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Pretty basic question here:

I am specifically designing a foundation for a pre-fab metal building, but
it could be uplift design for any foundation.

I have always applied a 1.5 FS to the size of the foundations to resist
uplift from my net wind loads. So if I get a reaction of 10kip, I supply
15kip worth of foundation. I have based this on a section in the Florida
BC - 1620. But now that I am looking closer that is in a section under High
Velocity Hurricane Zones, which only applies to Broward and Dade counties.
However, nowhere else does it mention in the rest of FL if you are to abide
by this 1.5 FS.

If I then go to  the ASD Factor Section 1609.4, #4:   0.6D+W, which would
mean for a 10kip uplift, you would need 10k/0.6= 16.67k of foundation.

Now, this is worse then the FS=1.5, since 0.6D equates to 1/0.6W=1.67. Now,
I do not think the code is implying that they want you to supply both
factors, just one or the other. So which one is it? I know most of you out
there are not using the FBC daily, but this is just a common sense factor
question I think. But why would the High Velocity factor be less then the
regular ASD factors. Or are they implying they want 0.6D + 1.5W in High
Velocity Zones, and just 06D + W in the rest of FL?

IBC 1609.1.3- Tells you to use the long used standard of 2/3 of the DL to
resist the Wind Load, which is the same as FS=1.5.

IBC 1605.3 ASD
0.6D+W - same as FBC (codes are essentially the same thing but no seismic or
snow in FL and slightly different wind)

So is everyone out there taking only 60% of their dead loads to resist the
wind uplift, or are most people still using 2/3?

I am assuming ASD factors are to be used for stability analysis as well as
actual member design. Maybe that is where I am getting screwed up.

Anyone care to shed some light on this subject or give their interpretation
or "what I do"s would be appreciated.

TIA

Andrew




Andrew D. Kester, EI
Structural Engineer
Bentley Architects & Engineers
665 W. Warren Ave.
Longwood, FL 32750
1-407-331-6116
andrew(--nospam--at)baeonline.com
www.baeonline.com




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