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RE: PEMB Foundation Uplift

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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

 

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From: Jeff Hedman [mailto:jeff_h(--nospam--at)lrpope.com]
Sent: Wednesday, December 01, 2010 8:27 AM
To: seaint(--nospam--at)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