Wood Truss
Wood Truss
(OP)
Got a job where they want a vaulted ceiling with no ridge beam. I designed 2x8 rafters with V- shaped plywood gussets. The span is only about 8.5 ft. The builder got a quote from a truss company and they are proposing what is attached.
Basically, using the nail plate to act as a moment connection at the peak. Seems a bit sketchy to me.
What do you think?
Basically, using the nail plate to act as a moment connection at the peak. Seems a bit sketchy to me.
What do you think?






RE: Wood Truss
Please remember: we're not all guys!
RE: Wood Truss
I noticed a 10 psf Bottom chord dead load mentioned that is not actually there since this is just a 2X8 instead of an actual residential open web truss...
I also noticed a horizontal force of 18# due to uplift, but no horizontal kick due to any snow or dead load - that seems really questionable.
I assume the 30 psf snow is for the site and is not their minimum design snow load for all trusses?
I, too, am a little leary regarding the ridge connector. Personally, I would like to see calculations verifying the connection.
Mike McCann, PE, SE (WA)
RE: Wood Truss
Not that it matters much but I have never seen a vaulted roof using a plate, a beam or scissor truss.
RE: Wood Truss
RE: Wood Truss
They are not showing any lateral thrust. That was my first inclination as well.
RE: Wood Truss
These press plates are designed, as far as I know, to take either all tension or all compression loads across the entire plate, with the forces normal to the press plate embedded shear tabs.
I have never seen these tabs used to transfer tension/compression top, and compression/tension bottom within the same press plate.
Mike McCann, PE, SE (WA)
RE: Wood Truss
Why no ridge beam? It will be hidden by the ceiling. If not you will need to design the wall for the thrust.
RE: Wood Truss
RE: Wood Truss
Use a W12 ridge beam and upset it. Can detail that to get a good ridge vent situation too using the 2X8 rafters.
After all, all they are worried about is the look on the interior, right?
Mike McCann, PE, SE (WA)
RE: Wood Truss
RE: Wood Truss
- I've never once seen this proposed in a real structural application. Back in my day, there wasn't a TPI method, nor a software option, for designing plated moment connections. Perhaps that has changed.
- All of the software packages that I used in the nineties had magic key combinations that you could use to hide the error messages in a bogus design. This may be that.
- In the plant, we used to make all kinds of pseudo structural stuff that involved non-designed, plated moment connections. It seemed to work quite well. One example in particular was a shelf system for plate storage that had 2x cantilevered out from such a joint.
- Used like this, the creep associated with each plate tooth will contribute to deflection in a manner much more severe than you'd get with a normal, axial only, truss connection. I'd like to know that was considered in the design. That said, my gut feel us that this would actually out perform your gusset plate solution with respect to stiffness. We never really saw any sag in those shelves that I mentioned.
- Rationally, one could simply count on only the lower third of the plate as a stand alone, tension only connector and prosecute the design on that basis using accepted TPI design principles. I doubt tthat the fabricator did that but Mitek could. The plate probably buckles at the top by the numbers anyhow. And, the rafters being as shallow as they are, the tension connection would be mostly parallel to grain anyhow which is helpful.
- Obviously, none of us wants to discourage innovation and economy. Because this is an unconventional application, I would ask for detailed calculation output on the joint and truss and ask that the design be revised and stamped by a Mitek rep. Mitek for two reasons. Firstly, like SLTA mentioned, they know their stuff. Secondly, they're a lot more liability cocious than the fabricators. 50/50 odds they veto it I bet.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Wood Truss
Thanks for everyone's help.
RE: Wood Truss
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Wood Truss
Mike McCann, PE, SE (WA)
RE: Wood Truss
RE: Wood Truss
RE: Wood Truss
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Wood Truss
Your a frickin Petroleum Engineer, why should I listen to you
My gut feel was that the laminated nature would negate the notch issue in a low stress application like this one anyway.
Here is an easier idea that i should be able to make work...
RE: Wood Truss
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Wood Truss
RE: Wood Truss
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RE: Wood Truss
>>There was an inappropriate joke here, which I have deleted shortly after posting<<
All in good fun IFR; Besides, you could be one of the "real" petroleum engineers who size pipes and the supporting brackets, rather than one of the process or chemical ones (those ones are more MBA than Engineer)... *smiles evilly and looks around for Snorgy*
RE: Wood Truss
I guess we could even simplify your idea and just lap the suckers. Would change the dynamics of the fasteners however. My moment is only 360 ft-lbs (ASD) so it could be made to work. Might want to add some Elmers on top of that :>
RE: Wood Truss
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Wood Truss
It's so nice to even be in this conversation with y'all.
If you overlap the rafters at the peak and put a single bolt through it I would call it a pinned connection. Does this simplify the analysis or create more problems?
RE: Wood Truss
Basically it would open up and go flat with all but the most careful detailing and, in my opinion, much stronger and stiffer walls than wood typically allows.
RE: Wood Truss
Damn Pet Engineers always thinking in bolts!
I was think a group of nails. I have not laid this out, but If you consider a nail circle of say 4 1/2" using 9 nails and 360 ft-lb moment, you end up with a shear of 107 lbs per nail. So theoretically possible, but I would prefer a different solution that would mot be as sensitive to installation errors.
RE: Wood Truss
Mike McCann, PE, SE (WA)
RE: Wood Truss
RE: Wood Truss
Too much oil in the old (head) gears I guess. I apologize for wasting y'all's time.
RE: Wood Truss
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Wood Truss
I would like to see the derivation of this equation. Please provide ASAP. :>