Study Of The Effects Of Near-fault Ground Motions On Special Seismic Response Of Cable-stayed Bridge

Compared with the far-field ground motion, near-fault ground motion has significant different special effects, such as disaster concentration, forward directivity effect, fling-step effect, hanging wall effect, footwall effect and vertical effect. Special ground motions of near-fault earthquake cause the more intense and complicated response of cable-stayed with long period located near the active fault or cross the fault. Based on the National Natural Science Fund Project "Cable-stayed bridge under near-field earthquakes ’active faults-soil-structure’ Integration of analysis theory and experimental research " (No.51378110), the unfavorable influence of spatial distribution characteristics, forward directivity effect and fling-step effect contained in the pulse-like ground motion, hanging-wall district and footwall effect on the dynamic response of the bridge is studied. The main research conclusions are listed as following:(1) Study the unfavorable influence of spatial distribution characteristics on the response of the structures.Study the influence of total spatial distribution, partial spatial distribution on the structure in Taiwan Chi-Chi earthquake, and intensity indices to evaluate earthquakes in different areas responses of the bridge. The response spectrum analysis illustrates that the seismic ground motions in MD district contain the notable abundant low-frequency component, the spectrum value of normal earthquakes is greater than that of parallel earthquakes. The bridge with the main length 406m under MD earthquakes has the maximum seismic responses, with the average value of longitudinal displacement at the top of the tower 4.03 times and the internal forces 2.0 time more than that in BD, respectively, the parameters PGV and PGA value should be selected as intensity indices to evaluate dynamic seismic responses of the structure; the average value of longitudinal displacement under FD earthquakes is 2.06 times of that under BD earthquakes, while the average value of moment and shear force decreases 35.5% and 32.2%, respectively. Numerical results illustrate that the seismic design of structures in FD should take into account higher displacement demand, and strength demand should be a priority for the structures in BD.Numerical results illustrate that the response under normal direction earthquakes has higher value than that under parallel direction earthquakes; the normal direction earthquakes is selected to be as seismic input preferentially when the PGVP/N is between 0.2～0.6.(2) Study the unfavorable influence of pulse-like ground motions with forward directivity effect and fling-step effect on cable-stayed bridge.It is important to study the unfavorable influence of pulse-like ground motions with fling step effect on the seismic reponse of strutures. The bridge under pulse-like ground motion with fling step effect has the maximum seismic responses, with the average value of longitudinal displacement at the top of the tower 1.61,2.92 and 17.27 times more than that in other earthquakes, respectively; the average value of moment at the bottom 1.04,1.15 and 2.88 times more than that in other earthquakes; with the average value of shear value at the bottom 1.22,1.26 and 2.38 times more than that in other earthquakes.Numerical results illustrate that the spectral characteristics of synthesis wave using Mavroeidis model has better agreement with that of the original wave. Numerical results illustrate that the displacement of the main tower, the internal force distribution of cable-stayed bridge under the composite wave has a better fit with that under the original wave.(3) Study the unfavorable influence of hanging-wall, footwall effect and vertical effect on the dynamic response of cable-stayed bridge.It is important that the bridge under ground motions in the hanging-wall district has the maximum seismic responses, with the average value of longitudinal displacement at the top of the tower, moment, shear at the bottom 2.61、1.41 and 1.30 times more than that in the Footwall district, respectively. Based on the analysis above, the demand of longitudinal displacement and internal force of the structures in the HW district should be satisfied. The average value of longitudinal displacement increased when taking into vertical effect in the HW district; the average value of shear under increases to varying degrees, while the momnet value of the structures needn’t take vertical effect into consideration.The paper researches the unfavorable influence of special spatial distribution characteristics, pulse-like ground motion contained rupture directivity effect and fling effect, hanging wall, footwall wall effect and vertical effect of near-field earthquake on response of cable-stayed bridge, the study reveales the influence of near-fault special effect of seismic performance on long-period structures.The results above can provide some advice and reference for seismic design of cable-stayed bridge under near-fault ground motions in the future.