Area of steel required for footing
Area of steel required for footing
(OP)
According to 10.5.4 (318-08) As-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 As-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 As-req using 0.0018bh, so I am hoping that was a correct assumption on both our parts.






RE: Area of steel required for footing
RE: Area of steel required for footing
RE: Area of steel required for footing
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: How to place minimum steel in a rectangular footing?)
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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RE: Area of steel required for footing
Mike McCann, PE, SE (WA)