In the ACI 350-20 Code, what is the purpose of the additional rebar spacing checks found in Section 11.9.9.5

[bblackmon]

I'm designing a process basin for a water treatment plant in Idaho, which is being constructed with concrete walls with vertical and horizontal reinforcement (rebar). I've been designing according to the ACI-350-20 standards. When selecting my rebar size and spacing, I had to check my maximum spacing requirements according to section 11.9.9.5. When examining the code, I found that I was always selecting the set minimum of 12 inches, even on relatively short walls (7-8 feet long). It states that the minimum spacing must be the smallest of the following, the length of the wall divided by 3, 3 times the thickness of the wall or member, or 12 inches. In order to have a spacing that is less than or equal to 12, then the thickness of the wall would either need to be 4 inches thick or the length of the wall would need to be 3 feet long. So now I'm wondering what's the point of these checks besides the very unlikely cases where there is a 3-foot-long wall or a 4-inch-thick concrete wall? Just thought I'd ask Eng-Tips for a solution, since we have been scratching our heads over this all day.

 

[TRAK.Structural]

I haven't used this version of ACI 350 before, but the first part of section 11.9.9.5 reads as this is only for shear reinforcement? Only in unusual circumstances have I used reinforcing for shear in concrete water bearing structures; the concrete thickness itself can be sized to handle the shear loads.

I think the idea is that 12" will basically always control, and this is in line with the more stringent reinforcing requirements in 350 to minimize cracking/leaking in these type structures.