| Earthquake disasters have great damage to building structures.Especially in the offshore area,where the terrain is complex and often located at the boundary of plates,earthquakes are frequent.However,the ground motion records detected are often limited by the topography of the monitored area,and cannot well reflect the interference effect of the complex seabed field on the ground motion propagation.With Marine resources development and major strategic step by step,towards the sea of human,cross-sea bridge,offshore platform and submarine pipeline structure is becoming more and more popular,and ignore the bottom of the sea complex field vibration transmission will make the influence of Marine seismic design of building structure has certain defects,which affects the safety of Marine structures.Therefore,it is of great theoretical significance and engineering practical value to study the seismic transmission mechanism of complex Marine site and improve the synthetic theory of offshore submarine seismic motion.Based on the trigonometric series method,the spatial coherence effect,traveling wave effect and local site effect are considered in this dissertation.Through optimizing and improving the site transfer function,the transfer function of undulated water layer and the combined reflection coefficient of ice-water layer are proposed to evaluate the influence of complex offshore site on spatial ground motion.Then a synthetic theory of multidimensional and multipoint space ground motion considering the influence of complex offshore site is concluded.Based on the generated man-made vibration acceleration time history,the random seismic response of a sea bridge is studied.The specific content is as follows:(1)The effects of different site conditions of soft soil,sand soil and hard soil on multidimensional and multipoint space ground motion are studied.Combined with a bridge engineering example,the influence of soft soil,sandy soil and hard soil on spatial ground motion is calculated by using the site transfer function.Then,the three-dimensional multi-point spatial ground motion time histories are synthesized by using the theory of artificial ground motion synthesis based on trigonometric series method.By applying the large mass method to the finite element model of the long-span cable-stayed bridge,the seismic response law of the cable-stayed bridge under multidimensional and multipoint excitation and the influence of site conditions on the seismic response of the long-span bridge structure are studied.(2)Based on the Crouse and Quilter transfer function models,the improved Crouse and Quilter transfer function was derived by considering the influence of the damping coefficient of soil and water layer on the transfer function of water layer.Furthermore,considering the water saturation in the soil layer,the influence of saturated soil and water layer on the ground motion propagation is studied.In this dissertation,the time-history of multidimensional and multipoint spatial ground motion is synthesized by the derived theoretical formula,and the structural response of a sea-crossing bridge under the excitation of multidimensional and multipoint man-made site motion is analyzed.The influence of seawater and saturated soil layer on the seismic response of the sea-crossing bridge structure is studied,and the attenuation ratio of ground motion under different sites is obtained.(3)Considering the effect of random wave of sea water on seismic reflection,a transfer function model of fluctuating water layer is proposed,which is composed of reflection coefficient of sea water layer and scattering coefficient of fluctuating sea surface.The sea surface fluctuation is a random event.According to Pierson-Moscowitz spectrum,the sea surface fluctuation shape is simulated by using harmonic superposition method,and then the scattering coefficient of the fluctuating sea surface is calculated according to the random wave height.Finally,the time history of multidimensional and multipoint space seismic movement is synthesized.The differences between the proposed transfer functions and Crouse and Quilter water layer transfer functions are compared and analyzed from the power spectrum and response spectrum of the synthesized ground motion acceleration time history.(4)A combined reflection coefficient of ice and water layer is proposed to evaluate the impact of floating ice layer and seawater layer on the seafloor ground motion.The coefficient includes three parts: water layer reflection coefficient,water layer transmission coefficient and ice layer reflection coefficient.According to the mechanical parameters of soil layer,water layer,ice layer and air,the coefficient comprehensively analyzes the reflection,transmission and refraction ways of seismic waves from soil layer to water layer,ice layer and air,and then reflects the attenuation principle of ground motion in the ice layer.Then,by using the combined reflection coefficient,the time history of multidimensional and multipoint spatial ground motion under the combined influence of ice and water layer is synthesized artificially.A synthetic random ice load time history method based on ice load spectrum is introduced,the ice load power spectrum of the vertical structure and the narrow cone structure interacting with floating ice is analyzed in detail in the case of extrusion failure and bending failure.Based on the ice load spectrum and the corresponding sea ice parameters,the random ice load time histories of extrusion failure and bending failure are synthesized artificially.Under the combined action of ground motion and random ice load time history,the random response of cable-stayed bridge is analyzed by using ANSYS finite element software,and the analysis results show that the random ice load has a great influence on the response of the bridge. |
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