Gambrel truss design questions

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Old 02-10-14, 08:24 AM
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Gambrel truss design questions

My father has a small (14' x 18') stick framed shed with a gambrel roof. The trusses look like this (this is a design I came up with, just scaled up):



Actually, I wonder if this is really even considered a truss since there is no internal structure to it (no chords or posts, etc...)-maybe it's more of a laminated arch? Anyway, what I'm trying to figure out is what the upper limit of a design such as this would be in terms of span. My father's shed is an Amish built building he's had for about 15 years. The span is 14' and the rafters are 2x4 with 1/2" plywood gussets spaced 24" OC. The gussets are fastened with staples and I don't believe any glue was used. They are also quite small-they cover about 8" out from the joint on each rafter. There is no collar tie or bottom chord-just the rafters and gussets. The rafters are toenailed into the 2x4 top plate on either side. Despite all that, and the fact that right now there is probably about 3500lbs of snow sitting on the roof, it hasn't sagged a bit or pushed the walls out a bit over the last 15 years.

I am planning a simple pole building into which I'd like to eventually put a large storage loft, hence the desire for the gambrel style roof. I'm trying to figure out how much I can span since the difference between putting up a 14' wide pole building and a 20' wide building is not all that much. The plan would be to build each truss on site using a jig and to glue AND nail gussets on both sides-they would be spaced 24" OC. I'm interested to hear from someone who knows more about truss design than me on whether or not these would work. As far as code, around here you can either buy trusses with an engineer's stamp on the drawing or work with the inspector on a homemade design. I've already talked with the inspector who said he doesn't see any issues but would try to read up on it. I suppose I could incorporate some kind of collar tie, but I don't want to lose too much headroom on top. I want to be able to walk down the center of the loft and store things (mostly bins of out of season clothing and holiday decorations) on either side.
 
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Old 03-05-14, 07:14 PM
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The structure in the sketch is not a truss. It's called a rigid frame, and can be made from timber, steel or concrete. All of the connection points are rigid, able to resist applied bending moments without yielding or deflecting.

For your situation, you should hire a local structural engineer, who will gladly perform the engineering calculations necessary to come up with a suitable frame design.
 
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