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RE: ASD Load Combinations

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The answer to your question depends upon what requirements are in the building code that is in effect for your project. Almost all building codes get you back to ASCE 7 for the loads and load combinations (or a set of loads and load combinations that are based upon the requirements in ASCE 7). But different jurisdictions have different revisions of that document referenced. So you'll have to check the specific requirements in the applicable building code.
As far as the latest and greatest goes, ASCE 7-98 is the most recent revision. It is referenced as the model code for loads and load combinations in both the International Building Code and the competing model code NFPA is writing. I'm not sure of when, but this version of ASCE 7 will trickle into the building code, jurisdiction by jurisdiction.
In the 1998 revision of ASCE 7, the ASD load combinations were significantly modified. A lot of verbiage was added disallowing the use of the 1/3 stress increase on the material side and replacing it with a reciprocal 3/4 factor on the load side. However, the application of the 3/4 factor is strictly limited in ASCE 7-98. It can't be applied to dead load in any combination. It can't be applied to a transient load acting alone. And it can't be applied to multiple transients if the reduced sum is less than one of the transients acting alone. After working all of the written restrictions into equation form, I've found that there are now eleven distinct load combinations in ASD.
-----Original Message-----
From: Larry Hauer [mailto:lrhauer(--nospam--at)]
Sent: Tuesday, November 14, 2000 5:52 PM
To: seaint
Subject: ASD Load Combinations

Can someone enlighten me on when to use Basic Load Comb. formulas 12-7 thru 12-11, (sec. 1612.3.1 of '97 UBC), in lieu of formulas 12-12 thru 12-16-1, (sec. 1612.3.2), when designing in ASD?
It seems to me that, for economical design, formulas 12-12 thru 12-16-1 should always be used since they allow a 1/3 stress increase for wind/seismic, while sec. 1612.3.1 formulas DO NOT. Granted, the 0.75 factor in formula 12-11 is the same as the 1/3 increase, but formulas 12-9 and 12-10 which also include wind/seismic forces, do not allow the increase.
Thanks in advance,
Larry Hauer SE