CFM Truss Hung from Side of CFM Wall

[KootK]

I've got a CFM truss supported (not cantilevered) from a CFM wall system. What's the best way to do this? Thoughts:

- I suppose one could do an LVL ledger.
- One could also do a light gauge ledger but that would require stiffening at each truss, right?
- I'd like to mess with the envelope as little as possible.
- Would like to tie the roof diaphragm into the wall diaphragm at least nominally.
- Sometimes I'll likely be hitting CFM headers instead of wall studs.
- This may not work so hot when a truss lands right on a walls stud.

I find CFM trusses to be a pain in that, until a supplier is chosen, I won't actually know what the chords etc will look like. With wood trusses, they make a big deal of truss-to-building connections being by the EOR. Here, it's tough to comply with that. Hard to know just how to connect something when you don't quite know what that something is. There's quite a variety in possible chord profiles.

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.

 

[sticksandtriangles]

Tough section. Are you trying to make this connection through the sheathing?

If so be cautions about the shear capacity of self drilling screws through sheathing. Values go to almost nothing very quickly. There is an OSHPD document that records some values in the 80 lbs range for shear to 18ga metal studs (I can find the document when I get back to work if you would like to see it).

Other aspects, it seems like you got a good into the page diaphragm shear transfer shown (diaphragm shear carried by the metal deck transfers through bent plate and the into wall sheathing).
Seems like roof pressure on the roof to right will propagate into bending of the wall studs, again no problem.

Overall looks like a good detail in my book. I got a little lost at your statement about requiring stiffening of the trusses. Care to explain?

S&T

 

[dhengr]

Koot:
Could you use a 12,16,18" wide piece of .125" thk. steel plate (+/-), 12' long; and brake form 2 or 3" wide flanges on the bottom edges? These would form an “L” shape ledger and the top ledger would have a bot. flg. bent to match the roof chord slope and be fixed in place on the CFM wall prior to truss erection. The bot. ledger would be bent to 90̊ to form a bot. flg. and would be pushed up under the trusses as they were erected. These ledgers would be strong/stiff enough to allow the trusses to fall anyplace btwn. the CFM studs, or at a stud, and the stud to ledger connections would each have to carry a bit more than one truss reaction. Then, how many lineal feet of each ledger do you need and what does the butt end connection look like at a stud every 12'?

 

[XR250]

What is your reaction and the stud and truss spacing?

 

[XR250]

Another detail to consider is to attach say an 8" stud horizontally to the face of the wall studs and cap it with a piece of track. This gives you a tall and skinny piece of light gage tube steel to attach your truss clips to.
Blocking between CFS studs is always sketchy as the flange gets in the way so you end up with extended leg clips etc.

 

[KootK]

Thanks for the help thus far gentlemen. I've sketched my interpretations of your solutions below. Please let me know if corrections are in order.

A big concern for me here is the extent to which I can delegate truss to structure connections. In wood trusses, those are EOR territory. Here, I don't know how I'd accomplish that as I don't know what form the chords will take and I'm woefully ignorant of CFM truss industry preferences in general. I have called Mitek and Alpine but they've been too slow getting back to me. Maybe I'm just too much of a CFM newbie to be doing this. Not worth considering at this point.

So, think I can supplier delegate some of the truss to supporting structure connections?

What is your reaction and the stud and truss spacing?

Spacing = 4'. Reaction ~1000 lb.

Are you trying to make this connection through the sheathing?

I am but I'm questioning whether or not that's necessary. If I interrupt the sheathing with the connection, am I messing with the envelope? I suppose the envelope probably wraps the truss. Perhaps not such a big deal.

I can find the document when I get back to work if you would like to see it)

I'd very much like to see it.

I got a little lost at your statement about requiring stiffening of the trusses. Care to explain?

My first instinct was to try to replicate what I often do with wood trusses (detD below). Not sure if that's really possible in CFM trusses. I thought that I'd create a ledger in CFM but then that ledger might need stiffening of the track/stud legs at the truss reaction. Not sure if it's necessary but kinda seems impractical. I'm showing the stiffener in detC below.

Blocking between CFS studs is always sketchy as the flange gets in the way so you end up with extended leg clips etc.

Good call.

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]

My guess is that with a 14ga stud and a 16ga track, bending would not be a problem. Might want to put the assembly hi and low to get two connections.

You could also just do the cont. stud and then add say a stiffener and a 12" long piece of track at each truss location to save some material.
However, my experience has been that trying to be economical in metal stud design always comes back to bite me in the butt.

 

[sticksandtriangles]

Attached is the OSHPD document referenced. Testing was done on gyp sheathing, but I am sure there is some sort reduction in shear capacity through sheathing would also need to be accounted for. I believe wood has tabulated reduction in shear values when wood ledgering through sheathing as well.

 

[DaveAtkins]

I recommend a hot rolled angle welded to every stud, similar to the sketch above, but with the vertical leg pointed down.

DaveAtkins

 

[txeng91]

@Kootk I think the angle for bearing is most likely to be compatible with whatever truss configuration the manufacturer uses, and should require minimum coordination. I don't think interrupting sheathing there would be too difficult to flash, however I do think the blocking is still called for to transfer shear. I understand the issue with the flanges of the studs being in the way, but the blocking to stud connection would be irrelevant if the angle is directly screwed or welded to the studs, and your blocking is there just to transfer shear between plywood panels. You would just need to use a leg on the angle that is a few inches shorter than the blocking.

 

[KootK]

Great points dnlv. Gawd... structural engineering is hard sometimes. My childhood experiences with Lego and Lincoln logs led me to be believe that it was just going to be stacking stuff on top of other stuff.

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.