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COEngineeer (Structural) (OP)
20 Oct 06 11:48
Do you know the minimum before you have to go to flat ceiling?  Can you use scissor truss on 3:12 slope?
MiketheEngineer (Structural)
20 Oct 06 13:04
Depends on span, loading etc. but I have done 3/12 over 1/12
Helpful Member!  CJJS (Structural)
20 Oct 06 13:19
A good rule of thumb is the inside pitch is 1/2 the outside pitch.
RARWOOD (Structural)
20 Oct 06 15:31
The answer to your question is partially a matter of sematic's.  You can have a wide range of truss shapes between a gable truss with a flat bottom chord and a scissor truss.  How about a French 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.
COEngineeer (Structural) (OP)
20 Oct 06 16:22
I just found on the internet that says the slope of top chord minus the bottom chord has to be equal or more than 2:12.  What do you guys think about this guideline?  I found this at:
http://www.mii.com/site/frameset.aspx?siteid=1&langid=1033&main=%2Fpage%2Fopen%2Easp%3Fpid%3D4618
CJJS (Structural)
20 Oct 06 16:36
That guideline is probably more consistent with RARSWC's comments on depth.  I think that there are conditions where that differential could be less than 2.  I have done 3/12 roof pitch with 1.5/12 ceiling pitch trusses for smaller spans and they work just fine.  If you know what the configuaration of the truss is, I would suggest contacting a local truss manufacturer and getting their advice.  The software they use allows them to analyze various configurations fairly quickly.  Good luck.       
Trussme68 (Structural)
4 Nov 06 11:24
Truss manufacturer's advice = Depends on span and loading. We have done 6 over 4.5 to a large degree of success, but very small spans. Some tricks that can help get the ceiling pitch up are raised heels on the trusses, or cantilever trusses instead of overhangs.

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.
structurebeton (Structural)
29 Nov 06 22:29
What about a 50' span, with a 6/12 top chord and 3/12 interior chord pitch?

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?
ctcray (Structural)
30 Nov 06 8:40
On a 50' span 6:12 truss, the overall height of the truss is 25'. You can't ship that, or for that matter, roll it through the presses so it would be built with piggy back trusses....
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.
NakedTestMonkey (Chemical)
2 Dec 06 1:39
It will be a little bit taller than 25' because you will need to have at least a 2x8 member and the associated heel. That leaves you with a rather tall piggy back truss to be installed.

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.

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