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

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.

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.

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.

Row 1
8' 4'

Row 2
4' 8'

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

--

dadiOH
____________________________

Winters getting colder? Tired of the rat race?
Taxes out of hand? Maybe just ready for a change?
Check it out... http://www.floridaloghouse.net

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

On Tue, 12 Feb 2013 12:06:22 -0500, "David L. Martel"
wrote:

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.

On Feb 12, 12:56*pm, Darro wrote:
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

On Tue, 12 Feb 2013 12:06:22 -0500, "David L. Martel"



wrote:
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.- Hide quoted text -

- Show quoted text -

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

" 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.

On Tue, 12 Feb 2013 07:34:28 -0800, 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.

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


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. .

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

On Tue, 12 Feb 2013 13:43:49 -0800 (PST), DerbyDad03
wrote:



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.

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.

"Darro" wrote in message
...
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

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

On Tue, 12 Feb 2013 16:36:04 -0800, "Pat" wrote:


"Darro" wrote in message
.. .
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

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

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

On Tue, 12 Feb 2013 17:40:57 -0600, Vic Smith
wrote:

On Tue, 12 Feb 2013 13:43:49 -0800 (PST), DerbyDad03
wrote:



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.

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.

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

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.

Doug wrote:

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

-snip-

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]

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

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

Jim Elbrecht wrote in news:2ivlh8lflr1nip8f2c5qnpgfls130mg47d@
4ax.com:

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

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 . . . .?

Steel. :-)

On Tue, 12 Feb 2013 09:56:49 -0800, Darro
wrote:

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

On Tue, 12 Feb 2013 12:06:22 -0500, "David L. Martel"
wrote:

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.


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

On Tue, 12 Feb 2013 17:40:57 -0600, Vic Smith
wrote:

On Tue, 12 Feb 2013 13:43:49 -0800 (PST), DerbyDad03
wrote:



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.

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.

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

I graduated with a degree in mechanical engineering, and the problem you've got are the joints between the 2X3's. In order for the beam to be strong, you need to carry stresses from one piece of lumber to the next all along the length of the beam. Joints between the pieces of lumber make that a problem and complicate the design and sacrifice portability.

Can you have a 12 foot 2X6 ripped into two pieces, each similar in size to a 2X3? That would eliminate joints between pieces of lumber along the length of the beam.

On Tue, 12 Feb 2013 07:34:28 -0800, 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.

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


I did a quick calculation between yawns g 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" .

Jim Elbrecht wrote:
Darro wrote:

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 say it can't be done.

Yes it can.

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.

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.
http://www.woodbin.com/calcs/sagulator.htm

--

dadiOH
____________________________

Winters getting colder? Tired of the rat race?
Taxes out of hand? Maybe just ready for a change?
Check it out... http://www.floridaloghouse.net

Darro wrote:
On Tue, 12 Feb 2013 16:36:04 -0800, "Pat" wrote:


"Darro" wrote in message
...
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

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

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.

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
____________________________

Winters getting colder? Tired of the rat race?
Taxes out of hand? Maybe just ready for a change?
Check it out... http://www.floridaloghouse.net

DerbyDad03 wrote:
On Feb 12, 12:56 pm, Darro wrote:
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

On Tue, 12 Feb 2013 12:06:22 -0500, "David L. Martel"



wrote:
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.- Hide quoted text -

- Show quoted text -

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

" 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"


Or, he could just use my 2 - 8' lengths and change my 2 - 4' lengths to 2 -
4' 3" lengths


--

dadiOH
____________________________

Winters getting colder? Tired of the rat race?
Taxes out of hand? Maybe just ready for a change?
Check it out... http://www.floridaloghouse.net

"dadiOH" wrote:

Jim Elbrecht wrote:
Darro wrote:

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 say it can't be done.

Yes it can.

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.

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.
http://www.woodbin.com/calcs/sagulator.htm


I won't argue with the calculator-- but I'd love for the OP to give it
a go and get back to us next year.

Jim

On 2/12/2013 4:43 PM, DerbyDad03 wrote:

" 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.

But that would allow perhaps someone to suggest a good alternative to
accomplish the task.

Its always a good idea to state what you are trying to accomplish
because quite often someone will say "did you consider x"

On 2/12/2013 11:56 AM, Darro wrote:
Thanks for your input, Dave.

I need to make up a beam because the actual length is 12 feet, 3
inches....

So, buy a 14 and field fit...(saw it to length, that is)

--

On 2/12/2013 7:00 PM, Darro wrote:
On Tue, 12 Feb 2013 16:36:04 -0800, wrote:
wrote in message
...
....
... beam ... spruce or hemlock ... 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.

....

... Any other information that would help me construct a strong and
sag-resistant assemble/disassemble beam of 2x3 lengths would be
welcome as well.

....


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

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.

....

Since softwoods are generally about 35 lb/cu-ft, your 3x3 (roughly) will
weigh about 25 lb or so while you're talking of only 30 lb load uniform
total load which is only 2.5 lb/ft.

An unsupported 12-ft 2x4 will sag some w/ time on edge and definitely
will laid flat--as somebody else said, you want the long edge down.

Your best plan would be to buy two 2x6 14-s and cut to fit for simplicity.

If you really want the smaller cross section use 2 tubafors and stiffen
'em w/ a 1/2" plywood gusset between them--nail and construction
adhesive or use one of the waterproof glues.

Alternatively, use 1x material and make it a box beam--cut the beam
weight by roughly a third.

