Characteristics Of Near Fault Ground Motions And Its Influence On The Seismic Performance Of Long-span Cable-stayed Brideg

Recent earthquakes have demonstrated that seismic disastrous areas along the fault rupture line presented an elongated and strip-shaped distribution.Near fault ground motions ofen possess distinct charactristics such as short-duration,high-amplitude,and large-pulse components.Since the common values of pulse periods are in the range of 1.0~13.0 s,near fault ground motions may have a significant influence(even lead the resonant response)on the seismic behaviors of long-span structures(such as cable-satyed bridges)wth high flexibility and long period.The near fault records,especially the pulse-like records,are scare and only 140 stations with the ruture distance of 40 km have recorded the pulse-like ground motions according to the latest database of the Pacific Earthquake Engineering Research Center(PEER).Hence,the characteristics of near fault records and their influence on seismic behavior of long-span bridges are not clear.Besides,considering the severe damage of structures in near-fault regions,it is necessary to control the seismic responses of long-span bridges.In this regard,this study mainly focuses on the characteristics of near fault ground motions,seismic behaviors of cable-stayed bridges,seismic control and performance evaluation of cable-stayed bridges.Firstly,the spatial distribution and pulse-like characteristics of near fault records are investigated.It provides an approach to select seismic inputs.Then,the seismic responses of cable-stayed bridges in different fault regions and under near fault records with different pulse types are analyzed.Lastly,a new generation of base isolation bearing,i.e.SMA-LRB,and the corresponding constitutive model are proposed.The seismic performances of isolated and non-isolated cable-stayed bridges are evaluated by using dynamic time history analysis and fragility analysis.The main works and conclusions are summarized as follows:1.Seismic responses of long-span cable-stayed bridges induced by ground motions in different sites relative to fault ruptureAccording to the location of stations relative to the possible position of the fault,three groups of near fault ground motions are selected in Forward Region(FR),Middle Region(MR)and Backward Region(BR),respectively.The Sutong cable-stayed bridge(SCB)is taken as an example.The influences of near fault regions,SSI effect,and orientation angles on seismic responses of the bridge are investigated.The results showed that the seismic behavior of SCB in those 3 regions presents significant differences.The SSI effects affect the bridge responses through a systematic decrease of modal frequencies and a substantial change in nature of dominant shapes especially for the higher modes.PGV and PGA correlate well with the responses of the bridge located in MR and BR.Velocity-related intensity measures show strong correlation with the seismic responses of the bridge located in FR.2.Effects of near-fault motions on long-span cable-stayed bridge systemsThe effects of forward-directivity pulses,fling-step pulses and non-pulse motions on the responses of girders,towers and cables are assessed for SCB.The results showed that the near-fault pulse-type ground motions generate larger responses compared with the non-pulse ground motions.The fling-step ground motions are more damaged to the bridge than those of the forward-directivity ground motions.A new method,record-decomposition incorporation(RDI),is proposed to synthesize the artificial near-fault pulse-type ground motions.The effectivity of the new method is assessed by using the spectrum analysis and dynamic time history analysis.The influences of the high-frequency and low-frequency components on seismic responses are evaluated.3.A new generation of SMA-based smart lead rubber bearing and its seismic control on cable-stayed bridgeA new generation smart isolation system,namely SMA-LRB,is proposed.The mechanical properties of SMA are investigated experimentally.Then,the constitutive model of SMA-LRB is coded and implemented into OpenSees as a new user element.The seismic responses of SCB with and without isolation bearing are compared.The results showed that implementing shape memory alloy wires into lead rubber bearings can effectively reduce the residual deformation in lead rubber bearings under near-fault ground motions.Shape memory alloy wire-based lead rubber bearings can effectively restrict the deck displacement and reduce the base shear of the towers.4.Seismic fragility analyses of long-span cable-stayed bridge in near fault regionsThe fragility curves of the bridge components including pier,tower,cable and isolation bearing are calculated according to probabilistic seismic demand model.Then,the fragility curves of the bridge system are obtained according to reliability theory.Lastly,the seismic performance of the cable-stayed bridge with and without isolation bearings is evaluated.The results showed that the damage probability of the bridge is higher than that of each component.The towers have a high probability to experience slight damage,whereas the isolation bearings are the most vulnerable component in the moderate,severe and collapse damage state under near-fault ground motions.The bridge system equipped with SMA-LRB undergoes damage with a smaller risk compared to LRB.The bridge tower,pier,and cable have low probability to occur the collapse damage under strong ground motions.In order to investigate the seismic performance and control the seismic responses of cable-stayed bridges near active fault,a series of studies are conducted in this study,including characteristics of near fault ground motions,seismic responses of cable-stayed bridge,seismic control devices,and fragility analysis of bridges.The results can provide a certain theoretical and technical support for the construction of long-span bridges near active fault.