Study On Design Method And Parameters For Seismically Isolated Railway Bridges

Seismic isolation technology is an efficient method of aseismic design for bridges. This technology has been widely applied to bridges in many foreign countries. In China, it is rarely used in bridges especially in railway bridges, where the theories and methods of seismic isolation for bridges have not been well established. According to the current shortages in the seismic isolation design for railway bridges, some important problems are systematically studied in this paper, including the influences of different dynamic parameters on seismic responses of seismically isolated bridge system (IBS), seismic isolation design for bridges based on energy concept, running safety assessment of trains in seismic design of isolated bridges and parameters optimization for isolated bridge system. The principal contents are as follows:1. An analytical model of soil-pier-isolation bearing-beam system is established, and the influences of various dynamic parameters on seismic responses of IBS are systematically studied. (1) Based on the TDOF (two degrees of freedom) model of isolated bridge considering soil-structure interaction, the approximate solving methods and applicable ranges for the vibration characteristics and seismic responses of TDOF system are obtained. The seismic responses of isolated bridge calculated by the approximate methods are coincident with those by FEA method. (2) The reasonability of taking equivalent isolation degree as important parameter for bridge seismic isolation design is verified, which should be larger than 3.0 in bridge seismic isolation design. (3) The assessment index for bi-directional coupling effects of isolation bearings on seismic responses of isolated bridge system is created. Through nonlinear time-history analysis, the influences of these effects are studied, and the numerical formula of assessment index and the first natural period of non-isolated bridge pier are obtained. The current combination method of such motions is evaluated, and some suggestions for bridge seismic isolation design under bi-directional ground motions are proposed when considering bi-directional coupling effects of isolation bearings. (4) Through nonlinear analysis for isolated bridge system considering soil-structure interaction, the influences of foundation stiffness variations on the seismic responses of isolated bridge and non-isolated bridge are compared, and the influences of soil-structure interaction on isolated bridges with various configurations in various types of sites are studied.2. The energy equations for IBS are deduced according to energy equilibrium theory, based on which bridge seismic isolation design by energy method is studied. Every forty earthquake waves are selected for three types of sites, and the dynamic magnification factors calculated by these earthquake waves are well coincident with those of Seismic Design Code for Railway Engineering. The influences of peak ground accelerations, principal dynamic parameters of isolated bridge system and bi-directional coupling effects of isolation bearings on input energy and their distributions are studied. Based on the energy dissipation properties of lead rubber bearing, the energy failure criterion for isolated bridge system is established. The procedures of bridge seismic isolation design by energy method are proposed. The isolation degree is taken as isolation objective, and the displacement ductility ratios of isolation bearing and bridge pier are considered as constrained conditions. The inelastic input energy spectrum is used for bridge seismic isolation design, and the isolation efficiency by energy method is confirmed by the spectral response method given by the Code and the nonlinear time history analysis, respectively.3. The spectral intensity (SI) is employed as an index for running safety assessment in seismic design of isolated bridges, and safety assessment of trains running on isolated bridges is completed, which provides simplified method and important foundation for safety assessment of trains running on isolated bridges under earthquakes. Under every forty earthquake waves of three types of sites, the spectral intensities for frequent earthquakes and rare earthquakes are calculated. Based on the safety limits (SI_Lim) of spectral intensity in Seismic Design Code for Railway Structures of Japan, the safe zones and dangerous zones for conventional trains and high-speed trains running on isolated bridges are determined by comparing SI and SI_Lim at the natural period of the bridge. Under frequent earthquakes, running safety can be assured by reasonable seismic isolation design for bridge structures. For design earthquakes, the dangerous zones of the three types of sites are located in middle-long periods (1.0s to 2.0s), and the two sides of dangerous zones are two safe zones. The range of safe zones in long periods increases from type I site to type III site in turn. Therefore, isolated bridges are not suitable for building in soft soil sites in view of considering running safety.4. A parameter optimization model for isolated bridge is established, and two optimization analysis methods are applied to study on the parameters optimization of isolated bridge system, which provides efficient paths and beneficial references for dynamic optimization design of isolated railway bridges. (1) The optimization model of isolated bridge system considering soil-structure interaction and running safety is established. The mechanical parameters of isolation bearing are taken as design variables, and the maximum moments at bottom of bridge piers are employed as objective functions when the displacement between pier top and beam meet with the limited values. In the optimization analysis, by considering the influences of the variation of Rayleigh damping factors on seismic responses of isolated bridge, both single earthquake wave and multiple earthquake waves are taken as excitations for the parameters optimization analysis, respectively. (2)The optimum yield ratio is defined as the yield ratio at maximum energy dissipation ratio of isolation bearing. The nonlinear energy response analyses are carried out for various isolated bridge systems under the El Centro earthquake wave, and the regression equation of optimal yield ratio for different earthquake levels is obtained. Finally, the applicability of regression equation for other types of earthquake waves is verified. Calculation process of determining the optimum yield ratio can be simplified by the regression equation, which is convenient to master and use for engineers.