Research On Seismic Design Of Long-Span Steel Box-Steel Truss Composite Continuous Girder Bridges

Long-span steel-box and steel-truss composite continuous beam bridge is a new type of structural bridge formed by steel box continuous beam with steel truss stiffening.As the span of the continuous support system is large and the distribution quality of the upper structure is small,the seismic response,damping and isolation of the structural system deserve attention.In this paper,the seismic design of the Aodang Forth Bridge in Zhuhai is studied systematically.The main research contents are as follows:(1)The structural characteristics and basic dynamic characteristics of long-span steel box-steel truss composite continuous beam bridge are analyzed.According to E2 level seismic fortification standard,the seismic performance of the bridge is evaluated by response spectrum method and dynamic time-history analysis method,and the vulnerable parts of the bridge under earthquake are identified.The analysis shows the necessity of using seismic isolation measures.(2)In order to meet the seismic isolation requirements of long-span steel box-steel truss composite continuous beam bridge,the friction pendulum support is selected by comparing various vibration isolation devices.The operating principle of friction pendulum mount is analyzed,and the low-cycle repeated hysteresis characteristics of friction pendulum mount are obtained through finite element simulation analysis and compared with the analytical results.On this basis,a dynamic analysis model of the full-span steel box-steel truss composite continuous girder bridge with friction pendulum support is established,and the internal force and displacement responses of key parts under E2 earthquake are analyzed and compared with those of conventional support systems.(3)In view of the randomness of seismic action,the vulnerability analysis of two kinds of steel box-steel truss composite continuous beam bridge systems with common support and seismic isolation support is carried out.A series of seismic waves suitable for bridge location characteristics were selected from PEER strong earthquake database in the United States and fitted.Based on the local strain condition of pier bottom section,the damage evaluation index of displacement ductility of pier is established and the vulnerability curve of pier Z2 is plotted.The damage probability of fixed pier under two support systems is obtained.The result shows that the damage probability of pier using friction pendulum isolation support is about 50% lower than that of common support system.(4)To further judge the working state of long-span steel box-steel truss composite continuous girder bridge under E2 level earthquake,capacity spectrum analysis is carried out for common support system and damping support system.Elastic-plastic demand spectrum is reduced and displacement ductility is selected as the evaluation standard of pier performance level.The performance points of the two systems are obtained by graphical method,and an evaluation method for seismic design of Bridges based on capacity spectrum is formed.