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RE: Lag Screws

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Depending on how deep your bottom chord is, you could drill all the way
through and use a nut and bolt or all-thread, with appropriate washers of
course. You should probably also get the truss manufacturers blessing for
the drilling.


Daryl H.

-----Original Message-----
From: Michael L. Hemstad [mailto:hemstad.ml(--nospam--at)tkda.com] 
Sent: Friday, October 22, 2004 1:06 PM
To: seaint(--nospam--at)seaint.org
Subject: Lag Screws

List,
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 chord.

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?

Thanks,

Mike Hemstad, P.E.
TKDA

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