Seismic Performance Assessment of Inadequately Detailed Reinforced Concrete Columns

Existing New Zealand building stock contains a significant number of structures designed prior to 1995 with inadequate detailing of the internal or 'gravity' reinforced concrete (RC) columns. Typically these columns have insufficient transverse reinforcement; lap-splices in the plastic hinge region; and longitudinal bars that are 'cranked' at the end of the lap-splice. Columns with such details have been shown to perform poorly when subjected to seismic demand, losing axial load carrying capacity at drift levels less than the building is expected to be subjected to during a design level earthquake. This paper outlines an investigative program to determine the susceptibility of these gravity columns to axial collapse. A drift based backbone capacity model for shear and subsequent axial failure is presented which has been verified by experimental testing performed to date. Such experimental tests have highlighted the susceptibility of these inadequately detailed columns to lose axial load capacity at drift levels significantly below the seismic demand on such structures due to a design level earthquake.