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Large span "free span" or scissor truss design.

Large span "free span" or scissor truss design.

Large span "free span" or scissor truss design.

(OP)
I am working with my brother which is also a structures PE but this is not an area of specialty for either of us. However, these roof trusses are for our private barn and seems a great opportunity and alleviates the issue of transporting large trusses since we will build onsite and erect.

Overall design goal is a 70-80ft free span (size of building dependent on truss) BUT with a lifted cross chord for a vaulted or scissor style to open up head room.

Using software, is is certainly not difficult to design something that will meet the requirements BUT being smart in design in using economical materials is where I am getting in trouble. I can sandwich 2x12" until I have what I need BUT probably end up costing more than a metal truss.


We are also considering a hybrid truss with "some" steel in it.

Pitch is targeted at 3-3.5:12 but will go to 4:12 if design requires it.


I know that is not much to go on yet but would like to discuss with everyday designers to get ideas here.

RE: Large span "free span" or scissor truss design.

I've been designing wood roof trusses for about 30 years, and have done many long span trusses.

My first concern is always erection bracing. Trusses over 60' long are very unstable until the plywood or metal is on the roof. They require a ton of bracing and a lot of care to install.

Deflection is also a concern on long span trusses. Truss designers typically use L/360 for live load and L/240 for total load deflection. (Around here anyway, and I think it's widely used) On an 80' truss that's 4" of allowable deflection.

If you're trying to keep costs down I'd drop the idea of a vaulted truss. They just don't work well on long span trusses. In addition to vertical deflection, you have horizontal deflection issues.

If you have a truss manufacturer around, I'd encourage you to stop by or call them and discuss your ideas. Some places are limited in the spans/heights they can build in one piece. Some will have designers who are willing to put out some effort and try to help you. Others will be somewhat less than helpful.

They'll know local codes and things like that. I can't help you there.

RE: Large span "free span" or scissor truss design.

(OP)
I appreciate the advice on the designs. I am afraid the vaulting or at least some type of lifted ceiling design would be a primary design parameter. Enough so that we would consider either adding steel or constructing complete from steel.

I can see your point and concerns on the horizontal deflection and getting proper wall loading to the adjacent wall for wind will be important.


What would you think of a lifted ceiling but still with a horizontal chord? (not sure what you would call that)


I also hear you on erection bracing. I already noticed that as a big issue. I figured we would need to fab a custom spreader bar and supports to stand them. Certainly on a low wind day!!!

RE: Large span "free span" or scissor truss design.

Could you build the sidewalls higher rather than doing a scissor truss? Might be less expensive overall.

You asked about a "lifted ceiling but still with a horizontal chord." That would work better than a scissor truss. But I doubt you could get much height out of it.

RE: Large span "free span" or scissor truss design.

"lifted ceiling but still with a horizontal chord? (not sure what you would call that) "


That would be a coffered ceiling.

RE: Large span "free span" or scissor truss design.

(OP)
Aellc, that is dead on! Thanks!

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