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General Dowel Equation Reduction Terms

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Title: General Dowel Equation Reduction Terms


In the 1991/1997 NDS(r) (and TR12), a factor of 0.75 was used which assumed a constant ratio of yield moment of the lag screw threaded portion to yield moment of the shank. Note that for yield modes III and IV, the Rd terms in the 91/97 NDS are approximately 3/4 of those used in the 2001/2005 NDS. TR12 section 1.2.2 and 1997 NDS Commmentary Part 9 provide more detail on the 0.75 factor:

The change is related to assuming root diameter (Dr) in the yield equations vs. shank diameter (D) for lag screws (see 2001 NDS section 11.3.6). Here's a link to more background on that:

To answer your question about which ones to use, stay consistent with the edition of the NDS you are working with. If using the 1991/1997 NDS, use the Rd terms there. If using the 2001/2005 NDS, use the Rd terms there.



John "Buddy" Showalter, P.E.
Director, Technical Media
AF&PA/American Wood Council
1111 19th Street, NW, Suite 800
Washington, DC 20036
P: 202-463-2769
F: 202-463-2791

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: "Joe Grill" <jgrill(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: General Dowel Equation Reduction Terms

I have noticed that the reduction terms, for lag screws, are different when
looking between the NDS 2001 Edition and Technical Report 12 by the AF&PA.
It seems that using the NDS values produces a lag screw value that is
conservative when comparing to the NDS 2001. Does anybody know the
background in this difference and which values should be used?
Joseph R. Grill, P.E. (Structural)
Shephard - Wesnitzer, Inc.
Civil Engineering and Surveying
1146 W. Hwy 89A Suite B
Sedona, AZ 86340
PHONE (928) 282-1061
FAX (928) 282-2058