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Re: load factors

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Andrew,

To my knowledge the 0.6D+W has nothing to do with the 1.5FS for uplift in
the High Wind portion of hte code.  The 0.6 on the dead loads is to
account for the "fudge" you apply when determining dead loads.  For
example, you might typically use 7 psf for mechanical (or something else)
but in reality you may only have 2 psf in a significant area.  Bascially,
the 0.6 factor is to account for overestimation of your dead loads.

Thus, as I see it for the high wind area, your overturning moment would
typically due to W with your resisting moment being due to the 0.6W.
Then, your overturning moment divided by the resisting moment would have
to be greater than 1.5.

Thus, for non-high wind areas, you would still have a "FS" of higher than
1 due to the 0.6 on the dead load.

HTH,

Scott
Ypsilanti, MI


On Wed, 16 Apr 2003, Andrew D. Kester wrote:

> 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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