Wood Truss 1

[XR250]

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?

 

[XR250]

Yup, using a 4' sheet. I like yours.

 

[IFRs]

I like KooTk's latest version, but I'm just a dumb Pet Eng so, never mind...
'

 

[CELinOttawa]

lol.... Look, the petroleum engineer's upset!

>>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*

 

[XR250]

If you've already got it designed and detailed, I'd stick with what you've got. It's a good detail and I assume that you went 4' to use strips out of a common sheet. The sketch below shows an alternate option for both kinds of plate.

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 :>

 

[KootK]

I'd considered that too XR250. I was concerned that you builder might object to the plywood joints being offset across the peak. Although, it's not as though the ply is continuous across the peak anyhow.

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.

 

[IFRs]

I yam what I yam.
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?

 

[CELinOttawa]

Well, it would certainly simplify the analysis.... But then you would need positive lateral restraint at both the lower supports, and most walls are rather bad at providing that support.

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.

 

[XR250]

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?

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.

 

[msquared48]

I noticed that IFRs bolted from this conversation...

Mike McCann, PE, SE (WA)

 

[CELinOttawa]

Mike: You truly are one with the daddy jokes...

 

[IFRs]

Stepping out of the box sometimes gives a new perspective, when I see a conversation about something that "might" work with "what about this" and "what about that" I like to go back to 30,000 feet and get a broad look at it. Yes, horizontal thrust is an issue but as I see it, wood is not particularly rigid or stiff and any small deflection in the peak will put huge forces on the top of the walls. Of course, they will give a little and everything gets redistributed. But, if the wall connections are rigid or the horizontal thrust (which was mentioned bu SEJohn and MSquared above) is handled elsewhere then struggling with a ridge detail that does not need to be fixed or rigid might be handled with a pinned connection.

Too much oil in the old (head) gears I guess. I apologize for wasting y'all's time.

 

[KootK]

@XR: While looking as something else, low and behold, I stumbled across TPI standard provisions for plates with moment. I figured that you'd be interested, at least in passing. If you're interested in knowing more about this highly intuitive equation, lemme know.

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.

 

[XR250]

KootK,

I would like to see the derivation of this equation. Please provide ASAP. :>