Study On Mechanical Parameters Of Anti-seismic Devices For Highway Bridge

Bridge is the key project on the disaster relief lifeline. The aseismic performance andserviceability of bridge during and after earthquake play a very important role in earthquakerelief and post-disaster reconstruction. Isolation design isolates the bridge from earthquake bylengthening the fundamental period of the structure and increasing the damping with the helpof anti-seismic devices. Compared with advanced countries in seismic research, Chineseguidelines for seismic of highway bridges is relatively backward in research of mechanicalmodel and parameters of anti-seismic devices. In this paper, lead rubber bearing and fluidviscous damper are taken as objects, the mechanical model and key parameters of them andthe isolation performance are systematically studied.Typical earthquake damages of highway bridges in Wenchuan Earthquake are analyzedin this paper. Based on the movement difference of south and north part of the fault fracturedzone, different characteristics of earthquake damages of highway bridges in south and northpart of the fault fractured zone are analyzed. It is shown that brittle thrust plays a major rolein the south part of the fault fractured zone, and the earthquake damages of bridges aremainly secondary disasters of landslide impact damages and strength failure. While in thenorth part of the fault fractured zone, the toughly slip plays a major role, and the earthquakedamages of bridges are mainly displacement failure such as unseating of beams and collapseof double curvature arch bridges.Fiber section method is used to study the mechanical parameters of lead rubber bearing.Change law of mechanical parameters under different lead rates and shear strains is studied.Example bridge analysis is carried out with different lead rates and different peak groundaccelerations. Difference of earthquake responses of bridge and the lead rubber bearing isanalyzed by using different mechanical parameters considering shear strain or not. The studyfound as follows:1. Lead rate and shear strain significantly affect the mechanical parameters of leadrubber bearing. The yield force, post-yield stiffness, rate of pre-yield and post-yield stiffnessand the damping ratio of lead rubber bearing are not constant and vary significantly withshear strain. The effect of shear strain on mechanical parameters increases with increasing of lead rate.2. When the peak ground acceleration and lead rate are low, there is little difference inearthquake responses of bridge and the lead rubber bearing by using different mechanicalparameters considering shear strain or not. While, When the peak ground acceleration andlead rate are high, there is considerable difference in displacement of superstructure and thebearing by using different mechanical parameters considering shear strain or not. Whether thelead rate is high or not, there is a considerable difference in earthquake response of the forcein piers of the bridge by using different mechanical parameters considering shear strain or not.The difference is about10~25%.The closed form expression of optimum damping ratio and the corresponding optimumdamping coefficient of linear fluid viscous damper for bridges is derived by stochasticvibration method. Single degree of freedom system and an example suspension bridge areused to verify the validity of the closed form expression of optimum damping coefficient oflinear fluid viscous damper. The study found as follows:1. There is a theoretical optimum damping ratio and the corresponding optimumdamping coefficient of linear fluid viscous damper for bridges. The efficiency of the fluidviscous damper reaches maximum with the optimum damping coefficient. The optimumdamping coefficient only depends on the modal participation mass and the frequency, and isin direct proportion to them.2. The efficiency of the fluid viscous damper is roughly equivalent to that of optimumdamping ratio when the damping ratio is0.4to0.6.Thus the practical optimum dampingcoefficient can be adjusted in this range according to some factors such as the earthquakeintensity，capacity of joints and the cost.