Several Discussions About Seismic Analysis Of Long-Span Suspension Bridge

The seismic isolation system as all floating or half floating system is usually taken by the suspension bridge to keep the tower in elastic state. These two systems will bring out large displacement in earthquake, the large displacement at the end of main beam will cause the collision between main beam and the approach bridge, the safety performance of whole bridge will be in danger. The seismic response of the bridge influenced by the spatial variation of ground motions is studied by many scholars. The spatial variation of seismic ground motions is produced primarily by the three factors:geometric incoherency effect, wave-passage effect and local site effect. The pile-soil interaction cannot be ignored in the seismic computation and the application of viscoelastic artificial boundary is little researched.In order to study the following points:how to realize the viscoelastic artificial boundary when considering the pile-soil interaction; the influence on seismic response of the pile-soil interaction, wave-passage effect and collision between the main beam and the approach bridge; optimal selection of nonlinear damper.The finite element model was established by the software Midas Civil, the soil was simulated by the solid element and the ground motion was input on the boundary nodes. Damper and the collision units were simulated by the software’s model. The large mass method was used to consider the wave passage effect and the seismic response under different apparent wave velocity was studied. For the long-span suspension bridge, structural response used to be analyzed includes the displacement at the end of main beam, shear force and bending moment at the bottom of tower, damping force and collision force.The results showed that:The main natural period of vibration was very long and influenced by the pile-soil interaction. It was safer but less economic when using the seismic response of fixed model to design. When using the viscoelastic artificial boundary model, it was more economic but safe. M method was a good way to consider the pile-soil interaction.When considering the collision between the main beam and the approach bridge, the displacement between the main beam and approach bridge was decreasing and same for the internal force of pier of approach bridge. Collision stiffness and recovery coefficient had big influence on the collision reaction; the reasonable recovery coefficient should be confirmed by the test. The width of initial gap was an uncertain factor.The geological condition was good and the wave-passage effect could need not to be considered.The seismic reduction effect was good for the suspension bridge with viscous dampers, and the reduction effect was mainly reflected on the displacement at the end of main beam and the internal force of main tower. The optimal damper parameters were C=2000kN(sec/m)ζ, ζ=0.2～0.4, rated damping force of one single damper could be2250kN.