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

The maximum drift at which an interior connection will fail can be estimated from the gravity shear ratio V^sub g^/V^sub o^ (Pan and Moehle 1989; Luo and Durrani 1995). The gravity shear ratio represents the unfactored vertical gravity shear V^sub g^ divided by the theoretical punching shear strength without moment transfer V^sub o^ determined using

V^sub o^ = v^sub c^b^sub o^d (22)

The term v^sub c^ is calculated using Eq. (6) or (7). A similar ratio can be computed for slabs with shear reinforcement by replacing v^sub c^ with v^sub n^ defined by Eq. (8) through (10).

Figure 5 provides a plot of peak drift as a function of V^sub g^/V^sub o^ for interior slab-column connection specimens with no shear reinforcement. The figure shows the direct influence of the gravity shear ratio on the lateral drift capacity of slab-column connections. It may be observed that punching shear occurs for a large range of V^sub g^/V^sub o^ values (approximately 0.1 to 0.9), while flexural failures primarily occur for V^sub g^/V^sub o^ values of 0.3 or less.

Figure 6 provides a similar plot for interior slab-column connection specimens with shear reinforcement. The experimental data indicates that larger drift ratios are possible when shear reinforcement is used. In particular, a number of slab-column specimens with stud-shear reinforcement (SSR) attained story drift ratios well over 3% before failure.

The data from slab-column connection tests, with and without shear reinforcement, are compared in Fig. 7, along with the ACI 318-05 limits for assessing the need for shear reinforcement. The line defined by ACI 318-05 is a reasonable lower-bound limit for the data corresponding to specimens without shear reinforcement. A strength reduction factor of [straight phi] = 1 is used when determining V^sub g^/V^sub o^ for the test data.

PERFORMANCE-BASED SEISMIC DESIGN RECOMMENDATIONS

Research studies and past structural performance have shown that slab-column frames provide lateral stiffness contributions to the overall LFRS and, as such, they do resist lateral loads during a seismic event even if they were designed for gravity loads only. For this reason, compatibility of deformations must be considered to calculate the demands at the slab-column connections. Likewise, the analytical model should include the strength and stiffness of the slab-column frames to ensure an accurate representation of the overall building stiffness and allow an evaluation of the magnitude of the lateral load that must be resisted by the slab-column frame members. The appropriate parameters that should be included in such a model were highlighted previously (effective slab width for equivalent beams, cracked section properties, and hysteretic behavior for nonlinear models).

Performance-based seismic design (PBSD) criteria are suggested in the following. The criteria are based on experimental data of interior slab-column connections under combined gravity and lateral load. The suggested criteria reference FEMA 356 performance levels (immediate occupancy, life safety, and collapse prevention) and seismic design requirements for slab-column connections that are adopted in ACI 318-05. As noted previously, in regions of high seismic risk, the slab-column connections of two-way slabs without beams must be checked for the induced effects caused by the lateral displacement expected for the design-basis earthquake. It is important to note the direct influence of the gravity shear ratio on the lateral drift capacity of slab-column connections without shear reinforcement illustrated by the test data in Fig. 5. As suggested by the FEMA 356 limits for slab-column connections, this relationship is critical to the development of appropriate PBSD criteria for slab-column connections. The ACI 318-05 seismic design limits for slab-column connections given in Eq. (16) also underscore the direct relationship between these two parameters.