Dynamic Response Analysis Of Metro Tunnel Under Near-fault Ground Motion

Near-fault ground motion is different from far-field ground motion.These characteristics of near-fault ground motion make the seismic response of tunnel structure in near-fault area more significant and strong.Based on seismic database,this paper uses statistical analysis method to divide the near fault area.The adverse responses of tunnels under near-fault ground motions such as hanging and foot wall effect,fling step effect,directivity effect and vertical effect are studied.(1)The attenuation law of ground motion is considered from three aspects: source characteristics,site conditions and propagation media.Considering the influence of magnitude,focal depth,fault distance,fault type and site type,the concept of relative number is proposed.Based on the principle of least squares,the curve fitting method is used to analyze and the critical relative value is selected as the criterion for dividing near-fault area.According to the site type and fault type,the attenuation curve,curve fitting formula and critical relative value of each parameter with relative number are obtained under different site types and fault types.(2)The measured wave and the equivalent pulse model proposed by Mavroeidis is selected to fit the near-fault pulse ground motion,and the synthetic pulse ground motion is obtained.The fitting effect of the synthetic measured wave and the fitting wave is good.By adjusting the parameters of the selected equivalent pulse model,the pulse peak number,pulse period and pulse peak value of the synthetic near-fault pulse ground motion are changed,and the near-fault ground motion with different sensitive parameters is selected as the seismic load input to analyze the response law of pulsed ground motion to the tunnel structure under the near fault.The change of the number of pulse peaks has little effect on the structure.The peak value of pulse velocity and pulse period has a great influence on the response of tunnel structure under pulsed ground motion.The strain increases greatly under the earthquake action with rupture directivity effect,and the peak stress is less affected.The earthquake action with rupture directivity effect has more significant effect on the structural strain response.The peak stress and peak strain of the structure under near-fault ground motion are greatly affected.The seismic design of tunnel structure should consider the near-fault ground motion and far-field ground motion differently.The seismic design of near-fault ground motion should be emphasized.(3)The responses of structures under near-fault ground motion with hanging wall and foot wall effect are significantly different.The response values of structures under near-fault ground motion with hanging wall effect are larger than those under ground motion with foot wall with the same fault distance.At the same time,considering the difference of fault spacing between the two groups,the difference of structural response under near-fault ground motion decreases with the increase of fault spacing.When the period is greater than 0.2s,the mean values of acceleration and velocity response spectra in the hanging wall are larger than those in the foot wall region.The speed of the hanging wall area increases faster than foot wall area.The structural response under normal component earthquake and parallel component earthquake is significantly different.The structural response under normal component earthquake is larger than that under parallel component earthquake.In acceleration response spectrum,velocity response spectrum and displacement response spectrum,the normal component spectrum of fault is larger than the parallel component of fault.Vertical seismic response is very obvious,and there are many high-frequency components in vertical seismic response.Therefore,the vertical component in near-fault seismic response will have more influence on the seismic response of tunnel structure.In near-fault seismic design,the change of near-fault vertical seismic response to tunnel should be considered.In the acceleration response spectrum,when the period is less than 0.15 s,the vertical acceleration is greater than the horizontal acceleration,so the influence of the vertical ground motion on the tunnel structure should be considered.