Effect of Cumulative Cycling on Shear Strength of Older Reinforced Concrete Bridge Columns

Abstract

This research assessed the effect of cycling on the shear strength of older reinforced concrete bridge columns in Washington State, particularly those designed prior to 1975. These columns typically have low transverse reinforcement ratios (<0.5%) and circular hoops with overlapped ends, making them susceptible to shear and flexure-shear failures. A prototype column representative of 1960s Washington State bridge construction was scaled to a 1:3 ratio and used to design three nominally identical reinforced concrete columns that were subjected to loading protocols with identical drift-ratio targets but different numbers of cycles between drift targets. All three specimens exhibited shear-dominated damage progression and were classified as shear failures. For the relatively low cumulative plastic displacement demands these columns experienced before failure, increasing the number of cycles between drift-ratio targets did not yield a clear reduction in the effective lateral force at failure. However, the specimen subjected to a higher number of cycles sustained greater deformation before failing in shear. The results from these tests were combined with a compiled sample of 13 existing experimental tests of circular reinforced concrete columns with representative properties of pre-1975 bridge columns that exhibited shear damage, obtained from the UW/PEER and ACI 369 databases. These data were used to evaluate the Kowalsky and Priestley (2000) shear strength model, which overestimated shear strength in most tests with overlapped hoops, including those tested in this study. This overestimation is attributed to the reduced effectiveness of overlapped hoops in contributing to shear strength, together with trends suggesting that concrete’s shear strength contribution degrades as cumulative plastic demands increase. Additional tests are needed to reach definitive conclusions regarding the effect of cumulative plastic displacement demand on shear strength.