Vaulted wood ceiling, no ridge beam, no trusses, HOW?!

[jplay2519]

Went out to review a structure that has a vaulted ceiling, 7 degree slope (1.5:12 approximately) 4x8 beams 6'-0" on center, 2x6 t&g diaphragm, 28' long (wall to wall) with 1' overhang at exterior walls, 18' wide (three 6' bays). So I'm already looking at this like it's crazy but the main issue is the two center beams (6' apart plan east-west) are identical but the one on the left has twisted (lateral torsional buckling maybe) and the one on the right is just fine. They both are built the same, the beams meet at the ridge and appear to "lean" on each other to stay up. The owner did inform me that th one on the left that has buckled is the location of the decking joints. This means of the 18' width the first two bays are continuous 2x6 t&g 12' long and they stop at the left beam then a single 6' span to the end of the roof. I was saying maybe they didn't attach the decking to the beam correctly, but the whole beams leaning against each other to keep from dropping is not working in my mind, it's just not something i've designed or seen designed or seen any engineer be like "well technically this could work". No ridge beam, no rafter ties mean collapse to me, but this thing has been standing like that for more than 5 decades. Anybody know anything about setting up supports like that?

 

[TLHS]

I've seen this before. From a stablity standpoint, my assumption was that it was getting the lateral restraint that a tie would provide from somewhere else. Diaphragm action to the endwalls is a good candidate. If there are walls at 90 degrees to the outside walls at moderate intervals, that could also help with the exterior wall's ability to resist spreading, or act as an intermediate support for a roof diaphragm.

 

[TLHS]

That's not to say that it was designed with that intention or that it's a good idea, just that it was my assumption regarding how the loads were resolving in the as-built condition to result in a stable system.

 

[MIStructE_IRE]

We designed something like this before (on purpose!!). The roof basically consisted of pitched rafters with a ply diaphragm on top of them. This diaphragm spanned laterally from gable to gable to resolve the horizontal components (the spread) while the rafters themselves took the vertical component. Shear interface connection between top of rafter and underside of deck is critical and the diaphragm must be suitably anchored to each of the gables. Worked well but required thorough detailing and comprehensive design.

 

[OldBldgGuy]

I would guess that the 4x8 beam twisting is simply a result of drying and shrinkage in a less than ideal lumber member, not torsional buckling. People in general (and some engineers & most building inspectors in particular) put way too much faith in ridge beams. The absence of a ridge beam is not a reason to question roof framing, and the presence of a long ridge beam in a design is often a case of misplaced optimism. As MIStructE says, the diaphragm of the roof can handle quite a bit, and even though your roof is very flat I would suspect there are mechanisms in it that you're not immediately seeing. How stiff are the walls? Is there plywood on top of the T&G that you can't see? etc.

 

[jplay2519]

Thanks for your input. The stiffness of the walls and diaphragm makes sense. i'm not sure what is above the t&g, could be plywood, they are replacing roofing so this could be a chance to review what is there. If the beam has twisted due to drying and shrinkage what kind of repair would be specified? Can't very well twist it back, replacement maybe.