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Re: Concrete Code Interpretation

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I am not sure your interpretation of the code is quite correct.  The 200bd/fy requirement can be easily derived and is the minimum amount of rebar that can resist the equivalent moment capacity of a plain (non-reinforced) section and applies to all moment resisting elements.  Anything less and you can have a sudden failure as your concrete is loaded and begins to crack.

The following sections are tweaks and exceptions to the above rule for specific applications.  Section 10.5.3 give you a slight break if you have a large cross section due to other than structural requirements (extra mass for buoyancy, etc).  Section 10.5.4 gives a break if you have a slab on grade that does not transmit vertical loads from other parts of the structure to the soil.  The feeling being that since the slab is on grade if it fails it is not going anywhere like the second floor of a parking structure might.  ACI has always had a rather weak interpretation of "structural slabs" and "footings".  To them a footing is a blob of concrete under a column with minimum or no moment and a structural slab on grade is something only people walk on.  I am being a little facetious but you need to look close at the types of loads applied to the slab and the level of shear and moment developed from those loads.

Thomas Hunt, S.E.
ABS Consulting

"Lutz, James" <JLUTZ(--nospam--at)>

09/25/2002 05:23 PM
Please respond to seaint

        To:        seaint(--nospam--at)
        Subject:        Concrete Code Interpretation

In ACI-318, section 10.5, if flexural steel ratio provided is less than
200/fy, you can only use 75% of the computed moment capacity. However, in
paragraph 10.5.4, a lower minimum steel ratio is provided for slabs and
footings per section 7.12. My question is this, can you ignore the 200/fy
requirement and still use the full computed moment capacity for a slab on
grade as long as you meet the minimums in 7.12? I am reviewing an existing,
lightly-reinforced thick concrete pad for possible mounting of some heavy
equipment. It computes if I can avoid taking the discount for the low steel

Jim Lutz, P.E., S.E.
Earth Tech, Inc.

10800 NE 8th Street - 7th Floor
Bellevue, WA 98004
(425) 455-9494
(425) 453-9470 FAX

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