My stubbornness in getting a scissor type truss in a bole barn.
My stubbornness in getting a scissor type truss in a bole barn.
(OP)
Still working on designs for a vaulted ceiling design for a pole barn. We are now talking with our local truss company to get a few more ideas as they would know where will will get into expensive trusses.
Right now we have designed to reduce the span to 60ft and targeting a rood pitch of 3:12. They estimate about half the pitch for the interior so about a 1.5:12. I would really like to see more.... closer to a parallel chord BUT I am trying to weight several things here such as build cost, structure, aesthetics, etc.
The main concerns here is as we increase roof pitch, we are causing more problems in the wind and the overall height of the building gets up there. I am curious if there are any smart ideas anyone has to achieve our goals including possibly paneling the trusses?
Or maybe a 3/12 roof with 1.5/12 int seems a very good compromise? Trying to keep the heel at around 12" tall and bearing surface will be on a double header system on int and ext sides on the poles.
Right now we have designed to reduce the span to 60ft and targeting a rood pitch of 3:12. They estimate about half the pitch for the interior so about a 1.5:12. I would really like to see more.... closer to a parallel chord BUT I am trying to weight several things here such as build cost, structure, aesthetics, etc.
The main concerns here is as we increase roof pitch, we are causing more problems in the wind and the overall height of the building gets up there. I am curious if there are any smart ideas anyone has to achieve our goals including possibly paneling the trusses?
Or maybe a 3/12 roof with 1.5/12 int seems a very good compromise? Trying to keep the heel at around 12" tall and bearing surface will be on a double header system on int and ext sides on the poles.





RE: My stubbornness in getting a scissor type truss in a bole barn.
RE: My stubbornness in getting a scissor type truss in a bole barn.
Mike McCann, PE, SE (WA)
RE: My stubbornness in getting a scissor type truss in a bole barn.
You might try putting something like 10 to 12' of flat BC in from each end, and then starting the slope. That should make them less expensive and should reduce the horizontal deflection problem.
RE: My stubbornness in getting a scissor type truss in a bole barn.
How would this be resisted?
The trusses in a pole barn typically bear on a cantilevered wood pole.
The wood pole would be resisting vertical loads and horizontal thrust loads at the top due to wind.
A slide connection would alleviate the load going to the pole, if the horizontal movement could be accounted for and not affect other framing.
RE: My stubbornness in getting a scissor type truss in a bole barn.
RE: My stubbornness in getting a scissor type truss in a bole barn.
Thaidavid
RE: My stubbornness in getting a scissor type truss in a bole barn.
For starters, the scissor trusses don't need it. If they're designed correctly they'll work fine without it.
Collar ties would put stress on scissor trusses in places and ways they weren't designed for, so it might damage the trusses.
RE: My stubbornness in getting a scissor type truss in a bole barn.
Another concern is how to properly merge 2 or 3 trusses without creating load concentrations and getting them to respond properly.
I think there was always the plan of lastly installing some type of collar beam at the top of the bottom chords to assist. There is space up there that is not needed so adding structure to the trusses make more sense.
RE: My stubbornness in getting a scissor type truss in a bole barn.
Indeed, horizontal deflection seems to be driving the design of this truss. With wood poles way up in the air and no structure to assist the deflection, there is effectively no way but to design the truss with very limited deflection.
What I am playing with right now is 2x10 construction but adding a collar tie about half way down. I can see why Ron mentioned issues with this. It does seem to build some load concentrations that I am not a fan of but seems to only get better, the lower I go.
The 60' span truss with 3/1.5:12 pitch would have a ceiling height raised to 3.5ft on the interior. I can accept 2ft of lift before adding the collar tie. This would be an approx collar tie length of 30ft.
I have pretty much taken any idea of using plywood in the webbing out. Too many unknowns in the glue lams of plywood and the shear interactions are unknowns without real testing. I think this will be best designed from tested and proven lumber that we can drive with more concrete data.
The design at a minimum is looking at pairing the trusses and probably spec a urethane glue and bolting configuration.
RE: My stubbornness in getting a scissor type truss in a bole barn.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: My stubbornness in getting a scissor type truss in a bole barn.