Area of steel required for footing

[conradlovejoy]

According to 10.5.4 (318-08) A_s-min for footings of uniform thickness shall be the same as that required by 7.12.2.1 (0.0018bh) and the commentary section R7.12.2 states that this area of reinforcement may be distributed near the top, bottom, or allocated between the two faces. I was using a program to check hand calcs for a porte cochere footing, and it seems the program is only attributing the bottom reinforcement to the A_s-prov and insisting that my ratio is off. I was under the impression, and feel the code verified this impression, that both top and bottom steel can be taken into account for minimum area of steel, but am confused why this software doesn't seem to be in agreement. Maybe there is some setting I don't know about causing this issue or maybe the software is just programmed to be overly conservative. From looking at details from previous jobs done by others in my company, it seems that most footings have been detailed with steel at the top and the bottom, and I tend to agree that this is a good idea in order to ensure temperature and shrinkage is properly mitigated throughout the section. I don't want to ignore this software if I am the one that is wrong, though. Also, since the eccentricity in the footing is less than B/6, I hope I am correct in assuming that only minimum steel is required due to the assumption that there will be no tension developed at the base of the footing. From digging into the results of the software I am using, it also calculated A_s-req using 0.0018bh, so I am hoping that was a correct assumption on both our parts.

 

[bootlegend]

ACI 318-14 has clarified this in section 7.6.1.1. It no longer references temperature and shrinkage steel, but simply calls it minimum flexural reinforcement. Table 7.6.1.1 specifies 0.0018Ag for 60,000 psi reinforcement.

 

[Lion06]

bootlegend is correct that ACI 318-14 has clarified this, BUT I've never understood reaching the conclusion that both top and bottom can be considered for As,min unless it is truly only for T&S because the footing works unreinforced. The earlier codes are pretty clear that the minimum FLEXURAL reinforcement required at any section shall be equal to the requirements for T&S. It's only relating the T&S quantity, not the purpose. The purpose is minimum flexural steel (you wouldn't consider top bars in a beam to satisfy the minimum flexural requirements of said beam) and the quantity of that minimum flexural steel happened to be the same as required for T&S.

 

[JAE]

Section 10.5.4 states that the [bold]AMOUNT[/bold] of reinforcement for As(min) be determined from chapter 7. It doesn't say that the purpose and placement of the steel is the same as chapter 7.

This is a very confusing part of 318 (and a very stupid code writing example I might add) that has generated a lot of confusion and discussion (check out this: thread507-251751)

The thing to remember is that section 10.5 is talking about minimum flexural reinforcement - i.e. that reinforcement in the tension side of a member that the code requires to ensure a ductile behavior vs. an abrupt failure. It is not talking about temp/shrinkage behavior. Thus, section 10.5.4 states that for slabs on grade and footings - the AMOUNT of that tensile reinforcement can be calculated based on 0.0018Ag or something similar.

So once you have that AMOUNT of steel, it makes no sense to put half on the tension side and half on the compression side.

Per bootlegend - it appears that 318-14 has cleared this up.

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[msquared48]

I believe you can go less than .0018 if you provide at least 1/3rd more steel than what is calculated for strength.

Mike McCann, PE, SE (WA)