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RE: PEMB Foundation Uplift

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The load combination of 0.6D+W will always give a factor of safety of
1.5 or greater as long as you include the weight of the footing in the D
and thus satisfy a "1.5 factor of safety" though I am not sure the 1.5
factor of safety is codified in the code I design too but maybe it is
and I don't know it.

I don't think it is the intent of the code to exclude the weight of the
footing from the load combination and separating it without factoring it
by 0.6 is just bad algebra.  The expression for required weight of
footing should be expressed as '5/3W-D' with D being the dead load less
footing weight and thus the total load would reduce to 5/3W-D+D or 5/3W
which will be greater than 3/2D mentioned.

Using the expression 'W-0.6D' to determine required footing weight or
even 150% of 'W-0.6D' will not satisfy the required load combinations...
at least that is my interpretation.

Donny Harris SE
Los Angeles, CA


Subject: RE: PEMB Foundation Uplift
From: Long Chen <longfchen(--nospam--at)gmail.com>
To: seaint(--nospam--at)seaint.org

--000e0cd5d020df1427049675ae86
Content-Type: text/plain; charset=ISO-8859-1

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(--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

9675ae86--

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