall effect will be more significant for a small fault dip.The slip distribution,slip velocity,buried depth,stress drop,spectral decay parameter,magnitude moment,site amplification factors can affect the amplitude of ground motions,but have minor influence on rupture directivity effect.The quality factor and geometric spreading have no significant influence on the synthetic ground motions.(3)The seismic response of bridges in active-fault regions considering spatial distribution of ground motionsThe spatial distribution of seismic response of two bridges with different natural periods were investigated using nonlinear time-history analysis based on the proposed broadband ground motion simulation approach.The spatial distribution of seismic response of the bridges for strike-slip earthquake and dip-slip earthquake with same magnitude moment(M_w 7.0)were compared.The simulation results show that the seismic responses of bridges along fault strike direction have siginificant spatial dispersion caused by rupture directivity effect and hanging/foot wall effect.The spatial dispersion is more siginificant as the observation closer to the fault.The spatial distribution of seismic response of the bridges differs significantly for strike-slip and dip-slip earthquakes.The seismic response of bridges is mainly affected by the rupture directivity effect for the strike-slip earthquake,whereas it is affected by the incorpation of the hanging/foot wall effect and the rupture directivity effect for the dip-slip earthquake.The hanging\foot wall effect plays domiment effect on seismic response of the short-period bridge,whereas the rupture directivity effect has domiment effect on the long-period bridge.(4)Seismic response analysis and restrainer design method of fault-crossing simply-supported bridges(1)A ground motion scaling approach was proposed for the seismic response analysis of fault-crossing bridges.The effect of fault crossing angle was evaluated using six scalled ground motions with permanent ground displacement for strike-slip earthquakes.The multi-criteria decision making method was used to determine optimal fault crossing angels.The results show that fault crossing angle from 60 to 90 degrees causes the lowest structural seismic responses.(2)A artificial synthetic method for ground motion with permanent ground displacement was proposed for generating the ground motion with varying pulse amplitude and pulse period.Then the effect of permanent ground displacement on the seismic response of fault-crossing bridges was conducted.The results show that the permanent ground displacement increases with the increasing of pulse amplitude and pulse period,which have similar effects on seismic response of the bridge.(3)Fault crossing ground motions for a normal fault were generated using the approach fault-rupture-based broadband groung-motion simulation proposed in this study.Effects of frequency ranges,fault-to-bridge locations,vertical ground motions were inverstigated on the seismic response of a simply-supported bridge crossing the normal fault.The results show that broadband ground motion should be as the input for the seismic analysis of fault crossing bridges.The seismic response is underestimated without considering vertical ground motions.(4)A seismic cable restrainer design method to control the large-displacement response for simply-supported bridges crossing fault rupture zones was proposed.Nonlinear finite element analysis show that the restrainers designed by the proposed method could efficiently limit the relative displacement within a designer-specified value for the fault-crossing bridges.Using SMA cables as seismic restrainers could noticeably reduce the required length compared with elastic steel cables.(5)Theory and methodology for regional-scale seismic risk assessment of bridge networks in active-fault regionsA new methodology for regional-scale seismic risk assessment of bridge networks considering spatial distribution of ground motions was proposed.The damage distribution and seismic risk for the Wenchuan earthquake were evaluated.The results show that the simulated seismic damage distribution using the method proposed in this study has great aggrement with the actual seismic damage.The simulated seismic damage can consider the effect of the rupture directivity effect and the fault crossing effect.The simulated seismic damage in rupture forward directivity zones is undereistimated using the traditional seismic fragility analysis method.The proposed regional-scale seismic risk assessment of bridge networks can also be incorporated with probalistic seismic hazard ayalysis.The probalistic damage level and seismic risk curves of the bridge networks in the Longmenshan fault region are investigated in the present study.The proposed regional-scale seismic risk assessment of bridge networks can provide scientific guidance for pre-earthquake planning and post-earthquake emergency rescue.