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Re: canopy footing

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Steve,

I think the key thing to keep in mind is that the method(s) in the UBC are
intended for use with "rigid" "pole-type" foundations.  In otherwords, the
foundation element (caisson, cube, whatever) needs to move in a rigid body
fashion.  If you get to higher of a depth to diameter ratio (such as deep
pile or caisson), then the foundation element would bend/flex and thus the
element would have to be designed for bending.  Thus, I believe that your
situation may be applicable to the UBC provision...a 5'x5'x5' cube will
definitely move as a rigid body and so the lateral effects of the soil
pressure should be able to be approimated for design purposes by the UBC
formulas.  In otherwords, the cube will want to just rotate (and
translate as a rigid body) in the soil with the soil resisting that
rotation (and translation), while a tall, narrow caisson would also want
to bend.  This change in behaviour would affect how the soil would "react"
to the foundation and thus change the forces on the foundation.  And it is
my understanding that the "flag pole" foundation formula(s) in the UBC are
predicated on the rigid body behaviour.

Regards,

Scott
Adrian, MI


On Mon, 13 Feb 2006, S. Gordin wrote:

> Scott,
>
> Plain concrete is not allowed for structural applications in California. Anyway, the specified reinforcement of the "cube" was inadequate (as many other things in this design were).  For example, the restraining slab (as designed) is not even in contact with the "cube."
>
> My main problem with the design is that - according to CBC/UBC Section 1806.8 preamble - the formulas 6-1 and 6-2 are both for distinctively column-like (3:1 aspect ratio) foundations.  Apparently, this is how it was tested back in the 1930s-1940s.  I am not sure that these formulas were intended for such use at all.
>
> When drawn to scale, the canopy on such footing looks scary.
>
> Steve Gordin SE
> Irvine CA
>
>
>   ----- Original Message -----
>   From: Haan, Scott M POA
>   To: seaint(--nospam--at)seaint.org
>   Sent: Monday, February 13, 2006 11:17 AM
>   Subject: RE: canopy footing
>
>
>   I have seen lots of canopy designers use pole sign formulas when I was a plan reviewer.  Formula 16-2 is the constrained formula.  If they are using that one they need to have an adequate portland cement concrete slab around the foundation to resist the thrust.  Up here they frequently put a small PCC slab around the pumps to resist spilled gas but have asphalt pavement close by.  If you do have an adequate slab I would tell them they need to use formula 16-1.
>
>    You are not allowed to use plain concrete to resist seismic forces for other than houses/duplexes.  Are they reinforcing the "cube"?
>
>
>
> ------------------------------------------------------------------------------
>   From: S. Gordin [mailto:mailbox(--nospam--at)sgeconsulting.com]
>   Sent: Monday, February 13, 2006 9:47 AM
>   To: Seaint@Seaint. Org
>   Subject: canopy footing
>
>
>   Good morning,
>
>   I am reviewing a "standard" design of a canopy - a steel-framed structure with two cantilevering columns 14' tall supporting a light "low-pitched V" roof measuring 16'x24' with 12' cantilever.
>
>   The engineer used UBC formula 6-2 (Section 1806.8) to justify the adequacy of the footings for the columns - 5'x5'x5' "cubes."  According to the UBC Commentary p. 297, the formulas of UBC 1806.8 are historically applicable to "pole-" or "column-" type footings.
>
>   To me, these "cube" footings do not even look right for the subject application.  Any comments on the situation will be highly appreciated.
>
>   TIA,
>
>   Steve Gordin SE
>   Irvine CA
>
>
>

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