Guide to Shear Reinforcement for Slabs (ACI 421.1R-08)

Description

In flat-plate floors, slab-column connections are subjected to high shear stresses produced by the transfer of the internal forces between the columns and the slabs. Section 11.11.3 of ACI 318-08 allows the use of shear reinforcement for slabs and footings in the form of bars, as in the vertical legs of stirrups. ACI 318 emphasizes the importance of anchor age details and accurate placement of the shear reinforcement, especially in thin slabs. Section 11.11.5 of ACI 318-08permits headed shear stud reinforcement conforming to ASTM A1044/A1044M.

A general procedure for evaluation of the punching shear strength of slab-column connections is given in Section 11.11 of ACI 318-08.Shear reinforcement consisting of vertical rods (studs) or the equivalent, mechanically anchored at each end, can be used. In this report, all types of mechanically anchored shear reinforcement are referred to as “shear stud” or “stud.” To be fully effective, the anchorage should be capable of developing the specified yield strength of the studs. The mechanical anchorage can be obtained by heads or strips connected tot he studs by welding. The heads can also be formed by forging the stud ends.

Extensive tests (Dilger and Ghali 1981; Andrä 1981; Vander Voet et al. 1982; Mokhtar et al. 1985; Elgabry and Ghali1987; Mortin and Ghali 1991; Dilger and Shatila 1989; Cao1993; Brown and Dilger 1994; Megally 1998; Birkle 2004;Ritchie and Ghali 2005; Gayed and Ghali 2006) haveconfirmed the effectiveness of mechanically anchored shearreinforcement, such as shown in Fig. 1.1, in increasing thestrength and ductility of slab-column connections subjectedto concentric punching or punching combined with moment.Stud assemblies consisting of either a single-head studattached to a steel base rail by welding (Fig. 1.1(a)) ordouble-headed studs mechanically crimped into a nonstructuralsteel channel (Fig. 1.1(b)) are specified in ASTM A1044/A1044M. Figure 1.2 is a top view of a slab that shows atypical arrangement of shear reinforcement (stirrup legs orstuds) in the vicinity of an interior column.