I tried this out on the PROS over at the JLC forum, and got some replies, so I thought I would see how you folks would respond.
A little structural exercise.
The attached files show a roof frame with a shed dormer on one side, with multi-ply rafters each side of the dormer, picking up the entire dormer roof load, part of the roof loads above and below the dormer, plus some small wall loads from the above-roof dormer sidewalls. There are no bearing walls under the dormer sides or face, so the rafters need to do all the work.
Rafter span is 15 feet nominally, the house has a 30-foot width. It’s a 12:12 pitch roof, with the dormer roof being half of that at 6:12. Dormer width is 12′-9 1/2″. Walls are 2×6, and the structural ridge is a 2-ply 18-inch LVL. Common rafters are #2 SPF 2x12s.
Are those rafters supporting the dormer doubled 2x12s, tripled 2x12s, or double or tripled LVLs?
Ground snow load is 85 psf, which resolves to a roof design loading of about 66 psf.
Half the span length only of the above and below cripple rafters are contributing to load, because their other end reactions are being picked up by the ridge and wall, respectively. Thus, the roof area being handled by the siderafters is dormer width x 11′-7 1/2″, plus a little negligible amount due to the rake overhangs.
Turn on your calculators, and begin, now!”
And, when you give your answer, tell us what role roof pitch played in the calculations, plus what duration factor was used.
Edited 7/4/2006 11:00 pm ET by Gene_Davis
Replies
"tell us what role roof pitch played in the calculations" :)
I am just trying to see what kind of a structural engineer you are. ;-)
Then some comments about resolving the load into shear forces along the surface and bending forces perpendicular to the surface are in order. Then more comments about beams in tension and compression.Perhaps comments about the end conditions of the rafters.Comments about the effects of the plywood in the dormer.We will wait and see who makes them. :)