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RE: 2003 IBC load combnations & wood design

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This is one of those poorly written clauses in the code carried over from the UBC. The adjustment factors still must be applied for wood design. Most do not help, but some, like repetitive member (Cr), do (are greater than 1.0) and are still appropriate to use.


The “Duration of Load” is a material specific term CsubD (as you know) in wood and that’s why they worded it that way. The intent was to remove the 1.33 increase for steel but not wood. Saying no other allowable increase is allowed is a little too broad and another point for plan checkers to squabble over.


One bone of contention I have is why is it okay to use Cd=1.6 in NDS design for seismic, but UBC is 1.33 except for a couple of fastener failure modes??? I think the IBC limits Cd to 1.33 as well….  Oh well…





-----Original Message-----
From: Dan Morrow [mailto:dmorrow(--nospam--at)]
Sent: Wednesday, November 19, 2003 9:43 AM
To: seaint(--nospam--at)
Subject: 2003 IBC load combnations & wood design


In looking at the 2003 IBC ASD basic load combinations and Alternate basic load combinations (1605.3.1 & 1605.3.2) for wood design, it looks like a stress increase for load duration for is allowed under the basic load combinations where wind or seismic loads are included, however no other allowable stress increases are allowed.  Under the alternate basic load combinations an allowable stress increase can be used where permitted by the material codes if wind or seismic loads are included.


My question is this: It appears that the other adjustment factors such as repetitive member factor, Cr, flat use factor, Cfu, which effectively increase allowable stresses are no longer permitted for ASD design.   What about when you’re designing for only dead and live loads? Am I missing something?  Are wood designers in effect being pushed into LRFD design?  Any insight would be appreciated.



Dan Morrow, P.E.

Swenson Say Fagét Structural Engineers

2124 3rd Ave. Suite 100

Seattle, WA  98121

206-956-3745 (direct)

206-443-4870 (fax)

email: dmorrow(--nospam--at)