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My stubbornness in getting a scissor type truss in a bole barn.

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.

RE: My stubbornness in getting a scissor type truss in a bole barn.

I would say that's a good compromise. On a 60' span you would still need significant truss depth to stand a chance of it working.

RE: My stubbornness in getting a scissor type truss in a bole barn.

Yea. I was thinking more like 2 to 3 feet, but that is adding to the lateral too...

Mike McCann, PE, SE (WA)


RE: My stubbornness in getting a scissor type truss in a bole barn.

With a span that long, adding a sloped BC is difficult. Doesn't take long to get in to problems with horizontal deflection.

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.

With a scissors truss, there is a horizontal reaction as mentioned.
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.

I don't think there's any practical way to resist that much force. Especially on a long span truss. You have to let the truss move.

RE: My stubbornness in getting a scissor type truss in a bole barn.

If you don't mind seeing exposed collar ties, you could still use a scissor truss, and take most of the lateral thrust out of the equation.

Thaidavid

RE: My stubbornness in getting a scissor type truss in a bole barn.

Putting collar ties on a scissor truss is a bad idea.

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.

(OP)
Thanks for the thoughts guys! As we get closer to this build, what we are hovering around is basically a top and bottom chord with COMPLETELY plywooded center sections with 3/4" plywood and sandwiched 2 or even 3 of these trusses together. A few problems arise such as proper attachment of the plywood in which one method discussed is to router a step along in the inside edge of both the bottom and top chord to lock the plywood in.

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.

(OP)
UPDATE: I figured I would simply expand on this post to see if someone can offer a little help with design ideas.

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.

Please post some photos of the construction I would like to see how this works out.

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.

(OP)
A similar construction is already being done by Starwood plywood webs only. Seems my lack of experience in wood structure is showing as there seems to be plenty of data on plywood.

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