Theoretical modeling of spatially varying ground motions under ice-seawater interface and earthquake analysis of cable-stay bridges

The ice-seawater interface has a significant effect on earthquake-induced ground motion. This paper proposed a new method of modeling spatially varying ground motion due to ice-seawater layer. The reflection function was derived to evaluate the effect of ice-seawater layer on earthquake ground motion. Moreover, the coherence function and site transfer function were used to calculate the loss of spatial coherent and the corresponding local effects, respectively. Furthermore, the effects of water saturation on ground motion was considered. Taking the above factors into consideration, the spatial variance of seismic ground motions in an area with an ice-seawater interface was synthesized. The ground motion was assumed to consist of SH and P-SV waves, which included horizontal out-of-plane SH wave, horizontal in-plane P-SV waves and vertical in-plane P-SV waves. By comparing simulated ground motions with different site conditions, we observed that the vertical in-plane motion at the seafloor was noticeably attenuated when an ice-seawater interface was present. At the same time, the soil layer greatly amplified the acceleration of out-of-plane and in-plane ground motions. In order to verify the ability of the above method for synthesizing seismic time history, a three-dimensional seismic analysis of a large span cable-stayed bridge was conducted. Three of such case studies were used to investigate the effect of the ice-seawater interface on the bridge. Analysis of the displacement and bending moment of the bridge deck and tower demonstrated that the ice-seawater interface had a great effect on the seismic response of the cable-stayed bridge. In particular, the maximum vertical deck displacement with the ice-seawater interface present was 74% of the case in which the interface was absent. Likewise, the maximum longitudinal displacement of the bridge tower with the interface present was 70% of the case in which the interface was absent.