How does CRSI avoid shrinkage and temperature reinforcement requirement in square footings?

[Recent Grad Josh]

Hello,

I was looking at the "Concentrically Loaded Square Spread Footings" Table in the CRSI Design Handbook (2008, pg 13-6), and the required steel which they tabulate falls short of ACI's 0.0018bh minimum. Their values come out to be around 0.00125bh and up. Is there an exception in ACI that I'm missing? Thanks in advance!

 

[BridgeSmith]

Reinforcement for shrinkage and temperature stresses shall be provided near surfaces of concrete exposed to daily temperature changes and in structural mass concrete.

If the ACI is similar, a footing may not be required to have S&T reinforcement if it isn't thick enough to be considered "mass concrete" and isn't exposed.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[JAE]

ACI does not require T&S reinforcement in footings. In Section 10.5.4 they require minimum flexural reinforcement on the tension side of the footing in the amount of 0.0018bd.

They do reference you back to the T&S requirement, but it is not T&S steel.

I'm not sure why CRSI is using something less but it could possibly be that they are interpreting the whole of section 10.5 (ACI's minimum flexural reinforcement section in 318-11) and applying the As(min) = 0.0018bd OR 1/3 greater than that required by analysis, which could be less than 0.0018.

 

[JoshPlumSE]

A couple of possibilities:
Difference between 0.0018bd and 0.0018bh.
In the past (don't ask me exactly when), it was common to interpret ACI as requiring flexural minimum as 0.00333. But, where this could be waived if you provide As = 4/3 * required by analysis. So long as the total As (top and bottom steel) was greater than 0.0018 bh.

Going back to first principals, the idea was to avoid brittle failure with the 0.00333 requirement. Which was waived if you used 4/3 more steel than you really needed for design. The extra steel providing a margin of safety against failure (even if it was brittle). That being said, the total reinforcement for the footing still obeying the standard T&S limit of 0.0018.

Now, I'm not getting into an argument for (or against) this older interpretation. However, I am pointing out that the document you were looking at is 11 years old at least and may be based on documents even older. Therefore, it may be using the older interpretation which is difficult to make considering the code language of today.

 

[JLNJ]

Maybe the thought is that an isolated footing can shrink inward in all directions towards the pier and be none the worse for wear.

It also might have to do with 100 years of lightly or non-reinforced footings performing satisfactorily. (And out of sight out of mind.)

 

[RPMG]

CRSI Design Guide for Square Spread Footings for Individual Columns 2014 now follows the 0.0018bh rule. I believe that the 4/3-rule now only applies to beams.