On Tue, 12 Feb 2013 18:29:22 -0500, Jim Elbrecht
wrote:

Darro wrote:

I'd appreciate advice on the best way to construct a 12-foot wood beam
that is strong, sag-resistant and easy to assemble and disassemble.

Oh c'mon your not telling us the interesting part. Why do you need
to take it apart? Why won't a 14' beam with 21" cut off it work?

Why are you calling it a beam? Is it a shelf?


The plan is to construct the beam with two rows of 2x3 spruce or
hemlock lengths with their sides bolted together to make up a beam
that is 2-1/2 inches in the vertical dimension and 3" in the
horizontal.

Personally, I don't think it can be done. 3" wide-- only 2 1/2"
high- spanning 12' [3"]. With nothing on it, it will sag 6" the first
year. [WAG based on experience]


The total load on the beam would be about 30lbs. and would be
distributed evenly along the full length of the beam.

I say it can't be done. There is a span calculator someplace on
the web. It has been posted here. See what they say you can build
a 12' span with.


I'm particularly interested in knowing how many lengths should be in
each row, how long each length should be and where they should be
positioned in relation to each other. No length can be longer than 8
feet. Any other information that would help me construct a strong and
sag-resistant assemble/disassemble beam of 2x3 lengths would be
welcome as well.


The *only* way I can imagine a board that long and narrow holding
across that span would involve a whole lot of bamboo, epoxy, and
engineering--- and it wouldn't come apart.

Jim
Thanks,
Darro


He's over worried but if it makes him feel better, use a flitch plate.