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Concrete: 3000 psi and special inspection

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I’m not a concrete expert by any means.

 

I’m getting all fogged up trying to continue avoiding special inspection any more in a region of “high seismic risk” (Pacific Northwest).

 

I used to specify 3000 psi, and then state “strength design basis is 2500 psi”, to avoid special inspection for typical small commercial jobs with nothing but footings and stemwalls.  (Just like lots of other engineers.)

 

But is there any way around the following?

-        ACI 318 21.2.4.1: “Compressive strength f’c of the concrete shall be not less than 3000 psi”

-        ACI 318 Table R21.2.1 states that Foundations are included in the scope of section 21.2

-        2003 IBC confirms and reinforces this, by stating that …Seismic  Design Category D, E, or F, …foundations complying with Sections [ACI] 21.2 through 21.10 shall be used to resist [seismic].

 

Let’s look at the 2003 IBC 1704.4 special inspection EXCEPTIONS now:

-        “1.  Isolated spread concrete footings on buildings three stories or less in height that are fully supported on earth or rock.”  Okay, so we can (or must) specify 3000 psi for the footings, but they don’t need special inspection.

-        “2.  Continuous concrete footings supporting walls of buildings three stories or less in height that are fully supported on earth or rock where…”  Okay, so we can avoid special inspection for the strip footings – great, but I saw no mention of stemwalls.

-        “4.  Concrete foundation walls constructed in accordance with Tables 1805.5…”  That would be great, but 1805.5.5.1 (3) says that I can’t use those tables in Seismic Design Cat. D.  Hey, wait, but 1805.5.5.2 (3) says that the same walls out of masonry are okay???  Oh, I sure don’t want to digress into masonry.

 

Can we not use the Tables 1805.5 simply because the bars are typically at 48” oc, and 18” oc is the typical max spacing in a high seismic region (don’t know where that is required – is that a UBC leftover)?

 

What is a stemwall anyway?  Is it a “ordinary reinforced concrete structural wall”?  I suppose we can’t call it part of the footing, can we?

 

Here’s the big question:  Are all the stemwalls for each and every commercial project in high-seismic regions requiring special inspection?  I note the 3000 psi requirement is in the UBC, also.  What have I missed?

 

Thank you so much for your time reading this,

 

Ed Tornberg

Tornberg Consulting, LLC

503-551-4165