Displacement-based uniform risk design of reinforced concrete bridges

Abstract

Reinforced concrete (RC) bridge columns are among the most important components in the seismic design of bridge structures given their role in resisting lateral loads imposed by earthquakes and transferring vertical loads. Although recent changes in the seismic design of bridges have adopted displacement-based design approaches to promote adequate performance under seismic loads, the current design philosophy adopts a uniform hazard perspective without explicit consideration of the risk of damage or collapse. This paper puts forward a method to design bridge RC columns to have a uniform risk of failure, rather than the variable risk resulting from the application of current design codes, which is desirable in practice to reduce the uncertainty in the performance of bridges across regions. To obtain the risk in practice, a method for computing the RC column fragility based on design parameters of the column (e.g. height, diameter, longitudinal reinforcement ratio, and axial load ratio) is derived in this paper. The fragility curve of the RC column is developed using a prediction function relating the median seismic demand and the ground motion intensity measure without requiring new non-linear time history analyses for each prospective design parameter combination. Using this tool, a method for uniform risk design of RC bridge columns is put forward in this paper based on identifying a target ductility factor, a widely used performance parameter governing the seismic response of bridge columns. Sixteen typical RC bridge columns involving variations of parameters are considered in the study and designed according to the direct displacement-based design method with target ductility levels. The relationship of the failure probability of the RC column with the ductility demand is investigated and a target ductility factor is selected based on acceptable failure probabilities under the design seismic hazard. The approach described in this paper can support seismic design of RC bridges with a uniform risk of failure across different regions of the United States.