Rafter Design Question...

[Jambruins]

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.

 

[MiketheEngineer]

That's pretty much the way to do it but -
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.

 

[DaveAtkins]

Does the Table also account for shear and deflection? I assume it does, but you should be sure.

DaveAtkins

 

[Jambruins]

MiketheEngineer,
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.

 

[MiketheEngineer]

Span would be 12' - but many engineers take it from center of bearing to center of bearing - might save you 6''+/-

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...

 

[msquared48]

"There will be a ridge board at the peak of the roof and the rafters will tie into from each side."

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

 

[RockEngineer]

If you have a ridge board (non-structual i.e.2x12) then your rafters also have a large axial load and you have to worry about spreading your walls or use collar ties as Mike suggested to take out spreading loads. Collar ties if not at the wall level acan add in big moments which complicate your analysis.

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.

 

[Jambruins]

thanks for all the help.

 

[chicopee]

A bit late but I'll try anyway. Also be carefull about differences in elevations whereby the snow laod may no longer be uniform but assume a triangular distribution at the roof discontinuities.