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Assessment of confinement models for reinforced concrete columns subjected to seismic loading Riederer, Kevin Allen

Abstract

Research conducted over the past several years has shown that factors such as axial load level and the amount and spacing of confinement steel influence the performance of reinforced concrete columns subjected to seismic loading. The aim of this research project was to investigate the performance of the current ACI 318 confining steel requirements and compare them to other codes and proposed models to determine their suitability for a performance based design equation for implementation in Chapter 21 of ACI 318. The investigation was performed by analyzing the results of multiple reverse-cyclic column tests presented in the UW/PEER Structural Performance Database. The condensed database used in this investigation consisted solely of columns which exhibited flexural failure and contained 145 rectangular and 50 circular columns. First, a scatter plot was used to compare the confining requirements of each model with the lateral drift observed for each column within the database. The plot showed the drift ratio achieved by the column test versus a ratio of lateral steel Ash provided over that which is required by ACI (Ash provided / Ash ACI)- A drift ratio of 2.5% was selected as the performance target for the evaluation. Columns were identified as those which satisfied the requirements of the model but failed the performance target ('unconservative') or those which failed the requirements of the model but satisfied the performance target ('conservative'). For each model, the percentage of columns falling into these classifications was calculated and compared. Two fragility curves were generated for each model which provided the probability of a column being classified as 'unconservative' or 'conservative' as a function of drift ratio. A third curve was a combination of the first two and provided insight as to the overall performance of the model. For both the rectangular and circular column evaluations, the ACI model was determined to be the least desirable of all models investigated. Based on the evaluation techniques developed, specific models were selected as recommended alternatives to the current ACI requirements. The recommended models minimize the potential of a column experiencing lateral strength degradation before reaching the prescribed lateral drift limit.

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