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Re: 3000 psi concrete and special inspection

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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 “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, correct?

Ed Tornberg

Tornberg Consulting, LLC


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

760.564.0884 (office - fax)

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