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Re: residential pier foundation design

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Jeff :
I suggest you take a look at the City of Los Angeles "Seismic Design Provisions for Hillside Buildings". R=4.5 and no plywood shearwalls in the downhill direction.
As far as your hoop design, check UBC97 section 1809.5.2.2, which refers you to section 1921.4 with an exception. Your hoops do not need exceed the amount required by formula 21-2 unless shear governs. Also, note the many exceptions when your axial load is less than Agf'c/10, which is typical for hillside residences as in your case.
Oshin Tosounian, S.E.
Los Angeles, CA
----- Original Message -----
From: Jeff Smith
Sent: Wednesday, May 09, 2001 8:47 AM
Subject: residential pier foundation design

I searched the archive and found a similar question but I could not find quite what I was looking for. I am working on a small frugal 2 story wood framed plywood shear wall, pier and grade beam residence on a 2 to 1 slope right next to the Hayward fault. My design is yielding a hefty foundation that has inspired me to review my approach.
Should the R be adjusted for the foundation design even though the building is 5.5, ie 2.2 for cantilevered fndn piers or 3.5 for a OMRF pier and grade beam (unlikely because of ductility requirements in grade beams).
My pier axial loads are only about 26K of which 13K is the weight of the foundation, eq lateral loads are from 4-7K including the grade beams, with soil fixity from 4 to 10 down from grade..
Section 1910.8.4 excludes zones 3 and 4 for reduced effective area Ag, therefore min long. reinforcing is 1% which is 6#6 in an 18" dia pier regardless of load.
I am unclear on how to design the hoops. I come up with #3 spiral hoops at 3" (or something.)
I have never seen hoops like this on a small residential building. I talked to the soils engineer and he said he had not seen that before either, usually just stirrups at 12" with a few extra at the top. Of course the contractor said the same thing but more emphatically. This seems to me like an example of standard practice for residential construction that takes exception to the code, or that I do not understand the code. Any thoughts on this subject would be greatly appreciated.

Jeff Smith, S.E.