RE: PEMB Foundation Uplift

Hello, Jeff,

I think that the 1.5 factor is needed. Let's say that the wind load is W and the dead load beside the footing is D. To make it simple, let's say that both are given in positive values. In order to provide an uplifting safety factor of 1.5 or greater, we will need to have "D + footing weight" be greater than 1.5W.

When you design the footing with the net uplift load 0.6D+W, you actually get a footing weight of W-0.6D since D and W counteract each other. Then the total dead load including the footing is D+(W-0.6D) = W+0.4D, which may or may not be larger than 1.5W. Then the safety factor achieved may be smaller than 1.5.

But if you design the footing with 1.5 times the net uplift load, you will get a footing weight of 1.5 * (W-0.6D) = 1.5W-0.9D. Adding this back to D you get a total dead load including footing of D + 1.5W - 0.9D = 1.5W+0.1D. This value will always be greater than 1.5W since D can only be positive. And accordingly you will get a safety factor larger than 1.5 as required, regardless how small D is.

Hope that can help.

Regards,
Long

--
Long F. Chen, P.E.
Civil/Structural Engineer
Honolulu, Hawaii


From: Jeff Hedman [mailto:jeff_h@lrpope.com]=20
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 =3D 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 =3D 1.5 over the net uplift force?  To me this is more
like a factor of safety of 1.67*1.5 =3D 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