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Re: Lag screws in withdrawal from end grain

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This thread has really caused a lot of concern and I felt that maybe I could
shed some light of the question.

I think at Bill, Dennis, Roger,  and Ed all had excellent comments and already
indicate some of what was considered in the background for using the phrases
that are being questioned.

 Having worked on the revision to the AITC Timber Construction Manual (3rd and
4th Edition) and on the AF&PA technical committee that revised the NDS, I have
some knowledge of the background for the withdrawal of lag screws from end
grain. Remember that some the wording in the code is taken from older
documents when the wording was not as explicit as is the case now.  Wording
now has to be defensible in a court of law (as seen by the code).

Until recently, most of the design values for bolts, lag screws, nails and
other fasteners was based on some testing of those fasteners at the Forest
Products Lab in Madison, WI. Unfortunately, the amount of testing was very
limited. Most of the published design values were extrapolated. Committees at
AITC and other wood organizations used their best judgement and conservatism
to justify the values.

In the case of withdrawal from end grain, they did not want to totally
disallow the use of lag screws in withdrawal since experienced engineers and
designers had already successfully used the fasteners in those cases and new
of the limitations. However, to discourage designers not experienced in these
applications, the term "should not be used" was selected. To allow the
experienced designer the latitude to use them, the phrase "when it can not be
avoided" was included. based on the reasoning that the experienced designer
new when to avoid that type of connection. This was the compromise that
allowed for engineering judgement. 

I recommend the NDS Commentary, available from AF & PA, that has quite a lot
of background information. Ed King, who was with the NFPA for a number of
years, wrote most the commentary. It is well written and straight forward. 

Bruce Pooley, P.E.
Timber Design
3448 South Newland Court
Lakewood, CO 80227
303-989-8701