| Summary: | 碩士 === 國立中興大學 === 土木工程學系所 === 103 === In current practice, the foundation of most bridges is strategically designed to remain elastic even under severe seismic demands. The bridge foundation is assumed to be equipped with large stiffness and strength, so that inelastic deformation due to earthquake excitation occurs in the column. However, for bridges located in regions with flood and earthquake hazards, foundation exposure caused by riverbed scour can significantly reduce the lateral stiffness and strength of the foundation. When the scour depth exceeds a critical level, the lateral strength of the foundation is insufficient to protect the piles from damage during a seismic event. The seismic performance of a bridge with foundation exposure differs completely from the original design. Since many bridges in seismic prone regions also suffer from serious riverbed scour, the assessment of their seismic performance becomes important.
In this paper, the seismic performance of bridges at different scour depths is assessed using the capacity spectrum method. The capacity spectrum of a bridge bent is constructed from the lateral pushover curve, which is given by the finite element analysis. The finite element model of the soil-structure system uses beam-column elements to model the reinforced concrete column and piles, and employs the beam-on-nonlinear-Winkler-foundation framework to simulate the soil-pile interaction. The ratio between the lateral force applied on superstructure and that applied on the pile-cap is calculated by the modal analysis of the bridge bent. During the pushover process, the lateral force ratio is adjusted after the formation of plastic hinges in the structure to account for the effect of stiffness change. To ensure a satisfactory seismic performance, the inelastic deformation of the foundation is controlled within the serviceability limit and the inelastic deformation of the column is kept within the damage-control limit. The seismic performance limit of the structure is identified as the column or the foundation first reaching its deformation limit. The seismic demand imposed on the structure is assessed using the acceleration-displacement response spectrum, which is constructed with the consideration of the influence from structural yielding. The maximum seismic demand, which the bridge is able to take can be defined when the demand spectrum intersects with the capacity spectrum of the bridge at its performance limit point. The influence of riverbed scour on the seismic performance of bridges is assessed by comparing the maximum allowed seismic demands of the bridge at different scour depths.
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