When building a unit for a beam or a header, are two boards (say (2) 2x12s) stronger if they are fastened together to form a 3″ by 11.25″ unit or does it make a difference if the two boards are separated by a space?
I seems since most of the time you see units fastened together that one woud ASS-U-ME it would be superior, but does any engineer type out there actaully “KNOW”? Not to make and AZZ OUT OF YOU OR ME.
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Replies
It makes no difference, since air has no structural strength. If you added a continuous piece of plywood between that would add something and make the unit 3.5" thick, but it would take calculations to figure the difference. If you really wanted to make a strong member, you'd stack the two and connect them with gussets to make a member 22.5 x 1.5.
When I made the girders for my boat lift to carry the roof I was advised to use 2 2x12s with 2x4s zig zagging between them.
This makes a beam 6.5" wide with pretty good resistance to twisting.
There are bolts about 3' on center along the length to hold it together along with a butt load of 16d nails
What I thought
And yes I understand that the strenght (stiffness) of a beam goes up by the cube of its depth and only proportionally to its width. So yes, a 21 deep beam is a stiff hog.
which is stronger
It make no difference as long as what it is supporting is bearing equally on both menbers.
Interesting comment. Are you an engineer? I ask because at 5;30 this afternoon I had a conversation with my engineer about a beam. The plans called out a flitch plate which in past experience has been steel and seemed like overkill for this beam. He corrected me by telling me the flitch plate should be 1/2" plywood.
Engineer?
No, I'm not a licensed engineer. Just schooling, worked for/with them and been in the construction business for ever. I suppose a plywood spacer could be called a flinch plate, but I wouldn't. The added strength that a 1/2" plywood spacer would add would be minimal even if even if it were properly nailed and glued to the 2x's on each side.
According to the American Plywood Association, only the ply's running parallel with the beam can be considered when doing the calculations. I used to do a lot of strees skined floor & roof panels and box beams. It was a pain doing the calculations on just the thin parallel vereers.
Big difference in lateral & torsional stability
No difference in pure bending, but that can only happen in a lab. Beams can fail by twisting out of plane, and bending across the narrow width. Lateral and torsional stability is a function of the cube of the narrow width. Two boards fastened together is four times stronger in lateral torsional stability than two separate boards.
Stiff vs Bending
It is my understanding that STIFFNESS goes up by the cube of the depth and BENDING RESISTENCE by the square of the depth.
Doubling width doubles either.
So fastening two units together is the same as two separate units holding the same load. It woudl seem that fastneing them together would mke things "stronger" for both measurements. But.....
What does a box beam buy you and how?
Stiff vs bending
You are correct on the stiffness and bending resistance, but that only applies to the pure bending load. But when a beam is loaded, it can becone unstable and twist out of pure bending. The load when this occurs depends on the depth and thickness of the beam, the length, on how the beam is supported and braced. For floor joists, bracing (bridging) is used to prevent this mode of failure. For unbraced beams, the resistance to the twisting is dependant only on the beam geometry, with torsional constant J, being the important factor. (Wikipedia has an article on this)
Box beams increase both the out of plane bending and torsional stiffness by moving the boards farther apart. (the top and bottom have to be properly fastened) making the beam a form of rectangular tube. Box beams typically use plywood for the sides and solid wood for the top and bottom chords, so you can tailor the dimensions of the parts to more exactly match the strength requirements.
Wood Beams
A wood beam (any horizontal member) can fail in one of 3 ways. 1) Sheer. The beam can just break in two. This is usually least likely, but depending on the load distribution can be a deciding factor. Sheer strength depends on the cross sectional area of the beam. 2) Horizontal sheer. When a wood beam is loaded the top fibers are in compression and the bottom fibers are in tension. Somewhere in between the beam is subjected to a tearing force. If it fails it will split longitudinaly. I believe resistance to horizontal sheer is proportional to the beam width and the square of the depth. 3) Deflection. Beams are limited by code to only deflecting 1/360 of their span. Resistance to deflection is proportional to the cube of the beam depth.
Two members nailed together side by side with or without a space between will be stronger than two seperate members, but you really don't get much credit for this added strength.