What would it take to span this?

[MushCreek]

My living room opening, that is. The house is a cross gable design, with the main gable being 20' wide, and 14' high at the eaves. On each side will be a 4/12 shed roof, one for the bedroom, one for the living room. On the bedroom side, there is a wall for the bathroom and closet which is directly under the upper wall, so support is not an issue. But on the living room side, there is a 15' opening. Keeping this open keeps the living room open without anything in the way. Above that opening is the upper wall, which will be 4' high. It has to support 12' long rafters for the living room roof, as well as the roof for the upper gable. Throw in another 30-40 lbs/sq/ft if I want to use the upstairs (I do).

I'm trying to figure out what the load will be on this, and what it would take to span the opening. I'll be supporting half the load on the 12' living room rafters, since the outboard end sits on the outer wall. I'll also be supporting half the load on the upper rafters, since there will be a ridge beam holding up the center. There's very little snow load in SC, but I like to err on the overbuilt side, so let's use 30 lbs. The living room load would be 1/2 of 12X15, or 90 sq ft. The upper roof would be 1/2 of 10X15, or 75 sq ft. That's 165 sq ft @ 30 lbs, which equals 4950 lbs. Then you've got 1/2 of the upstairs floor in that area, which equals 10X15, or 150 sq ft. We'll go 40# on that, for another 6000 lbs. So you've got 10,950 lbs distributed along that 15' span. Am I in the ball park, math-wise? Going with the minimums, it's down around 6500 lbs total.

Assuming I've got that sorta right- how do I go about figuring out what to span it with? A whopper LVL? I trust I can have my roof truss supplier run the numbers on it, and engineer a beam. I'm just wondering what size the beam might end up being.

[Don_P]

Just got back in from visiting family. It looks like they got about 4" still laying along 11 around Table Rock.
Looks like there's a snake in my woodpile..
This is a calculator for checking simple spans.
http://www.windyhilllogworks.com/Calcs/beamcalc.htm
The design values for regular LVL's are Fb 2900, E 2.0, Fv 285

I've just posted a link to formulas that are available in any elementary text. The design values are from GP's product literature. Hope this helps you do your own design, of course you should have a registered design professional check your work  .

[John Raabe]

Mush:

I just used 11,000 lbs of combined load on a 15' beam span and came up with a 7"x12" glu-lam.

This is a very rough calc but will give you a general size to consider. Your supplier can likely size it for the final plans.

[MushCreek]

Thanks for the help! That doesn't sound too bad- I was envisioning something much bigger. I realized last night after I posted that there won't be any significant second floor loading on the beam. The stairwell is only 4' away, running parallel to the beam. The resulting 4' wide nook will be under the slope of the upper roof, and thus unusable except for storage. The sides of the stairwell run clear down to the basement, so our beam would only be supporting 1/2 of 4X15, or 30 sq ft. of load. Even at 40#, that's only 1200 lbs. instead of the 6000 I originally posted. At 10#, it's only 300 lbs.

The bigger question for me is whether or not my load calcs are correct. I made the assumption that the roof loads would be half of the total area, since the other end of the rafter is supported by either a wall or ridge beam. I'll have to run this by my son, who actually IS an engineer. He gets his Masters in Mechanical Engineering this spring, so he should be able to check my work. Now all I have to do is pin him down.....