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RE: 3000 psi concrete and special inspection

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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

-----Original Message-----
From: Dennis S. Wish, PE [mailto:dennis.wish(--nospam--at)verizon.net] 
Sent: Wednesday, February 16, 2005 10:53 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: 3000 psi concrete and special inspection

Ed Tornberg wrote:

> For engineers in "high seismic risk" regions, including the West
Coast:
>
> Do you specify minimum 3000 psi concrete on all your foundations, and 
> thus trigger special inspection?
>
> You are required to do this per 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].
>
> So I understand that all foundations, including stemwalls, PEMB 
> pedestals, etc. require special inspection. The only exception would 
> be isolated and strip footings.
>
> If you state that "design basis is 2500 psi", you are violating ACI 
> 318 21.2.4.1, correct?
>
> Ed Tornberg
>
> Tornberg Consulting, LLC
>
> 503-551-4165
>
Ed,
In a sense, this is a loaded question. Most of the West Coast still uses

the 97 UBC and not the 2003 IBC. There is a section in, I believe, 
chapter 19 of the UBC that covers the use of 3,000 psi concrete. This is

especially true in areas that have specific municipal requirements based

upon modifications to the state adopted code (which in California is 
based on the 97 UBC). The use of 3,000 psi concrete, beyond its specific

need for strength in the design, triggers the requirements for special 
deputy inspection and structural observation. This can be required as 
part of the municipal code if in a region where the 97 UBC is not used, 
but the local municipality has deemed it to required.
The issue you raised is what governs. While I would like to say that 
ACI-318 governs, it is only the method, not the law. What is written in 
the code that you use is law - it was adopted and ratified by the state 
and local municipalities. You can use your judgment to argue the issues 
or to design to the more conservative, but you can not, without a change

in the code, design to a standard less than that required within in the 
code.
I think that some years ago when design methods were part of the UBC 
(prior to 1994) this was an easier issue to deal with as discrepancies 
were less common since the section of the code dealing with any material

contained the general verbiage of the material organizations such as 
ACI, AISC, MIA, AITC etc. I don't particularly think that the split to 
sell separate references was good from the position of a practitioner 
since it left the rhetoric open to confusion and possible error.
Finally, I don't specify a "design basis" but rather state that the 
minimum strength of concrete to be used must be 2,500 psi or as noted on

the plans. The contractor is free to use 3,000 psi concrete without 
special inspection and while I have a note that states that all grade 
beams and foundations resisting lateral forces are to be designed per 
section xxxx.xx.x.x of the current code. Furthermore, I specifically 
note the requirements for special inspection and structural observation 
on the plan and on the details. There are times when, for example, I 
need to show a retrofit of an epoxy anchor if omitted or changed during 
construction. If the anchor is for shear only, then deputy inspection 
requirements for epoxy are generally not enforced within most 
municipalities - the local building inspection can check the cleanliness

of the holes. The same is true when I use rebar dowels to tie a new 
foundation to an existing so as to prevent potential differential 
settlement. Here again, the anchor acts in shear and not tension so just

sticking a dowel into concrete without epoxy or grouting will still 
achieve the same goal when lateral movement (tension) is not the case.

Hope this helps - just remember that code is law and ACI-318 documents 
are not enforced by law but are assumed to be the basis for the law. If 
the law is worded wrongly and not corrected then theoretically you can 
design to the mistake and be covered. I think this is risky as your 
education and professional intuition can easily be questioned in a court

of law.

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

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

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