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wood truss and glue

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Mark,
You've gotten an earful (screen full?) from several other people about
how undesireable a commission this is, so I won't belabor the point.

I think the biggest concern, as Dennis alluded to, is counting on the
glue bond.  It doesn't have much give, and will be rather unforgiving of
repeated shrinkage/expansion movement across the grain of the chord
members.  As a woodworker, I think Gorilla Glue is wonderful stuff (it
really is), but as a structural engineer I don't think adhesives have a
place in truss joints.  Period.  What happens if the plywood pulls
apart?  The best glue bond in the world is still only to the face ply of
the gusset--you need something that engages the whole thickness of the
gusset.  First, obviously, see if the gussets are strong enough.  Then I
would calculate how much strength the nails represent (assuming your
client built them all about the same), then figure out how many screws
(or better, through bolts) he needs to add to make up the difference.
Finally, make sure the gussets are big enough to hold all this hardware,
and hope the chords don't split.

For what it's worth, I think your concerns about long-term performance
are reasonable.  I think the joints will be well-enough insulated by the
plywood to be reasonably fire-resistant (at least until somebody makes
them put a bunch of steel through the joint).  But all this is just from
the hip.

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota


-----Original Message-----
From: Mark Miller [mailto:milm(--nospam--at)chemeketa.edu]=20
Sent: Thursday, June 24, 2004 11:42 AM
To: seaint(--nospam--at)seaint.org
Subject: wood truss and glue- sorry second try

Hello-

I have a client that has built some wood roof trusses (normally I try to
=
avoid projects like this, but long story=E2=80=A6..). The trusses look =
very reasonable in their construction. I will have to analyze them for =
strength and deflection, but before I put the time into doing this I was
=
concerned with the way the plywood gussets were glued to the truss =
members. They used Gorilla Glue and nails to hold the joint tight while
=
the glue cured. I was concerned with how Gorilla Glue (one part =
polyurethane glue) compared with the resorcinol glue that we have used =
in the past. I have contacted the makers of Gorilla Glue and they were =
very helpful and pointed me to some tests done by the USDA Forest =
Service Forest Products Lab. The shear strength, in these tests, looked
=
as good or better than resorcinol for use on doug fir. I am still =
concerned with the long term performance and performance in a fire (high
=
temp). This is in Oregon and under the UBC.

Does anyone have experience with this?

What do you think?

=20

Thanks for your consideration.

Mark Miller, PE

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