Did You Know That… You Can Go Lower Than The Minimum Required Flexural Reinforcement?

This applies if the reinforcement provided is not driven by crackwidth limits and thermal requirements.

The minimum steel ratios of 1.4/fy and √fc’/(4*fy) of 10.5.1 of ACI318M-11 doesn’t seem much if you’re designing thin slabs ranging from 100mm to 300mm. But if you’re talking about a meter or more of thickness and you’re trying to get the reo rate down, the minimum reinforcement provided might solicit some unwanted comments such as “…dang man, that’s one helluva bars!” This is exactly the same reason why the ACI committee provided an exception which you can read why in the commentary on clause R10.5.3. More of that exception on my succeeding discussions below.

Say you have a meter and a half thick bending member which is not exposed to earth nor to any deleterious compounds, and by chance have a very low flexural utilization ratio which is way below the minimum As as stipulated in clause 10.5.1. The minimum steel ratio will give us approximately T25-100 of steel which is equivalent to 2705 kN-m per lineal meter of bending capacity.

calcs by yours truly

But what if the demand moment is half or even less of the minimum capacity and you’re not constrained by thermal and crackwidth requirements? It’s a good thing that the code, on clause 10.5.3 allows us to go lower than the said minimum if the provided reo is greater than a third of what is required. So for the demand bending moment equal to half of the minimum, the required reo is 2369 sqmm per meter. Multiplying it by 4/3 the reinforcement is now 3159 sqmm per meter which is equivalent to T25-150. In doing so, we can now supersede the requirements of clause 10.5.1.

continuation calcs by yours truly

Now T25-100 can go down to T25-150. If that aint a significant reduction then I don’t know what is.