Seismic Assessment of Slab-Column Connections in Walled Buildings

Abstract

Reinforced concrete flat plate construction is a popular floor system in buildings across the United States. Due to their high flexibility and low energy dissipation, flat plate systems must be used with other lateral force resisting systems (LFRS) to withstand strong ground motion in moderate to high seismic zones. Structural walls are one of the widely used systems along with flat plates to control displacements and dissipate seismic energy. However, based on recent earthquake observations, the performance of flat plates remains in doubt. Failures have occurred in flat plate buildings with structural walls as the primary LFRS.

The leading causes of flat plate building failure were punching shear failure, excessive lateral drift, and progressive collapse. Recent earthquakes demonstrated the inability of the flat plate systems to withstand large earthquake forces. Although structural walls were used as the primary LFRS in some buildings, design flaws resulted in structural collapses.

The motivation for this research was the lack of full-scale test data on slab-column (SC) connections when walls are present, as well as the fact that the members used in previous experiments on SC connections were not detailed based on the most recent code provisions. The purpose of this study was to evaluate the performance of SC connections as well as the overall expected damage of a flat plate building with walls as the primary LFRS designed in accordance with the current ACI code (ACI 318-19) provisions. While the walls are assumed to resist the entire lateral force, the drift demands for the SC connections must still be reviewed.

A seven-story prototype building designed based on ACI 318-19 was studied during this research. CSI ETABS was used for the nonlinear analysis of the prototype building. Due to the lack of documented studies for using ETABS for nonlinear analysis of structures, modeling parameters and approaches were verified for various structural elements. The verified modeling approaches were used to construct the nonlinear model of the prototype building. Nonlinear static and dynamic analyses of the prototype building were conducted to evaluate the performance of SC connections, as well as the overall building performance, such as the walls and interstory drift demands . The prototype building was evaluated under BSE-1N and BSE-2N hazard levels in accordance with ASCE 41-17. It can be concluded from the analyses and evaluation results that the slab-column connections have acceptable performance in the flat plate buildings as long as adequate shear reinforcement is provided. Although no shear reinforcement was required for the SC connections based on the ACI 318-19 provisions, some of the SC connection hinges failed when no shear reinforcement was included indicating the need for further review of the provisions of Section 18.14.5 of the ACI 318-19 for flat plates with core walls. However, a more detailed analysis is required to incorporate the torsional effects more accurately during the preliminary design of the building.