Minimum slope for scissor truss?
Minimum slope for scissor truss?
(OP)
Do you know the minimum before you have to go to flat ceiling? Can you use scissor truss on 3:12 slope?
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Minimum slope for scissor truss?
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RE: Minimum slope for scissor truss?
RE: Minimum slope for scissor truss?
RE: Minimum slope for scissor truss?
When I think of a scissor truss generally I evision a truss with a steep top chord 8/12, 12/12 ect. Often times the truss has two top chords, and two intersecting bottom chords that extend to the top chord and a single vertical center web.
With scissor trusses that have steep top chords CJSchwartz's rule of thumb is a good guideline.
The important thing in any truss design is having enough truss depth. CJSchwarz's rule is based on that concept. When I've designed scissor trusses the bigest problem I've faced is getting enough depth. It seems architects really like to push up the bottom chord slope.
The AITC recommendation for triangular or pitched trusses is a depth to span ration of 1/6 or greater.
RE: Minimum slope for scissor truss?
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RE: Minimum slope for scissor truss?
RE: Minimum slope for scissor truss?
We can do a 28' scissor at 3:12 over 1.5:12. 30' and that is pushing it. With 24" cants, we can do a 28' 3:12 over 2:12.
The major key to a lot of scissors is the horizontal displacement a lot or architects do not take into account. We can design a 8:12 over 6:12 all day long, but the same 28' truss will have a 1.4" horizontal displacement equal to 339 lbs. per truss. I doubt a 40' long 2x6 wall @ 9'4" tall can handle this without help.
RE: Minimum slope for scissor truss?
I thought that with scissor trusses, the wall is laterally braced at the top for horizontal displacement caused by the kick-out during erection, and when all the roof decking is installed, the braces are removed. Don't Simpson has a special connectin that allows for lateral displacement at bearing locations?
RE: Minimum slope for scissor truss?
The problem now is where to piggy back the truss since your 3:12 ceiling is 12.5' high at it's ridge.
I'd be looking at some sort of ridge beam or girder truss along the ridge supporting rafter framing or side mono trusses.
RE: Minimum slope for scissor truss?
I vote steel ridge beam and running monos down the span. A 6:12 / 3:12 25' mono can be made out of significantly cheaper material at longer spacings. It will also cut down on the size of the crane and setup time on site.
The issue then is getting the foundation and wall to live with the point loads from the beam.