A risk-targeted approach for the seismic design of bridge piers

Designing a structure to resist earthquakes by targeting an explicit failure risk has been a key research topic over the past two decades. In this article, a risk-targeted design approach is developed for circular reinforced concrete bridge piers, based on a probabilistic optimization procedure aimed at minimising the design resisting moment at the pier base. In order to reduce the computational effort, a surrogate model is developed to describe the influence of two key design parameter (i.e., the pier diameter and the longitudinal reinforcement ratio) on the structural behaviour and performance. The proposed approach is applied in a case study for Italy for target mean annual frequencies of failure selected according to European codes using a probabilistic seismic hazard assessment for average spectral acceleration across a wide range of structural periods. The variation in the design parameters across Italy is considerable because of the large variation in seismic hazard. It is found that in areas of low seismic hazard the level of seismic design required is near the minimum allowed by Eurocode 8 in terms of reinforcement ratio. In areas of the highest seismic hazard much higher reinforcement ratios and pier diameters are required to meet the risk targets. If both pier diameter and longitudinal reinforcement ratios are considered as design parameters then the optimisation procedure may mean adjacent sites have significant different pairs of these parameters as the target can be reached in multiple ways. This problem can be solved by fixing one parameter and optimising the other.