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Re: wood truss and glue

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Gentlemen,

        I have designed a few of these gusset plated trusses in the past;
but not in recent past.  At that time the gusset plate design was a function
of two variables: the length and thickness of the plywood along the shear
failure line; and the required contact area between the plywood gusset and
the various main members involved.

        The stress on the contact area had to be limited to a value known as
"rolling shear in the plane of the plywood plys".  The allowable value for
this rolling shear from my 1970 Canadian Timber Design Manual (yes, I still
have it!) is about 55 psi working stress value depending on plywood grade.
The value from the 1995 equivalent (it is referred to by a different name
now but it is essentially the same thing) is about 80 psi factored (limit
states) stress value again depending on plywood grade.  So not much has
changed!  Using higher stresses than the above would probably result in
joint failure by pulling wood fibres out of the parent material (whether
gusset or truss member) rather than by failure of the glue.

        Strength of the glue is not relevant (most glues probably have at
least 10 times the necessary strength) but rather durability of the glue and
workmanship in attaining good contact between the components.

        Design the joint as glued OR nailed: DO NOT ADD THE TWO VALUES
TOGETHER!  The nails will not take any load until the joint "slips into
bearing"; and by then the glue joint will have failed.

Respectfully submitted,

H. Daryl Richardson

"Michael L. Hemstad" wrote:

> 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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