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Beam Sizing??

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Topic by honeydonot posted 08-17-2013 07:21 AM 2373 views 0 times favorited 3 replies Add to Favorites Watch
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honeydonot

2 posts in 1739 days

08-17-2013 07:21 AM


I am looking to build out the bottom 8’ of the top floor of our cabin.
This is the front of the cabin, that is 2 stories of open vault space (16’ 9” high).
The build out will be 3’ deep x 19’ 7” wide.
I need to size a beam to span the 19’ 7” opening.

The roof of the cabin is supported by 4-3”x12” laminated (spruce or pine beams) 1 on each exterior side wall, and 2 spaced at 8’ centers (so cabin is 24’ wide). The beams are 27’ 4” between the front and back supporting exterior walls. The front has a 36” overhang, also supported on the same beams.
The exterior roof is a laminated asphalt shingle, likely on 3/4” plywood or OSB.
Although it is cold in Alberta, Canada, we don’t get a real heavy wet snow. I would think a snow load rating of 20-30 lb/sf would the maximum.
I have attached a picture of the interior side of the front pf the cabin. The 8’ height being removed, is about 7” down from the bottom of the upper windows. Assuming the height of the beam will need to be greater than 7” (unless it is made of steel), we will have to live with a noticeable beam, that will encroach down into the new opening.
I was figuring on either a 3.5”x12” LVL or 2 sistered 1.8”x12” LVL beams.
Does someone have the means to calculate the actual required beam size?

Thank you for reading the wordy post.



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Dan Lyke

338 posts in 3701 days

08-18-2013 03:14 PM

I think the LVL guys should be able to give you span calculations for their particular product. For solid wood stuff, I’ve always fallen back to the AWC span options calculator (if you poke around that site you’ll find various calculators for difference scenarios), and once or twice I’ve tried to work through the actual beam calculations with the span table data, but…

Anyway, seems to me that you’ve got a 3’ area you’re supporting, but you’re only supporting it on one side, so it’s like you’ve got 18” joist spans. It’s an exterior roof, so a little deflection is fine (this isn’t like a floor joist, or a gypsum board covered ceiling). I tossed in l/240, “rafter, snow load”, 24” spacing, and ended up with Doug Fir (‘cause that’s what’s cheap and available near me) 2×10 select structural or 2×12 #1 or #2 for fairly light roof loadings. So you should definitely be able to do this with gluelam, but part of what you’re paying for with LVL is for them to do the engineering.

-- Dan Lyke, Petaluma California, http://www.flutterby.net/

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honeydonot

2 posts in 1739 days

08-18-2013 04:57 PM

Thanks Dan, but if I am understanding your comments, you are referring to the structure that will be the “add-on”, or the new outside structure.
The span that I am trying to determine the beam size for, is for the new opening that I will be creating in the exterior wall of the cabin. If you look at my pictures, I will essentially be removing the 4 lower windows, and need to support the roof on the front side of the cabin.
I hope that makes it clearer. If there is more info required, please advise.
As far as the LVL guys go here (Alberta, Canada); when I last had to purchase a LVL beam for the last wall removal I did (wall removal on interior of house), the lumber yard engineers would only comment on engineered floor joists, not dimensional lumber or LVL beams.
Thanks again

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Dan Lyke

338 posts in 3701 days

08-18-2013 07:35 PM

Ah, yeah, I’m not the guy to do that calc. I suspect that to pull the building permit you’ll need an engineer’s stamp on the drawings anyway, any chance you can just punt this off to the engineer? They’ll have more experience with this stuff…

-- Dan Lyke, Petaluma California, http://www.flutterby.net/

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