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RE: uplift/overturning

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Read the commentary in ASCE-7 regarding these combinations.  The 2/3 issue
and the 0.6D don't occur together.  First, the 2/3 DL for overturning is to
be checked when you are using the 0.9D load combinations in IBC and UBC.
That way you still get a minimum F.S. for overturning.  The 0.6D+/-0.7E load
set was derived to give the same result as the LRFD set (which is supposedly
based on statistics).  If you use the 0.6D combination you don't need an
additional F.S. for overturning.  Be careful to use a complete set of load
combinations, don't mix and match sets.  It will only cause grief.  This is
based on load combinations in ASCE 7-98, 1997 UBC, and 2000 IBC.  I have
never used the FBC.

HTH,
Jake Watson, P.E.
Salt Lake City, UT

-----Original Message-----
From: Andrew Kester [mailto:andrew(--nospam--at)baeonline.com]
Sent: Friday, June 13, 2003 12:45 PM
To: seaint(--nospam--at)seaint.org
Subject: uplift/overturning


Karen:

"Seth - .9D +or- E/1.4 is the correct load combo for seismic overturning.  I
believe
that Andrew was referring to wind loads (since he is in hurricane country).
It's not expressed as a "factor of safety" in the UBC, but section 1621.1
stipulates that you can only use 2/3 of the dead load to resist wind
overturning."


You are exactly right (about wind loads), and I have talked about it on this
list before. The FBC 2001 is confusing on this issue too, and I have never
got what I would call a concrete answer, pun intended.

For ASD design, the code lists:

0.6D + W

So my question was/is ,to size the concrete footings for weight to resist
uplift, do you use this formula? I say yes, for lack of a better formula.
But I do not like or agree with it, because it gives you these obscenely
huge concrete footings, and for ex. pre-fab lightweight metal buildings it
just seems silly that the whole building would stay intact while the whole
footing is pulled out of the ground. I believe the intent is to reduce
building dead loads normally used in resisting wind loads. These DL may or
may not be in place, or may be overestimated. But a concrete footing is
THERE, you can count on it. That is why I think a seperate formula for
foundations needs to be in place, something more on the order of 0.9D.

Finally, the big curveball, in S. FL, Broward/Dade Counties, there is
another provision where stability of any building, structure, or part shall
have a FS=1.5 for uplift and overturning. However, the above ASD formula
already gives you a FS=1.67 (1/0.6), so are these to be applied
simultaneously?? I don't think so, but which one should you use?

Hope that clouded things up nicely for you. I guess for Seth that is not an
issue because you don't have to deal with this confusing part of the FBC....

Andrew Kester, EI
Longwood, FL



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