12-foot wood beam - How to construct?

[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.

Thanks, Darro

[David L. Martel]

Darro,

You can buy 3"x3"x12' posts. 30 lbs isn't much of a load. What are you leaving out? I don't see why you want to "make" a beam.

Dave M.

[dadiOH]

Row 1 8' 4'

Row 2 4' 8'

Bolts at 1', 3', 5', 7'

[Darro]

Thanks for your input, Dave.

I need to make up a beam because the actual length is 12 feet, 3 inches. I indicated a length of just 12 feet in an effort to keep responses focussed on the main issue -- the number of lengths and their placement relative to each other.

Darro

[DerbyDad03]

Please do not top post. This is a bottom post group. Thanks!

re: " I indicated a length of just 12 feet in an effort to keep responses focussed on the main issue.."

Problem is, telling us it's 12 feet gets us focused on wondering why someone would "make a beam" that is a standard length. That makes

*that* the main issue for us.

Since we now know why you need to make a beam (it's a non-standard length) it seems like dadiOH's concept ought to work, but you are going to have waste:

2 lengths at 6' 1.5" = 12' 3"

1 length at 6' plus 2 lengths at 3' 1.5" = 12' 3"

I did something similar in SWMBO's garden but I think mine was longer than 12' 3". I'll have to check tonight. I screwed the heck out of it using a couple of hundred screws coming at it from both sides. This spring I'm going to flip it over 'cuz even with all those screws, after 4 years there's a noticible sag. There's no weight on it, it's supported at each end, about 6" from the ends.

Gravity sucks.

[Doug]

With this load, you can do almost anything and get away with it. But in theory, don't splice it in the middle. Bolt it 18 to 24" o.c. .

[Jim Elbrecht]

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?

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]

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.

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

[Vic Smith]

Making such a small diameter wood "beam" without it sagging probably takes more engineering than it might seem to some. That why beam height increases with length. The best bet is to look for the technical specs somewhere. Different woods sag differently. Then you don't get disappointed. Glued plywood construction probably is the way to go for least sag. Just my 2 cents.

[Pat]

Change your plans to steel if you need that small a cross section. Or increase the size if you are going to use wood.

[Darro]

Since I hadn't considered the sagging that will inevitably occur as the wood fibres stretch over time, I'll go back to the drawing board and take a close look at steel alternatives.

My thanks to everyone who took the time to help me out!

Dennis

[Doug]

Not necessary but instead of glue, use a flitch plate with bolts because he wants to be able to undo the beam if necessary.

[Doug]

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

[Jim Elbrecht]

-snip-

Made out of what? Design me this softwood 'beam, 3" wide, 2 1/2" high, with flitch plate- 12' 3" long.

Now you're down to essentially a couple 2x2's with a fitch plate made of . . . .?

I can't see it.

Jim

[Doug Miller]

Jim Elbrecht wrote in news:2ivlh8lflr1nip8f2c5qnpgfls130mg47d@

4ax.com:

Steel. :-)

[Gordon Shumway]

OK, buy a 3 x 3 x 14' and cut off what you don't need. End of story.

[Gordon Shumway]

Why did you use the word "diameter?" I didn't see anyone say anything about "round."

[Doug]

I did a quick calculation between yawns and included the weight of the beams in addition to your 30 pounds. I calculated almost 1/2 " max deflection at mid span for a uniformly loaded beam length of 12' 3" .

[dadiOH]

Yes it can.

They say, using hemlock, that the sag would be 0.009" per foot, 0.11" total. Increasing the load 10X would increase the sag approximately 10X too.

[dadiOH]

Your problem wouldn't be stretching wood fibers, it would be warping. Keep it painted and wood should be no problem. Personally, I wouldn't hesitate using wood to do what you want to do. BTW, if you turn the thing 90 degrees it will be even stronger.

Also BTW, you still haven't said what the thing is for...inquiring minds want to know :)

[dadiOH]

Or, he could just use my 2 - 8' lengths and change my 2 - 4' lengths to 2 -

4' 3" lengths :)