Risk assessment and fragility curves for sea crossing monopile supported RC bridge under varying ocean current loading

Bridges are the structures that connect communities. Monopile foundations are preferred for sea-crossing bridges. As a result of tidal variations and global warming, the characteristics of waves and ocean currents change frequently. Even welldesigned monopiles are vulnerable due to such variations. Ocean current fragility curves are developed for an existing highway bridge in Mumbai by taking account of various scenarios affected by factors such as wave height, water depth (mean sea level), and the effect of scouring around the monopile and wind speed. Computational Fluid Dynamics (CFD) is used to obtain the response of the monopile, and with this response, fragility curves are developed for this monopile foundation for design limit state function by considering ocean current velocity as an Intensity Measure (IM). The threshold mean current velocities for the design limit state are derived for the scenarios affected by factors like wave height, scouring, wind speed, and mean sea level. It is noted that the combination of loads have more effect than considering each of them separately. The fragility of the monopile structure increases by 50% when the wave height is increased to 2.5 m from 1.5 m at an intense mean current velocity of 2.6 m/s. The damage to the bridge increases by 10% when wind speed is raised from 6 m/s to 12 m/s. The vulnerability of monopile enlarges by 67% for the change in water depth from 6.5 m to 7.5 m at a particular mean current velocity of 2.6 m/s. The threat to the bridge increases by an amount of 133% when a scouring depth of 1.5 m around the monopile is considered at an intense mean current velocity of 2.6 m/s.