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Old 12-16-2012, 12:11 AM   #31
Join Date: Dec 2008
Location: Fairbanks, AK
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Stacking beams

on a serious note... if i followed daniel's explanation, i ran the numbers for the original question, stacking a 3.5" W x 7.25" H beam atop another one. i have a tributary width of 23.5' and a roof psf of 65, giving a load of 1527 plf. for a beam of 6.7', V = (1527)(6.7)/2 = 5115 lbs. Q = (3.5)(7.25)(3.63) = 92.1. I =((3.5)(7.25)^3)/12 = 111.1

thus, t = (5115)(92.1)/111.1 = 4240 lbs/inch, which => forget nailing anything together to obstruct such a force. did i run the numbers correctly? thanks.

Last edited by jklingel; 12-16-2012 at 12:16 AM.
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Old 12-16-2012, 08:38 AM   #32
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Location: Boston
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Stacking beams

I refers to the moment of inertia of the composite system. In this case, you have two beams, each 7.25 inches deep, so the total depth is 14.5 inches. Therefore I = 889 in^4, not the 111 you computed. The maximum shear flow comes out to be about 529 lbs/in, which is probably too much for nails, but could be handled by bolts.

This member is carrying a lot of load, if you are seriously thinking about installing something like this, you should probably be thinking about a steel beam, not a composite wood beam.
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Old 12-16-2012, 01:37 PM   #33
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Stacking beams

Daniel: Thanks for catching my error and your suggestion of using steel. Steel is a last resort because of the thermal conductivity in our 14,000 heating degree days. A 3 1/2" x 9 1/2" Versalam LVL was engineer-approved for a 7' 8" span, using a 60 psf figure. However, that beam barely fails using the city code of 65 psf, so I will add a 2x10 and 1/2" plywood. There will be double 2x6 doug fir trimmers. Thus, the same "beam" on a 6' 8" span should be good. In 39 years, I have never seen a live load exceed 36 psf (by my crude method of cutting a 1' square column of snow, bagging and weighing it), so the 65 psf (50 live) is pretty extreme. Besides, when the snow starts to pile around 3', we generally get out and shovel roofs, just for kicks. Thankfully, that is rare. Again, I thank you for all the education. I've many times wondered if I should have gone into engineering instead of numbers and Greek symbols.

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