Axial failure of vulnerable reinforced concrete columns damaged by shear reversals

Columns in reinforced concrete buildings constructed before the enforcement of modern seismic design provisions may contain ties at large spacing (s \u3e d/2) and having 90-degree hooks. There are buildings still in service in seismic regions which have columns with ties that have these details. Columns containing ties at large spacing and having 90-degree hooks may experience shear and axial failure when subjected to displacement reversals caused by earthquake ground motions. To evaluate the likelihood that a building may collapse during an earthquake, engineers need to be able to determine which columns may experience axial failure. Guidelines have been developed to aid engineers in estimating the drift level at which a concrete column, containing ties at large spacing and having 90-degree hooks, will experience axial failure. These guidelines were calibrated using results from a limited number of experimental tests. The tests used to calibrate these guidelines examined the effects of several key parameters such as the amount of longitudinal reinforcement, the amount of transverse reinforcement and the level of axial force. Other important parameters such as aspect ratio, number of displacement cycles, and applying cycles along more than one axis have not been examined for columns containing ties at large spacing and having 90-degree hooks. This study includes results from eight full-scale concrete columns tested in double curvature. The columns had aspect ratios of 1.9 and 3.7, longitudinal reinforcement ratios of 1.5% and 2.5%, ties at large spacing (s \u3e d/2) and having 90-degree hooks with transverse reinforcement ratios ranging from 0.07% to 0.18%. All columns were designed to experience shear failure before developing plastic hinges. The columns were subjected to cycles of displacement reversals applied either along one axis (uniaxial displacement protocol) or along two orthogonal axes (biaxial displacement protocol). Each column was tested under a constant axial force. Axial force levels ranged from 150 kips to 500 kips. Analyses of the test results showed that increasing the number of cycles and applying cycles along more than one axis decreased the maximum drift ratio. Test specimens had little drift capacity beyond the point of shear failure. For the columns analyzed the difference between the maximum drift ratio and the drift ratio at shear failure ranged from 0 to 2-1/2% with a mean value of approximately 3/4%