Structural rated glue ????

[xtal01]

I am just doing a little "over kill" on some floor trusses.

They came from a certified manufacturer but the nail plates just look a little "iffy" to me.

So, being overly cautious ... and since this is for my own place .... I am putting a wood member over the joint for added strength.

I will also use mechanical fasteners.

Just trying to figure out what glue to use.

My "go to" is Titebond III. I use it on everything.

Problem ... when I go to their web site it says "Not for structural or load bearing applications"

So I went to my favorite place ... YouTube ... now I am scared. I see people building home made trusses using white glue ... Gorilla glue ... titebond ....

I look above me and have wood I joists. The top and bottom of the joist is obviously glued to the osb center with some kind of glue.

What is the "correct" glue to use that will have a structural rating?

I know any glue and even no glue just mechanical fasteners will be better than it is now but I like to do things right ... and even learn something in the process.

Thanks .... Mike

 

[Pmtottawa]

Liquid nails is an option as well as Advantage EP-925

 

[phamENG]

Be careful here. Those plates might look iffy (I tend to agree - the structural engineering equivalent of dark magic), but they're based on a rigorous set of destructive tests and are widely used throughout the residential construction industry. If you start modifying it, you may well modify your warranty out of existence.

 

[xtal01]

I agree ... I don't want to change anything.

I figure it is safe if I just add a backer.

I would be worried if this was for someone else but it is for myself.

I am thinking maybe an epoxy?

Mike

 

[SlideRuleEra]

What is the "correct" glue to use that will have a structural rating?

I Googled on "glue used for glulam beams" and got this:

Tracking down a specific product should be fairly straight forward knowing the generic adhesive.

http://www.SlideRuleEra.net

 

[phamENG]

Just adding a backer to the connections is modifying the truss. It's going to change the way in which the forces are transferred from member to member. Remember, these things are supposed to be able to flex. Gluing a structural panel to it is going to restrain that and alter the behavior.

Similar (but different) application is in light framed wood shear walls. Contractors are FORBIDDEN from gluing the wall sheathing to shear walls without a review by an engineer. If that path is taken, the building has to be designed for over 4 times the seismic forces that it would without gluing it on. Again, this is not the same application, but it's a good indication that gluing plywood onto something can and certainly does change not only the strength of a connection, but how that connection (and the connected members) behave.

Adding a backer will absolutely void any warranty you have on those trusses, and could redistribute the forces in a way that makes them, if not weaker, then less predictable and less ductile in their failure modes. I admit I haven't attempted to analyze this, but I wouldn't do it without checking the full implications of the change.

 

[SwinnyGG]

You should absolutely not modify the floor trusses in your house.

They're designed a certain way for a reason.

 

[xtal01]

Look flimsy and I would think just strengthening a joint would not change anything but I do have an engineer I can run it by ... just happens to be on vacation for two weeks.

So ... just for my own information ... any recommendations on structural glue?

Would epoxy be best (in any situation, not just this one) ... as it is structurally rated?

thanks

 

[phamENG]

Again, you may strengthen the joint, but the altered behavior may shift the load to an area that isn't designed for it and SNAP. For the system to function, the joints need to flex a bit. You're a machinist - say the shaft of a gear passes through a steel plate with a small gap, but that gear looks small and the shaft looks flimsy. If I weld it to the plate, it's going to be able to handle a lot more lateral force. But it's also not going to help the the overall operation of the gear train very much.

I have never had a need to specify structural glue. Glue has uses in structures, like between subfloor and joists to help minimize squeaking, but not as part of an actual load path. I've used certain adhesive anchor systems for concrete, but that's because they've been through extensive testing. And I usually only approve them after the contractor has screwed up doing it the right way.

 

[dauwerda]

There was a recent discussion of structural glue in this thread:

https://www.eng-tips.com/viewthread.cfm?qid=472596

It seems that most glues that are actually structurally rated per ASTM standards are more for the manufacturing industry (glulams, LVL's, etc.)and require huge amounts of pressure when applied.

 

[Craig_H]

dauwerda has it. Structurally rated wood adhesives are only really achievable in shop conditions with tightly controlled surface preparation, adhesive preparation and application (mixing and thickness), and curing (temperature and pressure). Caulking-tube style adhesives (ex. PL400) definitely play a role in the construction industry, but only as a secondary load path.

One thing that hasn't been mentioned is the potential for your plywood gussets and associated mechanical fasteners to actually damage the truss nail plates. If you drive a nail through the truss plate and further perforate the light gauge steel, it may compromise the plate's ability to carry the applied loads.