But, w/o knowing what it is you're actually trying to do, my initial
response is still to just go get a couple 2x6's, nail 'em together and
cut 'em to length and be done w/ it.

--

On Wed, 13 Feb 2013 08:10:49 -0500, "dadiOH"
wrote:

Jim Elbrecht wrote:
Darro wrote:

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 say it can't be done.

Yes it can.

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.

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.
http://www.woodbin.com/calcs/sagulator.htm


Using Eastern Hemlock, I confirm about 1/2 " total compared to my
earlier manual calculation. Remember to use floating ends
(conservative) and don't forget to include dead load with live load.

On Tue, 12 Feb 2013 21:48:27 -0500, Jim Elbrecht
wrote:

Doug wrote:

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

-snip-

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]

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

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

Metal but in practical terms, neither can I. Normally flitch plates
aren't used for such shallow beams. I merely was making a suggestion
if the OP must do it his way.

dpb wrote:

-snip-

But, w/o knowing what it is you're actually trying to do, my initial
response is still to just go get a couple 2x6's, nail 'em together and
cut 'em to length and be done w/ it.

That's the least I'd use for a 12' unsupported span-- And I'd specify
Doug fir just to make sure.

Jim

On Tuesday, February 12, 2013 6:29:22 PM UTC-5, elbrecht wrote:
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?

My money is on it's a beam for a science, engineering, or shop class. Darro wants us to do his homework for him.

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

On Wed, 13 Feb 2013 09:49:40 -0600, dpb wrote in
Re 12-foot wood beam - How to
construct?:

But, w/o knowing what it is you're actually trying to do, my initial
response is still to just go get a couple 2x6's, nail 'em together and
cut 'em to length and be done w/ it.

Good advice there.

On Wed, 13 Feb 2013 08:21:54 -0800 (PST), wrote:

On Tuesday, February 12, 2013 6:29:22 PM UTC-5, elbrecht wrote:
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?

My money is on it's a beam for a science, engineering, or shop class. Darro wants us to do his homework for him.

That would explain why it has to be easily dismantled-- and he hasn't
seemed concerned if it lasts more than an hour.

I wouldn't fault him [her?] for asking here. We *are* a resource--
sometimes a real good one.

Jim

My money is on it's a beam for a science, engineering, or shop class.
Darro wants us to do his homework for him.

If that were the case, I would think you could make a couple of torsion
boxes (thin plywood skins over internal webbing, like interior door slabs).
Then bolt them together with long brackets on each side.

Light weight, easy to dismantle, and should be able to support 30 pounds
easy enough (if it is evenly distributed as originally described).

Never tried it though. Just a guess for class...

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

On Thursday, February 14, 2013 12:48:13 PM UTC-5, HerHusband wrote:
My money is on it's a beam for a science, engineering, or shop class.

Darro wants us to do his homework for him.
Yup, clearly homework. I'd bet highschool, though, or maybe science for nonmajors.

The problem is trivial as stated, but in the real world there are always constraints - must weigh less than X, must cost less than Y, must resist dynamic loads, etc.

The safest solution is the double 2x6, on edge. If I were the teacher I'd fail that one, so it's safe only in the shop or backyard, not the classroom.

You can make your beam with much less material if you make what is commonly called an I-beam (or W-beam). But if you need to resist torsion, make it a box instead.

He didn't say how much sag was acceptable, did he?

Some sag is inevitable, even if the beam is a solid steel bar.

But it has to handle hardly any load. 30 pounds spread over 12 feet is nothing. and it has to be light, and assemblable.

So, why not an inflatable tube instead? Really impress your teacher by calculating how much pressure you need to inflate it to resist the sag. (build it slightly convex up, obviously)

On Wednesday, February 13, 2013 5:13:07 PM UTC-5, elbrecht wrote:
On Wed, 13 Feb 2013 08:21:54 -0800 (PST), wrote:
My money is on it's a beam for a science, engineering, or shop class. Darro wants us to do his homework for him.

That would explain why it has to be easily dismantled-- and he hasn't
seemed concerned if it lasts more than an hour.

I wouldn't fault him [her?] for asking here. We *are* a resource--
sometimes a real good one.

Yeah, except if he'd been paying attention in class they taught him how to do the relatively simple calculations to figure out what forces are involved and how to figure out how much of what material is necessary.

He's going to have to prove his design on paper to get a grade. How's he gonna do that if we give him the answer?

On Thu, 14 Feb 2013 13:39:40 -0800 (PST), wrote:

On Wednesday, February 13, 2013 5:13:07 PM UTC-5, elbrecht wrote:
On Wed, 13 Feb 2013 08:21:54 -0800 (PST), wrote:
My money is on it's a beam for a science, engineering, or shop class. Darro wants us to do his homework for him.

That would explain why it has to be easily dismantled-- and he hasn't
seemed concerned if it lasts more than an hour.

I wouldn't fault him [her?] for asking here. We *are* a resource--
sometimes a real good one.

Yeah, except if he'd been paying attention in class they taught him how to do the relatively simple calculations to figure out what forces are involved and how to figure out how much of what material is necessary.

He's going to have to prove his design on paper to get a grade. How's he gonna do that if we give him the answer?


Yep. I once saw a guy in college cheat and I was amazed he cheated as
long as he did according to some others. I remember saying to myself
that you are only cheating yourself because once you get into the
office, they'll know within a week or less, if you know your stuff. I
have no idea if he graduated or what became of him thereafter.