Seismic Design Criteria for Slab-Column Connections

ACI Structural Journal, Jul/Aug 2007 by Hueste, Mary Beth D, Browning, JoAnn, Lepage, Andres, Wallace, John W

Two-way slabs without beams are popular floor systems because of their relatively simple formwork and the potential for shorter story heights. Earthquakes, however, have demonstrated that slab-column frames are vulnerable to brittle punching shear failures in the slab-column connection region and dropping of the slab, which are costly to repair. This paper focuses on the behavior and design of slab-column connections under combined gravity and lateral loading and reviews current design procedures, performance-based design approaches, and relevant experimental data. An equation relating the gravity shear ratio at a slab-column connection to drift capacity is presented. Finally, practical recommendations are provided for defining specific performance objectives.

Keywords: deformation capacity; effective slab width; performance-based design; punching shear.

(ProQuest-CSA LLC: ... denotes formulae omitted.)

INTRODUCTION

Two-way slabs without beams are popular floor systems because of their relatively simple formwork and the potential for shorter story heights due to their shallow profile. This structural system is common in regions of low to moderate seismic risk, where it is 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. Earthquakes, however, have demonstrated that slab-column frames are not suitable as a main LFRS in regions of high seismic risk because they are relatively flexible and because of the potential for brittle punching shear failures in the slab-column connection region.

In the last 40 years, a significant number of experiments have been conducted to evaluate the performance of slab-column connections under cyclic lateral loading. This information has formed the basis of current code provisions and guidelines for the design of slab-column connections under combined gravity and lateral loading. As performance-based seismic design (PBSD) becomes more common in structural engineering practice, it is important to evaluate the recommended limits for various structural systems with respect to the latest experimental data and post-earthquake observations. This paper focuses on the behavior and design of interior slab-column connections under combined gravity and lateral loading and serves to review current design procedures, PBSD approaches, and relevant experimental data. Equation (23), for drift capacity of these systems in terms of the gravity shear ratio, is derived using the collected experimental data. Finally, practical recommendations are provided for the PBSD of slab-column connections.

RESEARCH SIGNIFICANCE

The objectives of this paper, developed by a task group within ACI Committee 374, Performance-Based Seismic Design of Concrete Buildings, are: 1) to review the current state of practice and PBSD approaches for slab-column connections; 2) to summarize experimental data for slab-column connections tested under combined gravity and lateral loads; and 3) to present a practical approach for PBSD of slab-column connections. The PBSD material is presented in a format consistent with the limit states suggested in FEMA 356 (ASCE 2000) and is intended to provide guidance primarily for new construction. The criteria, however, could also be applied to existing structures that contain subpar seismic details where a moderate seismic demand is expected. As a significant benefit for design approaches outside the PBSD framework, a practical equation that relates drift capacity to gravity shear ratio is presented (Eq. (23)).

SLAB-COLUMN FRAMES AND CONNECTIONS

Slab-column frame construction can deliver several desirable architectural features, including larger open space, lower building heights for a given number of stories, and efficient construction. The FEMA 356, "Prestandard and Commentary for the Seismic Rehabilitation of Buildings" (ASCE 2000) classifies slab-column moment frames as frames that meet the following conditions:

1. Framing components shall be slabs (with or without beams in the transverse direction), columns, and their connections;

2. Frames shall be of monolithic construction that provides for moment transfer between slabs and columns; and

3. Primary reinforcement in slabs contributing to lateral load resistance shall include nonprestressed reinforcement, prestressed reinforcement, or both.

This classification includes both frames that are or are not intended to be part of the LFRS for new, existing, and rehabilitated structures.

The connections between the slab and a column can be accomplished in several ways including direct connection (whether from solid or waffle slab construction), with column drop panels, and with column or shear capitals. Shear capitals are provided to increase the shear capacity at the slab-column connection and are defined by Joint ACI-ASCE Committee 352 (1989) as a thickened portion of the slab around a column that does not meet the ACI 318 plan dimension requirements for drop panels. A column capital is defined as a flared portion of the column below the slab that is cast monolithically with the slab.