Refined deformation limit states for circular reinforced concrete bridge columns

One hurdle against the widespread implementation of the performance-based design (PBD) for bridges is the lack of consensus among practitioners, researchers, and code committees on engineering demand parameter (EDP) limits defining the onset of various types of damage and their variability. This study seeks to bring consistency to the PBD methodology by establishing refined EDP limits at concrete cover spalling and bar buckling for circular reinforced concrete bridge columns. To this end, a database consisting of 118 previously tested flexure-dominant bridge columns was formulated and analyzed. The EDP limit considered at the member level was drift ratio, whereas, at the sectional level, the EDP limits considered were material strain and curvature ductility. State-of-the-art symbolic regression was adopted to fit the resulting data to mathematical expressions. At the member level, predicted drift ratio limits at the two damage states obtained with the proposed expressions were associated with lower root-mean-square error (RMSE) than those obtained from other similar expressions in the literature. At the sectional level, drift ratios at concrete cover spalling were adequately predicted with compressive strain limits in concrete ranging from 0.004 to 0.007 and a curvature ductility limit of 6.3. More accurate predictions of drift ratios at bar buckling were attained with variable sectional EDP limits, particularly for columns subjected to relatively high axial load. Two expressions predicting the tensile strain limit in the rebar and curvature ductility limit at bar buckling were proposed. The ratios of the measured drift ratio at bar buckling to the drift ratio predicted based on the proposed variable tensile strain limit had a mean of 0.99 and a coefficient of variation (COV) of 33%. Corresponding ratios based on the proposed variable curvature ductility limit had a mean of 1.02 and a COV of 31%. Fragility functions relating the likelihood of concrete cover spalling and bar buckling to the considered EDP limits were also developed.