Analysis Of Sea-spanning Cable-stayed Bridge Period In Service Based On Fragility

Chinese seismic belts are mainly distributed in Taiwan,southwest,northwest,north China and southeast coastal areas.Among them,the southeast coastal seismic belt is the most active one which is located at the intersection of the Eurasian plate and the Pacific plate.Bridges are an important component of the southeast coastal transportation line.If bridges are damaged in an earthquake,will cause unpredictable economic losses and casualties,Stayed-cable bridges are widely used in the construction of sea crossing bridges because of spanning capacity and diverse shapes.In this paper,a sea-crossing cable-stayed bridge is taken as an example.In terms of the influence of material durability damage on the seismic performance of the cable-stayed bridge,the seismic fragility of the main components and systems of the cable-stayed bridge is analyzed.Earthquake risk within service period is studied according to the seismic fragility analysis results.The main contents are as follows:(1)The marine environment is taken as a typical durability environment.As for the influence of chloride ion erosion on the deterioration of concrete performance,the degradation law of material performance during the service period of the bridge is obtained.The characteristic section of the towers and piers of the cable-stayed bridge during the service period is analyzed based on the XTRACT software.The bending moment-curvature is carried out to quantify the damage index.Curvature ductility ratio and relative displacement were selected as damage indexes of tower,pier and support respectively.(2)Based on OPENSEES,the beam,tower pier,support,stay cable and damper of cable-stayed bridge are simulated,the nonlinear finite element models of cablestayed bridge under different service periods are established,and theirs dynamic characteristics are analyzed.The results show that the first mode is the longitudinal drift of the beam and has a long period,which is in line with the characteristics of the cable-stayed bridge of the semi floating system.The structural response caused by horizontal ground motion is controlled by the contribution of low-order modes.(3)Considering the damping measures,through nonlinear time history analysis,the mid span displacement of beam,shear force and bending moment at the bottom of bridge tower and pier under different damper parameters are compared.With the method of incremental dynamic analysis,not only the fragility curves of cable-stayed bridge components can be drawing,but also the most fragile section and the most fragile supports components in the main towers and piers can be determined.The seismic fragility of cable-stayed bridge with different damper parameters is analyzed.According to the law of durability damage,the time-varying seismic fragility of cablestayed bridges in service are analyzed.(4)According to the site type of cable-stayed bridge,the seismic risk function is deduced.Combined with the seismic fragility analysis results,the seismic demand risk function of each component of cable-stayed bridge is established to analyze the seismic demand and risk of cable-stayed bridge in different service periods.The results show that the damping measures can reduce the displacement along the bridge direction of the floating system and reduce the seismic risk.With the increase of the damage degree of material durability,the damage risk of cable-stayed bridge system is increasing.