Evaluation of the critical plastic region length in slender reinforced concrete bridge columns

This paper presents how the length of the critical plastic region (Lpr) in slender columns is significantly larger than what is estimated from current models, which were developed for shorter columns. Experimental results from three large-scale slender reinforced concrete columns with aspect ratios (length to depth) up to 12 were used to explore the effect of slenderness on Lpr. Components of Lpr, including linear moment gradient, nonlinear moment gradient, and tension shift effect were extracted from the test data and the contribution from each component was determined. Experimental Lpr values were compared with American, European, and New Zealand design guidelines. All design codes considered in this study significantly underestimated Lpr except for seismic design provisions provided by Caltrans. Previous expressions for the length of the plastic region were reexamined in light of the new experimental data and strong disagreement between experimental and predicted Lpr was observed. A new expression for Lpr, which correlates well with test data is thus proposed. The expression varies according to the magnitude of inelastic demands, defined by displacement ductility, and includes the effect of nonlinear moment gradient due to P-δ effects.