Critique my self engineered beam

[James Suzda]

I’ve always wondered if my idea would work and maybe with this thread I could get an answer.
Would there be an advantage to laminate strips of sheet metal between the 2 by’s when assembling a “engineered” floor joist or in this case a wooden beam? I am of the belief that the tin would never bend if sandwiched and nailed between the wood and thus would add a lot of strength without much additional weight.
Jim

[Brian Walter]

You might want to just do 2 2x10s, as they will be yellow pine. 2x6s will likely be white pine (much softer/weaker), potentially with knots, unless you get treated lumber. I'd bet you get less flex with the 2 2x10s than with 4 2x6s. I'd also make sure you build some sort of "stand" for each end--like a triangular plywood piece to make sure you don't roll your beam at an inopportune moment.

Jason, you are correct that the pair of 2 x10's would be stronger, but you can't piece them together the way he wants to if you only use 2 of them. That's why I suggested using 4 pieces, you can distribute the forces between the laminations better with more pieces. Also, with something that would be 6" wide by 5.5" high, you wouldn't really need to build a stand for the end either.

[Montgomery Scott]

James,

Your idea would work to some degree but probably not as much as you would hope. What you really want is the strongest material on the outer faces to carry compression and tension stresses due to bending. The area in the middle should be designed to carry vertical, horizontal and transverse shear (VQ/I) which is why beams have a thin web and large upper and lower flanges. I don't think that having interlaminar metal members will help much in absorbing transverse shear.

[Jason Roehl]

Ahh..I see what you thought I said/meant/typed, Brian. I really meant two LAYERS of 2x10s vs. 4 LAYERS of 2x6. He does need to fit the pieces in an elevator. 4 chunks of 2x10 glued and screwed together with significant overlap to make a 4x10 beam would be what I envision.

[Brian Walter]

What I was getting at Jason is that your weakest point in the beam would occur at the joint, and if you are only using 2 members, you only have 1/2 of your maximum strength at that point and if you use 4 members and stagger your joints correctly, your weakest point will be 75% of maximum. Obviously you would want to locate your joints to result in the weakest point not occurring in a critical location. In a narrow beam such as he is looking at, it would also be best to use an odd number of laminations, it allows you to stagger your joints more uniformly. All in all, we are way over engineering this rather simple problem, but it does provide a good mental exercise.

[Jason Roehl]

What I'm getting at is that ONE 2x10 is probably sufficient, but he wouldn't be able to get it up to the room or office. Two 2x10s merely allows him the luxury of smaller parts. I've walked on a single 2x10 spanning that distance laid flat. There was a lot of deflection, though! Stand it up, and you won't notice a 200-lb load over a 13' span. Glue and screw 2 of them together, and it may not be quite double strength, but it will be a heck of a lot stronger than a single. The glue and screws go a long ways in overcoming the weakness at any joints.

[Paul Simmel]

James, it’s called a “flitch plate beam”, but tin wouldn’t add any appreciable strength IMO. See these links for further info.

http://en.wikipedia.org/wiki/Flitch_beam

http://www.betterheader.com/

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IMO, Jason is correct. But, going along with the mental exercise, a truss could be constructed with many small internal parts, sandwiched between sections of bb ply skins, all glued up with construction adhesive/screwed. That would be one stiff “beam”.