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Lag Screws

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I have a project where I need to hang a small steel beam from the bottom
of some prefab wood (2x) trusses to carry the track for a moveable wall.
The beam will cross about 7 trusses.  It's stiff enough to spread out
the load to the trusses.

The simplest idea I have come up with is to use lag screws through holes
in the top flange of the beam, into the bottom chord.

Simpson, for example, makes a 1/4 inch diameter lag screw, with catalog
values for strength in shear.  However, they corporately refuse to give
me a withdrawal value for it.

The NDS has values--rather large values--tabulated for generic lag
screws.  I could just use those.  However, in my earlier days (say, 20
years ago) I occasionally fastened pieces of wood together with lag
screws.  We shied away from the smaller ones because they often broke
during installation.  We drilled pilot holes, we waxed threads, we
sacrificed black chickens before we screwed them in.  5/16 screws broke
maybe one time out of four; the 1/4 inch ones broke more often than not.

Relating this fascinating story to the Simpson engineer, she said that
her understanding was that current lag screws were better than those of
twenty years ago, and theirs, being of hardened steel, were better than
the ones we bought at the hardware store.  She simply was not allowed to
give me a withdrawal value.

It would be easy to just use a bigger screw, but the truss manufacturer
is going to be excited enough as it is with the 1/8 inch pilot hole I'm
calling for in his bottom chord.  And I could bring straps up the side
of the truss chord, but it's at about midspan and there is probably a
metal truss plate there that doesn't want holes drilled in it, and
vertical or diagonal members that preclude strapping over the top of the

So, finally, the question:
Can anyone comment on the quality and viability of 1/4 inch lag screws?
Or does anyone have a better way to solve this simple problem?


Mike Hemstad, P.E.

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