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RE: ASD Load combinations (was UBC Section 1512.4 Equation12-17

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This is discussed in the commentary of ASCE 7.  The "Basic" ASD set in ASCE 7 is derived to give the same results as the LRFD set.  If you look at the two sets you can see the parallels.  The "Alternate" set comes from UBC land and tradition and has now been deleted outright in ASCE 7 (as of ASCE 7-98 or 95 I believe).  ASCE did a statistical analysis to come up with the factors for LRFD.  So it only made sense to extend that analysis to give the same results with ASD.  

This "Alternate" set discrepancy brings up interesting situations when you are designing mixed materials.  For example: if you size a grade beam foundation based on ASD, then try to design the concrete using LRFD, the condition is very likely unstable using LRFD.  It's even more interesting in UBC land where you get to neglect vertical acceleration due to seismic in ASD, then try to design your braced frame using LRFD.  All of the sudden your frame with zero uplift in ASD needs significant anchorage in LRFD.

Ahhh, the joys engineering.....whatever happened to the laws of physics anyway?

Jake Watson, P.E.
Salt Lake City, UT


---------- Original Message ----------------------------------
From: "Ben Yousefi" <Ben-Yousefi(--nospam--at)ci.santa-monica.ca.us>
Reply-To: <seaint(--nospam--at)seaint.org>
Date:  Thu, 20 May 2004 15:19:18 -0700

>Bill
>
>Excellent point. I was just looking at the two load combinations in the IBC. And, noticed some discrepancies that seem very odd. So, I contacted my guru, Dr. Ghosh to seek his input. The following is his response (I hope he doesn't mind that I am posting here):
>
>Q: In the 2000 IBC load combinations for ASD, the first one has 0.6D + 0.7 E and also prohibits using the additional increase allowed by material standards. The second one has 0.9D + E/1.4, which seems more liberal since you can use more of the dead load to resist overturning, and also allows the 1/3 increase. Am I missing something? Why would anyone use the first one?
>
>
>A: I noticed the same thing.  I do not believe you are missing anything.  It doesn't make sense.  I believe the reason why things are the way they are is that the alternate basic load combinations, taken from the UBC, remain what they were in the UBC, except for the addition of the omega factor.  The basic load combinations on the other hand, have evolved within ASCE 7.  SEAOC had modified the basic ASD load combinations of ASCE 7-95 to make them somewhat comparable to the alternate basic combinations.  However, the basic ASD load combinations changed drastically in ASCE 7-98.  That's when 0.6D came in, reflecting (I believe) the factor of safety of 1.5 that was often required in checking stability against sliding and overturning.  IBC simply adopted the ASCE 7 load combinations.  at this point the two sets of ASD load combinations are farther apart than ever before.  I agree with you - anyone who catches on will be using the alternate basic load combinations, unless he or she is a real believer in ASCE 7.
>
>Ben Yousefi, SE
>Santa Monica, CA
>
>
>
>
>
>
>>>> T.W.Allen(--nospam--at)cox.net 05/20/04 02:41PM >>>
>Ben -
>
>That's for the alternative basic load combinations (and what I'm used to),
>but for the basic load combinations I see that (gulp), roof live load is
>actually combined with wind or seismic (see 12-11). All this w/o 1/3
>increase!
>
>Sheesh!
>
>This brings me to another question. I know there might be building codes in
>the world other than the 1997 UBC which may not have the alternative basic
>load combinations, but are there any situations where one would use the
>basic load combinations in lieu of the alternative basic load combinations?
>Just a quick glance, it doesn't appear that, if one has a choice, this a
>very difficult decision.
>
>Thanks,
>
>T. William (Bill) Allen, S.E. (CA #2607)
>ALLEN DESIGNS (http://www.AllenDesigns.com)
>San Juan Capistrano, CA
>
>
>
>
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