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RE: Wind Uplift on Footings

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The controlling ASD combination for uplift design is 0.6D-W, which already includes the required safety factor. Therefore the footing size designed for this resulting uplift force is adequate with FS=1.0.
Javier Encinas, PE
-----Original Message-----
From: Harold Sprague [mailto:spraguehope(--nospam--at)]
Sent: Wednesday, November 21, 2007 5:09 PM
To: seaint(--nospam--at)
Subject: RE: Wind Uplift on Footings

Years ago, I designed a lot of foundations for pre-engineered metal buildings.  PEMB foundations are often designed for a net uplift.  The practice (not the code) was to design for a SF of 1.5.  Since the foundation size was predicated on the gravity load, the reinforcing for uplift (in the top of the footing) was sized based upon the uplift foundation moment and 2 #4's each way in the top was generally adequate for most conditions. 
The book "Foundation Design and Construction Manual" by Butler and CSD "recommends" a SF of 1.5. 

Harold Sprague

> From: t.w.allen(--nospam--at)
> To: seaint(--nospam--at)
> Subject: Wind Uplift on Footings
> Date: Tue, 20 Nov 2007 12:57:19 -0800
> Am I hallucinating or is there a provision in the UBC for a F.S.>1.5 against
> uplift?
> For some reason, I can't find that requirement. I found it for sliding and
> overturning relative to retaining walls, but not uplift on footings. I'm
> reviewing shop drawings for a lightweight shade structure.
> TIA,
> T. William (Bill) Allen, S.E.
> Consulting Structural Engineers
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