Tricky wood truss design for pole barn
Tricky wood truss design for pole barn
(OP)
I am working on some designs for a pole barn for my own place. I want additional headroom in an already tricky free span of 80ft. We have pretty much written off a parallel chord due to the way they load. A scissor truss looks better with some really beefy lumber.
I will certainly consider a raised ridge beam so a hybrid design might be a scissor with an additional raised ridge beam. The ideal outside pitch is 3/12 but might have to settle for 4/12. trying to keep that profile down due to wind concerns. Snow will be 25lb target. I don't have the other data in front of me. Roof will be 26ga corrugated metal PBR panel.
These will be designed, then blessed by a PE that is more familiar, then manufactured on site due to the size of them. I realize what I am asking might be difficult and open to discuss ideas here. Just want to maintain approx 2/12 interior pitch for added head space.
I am interested to see where an experienced truss designer would go with this. I have no problem matting trusses together in pairs if required.
I will certainly consider a raised ridge beam so a hybrid design might be a scissor with an additional raised ridge beam. The ideal outside pitch is 3/12 but might have to settle for 4/12. trying to keep that profile down due to wind concerns. Snow will be 25lb target. I don't have the other data in front of me. Roof will be 26ga corrugated metal PBR panel.
These will be designed, then blessed by a PE that is more familiar, then manufactured on site due to the size of them. I realize what I am asking might be difficult and open to discuss ideas here. Just want to maintain approx 2/12 interior pitch for added head space.
I am interested to see where an experienced truss designer would go with this. I have no problem matting trusses together in pairs if required.





RE: Tricky wood truss design for pole barn
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Tricky wood truss design for pole barn
If you are just visualizing shapes, try to keep the heel height of the truss (the distance from top of support to top of truss at the outside edge) as high as possible.
A heel dimension of 2 to 3 feet works very well with large span scissors from my experience, however this does affect the overall architectural look of the building.
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RE: Tricky wood truss design for pole barn
Dik
RE: Tricky wood truss design for pole barn
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Tricky wood truss design for pole barn
We can raise the pitch to 4/12 but I would really not want to go above that for wind concerns.
RE: Tricky wood truss design for pole barn
Also - scissor type trusses induce a lot of horizontal force/deflection to the columns/poles. This will need to be addressed.
Get with a truss mfg. Much cheaper, faster and better designed in the long run!!
Also, at this span - I would look HARD at steel frames!!
RE: Tricky wood truss design for pole barn
1ft of truss depth for each 10ft of truss span. better if this depth starts at the heel of the truss.
a flat girder truss (equal scissor on each side 40ft) with some depth placed at the center under a ridge may be a way to go too. similar rules of thumb apply ( 3 ply is usually good up to 60-70 ft.(nailed) ,4-ply(bolted) beyond that(80ft is pushing it for manufacturing though-maybe a tall steel beam is cheaper).
girders spans are part of another rough rule span estimate .
2-40ft is a lot cheaper than one 80ft and ships well too.
holddown connectors are pretty cheap- why the low pitch? I'd be thinking a church sanctuary clg with room for a mezzanine for storage if needed.
40ft would also get you into lvl (lsl) roof framing in leiu of trusses for most of it.
anyway my.02$
Mike
Murphys law 28th corollary- If there are five ways for something to go wrong, and you circumvent all five, a sixth will promptly develop.
Kwazai's addition- if you circumvent the sixth a seventh will develop, etc. etc.