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RE: PEMB Foundation Uplift

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I recall a Structural Engineer or Structure magazine article that laid this
out a bit more clearly.  I tried to find the article on-line, but have had
limited success. To summarize my memory:  

The older codes used a 1.5 overturning / uplift safety factor.  Meaning that
your stabilizing dead load had to be 1.5 times the uplift wind load.  But,
that was with a 0.9 factor on the dead load.  Therefore, the total safety
factor was 1.5 / 0.9 = 1.67.  

The new codes use a 0.6 Dead Load factor.  Therefore, if you use a 1.0
safety factor with these LC's then you will get a total safety factor of 1.0
/ 0.6 - 1.67.... The same as the older codes.  

Here is a simple example using an 11 kip wind uplift and a 2 kip stabilizing
dead load:

Old method: 
0.9 (DL + SW) > 1.5 * W
SW reqd > (11*1.5 - 0.9*2) / (0.9) = 16.3 kips

Similarly, using a 1.0 safety factor with the new code LC's would be: 

0.6 (DL+SW) > 1.0 *W
SW required > (1.0*11-0.2*0.6) / 0.6 = 16.3 kips 

Therefore, you get the same requirement using the old and new codes.  No
real changes, just a reconfiguration of where the safety factors are placed
(in the LC rather than in a separate safety factor).  

Sincerely,  

Josh Plummer

-----Original Message-----

From: Jeff Hedman [mailto:jeff_h(--nospam--at)lrpope.com]=20
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 =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




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