Seismic Fragility Of Multi-tower Cable-stayed Bridges With Different Structural Systems

With the increasing number and span of cable-stayed bridges,their behavior under seismic action has become a growing concern.Seismic fragility,which describes the conditional probability of a structure or component attaining or exceeding a specific damage state for a given ground motion intensity,has been an essential tool for assessing the seismic performance and potential seismic disaster risk of bridges.In this paper,seismic fragility analysis of three-tower cable-stayed bridges with three different structural systems,including rigid system,floating system,and passive energy dissipation system,is conducted to study the effects of different structural characteristics on the seismic fragilities of cable-stayed bridges.In addition,a simplified model of viscous damper that can take into account the limit states of viscous damper is established to study the effects of damper failure on the seismic fragility of cable-stayed bridges with energy dissipation damping system.The main research work and conclusions are as follows:(1)The three-dimensional finite element model of a cable-stayed bridge is established using OpenSees software,and the elements and material constitutive models in modeling are described in detail.Then,the tower-girder connection configuration is modified based on the built model of the cable-stayed bridge to obtain finite element models of cable-stayed bridges with three structural systems,including rigid system,floating system,and passive energy dissipation system.(2)The modeling method of viscous dampers with limit states is provided,and the model of viscous dampers considering limit states is established.The effects of damping coefficients and damping indices of viscous dampers on seismic responses of cable-stayed bridge with passive energy dissipation system are analyzed.The results show that a reasonable installation of viscous dampers between middle tower and girder of a threetower cable-stayed bridge with floating system can coordinate responses between middle tower and edge tower under earthquakes,improve the force conditions at the base of edge tower,and enhance the seismic resistance of the whole bridge.(3)25 ground motions are selected and incremental dynamic analysis is carried out for cable-stayed bridges with three structural systems,including rigid system,floating system,and passive energy dissipation system.The fragile parts of different components of cable-stayed bridges are analyzed in terms of structural responses.Combining damage indexes and limit states of the components of cable-stayed bridges,seismic fragilities of cable-stayed bridges with different structural systems are calculated.The results show that the bridge tower is the most affected component by different structural systems.For rigid system,the fragility of middle tower is significantly higher than other components,and the bridge failure starts from the middle tower,exhibiting a characteristic of local failure.For floating system and passive energy dissipation system,the fragility of edge tower is higher than middle tower,and the failure comes from the edge tower first.(4)The effects of damper failure on the seismic fragility of cable-stayed bridge with passive energy dissipation system are investigated.Although reasonable viscous dampers can reduce the seismic fragility of cable-stayed bridges,the seismic demand of cablestayed bridge with dissipative damping design may be underestimated,especially when the bridge is subjected to large earthquakes exceeding the design level,if the failure of dampers is not considered.Taking the failure of dampers into account is necessary to get reliable seismic fragility of cable-stayed bridge.