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

For both prestressed and nonprestressed slabs, ACI 318 mandates continuity reinforcement to give the slab some residual capacity following a single punching shear failure at a single support. Thus, in nonprestressed slabs, all bottom bars within the column strip shall be continuous and at least two of the column strip bottom bars in each direction shall pass through the column core (ACI Committee 318 2005, Section 13.3.8.5). In prestressed slabs, a minimum of two tendons shall be provided in each direction through the critical shear section over columns (ACI Committee 318 2005, Section 18.12.4).

ACI 421.1R-99 refinements

ACI 318 sets out the principles of design for slab shear reinforcement but does not make specific reference to mechanically anchored shear reinforcement, also referred to as shear studs. ACI 421.1R-99 (Joint ACI-ASCE Committee 421 1999) gives recommendations for the design of shear reinforcement using shear studs in slabs. This report also includes equations for calculating shear stresses on nonrectangular critical sections.

Shear studs have proven to be effective in increasing the strength and ductility of slab-column connections. ACI 421.1R-99 suggests treating a shear stud as the equivalent of a vertical branch of a stirrup and to use higher limits on some of the design parameters used in ACI 318. In particular, ACI 421.1R-99 suggests higher allowable values for v^sub n^, v^sub c^, s, and f^sub yv^, as follows

... (11)

... (12)

... (13)

f^sub yv^ ≤ 72,000 psi (500 MPa) (14)

The justification for these higher values is mainly due to the almost slip-free anchorage of the studs and that the mechanical anchorage at the top and bottom of the stud is capable of developing forces in excess of the specified yield strength at all sections of the stud stem.

ACI 352.1R-89 recommendations

ACI 352.1R-89 (Joint ACI-ASCE Committee 352 1989) includes recommendations for the determination of connection proportions and details to ensure adequate performance of monolithic, reinforced concrete slab-column connections. The recommendations address connection strength, ductility, and structural integrity for resisting gravity and lateral forces.

ACI 352.1R-89 only applies to nonprestressed slab-column connections with f'^sub c^ less than 6000 psi (42 MPa), with or without drop panels or shear capitals, and without slab shear reinforcement. The provisions are limited to connections where severe inelastic load reversals are not expected, and do not apply to slab-column connections that are part of a primary LFRS in regions of high seismic risk because slab-column frames are generally considered to be inadequate for multi-story buildings in these areas.

ACI 352.1R-89 classifies slab-column connections as one of two types: 1) Type 1-connections not expected to undergo deformations into the inelastic range; and 2) Type 2- connections requiring sustained strength under moderate deformations into the inelastic range. In structures subjected to high winds or seismic loads, a slab-column connection should be classified as Type 2 even though it is not designated as part of the primary LFRS.