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RE: IBC Load combinations

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I agree that the earthquake load effects are essentially the same in each formula.  I don't know "why" the dead load coefficients are different.  But, unless I'm missing something, this section of the IBC permits (contingent upon material code, etc.) 1/3 increase in allowable stress, not 1/3 increase in the load combination.  Thus, the effect of Formula 16-18 would be reduced to 75 percent (1/1.33) by application of a 1/3 increase in allowable stress.  By doing a some arithmetic, this brings the results of Formula 16-18 to within 7% of Formula 16-12.
-----Original Message-----
From: Jake Watson [mailto:jwatson(--nospam--at)utahisp.com]
Sent: Friday, January 25, 2002 4:56 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: IBC Load combinations

The "E" numbers are roughly equal in each equations (0.7E ~= E/1.4), why are the dead load factors so different?  The 1/3 increase will only make matters worse with the higher dead load number.
-----Original Message-----
From: David Williams [mailto:dwilliams(--nospam--at)snydereng.com]
Sent: Friday, January 25, 2002 2:36 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: IBC Load combinations

As stated in paragraph 1605.3.2 Formula 16-18 allows allowable stress increases or load combination reductions where permitted by the material section of IBC or referenced code.  Formula 16-12 does not permit allowable stress increases. I believe this accounts for the difference you see in the equations.
 
HTH
David L. Williams, P.E.
Vice-Pres., Snyder Engineering, Inc.
409 Vandiver Drive, Bldg. 5, Ste 203
Columbia, MO  65202
(573)449-9177 
-----Original Message-----
From: Jake Watson [mailto:jwatson(--nospam--at)utahisp.com]
Sent: Friday, January 25, 2002 3:08 PM
To: seaint(--nospam--at)seaint.org
Subject: IBC Load combinations

For those of you using the IBC, I am hoping you can clarify a load combination.  Equations 16-12 (0.60D+0.7E) and 16-18 (0.90D+E/1.4) give dramatically different results.  This can't be intented.  Any guidance would be appreciated.
 
Jake Watson
Salt Lake City, UT