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RE: uplift/overturning

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In my opinion, load combinations in codes generally relate to design of
structural materials and not to stability considerations, i.e., they apply
to "strength design" and "allowable stress design". For stability
considerations, I would use realistic loads and design to meet prescribed
stability safety factors (such as 1.5), without any load factors applied. 

William C. Sherman, PE
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com


> -----Original Message-----
> From: Kestner, James W. [mailto:jkestner(--nospam--at)somervilleinc.com]
> Sent: Friday, June 13, 2003 1:30 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: uplift/overturning
> 
> 
> 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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