Far-field Seismic Response And Seismic Isolation Analysis Of Arch Tower Cable-stayed Bridge

Cable-stayed bridge has good seismic performance under the action of common ground motion,which is due to its large flexibility and long natural vibration period.However,in recent years,with the frequent occurrence of large earthquakes,engineers have recorded many earthquakes with special spectral features,which are very different in nature from common ground motions and have rich long-period components.Far-field ground motion is one of these earthquakes,which usually occurs in the site far from the epicenter of the earthquake and has deep overburden.The response of long-period structures such as cable-stayed bridges often exceeds the design ground motion due to the low-frequency component and long duration of far-field ground motion.In this paper,the features of common and far-field ground motion are discussed,and the responses of a cable-stayed bridge with arch-tower under two kinds of earthquakes are compared.Then two different seismic isolation devices such as viscous dampers and lead rubber bearings are introduced to analyze the seismic isolation of cable-stayed bridges under far-field earthquakes,which provides a basis for the design of such bridges.The primary contents of this article are as follows:(1)The features of time-history parameters,time-frequency characteristics and response spectrum of far-field earthquakes and the characterization of intensity indexes are explored.By comparison of 20 far-field seismic records and 5 common seismic records,the results show that:in terms of time-history parameters,the PGA of far-field ground motion is usually small due to the distance factor,while the PGV and PGD are close to;in terms of time-frequency characteristics,the predominant frequency distribution of far-field ground motion is more uniform,which is mainly distributed in 0.2-0.5 Hz;in terms of response spectrum,all kinds of response spectrum values of far-field ground motions are several times larger than those of common ground motions in the long period,and the acceleration response spectrum has obvious’double peaks’phenomenon.It is also found that the dynamic coefficient spectrum curve of far-field ground motions is poorly adapted to the current code spectrum by comparison.Finally,in terms of the characterization of ground motion intensity index,the velocity-type intensity index is more reasonable for the intensity value of far-field ground motion.(2)Combined with the dynamic characteristics of cable-stayed bridge,the time-history response results under far-field and common earthquakes are analyzed by FFT transform.Midas/Civil is utilized to bulid a model of the arch-tower cable-stayed bridge,and the dynamic features are analyzed and calculated.And then,the selected far-field and common ground motion records are used to excite the cable-stayed bridge longitudinally+vertically and horizontally+vertically.And the time history response results of key parts such as beam end,tower top,tower bottom and main beam at the junction are obtained.Finally,the consequence indicated that the response results of the key parts of the cable-stayed bridge under the longitudinal+vertical seismic excitation are controlled by the first-order vibration mode,and the far-field seismic action will lead to the damage of the tower bottom section,while the ordinary seismic action will not.And the participation of the high-order vibration mode of the bridge in the response results of the key parts under the transverse+vertical seismic excitation is significantly increased,but the tower bottom section can maintain the complete elastic state under the far-field and ordinary seismic action.(3)Study on the optimization of parameters under different seismic isolation methods and comparison of damping effect of cable-stayed bridge.The parameters of viscous dampers and lead rubber bearings arranged between the main beam and the lower beam of the cable-stayed bridge tower are optimized,and the mitigation effect is compared.The results indicated that:under the far-field ground motion,when the velocity index of viscous damper is 0.4 and the damping coefficient is 4000 kN·s/m;when the diameter of lead core rubber bearing is 275 mm,both devices can effectively reduce the displacement of beam end and tower top,however the viscous damper effect is better than that of lead rubber bearing in reducing the internal force of tower bottom.