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.  
 

For 80' trusses you should talk directly to the truss manufacturer/engineer about what they can do for you.

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.

An 80 ft scissor profile truss is doable...although you should contact a design professional to assist you. The truss fabrication facilities will have specialized software and access to PE's for this purpose. 3.5/12 upper with 2/12 lower may be a costly roof to design though....

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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C

With those proportions, shipping may pose a problem.

Dik

You will have to construct the truss in two sections and bolt them together in the field regardless of the configuraton.  For this, a steel truss would be more appropriate.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
 

Thanks guys.  Yes, we planned to get an experienced PE involved here but would like to take a couple stabs at this as a learning exercise and to work through other design considerations in the building.  Th trusses would have to be built on site and we have an appropriate plate press but I am thinking this might be outside the range of typical mending plates.

We can raise the pitch to 4/12 but I would really not want to go above that for wind concerns.   

Other than field splicing I WOULD NOT even consider building them by hand.  You will not be able to reconcile a lot of the joint loads without truss plates.  Even a field splice is going to be iffy.

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!!

rough rule of thumb from a mitek jockey-
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.