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Re: Lag screws in withdrawal from end grain

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Nels, like Dennis, I don't know how the "Cannot be avoided" relief provision
got into UBC or on what authority the 0.75 factor is based. 

I cannot imagine any reliable basis when I imagine blunt, dull lag screw
threads trying to create for themselves a threaded receptacle for the thread
points to engage securely. Unlike threading them into side grain, where the
wood fibers are already severed in predrilling, and the threads can fit in
like bookmarks among the leaves of a book, going into end grain has the
parallel wood fibers simply being pressed sideways out of the way in a
cross-grain compression failure, and broken in flexure over the thread
points. Some sort of residual frictional grip on the lag's points might
remain for a while, pending seasoning, weathering, and creep-like relaxation.

If the threaded fastener had some precision to the threads and the hole
could somehow be cleanly tapped in advance, then the withdrawal mechanism
would be horizontal shear failure on the cylindrical area of the outside
diameter of the threads.  Not for me, thank you, not under any of the
typical conditions found in rough framing. 

Besides, my late grandfather told me pointedly not to use any fasteners in
end grain in withdrawal. He was an expert machinist and woodworker, built
and operated a hardwood flooring mill, built prefab and custom houses as a
designer- contractor, and as a fallback practiced SE and obtained Calif SE
license No.25 in the first offering in 1931.

Your later reply to Dennis clarified your end grain location as seasoned and
sheltered. The round, perpendicular hole you mentioned could have a
full-round metal pin if the pin were tapped in advance and the alinement of
the hole marked on one end. A threaded tie rod in an oversized hole parallel
to grain, and with a pre-tapered point to locate the tapped hole, would
avoid that loose nut. Of course, a tapped half-round allows for visual
inspection of the thread engagement later. 

Another mystifying addition to the 1991 NDS etc., is the provision allowing
built-up lumber columns by nail-laminating a stack of two-inch lumber, faces
to faces. Bowstring truss top chords have been done this way, where through
bolting only occurred at the web connections. I have pictures of top chords
of not-overloaded 100-ft span trusses made this way in a warehouse, and the
chords buckled like the edge of a leaf of lasagna-- one panel curved up, the
next down, and so on. The nails completely failed to make the stack of 2x6's
monolithic for l/d purposes. Resulting truss sag caused the solid 6x8 bottom
chords to split open along the tension splice bolt holes by the prying
action of their long, rigid steel side plate splice straps, that could not
follow the sag-caused joint rotation. What saved these tension splices (and
the building from collapse) was a couple of half-inch vertical "stitch"
bolts at each  joint, with ordinary cut washers, that stopped the split
6x8's from separating more than a half inch along a long line of horizontal
one-inch bolts. (The stitch bolts were loose by nearly that same half-inch,
due to seasoning shrinkage on the 8-inch dimension, at all the unsplit joints.) 

With all the noise about bolt-on hold downs tearing their bolts out of the
vertical posts by splitting them, and anchor bolts splitting the mudsills,
you would think some provisions to allow stitch bolts to catch the splits
might be offered, instead of relying on plate washers that can't catch
anything moving parallel to them.

Charles O. Greenlaw, SE   Sacramento CA         

__________________________________
At 12:17 PM 9/7/98 EDT, you wrote:
>The 1994 Edition of the UBC says "lag screws should not be loaded in
>withdrawal from end grain.  When this condition cannot be avoided, the
>tabulated nominal withdrawal value, W shall be multiplied by the end grain
>factor, Ceg=0.75."                (etc.)
>