I think the reason the net uplift equation factor on the dead load was changed to .60 instead from .90 is precisely to guarantee a 1.5 uplift safety factor. Adding another 1.5 safety factor on top of this should not be required.
Jim Lutz, PE, SE
Senior Structural Engineer
BHC Consultants, LLC
1601 5th Avenue, Suite 500
Seattle, WA 98101
Ph: 206-505-3400
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From: Jeff Hedman [mailto:jeff_h@lrpope.com]
Sent: Wednesday, December 01, 2010 8:27 AM
To: seaint@seaint.org
Subject: PEMB Foundation Uplift
I designed a foundation for a PEMB. I want to know peoples methods for designing the footings for uplift. I have typically designed the footing to weigh as much as the net uplift load (0.6D+(W or 0.7E)). The calculated net uplift force has a factor of safety of 1/0.6 = 1.67 in it from the reduction of the dead load. I received a plan check comment that states that with my calculations there is only a factor of safety of 1. But this is a factor of safety of 1 in regards to the net uplift force. Should we really be increasing the footing weight to maintain a factor of safety = 1.5 over the net uplift force? To me this is more like a factor of safety of 1.67*1.5 = 2.51. Not to mention we know what the actual weight of the footing will be. This is a commercial building and the city inspectors are very good in requiring construction matches the plans, especially foundations. Am I wrong? Do others design PEMB footings that weigh 1.5 times the net uplift force?
Thanks in advance for your replies
Jeff Hedman
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