SLAB-COLUMN FRAME’S CONNECTIONS

Two-way slabs structural system without beams is common in regions of low to moderateseismic 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. 

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.

Slab-column connections in structures subjected to earth quake or wind loading must transfer forces due to both gravity and lateral loads. This combination can create large shear and unbalanced moment demands at the connection. Without proper detailing, the connection can be susceptible to two-way (punching) shear failure during response to lateral loads. The flexibility of a slab-column frame can lead to large lateral deformations, which may increase the potential for punching failures; therefore, in regions of high seismic risk, slab-column frames are used in conjunction with beam- column moment frames or shear walls. Compatibility of lateral deformations between the slab-column frame and the LFRS, however, must be considered to determine the demands on the connections.

The seismic performance of reinforced concrete structures with flat-slab construction hasdemonstrated the vulnerabilities of the system. For example, following the 1985 Mexico City earthquake, punching shear failures were noted in a 15-story building with waffle flat-plate construction (Rodriguez and Diaz 1989). This failure was partly attributed to a high flexibility combined with low-ductility capacities of the waffle slab-to-column connection. In a department store during the 1994 Northridge earthquake, discontinuous flexural reinforcement at slab-column connections led to punching failures at column drop panels (Holmes and Somers 1996). Punching failures around shear capitals were also noted in the post-tensioned floor slabs of a four-story building during the same event (Hueste and Wight 1997).

CURRENT DESIGN APPROACH

General

The shear strength of slabs in the vicinity of columns is governed by the more severe of two conditions, either beam action or two-way action. In beam action, the slab acts as a wide beam with the critical section for shear extending across the entire width of the slab. This critical section is and assumed to be located at a distanced (effective slab depth) from the face of the column or shear capital. For this condition, conventional beam theory applies and will not be discussed in detail herein. For the condition of two-way action, the critical section is assumed to be located at a distance d/2 from the perimeter of the column or shear capital, with potential diagonal tension cracks occurring along a truncated cone or pyramid passing through the critical section (refer to Fig. 1, where d1 is the effective slab depth within the thickened shear capitalregion and d2 is the effective slab depth). 

Existing methods for calculating the shear strength of slab-column connections include applications of elastic plate theory, beam analogies, truss analogies, strip design methods, and others. The design method specified by ACI 318-05(ACI Committee 318 2005) provides acceptable estimates of shear strength with reasonable computational effort. The procedure is based on the results of a significant number of experimental tests involving slab-column specimens.

The eccentric shear stress model is the basis of the general design procedure embodied in ACI 318 for determining the shear strength of slab-column connections transferring shear and moment. The model was adopted by the 1971 version of the ACI 318 and only minor modifications have been included in subsequent versions. Recently, ACI 318-05 has incorporated special provisions related to the lateral-load capacity of slab-column connections in structures located in regions of high seismic risk or structures assigned to high seismic performance or design categories. 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).