Dynamic Response Analysis Of Cable-stayed Suspension Bridge Under Near-fault Pulse Ground Motion

With the rapid development of traffic network construction,many long-span bridges will inevitably be located in the active fault area due to the limitation of topographic conditions.In recent years,many studies on strong earthquake disasters at home and abroad have shown that the near-fault ground motion has the characteristics of long period and high peak value,which are obviously different from the far-site ground motion.At present,cable-stayed suspension bridge is a very competitive bridge type in cross-river and sea-crossing bridges,and this kind of long-span flexible bridge structure is very sensitive to long-period ground motion.In addition,the problem of insufficient number of measured near-fault ground motion records has been hindering the study of seismic response of bridge structures under near-fault ground motion.When an earthquake occurs,the pier foundation in deep water will change the natural vibration characteristics of the bridge structure because of the hydrodynamic pressure,which will affect the seismic response of the deep water structure.The bridge structure of cablestayed suspension system is relatively novel,and there are few domestic research achievements at present.Therefore,the dynamic response of cable-stayed suspension bridge under near-fault pulse ground motion is analyzed.The effects of different types of near-fault ground motions on the dynamic response of cable-stayed suspension bridges are studied,and the sensitivity of structural seismic response to pulse parameters is analyzed.It is necessary to explore the effect of hydrodynamic pressure on the dynamic response of long-span bridges in deep water.This thesis takes a cable-stayed suspension bridge with a main span of 1488 m as the research background,establishes a three-dimensional finite element model based on ANSYS software platform,considers the geometric nonlinearity of the structure through nonlinear dynamic time-history analysis,and studies the near-fault seismic response law of this kind of bridge.The main research work and conclusions of this thesis are as follows:(1)Based on the analysis of the response spectrum characteristics of near-fault pulse ground motion and the identification of different types of near-fault ground motion,two classical quantitative identification methods of near-fault pulse seismic waves are summarized.(2)Based on Tian Yuji’s simplified pulse mathematical model,by adjusting the pulse shape parameters to simulate the near fault rupture front effect type and slip impact effect type ground motion,the obtained low frequency velocity pulse and high frequency pulse free time history are superimposed,and the artificial synthesis of pulse type ground motion is realized.By using the comparative analysis of response spectrum,the rationality of artificial pulse ground motion is verified.(3)Based on the cable-stayed suspension bridge with a main span of 1488 m,a threedimensional finite element model is established.A total of 30 near-fault ground motions of three different types,namely,fling-step effect,forward-directivity effect and non-impulse effect type,are selected and input from longitudinal or transverse bridge directions.the dynamic response of cable-stayed suspension bridges under different types of near-fault ground motions is analyzed.The results show that the dynamic response of cable-stayed suspension bridge under pulse effect ground motion is obviously higher than that of non-pulse near-fault ground motion.Under the excitation of longitudinal bridge ground motion,the seismic response of the main tower induced by fling-step effect ground motion is the largest among the three types of ground motion,while under the excitation of transverse bridge ground motion,the response of the lateral displacement at the top of the main tower and the lateral bending moment at the bottom of the tower caused by forward-directivity effect is more obvious.(4)In order to study the sensitivity of seismic response of cable-stayed suspension bridge to near-fault seismic artery impulse parameters,five kinds of ground motions with different pulse peaks and pulse periods were input to analyze the seismic response.The results show that the peak value of pulse velocity is positively correlated with the seismic response of cablestayed suspension bridge,and the internal force response of cable-stayed suspension bridge is not sensitive to the change of pulse periodic parameters.the longitudinal displacement of the main tower under pulse ground motion decreases with the increase of pulse period.(5)The additional mass of pile foundation considering hydrodynamic pressure is calculated by using Morison equation and simplified formula of radiation wave theory respectively,and the additional mass distribution maps based on the above two methods are compared and analyzed.The results show that the additional mass based on Morison equation is larger than that based on radiation wave theory,and the additional mass is fixed along the height of pile foundation.The seismic response analysis after considering the additional mass shows that under the action of near-fault pulse ground motion,the hydrodynamic pressure will obviously increase the displacement response of the main tower and girder of the cable-stayed suspension bridge.The research results of this thesis can provide reference and enlightenment for dynamic response analysis and seismic design of cable-stayed suspension bridges under near-fault pulse ground motion.