| There are currently most active land-land collision orogenic belts,intense crustal deformations,complicated geological conditions,numerous seismic belts and densely distributed seismic activities along the railway lines in Western China.Thus a great number of railway bridges in these southwest high-intensity earthquake zones are now seriously threatened by the potential near-fault earthquakes.However,the railway bridge design codes currently adopted in China are almost blank regarding the seismic fortification in the nearfault area.There are no mature seismic isolation methods for the railway bridge in the nearfault area.Considering that the simply-supported girder bridges have been widely used in the railway system in this area because of their relatively simple force features,a typical simplysupported girder railway bridge is taken as an example in this paper to carry out the systematic research on the seismic responses and isolation design of the railway bridge under the nearfault earthquake.The main research work and achievements are listed as follows:(1)A total of 1701 near-fault seismic records,whose epicentral distance were less than90 km,were collected by domestic and foreign seismic networks.Subsequently a baseline adjustment method for near-fault seismic records was proposed.Then the characteristics of near-fault ground motions(including the velocity pulse and the vertical ground motion)were systematically analyzed,and near-fault seismic response spectrum was established.Finally,a program for artificially synthesizing near-fault ground motions was complied,and the reliability of the program was verified.(2)A numerical analysis model of a 32 m span simply-supported girder railway bridge was established,and the effects of far-field vibrations and near-fault ground motions on the seismic responses of isolated and non-isolated railway bridges are compared and analyzed.It is found that near-fault earthquakes with pulses and rich long-period components will cause more serious damage to the bridge and cause larger relative displacement of the friction pendulum bearings.The effect of vertical ground motion on the seismic performance of the bridge was studied,and it is found that when the ratio of peak vertical acceleration to the peak horizontal acceleration is greater than 1.5,there may be a negative reaction force in the bearings,and piers may have an axial tension without seirous damage.However,the vertical ground motion will lead to large fluctuations in terms of the pressure of bearings,significantly reduce the energy dissipation capacity of friction pendulum bearings and extensively enlarge the relative displacement of piers.In view of this,limit measures should be taken in the seismic design.Finally,the advantages and disadvantages of the existing seismic isolation measures were compared and a seismic isolation strategy for the railway bridges in the near-fault zones was formed.(3)A seismic isolation technology,combining the friction pendulum bearing and the metal damper(i.e.,damping tenon and block),for railway bridges was proposed.The structure form and working mechanism of the metal damper were expounded,and the crucial theoretical calculation formula of the mechanical parameters of the metal damper were derived.The mechanical properties of the metal damper were simulated by the numerical analysis method,the specific structure of the device was developed,a sample was manufactured,and the corresponding quasi-static mechanical performance test was carried out for validation.Finally,the obtained numerical analysis results were compared with the theoretical calculation values and quasi-static test results.The results show that the new metal damper proposed in this paper has a good overall performance with sufficient energy dissipation,full hysteresis curve and low cycle fatigue performance.(4)A total of 6 sets of shaking table tests were carried out for two-span simply supported railway bridges(including three seismic isolation schemes: typical bearing bridge,friction pendulum bearing bridge and friction pendulum bearing & clearance metal damper bridge,and two kinds of pier heights: 8m and 25m).The model manufacturing,construction and loading conditions of the shaking table test were designed.Based on the shaking table test results,the seismic performance of the friction pendulum bearing & metal damper bridge under multidirectional inputs(including longitudinal,longitudinal & vertical,transverse,and transverse & vertical directions)of near-fault and far-field ground motions were analyzed.The results show that the bridge with a combined seismic isolation system can significantly reduce the seismic response of the bridge and achieves an excellent seismic isolation performance.(5)A scaled numerical model was established using Open Sees software,and numerical model results were compared with the shaking table test data in order to verify the rationality of the finite element model.Then a full-scale four-span simply-supported bridge model was built based on the similarity relations to further analyze the damping efficiency of friction pendulum bearing & metal damper on the actual bridge under the 9-degree design earthquake.The results show that the combined seismic isolation technology can not only greatly improve the seismic isolation efficiency of the bridge,but also be insensitive to vertical ground motions,which is very suitable for the seismic fortification of bridges in near-fault seismic regions.(6)The Open Sees finite element analysis model of a full-scale four-span simplysupported bridge was established,and the nonlinear dynamic time history analyses were carried out.The design parameters of the friction pendulum bearing and metal damper on the seismic performance of the bridge were analyzed in depth.The results show that under the 9-degree design earthquake,reducing the yield strength of the shock-absorbing tenon,reducing stiffness ratio after yielding,increasing the radius of the friction pendulum bearing,and reducing the friction coefficient of the friction pendulum bearing are conducive to improving the seismic isolation efficiency of bridge piers,while the displacement of bearing will increase at the same time.Reducing the clearance and increasing the initial stiffness of damping tenon will reduce the displacement of bearing,but has a small impact on the bridge pier.Under rare earthquakes in the 9-degree zone,reducing the yield strength and stiffness ratio after yielding of the block will help improve the seismic isolation efficiency of the pier,while will increase the displacement of bearing.Reducing the clearance and increasing the initial stiffness of the block will reduce the displacement of bearing,while have a small impact on the pier.Under rare earthquakes,the key problem is limiting the displacement of bearing,which can be done by reducing the gap between blocks and increasing the yield strength of the block.Finally,the recommended value range of design parameters of the combined seismic isolation system of a typical railway simply-supported girder bridge is given. |
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