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Re: seaint Digest for 16 Apr 2003

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I can't speak to the Florida Building Code, but I am pretty sure of the intent of the IBC.  The 2/3 of dead load you
refer to in section 1609.1.3 only applies if you are using the alternate basic load combinations of section 1605.3.2
which will require the wind load to be factored by 1.3.  I believe this results in net F.S. of 1.95.  Using the basic
load combinations of 1605.3.1, the load factor on dead load is indeed 0.6 resulting in a F.S. of 1.67.  Note that
whichever load combinations you use, you must use the "minimum dead load likely to be in place during a design wind
event."  I think this takes care of the "fudge" in the dead loads that Scott Maxwell refered to.

The Standard Buildning Code previously required the 1.5 F.S. with no load factor on the dead load.  So the IBC, results
in larger total F.S.  In short, you do not have to design for a 1.5 F.S. in addition to reducing the dead loads by 2/3 or
0.6 depending on the load combinations you are using.

Adam Vakiener, P.E.
Southern A&E, LLC
Austell, GA

>From: "Andrew D. Kester" <andrew(--nospam--at)>
>To: "SE Web List \(E-mail\)" <seaint(--nospam--at)>
>Subject: load factors

>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.



>Andrew D. Kester, EI
>Structural Engineer
>Bentley Architects & Engineers
>665 W. Warren Ave.
>Longwood, FL 32750

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