Collar Tied Roof Fail 3

[XR250]

So this is a job one of my competitors did a few years ago. 60 ft. span trusses converted to a vaulted ceiling by sistering with 9 1/4" LVL's and adding LVL collar ties. 6" of measured sag. The whole roof is getting ripped off and replaced with trusses again. I guess he used the "1/3 from ridge" collar tie rule without checking in 2D software. Ironically, the owner is not seeking legal action.

 

[Ron247]

I think that part of the article is just trying to explain Tension since that is the heading. If you held one wall stable and only pulled outwards on the other wall, you would still have tension in the collar tie above the rafter tie that is created by the stretch of rafter tie, I think. Inward movement would cause the compression.

 

[BAretired]

If the rafters are infinitely rigid, I would agree that the collar tie feels tension. In the majority of actual roofs, I expect collar ties feel compression from snow load, not tension.

BA

 

[JoshPlumSE]

Ron and GC_Hopi -

Thank you both for the clarification on our terminology! Seriously, I appreciate you guys taking the time to explain the reasoning behind your terms as well as that link to the article.

 

[Ron247]

Oh BA, I see what you are saying. The Snow Load would push down on the rafter and cause compression in the collar tie. That force would do exactly what you stated. I thought you were talking about wall spread at the bottom. So the final collar tie has some compression from snow down and some tension from wall spread out. Roof pitch, span, snow load etc would have to be calculated to determine if the net effect would be C or T. Makes sense to me.

My comment on people who put collar ties in the bottom third did have to do with collar ties intended for the upper third, not rafter ties that were put in the bottom third because of ceiling joist issues. As roofs got steeper, carpenters started nailing collar ties at shoulder height or lower for ease of hammering. A lot of times they were in the middle third. That is who gets cussed.

Your welcome Josh. I had a meeting last week that lasted twice as long as needed because everyone seemed to call something a different name. Everyone just stayed confused. A rose by an other name is not a rose, it is a pain in the neck. And my other favorite occurred there, we are standing in a circle talking in the middle of 20,000 building and people are saying, "That beam on the left". Everyone looks to their left for it. I always carry a laser pointer for those kind of meetings. And I lose more laser pointers because people borrow it and forget to give it back. Some of us had different books we were taught out of, the area we live in uses different terminology, so we all deal with it best we can.

 

[kissymoose]

Why in the world would the owner not be seeking legal action? This should intuitively not work, and the Engineer who was paid to say it does should be held accountable.
Any case anyone is still curious/hasn't seen this before, here's load diagrams for the rafters as the horizontal tension member gets higher. The tension member ends up seeing >4x the tension, which creates quite the spike in the rafter shear and the moment in the rafter due to this spike becomes so much higher than the moment caused by design loads like snow. Yet still the weakest link: tell me how you would transfer that tension from the horizontal member into the rafter. Nails? Bolts? Plates? Good luck.

I encounter this a lot with residential, wanting to vault roofs and not understanding the purpose of ceiling joists. Roofs aren't magic, just because it's fine when you're building it doesn't mean it'll be fine when you have a couple feet of snow on it. This has become more of a rant to contractors, sorry.

 

[XR250]

I find serviceabilty ends up controlling - more than strength in these designs. Obviously, in my original post, neither were checked.