Immersed tunnels, as a form of underwater transportation engineering offering numerous advantages, have been widely deployed in coastal and riverside cities. However, due to the shallow burial and underwater characteristics, immersed tunnels present significantly different surrounding soil and water environments compared to land-based tunnels. Currently, there is limited research on the seismic analysis of submarine immersed tunnels, raising questions about the direct application of the methods of land-based tunnels. In this study, the Davidenkov soil constitutive model is introduced to simulate the strong nonlinearity of deep sedimentary soil in marine areas. The Coupled Acoustic-Structure (CAS) method is employed to simulate the dynamic interaction between seawater and seabed. A time-history analysis model is developed to capture the coupling interactions between seawater, seabed, and tunnel structure. The effects of the soil-tunnel contact mode and seismic input method on the seismic responses of immersed tunnels are investigated in detail. Seismic response characteristics of immersed tunnels are analyzed from four perspectives: distribution of tensile damage in the tunnel, maximum inter-story drift ratio, maximum bending moment, and tunnel inclination angle in the cross-sectional direction. The results indicate that the overlying seawater and sand compaction piles negatively impact the seismic performance of immersed tunnels in the scenarios of this study. Furthermore, their impact pattern and extent are closely correlated with the intensity of the input seismic motion.

The objective of this study is to explore the seismic fragility of reinforced concrete bridges, specifically in response to the vertical components of ground motions, utilizing fragility surfaces. The examination of bridge responses involves the application of optimally selected intensity measures through three-dimensional nonlinear time-history analyses, encompassing uncertainties in both superstructure materials and soil-structure interaction effects. In this investigation, an extended Probabilistic Seismic Demand Model (e-PSDM) is employed, leveraging fragility surfaces to concurrently consider vertical and horizontal excitations. The results obtained from this approach are compared with traditional fragility curves. This study emphasizes Pile-cap displacement and drift ratio as pivotal engineering damage parameters, acknowledging their sensitivity to the influences of both soil-structure interaction effects and vertical ground motion. The fragility surfaces derived from the study reveal a correlation between increased vertical spectral accelerations and elevated probabilities of surpassing both slight damage and collapse limit states. These observations underscore the critical significance and practical utility of fragility surfaces in the context of performance-based seismic assessment and design for reinforced concrete bridges. The findings from this research contribute valuable insights into the nuanced behaviour of reinforced concrete bridges under seismic conditions, emphasizing the relevance of incorporating vertical components in fragility assessments for a more comprehensive understanding of structural vulnerability.