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RE: IBC Question

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If I recall correctly, the "change" in the factors comes from the
"history" of where the two different load combinations come from.  The
default combinations, if I recall correctly, are the "newer" ones and are
basically from ASCE 7.  And the conventional wisdom now on ASCE 7 is that
one should not use the "traditional" 1/3 increase.  The alternative
combinations are legacy combinations that came by way of the UBC code, and
they have not been "updated" to eliminate the permitted use of the 1/3
increase.  However, since AISC has in effect "abandoned" the 1/3 increase
(per the 2001 supplement to the ASD spec), the only material standard/code
that still permits the 1/3 increase even when using the alternative load
combos is the MSJC (masonry code)...as Scott Haan pointed out
(technically, WSD no longer exists for concrete, so there is no need to
use either load combination with concrete as you will use the factored
load combos).

HTH,

Scott
Adrian, MI


On Mon, 20 Jun 2005, Jason Christensen wrote:

> I understand all that, I was just wondering what inspired such a large
> change in the factors?
>
> Jason
>
> -----Original Message-----
> From: Haan, Scott M POA [mailto:Scott.M.Haan(--nospam--at)poa02.usace.army.mil]
> Sent: Monday, June 20, 2005 10:04 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: IBC Question
>
> You are not allowed to use a 1.33 ASD increase with the basic combinations
> where you are with the alternative load combinations.  This does not help
> with anything other than working stress design of masonry.  You can still
> use
> a 1.6 load duration factor for wood connections.  Where you get reamed with
> the basic combinations is wind - a 1.3 increase is required on your wind
> loads and the wind chapter says you can only use 2/3 of the dead load to
> resist uplift / overturning.
>
> -----Original Message-----
> From: Jason Christensen [mailto:jason.christensen(--nospam--at)es2eng.com]
> Sent: Friday, June 17, 2005 10:38 AM
> To: seaint(--nospam--at)seaint.org
> Subject: IBC Question
>
> Does anyone know why there is such a difference in dead load factors between
> the IBC ASD Basic load combo Eq. 16-12 (0.6D + 0.7E) and the IBC Alternate
> ASD load combo Eq. 16.18 (0.9D + E/1.4)?
>
> Jason
>
>
>
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