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RE: wood truss and glue- sorry second try

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I'll add a technical bit here from my product days: adhesives are generally specified to have a particular shear strength at a specified film thickness under controlled curing conditions. There are several key variables in the process, and almost none of them are controlled in a construction environment. For example, one of the popular epoxies, 3M 2216 B/A Grey requires (iirc) a 5 mil, +/- 10%, film thickness cured at room temperature for 24 hours or (I don't remember the baked duration) to achieve the rated strength - and that's only on properly prepared, particular substrate materials. Many adhesives have absolutely lousy bulk properties, and therefore have very little strength when applied with too thick a film.

While construction adhesives may be a bit more tolerant, I generally view them the same as I view drywall for shear resistance: They'll probably help a bit if you have a borderline event, and they'll keep the structure from failing when you don't do everything correctly in the field, but I won't use them in the load path if my seal is going to be on the drawing.


At 02:12 PM 6/27/2004 -0400, you wrote:
Mark:

I know your questions here are to solicit good suggestions on how to get your job done, to follow Dennis's gut feeling, mine is to run don't walk from this. I run into so many contractors with the "I glued and screwed it, it has to work" attitude. First, what plywood did he use? was the main members dry enough to not shrink, was it clamped properly? are the plywood gussests big enough. I have ususally found the the amount of plywood and labor required quickly justifies the the use of pre-built, pre-engineered wood trusses. Sorry that this is not helpful I just have to rant about the contractor with the "i don't need no stinkin engineer" attutude leaving you to bless this mess when you would probally never have designed it the way he built it.

Jeff Fertich, PE
Structural Engineering Resources, LLC
Gettysburg, PA

From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: wood truss and glue- sorry second try
Date: Sat, 26 Jun 2004 18:39:28 -0700

Mark,
I can't respond to this with any knowledge or experience in the creation of a site built wood truss using glued gusset plates and nails. However, I would like to comment from a Professional "Gut" feeling and see how far off track I am. Nailed plates (or even pressed plates) are intended to allow some rotation at the joints. Even though the chords may be continuous, the connection at the struts allows for bending and the nodes have been historically considered pinned. When applying a glued gusset plate my 'gut' tells me that you reinforce the rigidity of the node and change the distribution of forces on the truss members. More to the point, the nails have no value until the more rigid glued connection fails. If the glue separate or the ply of the plywood gusset splits, the load to the nails could cause the truss to fail without warning.

I've seen site built trusses, but most of them are simply nailed, bolted or in the Midwest (more than out West) mechanical connectors such as shear plates are used.

FWIW, I know of Gorilla Glue as a recommendation used to repair fine instruments because of its ability to resist changes in humidity and to maintain its strength over long periods of time. The only difficulty I have heard (about most glues) is when used on woods that have an oily residue such as Rosewood.

Now that I let out my ignorance of site-built trusses, I would be happy to hear how far off my 'gut' feeling is.

Regards,
Dennis


Dennis S. Wish, PE


California Professional Engineer

Structural Engineering Consultant

dennis.wish(--nospam--at)verizon.net

http://www.structuralist.net




-----Original Message-----
From: Mark Miller [mailto:milm(--nospam--at)chemeketa.edu]
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 storyâ?¦..). 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?



Thanks for your consideration.

Mark Miller, PE

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