| In recently years,many bridges have suffered serious damage or even collapsed due to crossing faults in several major earthquakes.In order to avoid damage to the structure by faults,the regulation have show clearly that prohibit bridge structure from crossing faults or setting a certain avoidance distance between the faults in domestic and foreign.It is inevitable to use cross-fault bridges because of the limitation of objective factors such as topography,construction cost and construction period respectively.At present,China's design on cross-fault bridge structures is still in the initial stage,while lacking the reference literature and design specifications.In the design of cross-fault bridges,the main way to resist earthquake is improving the intensity of fortification.With the development of bridge structures are launched in high-intensity areas,therefore,it is of a very important practical significance to carry out seismic research on cross-fault bridges.For these reasons,the dissertation disscuss the perspective about seismic design of cross-fault bridge structures,including the ground motion input,analysis model,structural seismic response and seismic control measures of cross-fault bridges.This dissertation mainly researched the following aspects:1.The time-history simulation method for ground motion of a cross-fault bridge based on artificial synthesis to be proposed.Firstly,the directional pulse model proposed by Dabaghi M is used to simulate the directional pulse component.The fling-step pulse model proposed by Vaez SRH is used to simulate the fault fling-step pulse component.Secondly,the design response spectrum is used as the target spectrum to obtain the high-frequency component of the ground motion.Then,the pulse components are respectively superimposed with the highfrequency components to synthesize a cross-fault pulse-mode ground motion time history with multiple frequency components.The ground motion time history obtained by the above simulation method can simultaneously consider the characteristics of the bridge site and the different pulse-type ground motion characteristics of the support on both sides of the fault.Finally,the influence of fault interval on the cross-fault pulse ground motion is discussed.The results showed that the directional effect and the fling-step effect are not obvious when the fault distance R>50km,and can be treated according to the far-field earthquake.2.The non-uniform excitation model suitable for ground motion input of cross-fault bridges has been studied.Based on the displacement input model and the acceleration input model,the structural dynamic equations under non-uniform seismic excitation are derived.Taking the KuoKeSa Bridge of Dunge Railway as an example,the applicability of two input models in seismic response analysis of cross-fault bridges is analyzed.The results showed that the displacement input model can consider the influence of the permanent displacement of the ground caused by fault displacement on the seismic response of the structure,and can truly reflect the residual deformation and internal force of the pier after the end of the ground motion of the cross-fault bridge,which is suitable for multi-point excitation response analysis of the cross-fault bridges.However,the acceleration input model cannot consider this effect and may lead to unreasonable calculations.3.The seismic response characteristics and influence laws of cross-fault bridges are studied.Based on the ground motion time-history simulation method and input model determined in this paper,the non-uniform time-history analysis method is used to calculate the bridge as an example.The seismic response characteristics of the cross-fault bridge are studied around two aspects which are internal force and displacement,and the calculation results are discussed.The results showed that the seismic response characteristics of the cross-fault bridges have significant differences with the near-field and far-field seismic bridges.If the characteristics of the fault sites are neglected,it is unreasonable to improve the seismic design of the bridges by improving the fortification intensity.The influence of the fault distance and vehicle load on the cross-fault bridge is analyzed.The influence of the above factors on the seismic response of the bridge is obtained and put forward the preliminary suggestions for the design of the cross-fault bridges.4.For the significant problem that the low-frequency pulse component of the fault ground motion is rich and will cause serious damage to the structure,a viscoelastic damper that can be applied to the fault ground motion is designed.The performance test of the damper of different temperature,frequency,strain amplitude and thickness is carried out in order to study its energy consuming capacity and the influence of various characteristic parameters on its energy consuming ability.The influence of different parameters on the performance of viscoelastic damper is also studied.5.The viscoelastic damper is applied to passively control the cross-fault bridge.The seismic control equation of the bridge structure with viscoelastic damper is established.The method of solving the viscoelastic damping control system is proposed based on the modal strain energy method.The two methods for designing the viscoelastic damper parameters are discussed and analyzed respectively.The design method and general steps of the seismic control system of viscoelastic damping are proposed.Finally,the viscoelastic damper is used for seismic control in the KuoKeSa bridge,and the pier top damping system and the pier bottom damping system are analyzed.The results showed that the viscoelastic damping seismic control of the cross-fault bridge can reduce the seismic response of the structure,and the shock absorption effect of the pier bottom damping system is better than the pier top damping system.However,special treatment is required when designing pier bottom damping system,and it cannot be repaired or replaced later.But the pier top damping system can avoid these problems.It can not only control the vibration of new bridges,but also strengthen the existing bridges. |
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