Evaluation of a reinforced concrete building damaged during the Northridge earthquake and development of a punching shear failure model for nonlinear analysis.

Abstract

A four-story reinforced concrete (RC) frame office building damaged during the Northridge Earthquake was evaluated. The main focus of this study was to test and improve the analytical modeling procedures for the structural system represented by this building. The lateral force resisting system consists of a continuous perimeter ductile moment-resisting frame. The gravity load system for the interior portion of the building is composed of slab-column frames with shear capitals at the connections. The perimeter frames had no serious damage during the earthquake and appeared to have responded in a ductile manner, as designed. However, punching shear failures occurred at several of the interior slab-column connections. It is believed that lateral displacements in the slab-column frame system, due to seismic loading, caused an increase in the transfer moments at the slab-column connections. These moments increased the shear stresses around the critical shear perimeter to a point where punching shear failures were initiated. The building was evaluated using: (1) a code level strength analysis, (2) a static nonlinear push-over analysis, and (3) a dynamic nonlinear response analysis using a ground motion recorded within 1 km (0.6 mi.) of the building. The code level analysis confirmed that the perimeter frames had good ductile detailing and adequate strength to essentially satisfy current code requirements, as well as those in place when the structure was designed in the mid 1970's. Based on design material properties and field observations, both the static and dynamic nonlinear analyses successfully post-calculated the observed punching shear failures at approximately 1.25% average building drift. Prior to this study, the ability to model the unloading that occurs at slab-column connections following a punching shear failure was not possible using the nonlinear analysis program, DRAIN-2DM. Therefore, a model for predicting punching shear failures was developed based on experimental results obtained at various universities. This model has been incorporated into the DRAIN-2DM program, along with the desired unloading behavior when a punch occurs. The punching shear failures that occurred in the RC building were successfully post-calculated using the punching shear failure model.