glued plywood to use as rafter 1

[COEngineeer]

So I have this curved roof and it looks like the easiest to frame this is to use (2) 3/4" plywood nailed and glued together to create curved rafters. I only need probably 4-5 rafter at this particular location of the roof. What value can I use? Is it conservative to say it is D-F 2x12 #2 (I tell them to cut it 11.25", so it will have the same area as 2x12 so I can use the same hanger).

 

[SlideRuleEra]

Plywood Design Specification, Supplement #5, "Design & Fabrication of All-Plywood Beams". Free APA download, after registering:

http://www.apawood.org/level_c.cfm?content=pub_ply_libmain

http://www.SlideRuleEra.net

 

[SperlingPE]

Not big enough project to consider glulams?

 

[COEngineeer]

It is curved almost like 1/4 circle. Can you order curved glulams? But I think it is still cheaper to just use plywood. Thanks for the link slideruleera!

 

[structuralaggie]

Not sure how the cut is going to come out of the plywood, but only veneers running parallel should be counted on, so around 50% of section? Maybe less. Also, I am guessing less than 8 foot long rafter?

 

[haynewp]

You can get cambered glulams. Check for axial stress due to the curvature, I would also consider glulams instead.

 

[COEngineeer]

its only spanning 5 ft. I will probably be using the same plywood as the floor. What do you mean veneers running parallel? How much can you camber the glulams? Can you camber it almost like 1/4 of a circle? (5ft radius).

 

[haynewp]

5 ft may be a little too short for cambering a glulam. I had a larger span in mind.

 

[Loui1]

Yes, gluelams are curvable....much more than small cambers. Call your local rep, they can get some pretty tight radiuses. Also, look at

http://www.apawood.com,

there are case studies showing curved gluelams.

 

[UcfSE]

The NDS provides design stress values for sheathing in it's supplements. I would check there also.

 

[structuralaggie]

The veneers in plywood are turned 90 degerees for each laminiation. I think the two exterior laminations run parallel with the 8' dimension. A veneer by itself can literally be torn apart by hand perpendicular to grain. I think the glue between the veneers helps some, but I wouldn't count on it to be able to take any tenion (bending). So the section to design for would roughly be 50% of the gross section due to the alternating orientation of the veneers.