4. Two-Way Slab Shear Strength

Shear strength of the slab in the vicinity of columns/supports includes an evaluation of one-way shear (beam action) and two-way shear (punching) in accordance with ACI 318 Chapter 22.

4.1. One-Way (Beam Action) Shear Strength

One-way shear is critical at a distance d from the face of the column. Figure 16 shows the V_u at the critical sections around each column. Since there is no shear reinforcement, the design shear capacity of the section equals to the design shear capacity of the concrete:

Figure 16 – One-way shear at critical sections (at distance d from the face of the supporting column)

4.2. Two-Way (Punching) Shear Strength

Two-way shear is critical on a rectangular section located at d_slab/2 away from the face of the column. The factored shear force V_u in the critical section is calculated as the reaction at the centroid of the critical section minus the self-weight and any superimposed surface dead and live load acting within the critical section.

The factored unbalanced moment used for shear transfer, M_unb, is calculated as the sum of the joint moments to the left and right. Moment of the vertical reaction with respect to the centroid of the critical section is also taken into account.

For the Exterior column:

Figure 17 – Critical Section of Exterior support of Interior Frame

For the exterior column in Figure above the location of the centroidal axis z-z is:

The polar moment J_c of the shear perimeter is:

For the Interior Column:

Figure 18 – Critical Section of Interior support of Interior Frame

For the interior column in Figure above the location of the centroidal axis z-z is:

The polar moment J_c of the shear perimeter is: