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2003 IBC load combnations & wood design

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Title: 2003 IBC load combnations & wood design

The load duration factor, C_D=1.6, for wind and seismic is permitted in the IBC for all connection yield modes. If you're interested in reviewing the cyclic tests done on connections to verify this, take a look at the following webpage and follow the link to Shearwall and Connection reports at the bottom of the page:

Also, as Gerard accurately indicates, the load duration factor is a material specific term for wood, which is distinct from the 1/3 "load probability factor." There's a thread on this topic in the archives.

All other adjustment factors for wood including repetitive member, wet service, temperature, etc., are applicable as well.



John "Buddy" Showalter, P.E.
Director, Technical Media
AF&PA/American Wood Council
1111 19th Street, NW, Suite 800
Washington, DC 20036

The American Wood Council (AWC) is the wood products division of the American Forest & Paper Association (AF&PA). AWC develops internationally recognized standards for wood design and construction. Its efforts with building codes and standards, engineering and research, and technology transfer ensure proper application for engineered and traditional wood products.

>The guidance provided herein is not a formal interpretation of any AF&PA standard.  Interpretations of AF&PA standards are only available through a formal process outlined in AF&PA's standards development procedures.


From: "Gerard Madden, SE" <gmadden(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: RE: 2003 IBC load combnations & wood design
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 =93Duration of Load=94 is a material specific term CsubD (as you =
in wood and that=92s why they worded it that way. The intent was to =
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=3D1.6 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=85. Oh =
-----Original Message-----
From: Dan Morrow [mailto:dmorrow(--nospam--at)]=20
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=92re 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 Fag=E9t Structural Engineers
2124 3rd Ave. Suite 100
Seattle, WA 98121
206-956-3745 (direct)
206-443-4870 (fax)
email: dmorrow(--nospam--at)