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 design approach presented in this section of the paper is based on the design procedures given in ACI 318-05 complemented by ACI 421.1R-99 (Joint ACI-ASCE Committee 421 1999) and 352.1R-89 (Joint ACI-ASCE Committee 352 1989).

ACI 318 eccentric shear stress model

Slab-column connections experience very complex behavior when subjected to lateral displacements or unbalanced gravity loads. This involves transfer of flexure, shear, and torsion in the portion of the slab around the column. Combined flexural and diagonal cracking are coupled with significant in-plane compressive forces in the slab induced by the restraint of the surrounding unyielding slab portions.

Relatively simple design equations have been derived by considering the critical section to be located at d/2 away from the face of the column and by assuming that shear stress on the critical perimeter varies linearly with distance from the centroidal axis. This eccentric shear stress model is based on the work by DiStasio and Van Buren (1960) and reviewed by Joint ACI-ASCE Committee 326 (1962).

For a slab-column connection transferring shear and moment, the ACI 318-05 design equations for limiting the shear stresses v^sub u^ are given by

v^sub u^ ≤ [straight phi]v^sub n^ (1)

... (2)

where v^sub u^ is the factored shear stress; [straight phi] is the strength reduction factor for shear; v^sub n^ is the nominal shear stress; V^sub u^ is the factored shear force acting at the centroid of the critical section; M^sub u^ is the factored unbalanced bending moment acting about the centroid of the critical section; d is the distance from the extreme compression fiber to the centroid of the longitudinal tension reinforcement; b^sub o^ is the length of the perimeter of the critical section; c is the distance from the centroidal axis of the critical section to the point where shear stress is being computed; J is a property of the critical section analogous to the polar moment of inertia; and γ^sub v^ is the fraction of the unbalanced moment considered to be transferred by eccentricity of shear, defined by

... (3)

where b^sub 1^ and b^sub 2^ are the widths of the critical section measured in the direction of the span for which M^sub u^ is determined (Direction 1) and in the perpendicular direction (Direction 2).

For an interior column and a critical section of rectangular shape, b^sub o^ and J are determined by

b^sub o^ = 2(b^sub 1^ b^sub 2^ ) (4)

... (5)

The first term of Eq. (2), the shear stresses due to direct shear, is assumed uniformly distributed on the critical section, and the fraction γ^sub v^M^sub u^ is assumed to be resisted by linear variation of shear stresses on the critical section. The portion of the moment not carried by eccentric shear is to be carried by slab flexural reinforcement placed within lines 1.5h on either side of the column (h is the slab thickness, including drop panel, if any). This flexural reinforcement is also used to resist slab design moments within the column strip.

The provisions of the ACI 318 specify that in absence of shear reinforcement, the nominal shear strength (in stress units) carried by the concrete v^sub c^ in nonprestressed slabs is given by