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Re: Pier and grade beam foundation systems

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I'll take a shot..

I would consider the drilled pier and grade beam system a "frame" if the
analysis uses any kind of fixity at the tops of piers for reduction of bending
moments. Otherwise, I'd see the piers as fixed embedded cantilevers that
transfer loads from the building (grade beam) by dowel action. This joint
however should be appropriately detailed at the pier tops to prevent (reduce?)
pulverized concrete failure by shear forces. Typically, I'd space ties closely
in this region.

A variant of this Building Department interpretation for our office was a
requirement to detail the piers as columns for shear consideration over their
length by placing ties at a maximum of d/2.

Barry H. Welliver
Draper, Utah

John Buchanan wrote:

> To all:
> I have a general question regarding a foundation system for residential
> wood structures founded on a pier and grade beam foundation system.
> A college of mine has received a plan check comment from a locale building
> official stating that the pier and grade beam system should be considered a
> reinforced concrete frame structure and designed per the requirements of
> section 1921 of the 1994 Uniform Building Code.
> The standard of care for the area has always been to use an 8" x 18" or 24"
> deep concrete grade beam with #3 ties and 2 #5 bars top and bottom. The
> beam reinforcing varies per the vertical load. Typically the piers are
> spaced at eight to twelve feet on center.
> The grade beams are tied to the piers with four #5 bars from the grade beam
> to the top of the pier.
> The piers are reinforced with six number five bars and #3 ties at 10" o.c.
> with 4 #3 ties at two inches o.c. at the top of the pier.
> So the question is should the pier and grade beam system be considered a
> frame system a be detailed per the requirements of chapter 21?
> If the consensus is no, how does one confront the building official? (There
> is a board of appeals)
> My college has called the ICBO and they do not offer any help.
> Calls to ACI have been relatively unresponsive and they try to defer the
> question to others.
> (makes you want to be a member Ha!!!!)
> Have a call into EERI to discuss the project but no return phone calls as
> of yet.
> So does any one have any comments?\
> John Buchanan