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

If the DR exceeds the limit given by Eq. (16), shear reinforcement must be provided (or the connection can be redesigned). When adding shear reinforcement, ACI 318-05 prescribes that the term v^sub s^ , defined by Eq. (10), must exceed ... and the shear reinforcement must extend at least four times the slab thickness from the face of the support. Given that this approach is relatively simple, and that the added cost of providing shear reinforcement at connections is not significant for structures designed for high seismic performance categories, use of this prescriptive approach is likely to be common. The representative design steps are shown in Fig. 3.

If shear capitals, column capitals, or drop panels are used, all potential critical sections must be investigated. ACI 318-05 does not prescribe a minimum extension of shear capitals. Wey and Durrani (1992), however, recommend a minimum length equal to two times the slab thickness from the face of the column.

ANALYTICAL MODELING

The shear stresses due to the combined factored shear and moment transferred between the slab and the column under the design displacement can be determined by creating an appropriate analytical model of the slab-column frame and directly assessing the potential for punching. Recommendations by Hwang and Moehle (2000) may be used to establish the effective stiffness of the slab and to include the impact of cracking. Hwang and Moehle (2000) recommend that the uncracked effective stiffness for a model with rigid joints, for ratios of c^sub 2^/c^sub 1^ from 1/2 to 2 and a slab aspect ratio l^sub 2^/l ^sub 1^ greater than 2/3, be determined using an effective beam width represented as

... (18)

where b^sub int^ is the effective width for interior frame connections (interior connections and edge connections with bending perpendicular to the edge); c^sub 1^ and l^sub 1^ are the column dimension and slab span parallel to the direction of load being considered; and c^sub 2^ and l^sub 2^ correspond to the orthogonal direction. For exterior frame connections (corner connections and edge connections with bending parallel to the edge), half the width defined in Eq. (18) is used. Effects of cross section changes, such as slab openings, are to be considered. One way to accomplish this is to vary the width of the effective beam along the span (Hwang and Moehle 1990).

To account for cracking, a stiffness reduction factor ß has been proposed by Hwang and Moehle (2000) for nonpre-stressed slabs and is given by

... (19)

where c and l are the column dimension and slab span parallel to the load direction. Kang and Wallace (2005) recommend ß = 0.5 for post-tensioned floor systems with approximate values for span-to-slab thickness ratios of 40, c^sub 1^/l^sub 1^ of 1/14, and precompression of 200 psi (1.4 MPa).

The analytical model of the slab-column frame should capture the potential for both slab yielding and connection failure due to punching as recommended in FEMA 356. Figure 4 shows an approach where yielding within the slab column strip is modeled using slab-beam elements (in this case, an elastic slab-beam with stiffness properties defined by the effective beam width model, and zero-length plastic hinges on either side of the connection). Further details of this model are described by Kang et al. (2006). Punching failures can occur if the capacity of the connection element is reached or if a limiting story drift ratio is reached for a given gravity shear ratio. Hueste and Wight (1999) suggested an approach for incorporating this behavior into a nonlinear analysis program, where, after prediction of a punching shear failure, the member behavior is modified to account for the significant reduction in stiffness and strength. Kang and Wallace (2005) suggest a direct approach by employing a limit state model.