Hi Darro,

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

Sounds like an interesting situation, but there's not much information to
go on.

1. What is the load being supported?

2. Why is the height limited to 2-1/2 inches?

3. Why can't the lengths be longer than 8 feet?

4. Why can't you use a single 12' board instead?

5. Does it need to be a square beam, or can a round beam work?

6. Why does it need to be disassembled?

A 2-1/2" tall wood beam of any width isn't going to carry much weight,
and will probably sag under it's own weight over a 12' span.

Without knowing the intended purpose it is difficult to offer an
appropriate solution. Based solely on the limited information you
provided, there are a few solutions I can think of:

1. A single 12' long, 3" round, steel pipe. You could probably gain the
extra 3 inch length with appropriate brackets at each end. And you could
take the pipe down when needed. It's going to be heavy though. You might
look for thin wall steel conduit, but I don't recall seeing lengths over
10' unless you want to cut down a 20 footer.

2. A steel cable. If you have solid anchors at each end you could stretch
a cable between them, probably with turnbuckles to get it nice and tight.
You may get some sag under load, but it's easy to install and take down,
and can easily support 30 pounds over that distance. Of course, this is
really only an option for loads that "hang". Clothes, hanging flower
pots, etc.

3. A cable supported beam. Similar to a suspension bridge, you would have
a cable anchored at each end with the "beam" essentially hanging from the
cable. Of course, the anchor points on each end would have to be higher
than the beam and you would need midpoint supports between the beam and
suspension cable.

4. The obvious. A temporary support post at mid-span. Grab a couple of
6' boards, anchor at each end and the other ends meet over the temporary
center post. You would probably have to build some kind of bracket or
have a hole you could drop the post into so the whole assembly doesn't
fall over sideways, but it's doable.

Clearly, we can come up with solutions all day to theoretical problems,
but they won't be of much help if we don't know the intended goal.

Good luck!

Anthony Watson
Mountain Software
www.mountain-software.com/about.htm