Seismic Vulnerability Analysis of Long-Span Multi-tower Suspension Bridge

Analytical methods for comprehensive evaluation of the seismic vulnerability of bridges have only recently become feasible due to a combination of advancements in the field of seismic hazard assessment and structural response analysis. However, current analytical methodologies, mainly based on fragility curves, may not be appropriate for the long-span multi-tower suspension bridge which belongs to a new type of bridge. In combination with Taizhou Yangtze River Bridge which is a tri-tower suspension bridge over 1000m in the main span, the paper attempts to present a large-scale assessment of the seismic vulnerability for this bridge. The capacity/demand ratio is introduced as fragility index for various key elements. By construction of the 3-D FEM dynamic model of Taizhou bridge, the investigation incorporates detailed nonlinear time history analysis. It can be found among all the components the tower, the pile foundation and the connection between the main cable and the saddle are most likely to be damaged by the severe earthquake. It is also observed that the most vulnerable locations/components may vary due to different directions of ground motion inputs. For Taizhou bridge, the smallest C/D ratio values are found to be at the bottom of the tower, at around 1/4 height of the tower-column from the top of the tower, at the pile foundations and for the slip resistance at the saddles under the longitudinal earthquake, and in a transverse ground motion the vulnerable locations may be the bottom of the tower, the joint between the tower-column and the lower crossbeam and the pile foundation.