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steel required for a column and wall footing

steel required for a column and wall footing

(OP)
Questioning the amount of steel required for a column and wall footing and difficulty interpreting ACI code 10.5.  I had a professor that designed flexural footing steel for 3 things:

a) Min steel based on ?
As - ?bd
        
b) Min steel required for Flexure (ACI 318-02 - 10.5)        
As=200/Fy*bd
        
c) Min steel based on shrinkage    (ACI 318-02 - 7.12.2)
As=.0018bh

The max of these 3 would govern the steel used in flexural direction with 7.12.2 used as minimum for longitudinal reinforcement.   After reading ACI 318-02 - 10.5 closer, 10.5.4 states that structural slabs and footings require min reinforcement in the direction of the span as that required for 7.12 (shrinkage and temperature).  After reading this, it would seem that 10.5 (As=200/Fy*bd) does not apply for column and wall footings.  In addtion, 'Notes on ACI 318-02' chap 22 examples only uses 7.12, not taking 10.5 into account either.  is using 10.5 over reinforcing the footing and not necessary?

RE: steel required for a column and wall footing

You are correct! This is how I interpret it also. ACI 10.5 does not apply for footings.

RE: steel required for a column and wall footing

It is important to understand that the code does not talk in terms of columns or beams. It provides design practices for tension-controlled and compresion-controlled members. Min reinf ratio for compression members is 1% (10.9.1).

RE: steel required for a column and wall footing

If the plan dimensions are sufficient that shrinkage stresses are not an issue, the code also recognizes the use of plain concrete.

Dik

RE: steel required for a column and wall footing

Also .0018 only applies to slabs reinforced with grade 60 rebar. For footings the code allows you to use .0014.

RE: steel required for a column and wall footing

Question, now isn't there somewhere where for slabs greater than 8" you need two mats of steel?  So for 12" strip or raft footing, you need to mats in the 12" portion?

Or is this also overestimating the ACI code?

RE: steel required for a column and wall footing

Oh, and for seismic regions, is there not more of a need for reinforcement in footings?  It is difficult to follow the code on these issues.

RE: steel required for a column and wall footing

I have seen textbooks whoe authors design retaining wall footings as "beams" since they act as beams.  

The ACI code is woefully deficient in this area and the mere fact that engineers get conflicting guidance is testimony to my assertion.  I feel it is long overdue to promulgate proper guidance in thie area.

RE: steel required for a column and wall footing

We routinely design wall footings up to 2'-6" thick before we start adding rebar in the top, unless it is needed for top tension.

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