| The sea-crossing bridge has been developed rapidly during the past decades. However, seismic damages of the sea-crossing bridge have been reported frequently, such as the damages of the Higashi Kobe Bridge and Akashi Kaikyo Bridge (under construction during the earthquake) in Kobe earthquake in Japan in 1995. Therefore, it is of great importance to focus on its seismic safety. Currently, due to lacking of research on the characteristics of offshore ground motions, ground motions recorded on the land are still applied for seismic analysis of sea-crossing bridges, which remains to be studied and verified. In this study, the characteristics of offshore ground motions and influence factors are investigated based on the statistical analysis of recorded strong motions and the numerical simulation of motion propagation. A new seismic analytical model is proposed for consideration of pile-soil-seawater-bridge interation. Model of a cable-stayed bridge is established to illustrate the analytical model. Furthermore, a new aseismic method for the long span cable-stayed bridge is proposed, that installing buckling-restrained brace (BRB) between the tower (pier) and the beam. Numerical study has demonstrated the feasibility of this aseismic method (structure). The main contents are listed as follows:(1) The seismic damages of the sea-crossing bridges all over the world has been summarized. This study concludes the state of the earthquake resistance technology of sea-crossing bridges, based on analysis of aseismic design and seismic reduction design for some typical sea-crossing bridges during different periods of history in the world. The study on offshore strong motion observation records is presented in detail. Moreover, this study summarizes the main achievements of theoretical analysis and numerical simulation of offshore ground motions.(2) The strong motion records in the K-NET strong-motion seismograph networks (K-NET) in Japan and the Seafloor Earthquake Measurement System (SEMS) in the USA are utilized to compare the onshore and offshore ground motion from the following aspects:the acceleration time history, the peak ground acceleration (PGA), the horizontal elastic response spectrum, vertical-to-horizontal response spectral ratio (V/H spectral ratio) and the inelastic response spectrum. The influences of magnitude and epicentral distance are also discussed. The results indicate that the V/H PGA ratio for offshore ground motion is only half of the onshore ground motion, and the site condition can influence the V/H PGA ratio for offshore ground motion obviously. The characteristic of horizontal elastic response spectrum at the offshore site is similar to that at the onshore soft-soil site. The characteristic period for offshore ground motion is larger. Comparing to the horizontal elastic response spectrum for onshore ground motion at stiff-soil site, the long-period composition of horizontal response spectrum for offshore ground motion is more abundant. The influence of water depth on horizontal response spectrum for offshore ground motion is negligible. It can be observed that V/H spectral ratio for offshore ground motion is obviously less than onshore ground motion for the periods less than Is. According to different earthquake magnitudes, the simplified design equations of the V/H spectral ratio for offshore motions with different earthquake magnitudes are proposed. Moreover, the differences in horizontal inelastic response spectrum between onshore and offshore ground motion are limited.(3) Numerical model of offshore site including seawater and offshore overburden is built in ADINA software, and it considers the exterior-source wave input. The seawater layer is simulated by potential-based fluid element. Besids, the Fluid-Structure Interaction Boundary and Fluid-Infinite Region are used in the model. The viscous-spring artificial boundary is also used to consider the radiation damping effect of infinite foundation. The influences of sludge soft-soil layer on the seabed, topography (slope site), seawater depth, stratified site and the incident angle of P wave and SV wave on offshore ground motion are analyzed. The study indicates that the horizontal ground motion for SV wave is significantly amplified by sludge soft-soil layer on the seabed, and the sludge soft-soil layer is the main reason why the V/H spectral ratio for offshore ground motion is reduced for frequency more than 1.0 Hz. The influence of topography on offshore ground motion cannot be ignored. The vertical component of P wave can be amplified by slope site. On flat seabed, the vertical response of offshore ground motion is reduced at the resonant frequency of P wave propagating in seawater. The influence of seawater layer on horizontal ground motion of SV wave is little.(4) Based on the simulaiton technigue of the offshore ground motion, a numerical model for considering the pile-soil-seawater-bridge interation is established. A novel aseismic method (structure) that BRB installed between the tower (pier) and the beam are proposed. The earthquake responses of a cable-stayed bridge structure with BRB, a cable-stayed bridge in a floating system and a cable-stayed bridge structure with viscous damper are calculated. The case study indicates that the displacement between the bearing and the tower top is hardly reduced by the BRBs. Wheras, the bending moment of the tower (pier) bottom can be effectively suppressed. The viscous damper can effectively reduced the relative displacement between the bearing and the tower top. However, the bending moment of the tower (pier) bottom is amplified by the viscous damper. |
PDF Full Text Request