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RE: uplift/overturning

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I think that most of us are now dealing with the 1.67 factor, since IBC refers to ASCE 7-98 which uses the .6D+W+H (where H=0)load combination which is more severe than the old 1.5 factor. 

Jim K.

-----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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