Fatigue Reliability of Hammerhead Bridge Piers

Abstract

This paper aimed to determine safety levels involved in bridge pier design when the fatigue nature of traffic load is included. Reliability analysis was used to estimate values of reliability index for the ultimate limit state (pier’s capacity). The analysis was performed in two steps. In the first step, reliability indexes were calculated using statistical information on traffic load and resistance of piers, as it was used during the AASHTO, Load and Resistance Factor Design (AASHTO LRFD) code calibration. The purpose was to estimate design safety according to code provisions. The second step also involved estimating reliability indexes, but with different statistical parameters for live load and resistance. A new, recent live load model with new statistics was used. Statistical parameters of pier resistance were established, including different statistics for concrete strength when fatigue of material is considered. The study was performed for a range of design cases, including different span lengths, sizes of a pier’s cross section, three typical strengths of concrete, and three reinforcement ratios. In the resistance analysis, piers were considered as short columns under biaxial bending. The results did not show a drastic drop in piers’ capacity when the fatigue nature of the traffic load is considered. The resulting reliability indexes when fatigue was included were, on average, 12% smaller compared with the values of the index associated with the current code design.