Rafter Design Question...
Rafter Design Question...
(OP)
My snow load is 55 psf. The building is 16' x 24' and the rafters are going to run parallel to the 24' length. Is below the correct way to size my rafters (spaced 24" o.c.)?
W= 55 psf (24"/12) = 110 lb/ft
M = wl^2/8 = (110 lb/ft x (12^2))/8 = 1980 lb-ft = 23,760 lb-in.
From Table M4.5-3a ASD Bending Member Capacity I follow the No. 2 Cr M' (Repetitive) column down to Spruce-Pine-Fir and see that a 2x10 has a capacity of 23,700 so I would need to use a 2x12 that has a capacity of 31,800.
Is this the correct way to do it? Are there any other steps?
Thanks.
W= 55 psf (24"/12) = 110 lb/ft
M = wl^2/8 = (110 lb/ft x (12^2))/8 = 1980 lb-ft = 23,760 lb-in.
From Table M4.5-3a ASD Bending Member Capacity I follow the No. 2 Cr M' (Repetitive) column down to Spruce-Pine-Fir and see that a 2x10 has a capacity of 23,700 so I would need to use a 2x12 that has a capacity of 31,800.
Is this the correct way to do it? Are there any other steps?
Thanks.






RE: Rafter Design Question...
Is the span 12' or 24' - you never talked about a center bearing but used 12' for the Length. This will reduce max moment somewhat but you would need a 24' piece - not easy to find.
You haven't added in any dead load
You can possibly increase the allowable by 15% for short term loading (snow) if that is not built into the table.
It may be cheaper to stay with the 2x10 at 19.2'' oc or 16'' oc.
RE: Rafter Design Question...
DaveAtkins
RE: Rafter Design Question...
There will be a ridge board at the peak of the roof and the rafters will tie into from each side. Therefore, the span would be 12' correct?
So I should change my load to 55 psf + 10 psf = 65 psf?
How does the 15% short term increase work? If that is not built into the table can I just add 15% to the numbers use the new numbers?
DaveAtkins,
There is another column that has the shear values. I will also be calculating deflection from 5WL^4/384EI.
Thanks for all the help.
RE: Rafter Design Question...
Typically 10 psf is added for roof - plywood+ shingles and 10 psf for ceiling, insulation, lights, etc (sometimes, I use 7 for ceiling). Tile roofs are something else.
Yes - you can add 15% to the table numbers if not included and another 15% (1.15 x 1.15 - not 1.3) if there are 3 or more rafters at 24'' oc or less. Called repetitive bending stress alllowable. Not sure what the table has included.
THAT said - your ridge rafter must now be designed to carry these loads.
And as suggested - you always want to check shear and deflection.
You might want to get a SE involved at this point...
RE: Rafter Design Question...
I assume the ceiling joists will be tied into the bottom of the rafters as you are talking about a tied three hinge arch here. Some call it a collar tie arrangement.
You need to involve a structural engineer...
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Rafter Design Question...
If you have a ridge beam (structural i.e. 6x12 or glulam) then you can size the rafters as you have stated.
If all this seems overwhelming or confusing, get experienced help.
RE: Rafter Design Question...
RE: Rafter Design Question...