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Re: Lag Screws

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I've used 1/4" lag screws in my designs with no reports of damage because of
installation torque.  However, I have found no way to specify lag screws by
a recognized industry standard.

I always specify pilot hole sizes for lag screws using Table 7.19 of the
AITC Timber Construction Manual , 4th edition.  It recommends different
diameters for the shank and for the threaded length.  I understand that
step-drills are available that allow the pilot hole drilling to be a single
operation, but I have not researched the availability of step drills
appropriate for lag screws.

The Simpson SDS Screw does not require drilling of pilot holes -- apparently
because the cutting-tooth at its tip probably damages the zone of wood
around the threads as the screw advances into the wood.  It is probably this
damage to wood that you would count on to develop tension capacity that
makes the SDS Screws unreliable in tension.  I would not use the SDS Screws
to resist design tension loads.

I've measured with a micrometer the diameter of some SDS Screws -- the
shanks measured slightly smaller than 1/4" diameter.

I try to avoid using in tension any wood connectors without positive
restraint to the member -- I don't like to count on threads.  Can you design
a bolted connection to develop the needed tension capacity?

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net
----- Original Message -----
From: "Michael L. Hemstad" <hemstad.ml(--nospam--at)tkda.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, October 22, 2004 1:06 PM
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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