ABSTRACT
It has been well established by experimental evidence that many existing reinforced concrete columns are vulnerable to shear failure after flexural yielding (Kokusho 1964, Ikeda 1968). Several models have been developed to represent the degradation of shear strength with increasing inelastic deformations (Priestley et al.1994, Sezen 2002). While these shear strength models are useful for estimating the column capacity for conventional strengthbased designed assessment, the recent move toward displacement-based design and assessment methods (ATC 1996, ASCE 2000) requires a model for the drift beyond which shear failure is expected. Furthermore, after flexural yielding, the force demand on a column will be approximately constant, while the displacement demand will increase substantially, suggesting that a drift capacity model is more useful for columns experiencing flexural-shear and flexural failures. Such as these considered in this study, although the shear strength models relate the degradation of shear strength to displacement ductility, these models may not be appropriate for assessing the drift at shear failure. Most models for estimating the drift capacity of reinforced concrete columns are based on performance of columns with good seismic detailing. Such models assume the response is dominated by flexural deformations and use estimates of the ultimate concrete and steel strains to determine the ultimate curvatures that the section can withstand. These models are not applicable to older reinforced
concrete columns with limited transverse reinforcements since the degradation of the shear strength begins before the flexural deformation capacity can be achieved. Furthermore, the calculation of ultimate strains assumes good crack control, provided by reasonably distributed reinforcement, such that deformation can be averaged over finite distances. Experimental studies and earthquake reconnaissance have shown, however, that the shear failure of older reinforced concrete columns often is associated with concentrated deformations along a limited number of primary cracks (Pantazopoulou, 2003). Hence, such models based on flexural performance can not be used.
