Lateral Drift Limits for Structural Concrete Slab-Column Connections Including Shear Reinforcement Effects

Abstract

Floor systems composed of two-way slabs without beams are frequently used because they offer several advantages, including relatively simple formwork and a shallow profile that can accommodate shorter story heights. Slab-column frame systems are common in regions of low to moderate seismic risk, where they are allowed as a lateral-force-resisting system (LFRS), as well as in regions of high seismic risk for gravity systems where moment frames or shear walls are provided as the main LFRS. However, past earthquake damage has shown that slab-column frames are not a suitable main LFRS in regions of high seismic risk because of their relative flexibility and potential for brittle punching shear failures in the slab-column connection region. Slab-column connections experience very complex behavior when subjected to lateral displacements or unbalanced gravity loads. The portion of the slab around the column must transfer a combination of shear, flexure, and torsion. Flexural and diagonal cracking are coupled with significant in-plane compressive forces induced by the restraint of the surrounding unyielding portions of the slab. A number of experiments have been conducted over the past forty years to evaluate the performance of slab-column connections under cyclic, lateral loading. In 1989, Joint ACI-ASCE Committee 352 published the report ACI 352.1R-89 to provide recommended practices for the design of slab-column connections in reinforced concrete structures [ACI-ASCE 352, 1989, Reapproved 1997]. The existing report covers design considerations, methods for analysis to determine connection strength, and reinforcement recommendations (not including shear reinforcement). This report is currently being updated by the committee to include more specific recommendations for slab-column connections with respect to lateral drift limits. The proposed recommendations reflect recent research, as well as changes to relevant ACI 318 provisions. The scope has also been expanded to discuss both reinforced concrete (RC) and post-tensioned (PT) slabs, for cases with and without shear reinforcement. This paper provides an overview of new recommendations being considered for the updated report, with specific emphasis on the lateral drift limits for slab-column connections, accompanying shear reinforcement recommendations, and the basis for these updates.