ACI 318-19 Sect. 22.5.5 Concrete Shear Capacity

[AHEng]

Hello, I have a question related to ACI 318-19 Section 22.5.5 and in particular Table 22.5.5.1.

I'm looking at slab design in an FEA program and running into a situation when designing acc. to the ACI 318-19 where no shear reinforcement is defined (i.e. Av is less than Av_min). There are FE mesh point locations where we may have zero bending moment but the concrete slab is in compression.

Following Table 22.5.5.1 Eqn. (c), if we have no bending moment then rho_w in the equation = 0.00 due to the fact that there is no longitudinal reinforcement in tension. This leads to a very, very small shear capacity in the concrete and we are failing at these locations. This is a bit questionable as the shear capacity in the concrete should be higher for this type of scenario simply using common sense.

The Eurocode address this similar situation with a minimum value for the concrete shear capacity, Vc, whereas the ACI simply does not. Is anyone aware of additional provisions in the code that address when the slab is in compression but has zero bending moment for a higher shear capacity of the concrete alone?

 

[ClearCalcs]

I suppose you could perhaps use the plain concrete provisions as a lower bound - in which case Table 14.5.5.1 would be your friend to find a shear capacity.

But thinking further - you've got to have some minimum steel in your slab per the relevant provisions. It sounds like you're concerned because max shear occurs at a point with 0 moment, so it's not clear whether you can consider your steel into rho_w, which by definition considers As as the area of steel in tension, and there's theoretically no tension there. But considering that moving 0.001" either way from your line of zero moment would put some steel in tension, I think you can justify including As in there (maybe pick the more conservative of top or bottom steel). Otherwise, a simply supported beam would technically have rho_w = 0 at its ends where shear is maximum - doubtful that this is the intent of the code.

I hope that makes sense!

-Laurent

http://www.ClearCalcs.com

 

[AHEng]

Thank you for the feedback Laurent! Unfortunately, the provisions of ACI 318-19 14.1.3 don't appear to apply to elements such as elevated slabs. It's difficult to justify then Table 14.5.5.1 to fall back on. As you also mention, I'm directly interpreting As as the longitudinal steel in tension. Without any tensile forces, As = 0 which leads to rho_w = 0 which further leads to Vc = a very small value. It would be easy enough to make some general assumptions on a case-by-case basis. However, I was hopeful for a specific code reference or workflow to apply to all such cases.

The little feedback I received from the ACI technical staff was, "For prestressed members, see 22.5.6. Nonprestressed slabs are required to have minimum flexural reinforcement (7.6.1.1); this will make rho_w nonzero." This makes me think that rho_w is always calculated from As_provided regardless of tensile forces present. Perhaps we use your suggestion to then take the more conservative top vs. bottom steel.