# Technical Questions

## Shear Design Provisions in ACI 318-19

Q. Do I have to use equation (c) from Table 22.5.5.1 in ACI 318-19 if my analysis shows that I don’t need to use shear reinforcement? It seems to me that if I don’t need stirrups, then Av,min=0. From that, it would follow that AvAv,min, so I could use equation (a) from Table 22.5.5.1, even in a slab with minimum flexural reinforcement.

A. While it is correct that Av=0 in members without shear reinforcement, Av,min does not equal zero. Av,min is determined from the geometry of the design section per Section 9.6.3 in ACI 318-19. Thus, if the design section does not have shear reinforcement, Av< Av,min, and equation (c) from Table 22.5.5.1 is required. This is true even when the design section is lightly loaded and Av,min is not required.

The resulting shear capacity will be less than calculated in ACI 318-14, especially if the design section has a low ρ as is likely in double-tee flanges, other precast slab elements, or cast-in-place slabs. ACI Committee 318 updated the Code provisions for shear capacity to reflect research findings on the effects of flexural reinforcement and section depth, using test data compiled and analyzed over the past two decades. Figure 1, for example, shows the impact of longitudinal reinforcing ratio on shear strength as calculated using ACI 318-14 and ACI 318-19. For further discussion of the development of the one-way shear design provisions in ACI 318-19, refer to Kuchma et al. 2019.

Fig. 1: Ratio of test shear strength Vtest and nominal shear strength Vn versus longitudinal reinforcement ratio ρw, for members without Av or axial load Nu (after Fig. 5 in Kuchma et al. 2019). Note that for many of the test specimens with ρw< 0.01, Vtest/ Vn< 1.0 when Vn was calculated per either the simplified or detailed equations provided in ACI 318-14

References: ACI 318-19; ACI 318-14

Topics in Concrete: 318 Building Code