Spread Footing Min Reinf 2

[iv63]

I am evaluating existing spread footing built in 1973 for new horizontal vessel (boiler). Everything works but min reinforcement required for shrinkage and temperature. Per ACI318 Section 7.12. for steel Grade 40 (from 1973) it is As,min = 0.0020bh = 0.0020*20*12 = 0.48 in2. Reinforcement provided is 0.31in2 (#5 @ 12” OC). Do you know what was code requirement in 1973? Is there any “trick” to get around this requirement? If not what is the quickest fix (the client has only 20 days to replace this boiler)?

 

[KootK]

In this situation, I'd wave the T&S requirement. Certainly, she's done shrinking.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

Actually, for #5 and smaller bars, the minimum was .0018bt.

Moreover, if you provided 1/3 more steel than was actually required, you could use less than the minimum steel mentioned above.

Mike McCann, PE, SE (WA)

 

[DaveAtkins]

KootK,

I did not know footings are female

DaveAtkins

 

[Ingenuity]

Certainly, she's done shrinking.

I did not know footings are female

...and usually where 'shrinkage' is involved it is a 'male thing'...Happy New Year!

 

[JAE]

ACI 318-77 (my old first copy) states in 10.5.3:

[blue]"For structural slabs of uniform thickness, minimum area and maximum spacing of reinforcement in the direction of span shall be as required for shrinkage and temperature according to Section 7.12"[/blue]

Section 7.12 states the following:
[blue]for Grade 40 or 50 rebar - use 0.002
For Grade 60 rebar - use 0.0018
Max. spacing = 5 times slab thickness or 18 inches.[/blue]

In Chapter 15 there is no mention of minimum reinforcement for footings, only comments about distribution of rebar in rectangular footings.
So in 1977 there was no mention of footings, only slabs.

Section 10.5 in the 1983, 1989, 1992 codes all are similar.

The word "footings" shows up in 10.5.4 in the 1995 edition.



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[hokie66]

We have had this discussion several times before, and the answer really is that for normal size spread footings, temperature and shrinkage reinforcement is not required. Flexural reinforcement is required, and for some obscure reason, the code writers linked the required amount of flexural reinforcement to the T&S requirement for slabs. Confusion about this has carried through over the years. As Mike said above, you just need to be sure there is enough for flexure, or 1/3 more than required if the calculated amount is small.

 

[msquared48]

Yes. ACI 318-71, section 10.5.1 pertains to all flexural sections needing positive reinforcing, which spread footings are. 10.5.2 pertains to slabs. There was no 10.5.3 until 1978.


Mike McCann, PE, SE (WA)

 

[msquared48]

Also, in section 7.13 of ACI 318-71, minimum temperature reinforcing is mentioned as .0014, with that limit applying to steel reinforcing greater than 60,000 ksi.

Mike McCann, PE, SE (WA)

 

[iv63]

Thank you all for the valuable contributions. As KootK suggested I will waive T&S requirement since "she" has done shrinking.
iv

 

[racookpe1978]

"Does this bar make my shrinkage look good?"

Answer that question the wrong way and you may get a footing inserted past your shrinkage ...