Research On Earthquake Response Characteristic And Seismic Control Of Long Span Arch Bridges

Recently, long-span arch bridges are widely used in our country. And being traffic lifeline, the safety of long-span arch bridge by seismic excitation is very important. Once traffic line damaged, loss on life and property may be serious. However, the seismic behavior of arch bridge is poor, and traditional ductile design is unsuitable because of the high axial compressive ratio of arch rib. Moreover, it is lacking in research on seismic control of arch bridge. So making an investigation on some new, proper, effective, economical approaches of seismic control of arch bridge is the principal objective of this paper .The research is mainly focused on the following aspects.①The spatial variable seismic acceleration time history is simulated. Based on a power spectral model of spatial nonstationary earthquake ground motion and considering the spatial partial coherence and traveling wave effect , the seismic acceleration time history are simulated by the wave superposition method.②Establishing finite element model of long-span arch bridge and dynamic equation, and selecting analysis methods, the finite element dynamic analysis program is realized to make analysis on seismic response of arch bridge by multi-support excitation.③In order to solving the primary difficult in seismic control of long-span arch bridge that designing control system based on original model of long-span arch bridge is almost impossible because of the very large degree of freedom (DOF), methods of model reduction including critical modal analysis and balanced model reduction are proposed. As for the critical modes selection, an approach based on the maximum modal displacement is presented, which consider the effect of seismic excitation and is more reasonable than the method based on modal contribution ratio that only considers dynamic characteristics of bridge.④The seismic control schemes for arch bridge are presented after considering the structural characteristics of arch bridge. For top-through arch bridge, diagonal bracing with damper can be mounted on bridge. As for half-through arch bridge, dampers can be installed at the connection of deck and bridge pier, and the dampers can also be set at the end of suspenders. Numerical results show that the seismic control schemes mentioned above are effective for seismic control of arch bridge.⑤For the long-span arch bridge with nonlinear viscous dampers, an efficient method considering nonlinear damping force is presented. In the nonlinear time-history analysis of long-span arch bridge, this paper regards the nonlinear damping force as external control force which act upon bridge, and arch bridge is in the state of elasticity where the initial critical modes of the bridge are used in the model reduction. Then the state space method based on modern control theory or the precise integration method is used to command the state equations. The solving process for the nonlinear dynamic equation has high computation efficiency and realize a program which can perform passive control as well as semi-active control analysis .Taking Nimu arch bridge as an example, analysis are carried out to determine the optimal values of damping coefficient and velocity exponent for viscous damper for which the Maxwell model is used as computation model .⑥For semi-active control, Magnetorheological Fluid damper (MRFD) is used for seismic control of arch bridge. Modified phenomenological model is selected as the mechanical model of MRFD, and H∞-based Sign function control algorithm is proposed to command MRFD. Optimum analysis for semi-active control system consisting of installation position and number of MRFD and comparison between different semi-active control algorithms are performed. About the problem of overflow in the model reduction control, equivalent normal H∞control system of mixed sensitivity optimization is established to eliminate destabilization due to overflow. The analysis results demonstrate that this method can suppress overflow efficiently and has better robust performance.⑦At last, taking a top-though arch bridge, Nimu arch bridge in Tibet, and a half-though arch bridge,CaiYuanBa arch bridge in Chongqing , as examples, the seismic responses of the two bridges under uniform excitation and traveling wave excitation and multi-support excitation, and seismic absorption with viscous dampers and semi-active control with MR dampers, are numerically simulated. The traveling wave effect on the seismic control is investigated, and some valuable suggestions have been got. These works provide an important theoretical reference for improving seismic safety of the long-span arch bridge.