Axial Load Failure of Shear Critical Columns Subjected to High Levels of Axial Load
This paper discusses the experimental behavior of two lightly reinforced (shear-critical) columns subjected to high levels of axial load and lateral load reversals. The two full-scale experiments were carried out at the NEES-MAST faciity at the University of Minnesota as part of a large study on the risk of collapse of older concrete buildings in which partial collapse and redistribution of vertical loads between columns takes place. Tests evaluated the behavior of two 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 of each column was 0.3 and 0.2 f'cAg, respectively.The two columns had a height of 2945 mm and cross section of 457 x 457 mm. The longitudinal reinforcement ratio was 2.5% and 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. It was found that the behavior of the two columns at drift demands higher than the drift at axial failure was significantly different. The lateral stiffness and the residual axial load capacity of the columns were found to be related to the axial load prior to axial failure.
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