Effect of Longitudinal Reinforcement Ratio on the Failure Mechanism of R/C Columns Most Vulnerable to Collapse
Models to calculate the drift ratio at axial load failure of reinforced concrete columns such as that proposed by Elwood and Moehle indicate that the drift ratio at axial load failure is primarily a function of axial load demand and the detailing of the transverse reinforcement. A series of column tests recently completed at the MAST facility at the University of Minnesota showed that for R/C columns most vulnerable to collapse the longitudinal reinforcement ratio had a very ignificant effect on the drift ratio at axial failure. Three full-scale experiments were carried out at the NEES-MAST facility at the University of Minnesota as part of a large study on the risk of collapse of older concrete buildings during major earthquakes. Tests evaluated the behavior of columns with ratios of nominal shear strength to plastic shear demand on the order of 0.85. The longitudinal and transverse reinforcement ratios were maintained constant, while the axial load ratio and the longitudinal reinforcement ratio were varied. Axial loads were 0.3 and 0.2 f’c Ag, and the longitudinal reinforcement ratios were 2.5 and 3%. All columns had a height of 2945 mm and cross section of 457 x 457 mm. The transverse reinforcement consisted of No. 3 bars (9.5 mm diameter) spaced at a distance equivalent to the column size (457 mm). The loading protocol consisted of cycles with increasing maximum lateral displacement under constant axial load. The paper analyzes the load resisting mechanism and behavior of the specimens, and compares the experimental results with existing axial failure models and modeling and acceptance criteria in the ASCE-41 Standard.
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