Reliability analysis has been widely used in recent years to evaluate the safety of a structure. Among varied reliability approaches, fragility curve defines the failure probability of a structure for a given condition which agrees with the concept of performance-based design and is adopted in this study. Among many hazards, earthquakes and floods are two major threats for a river bridge in Taiwan and are considered here.
Nanyun Bridge, located in central Taiwan is used to demonstrate the proposed safety evaluation procedure. SAP2000 is used to simulate the nonlinear structural response in which the soil modelled by a bilinear link. Plastic hinge is predefined to simulate the non-linear behaviour of the bridge pier and caisson. Because Nanyun Bridge is supported by a single pier for each foundation, the plastic hinge is allocated at the bottom of the pier and the point with maximum moment on the caisson.
A series of non-linear time history analysis is performed to obtain the structure performance under different PGA values and varied scour depths. Displacement ductility from the response of non-linear time history analysis is calculated to construct the fragility curve for various limit states. Scour depth is estimated through probabilistic approach using the water depth and water velocity in which follow lognormal distribution is assumed.
The final result of this analysis is to establish the joint failure probability of the Nanyun Bridge against two major hazards. Based on results found, the Nanyun Bridge is likely to have a scour at 3-5 meters. The failure probability under a scoured condition for each limit state is usually small which found to be consistent with the on-site observation and other studies conducted for different bridges.

Reliability analysis has been widely used in recent years to evaluate the safety of a structure. Among varied reliability approaches, fragility curve defines the failure probability of a structure for a given condition which agrees with the concept of performance-based design and is adopted in this study. Among many hazards, earthquakes and floods are two major threats for a river bridge in Taiwan and are considered here.
Nanyun Bridge, located in central Taiwan is used to demonstrate the proposed safety evaluation procedure. SAP2000 is used to simulate the nonlinear structural response in which the soil modelled by a bilinear link. Plastic hinge is predefined to simulate the non-linear behaviour of the bridge pier and caisson. Because Nanyun Bridge is supported by a single pier for each foundation, the plastic hinge is allocated at the bottom of the pier and the point with maximum moment on the caisson.
A series of non-linear time history analysis is performed to obtain the structure performance under different PGA values and varied scour depths. Displacement ductility from the response of non-linear time history analysis is calculated to construct the fragility curve for various limit states. Scour depth is estimated through probabilistic approach using the water depth and water velocity in which follow lognormal distribution is assumed.
The final result of this analysis is to establish the joint failure probability of the Nanyun Bridge against two major hazards. Based on results found, the Nanyun Bridge is likely to have a scour at 3-5 meters. The failure probability under a scoured condition for each limit state is usually small which found to be consistent with the on-site observation and other studies conducted for different bridges.

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