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Re: 3000 psi concrete and special inspection

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Ed Tornberg wrote:

Dennis,

Sorry I don't have a copy but doesn't the current CBC (aka 97 UBC) say
in 1921.2.4.1 that "compressive strength f'c shall not be less than 3000
psi"?  Note the only exception to this is for "footings".  (Note I agree
with Scott's inclusion by reference anyway.)

Doesn't this force you to specify 3000 psi concrete for a stemwall?

Do you draw a distinction between when you "specify" per the code and
"design" per the code?  I'm referring to the subtle wording difference
between the above and UBC 1701.5 or IBC 1704.4.

I ask the question again: is everybody requiring special inspection on
simple stemwalls?  If not, what is the reason?  I was trained by the
oldtimers to always specify and design with 2500 psi for the
"foundation", but I see that as only permissible for "footings".
Ed Tornberg
Tornberg Consulting, LLC
503-551-4165

Ed,
Please read my response to Scott as I think it better explains my comments. In short, whatever is referenced in the code (such as a section or chapter of ACI-318) would be considered the law. However, if the CBC omitted a section or revised or replaced it with whatever they adopted, then what remains in the ACI-318 would not be lawful in the state (and also no unlawful if more conservative). This is simply rhetoric of the interpretation of a legal document that becomes code or law. I think we are mincing words on this topic.

The section you refer to is referenced back to section 1921.2.4: Concrete members resisting earthquake-induced forces. This is the section I originally referred to that would trigger the use of special inspectors and structural observation. However, this is not all inclusive. Section 1922.2.4 indicates that for Plain Concrete the minimum strength is 2,500 psi.

The bottom line is what are you designing for. When we design a residence constructed upon a slab on grade, the minimum compressive strength is f'c=2,500 psi whereas if I need to introduce a moment frame embedded into a laterally resisting grade beam, then the provisions of Section 1921.2.4 comes into play.

A simple stem wall falls into the Structural Plain Concrete (Section 1922) in my opinion. I am not sure, but I think you are referring to a simple concrete foundation stem wall that supports a wood or masonry (or concrete) shearwall or a cripple wall with a raised first floor. In my opinion this is not defined as a concrete member resisting earthquake induced forces as would be a grade beam.

*Dennis S. Wish, PE*
*California Professional Engineer
Structural Engineering Consultant

dennis.wish(--nospam--at)verizon.net*
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760.564.0884 (office - fax)
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