Beam Strength

I am in the process of designing a playset for my kids. I am going to have a 12 foot span that will support a tube (tube will be on top of the supports) as well as have two swings attached to one of the two beams. I am having trouble finding a 4x6 beam that is long enough. Would 2 2x6s (or 2x8s) glued and bolted together be as strong as 1 4x6 assuming that they are mounted vertically?
Thanks
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Find a commercially-available unit similar to what you intend to build and buy the replacement (as though you owned one) beam.
On 5 Jun 2004 11:30:29 -0700, snipped-for-privacy@kmfms.com (Ringo) wrote:

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Confused here... When you say you will have a 12 foot span that will support a tube, what do you mean? It seems you cannot mean that the tube will span 12 feet from upright to upright, since you're also talking about having two swings attached to one of the "beams".
- What do you mean when you use the term "beam"? Are you talking about uprights or about horizontal elements of this structure? - Can you either draw an ascii art picture of what you're doing (ugh!) or preferably, post a line art drawing on the binaries page and give us a heads up after you do so?
Absent those pieces of information, a simple answer to your question is that for all practical purposes, you can expect a near same strength out of two 2x6's nailed together as out of a single 4x6. Remember that you won't really have the same dimensions since two 2x6's will only be 3 inches wide and a 4x6 is 3 1/2 inches. That said, you'll realize a near enough equal strength by putting two together. You don't have to go crazy with glues and bolts, unless you really want to. Spiking the 2x6's together with 12's driven in on a slight angle will hold them together very nicely.
--
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K... I need to be more clear...
There are two tower type of structures. They will be connected by the tube which will be perfectly horizontal and about 8 feet off the ground. The 2 beams I am talking about will be horizontal and will be holding the weight of the tube (the tube will be mounted on top of them). In addition, the front beam (closer to you if you were looking at it in front of you) will support the two swings.
The thickness is something I don't care about. I do care about sagging over time and the ability to hold the weight of 2 kids in the tube. There was a mention of loads in various directions...Since the swings will be attached to one of the beams, that one will be required to take loads in many directions and the other will not.
BTW...what epoxy?
Thanks..
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OK - I think I'm seeing it in my feeble mind now. So tell me - how will the tube attach to the two beams? If it bolts to them or screws to them in some manner, it then becomes a structural part of the assembly and will add rigidity to the whole thing. That's just an added plus in the scheme of things. Think of a stud wall, and the way that sheetrock - a relatively crumbly material, adds strength and rigidity when screwed or nailed in place. Even if you don't rely on just the tube for this, you can always run a stringer or two or three between the two beams and suddenly find yourself the proud owner of a very rigid, very strong assembly.
Again, assuming either some stringers or using the tube as the web between the two beams, you have plenty of strength in a simply nailed together construction of two 2x6's per beam. You can over engineer even the simplist of things...
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I'd not consider the tube as part of the structure. It's flexible compared to the wood.

If there are two beams, putting a brace between them is a good idea. If the brace is also 2x6 (or 2x8) and is solidly connected. you'll not have to worry much about lateral loads - at least in the difference between two 2x and one 4x.
Mike
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wrote:

some
compared
Even if the tube is more flexible - to a limit, of course, it will still bring rigidity and strength to the overall structure since it serves to unite the structure. That's really what is happening when we bridge things or sheet them as we do with sheetrock - we're uniting the structure and that increases the overall strength and rigidity by large amounts. Notice the clever use of the phrase "large amounts" which is a clear give away that I have absolutely no clue exactly how much...
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I agree - but since it's an unknown, ignore it and deal with the wood alone. If the wood works, the plastic is a bonus. If the plastic is included and doesn't work like you expect, you lose.
When you calculate the strength of a wood building, you don't usually include the capacity of the drywall for the same reason. It's real contribution is due to the quality of installation and that isn't standardized like wood construction is. In fact, non-load bearing walls are not included in determining the strength of a building, though they can contribute a lot.
Mike
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wrote:

in situations where structural integrity is critical, all kinds of stuff can sometimes be engineered in. Sometimes the structural engineers will call for glued drywall, for instance.
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On 5 Jun 2004 11:30:29 -0700, snipped-for-privacy@kmfms.com (Ringo) wrote:

if it's standing vertically it's a post, not a beam.
2 2x6 will be about as strong as a 4x6 when used as a post (vertical load, not deflection)
2 2x6 will not be as strong as a 4x6 when used as a beam.
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(Ringo) wrote:

although if done right (flattened, glued, and bolted), it will be almost as strong...
too much work though.
randy
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On 5-Jun-2004, snipped-for-privacy@kmfms.com (Ringo) wrote:

Two 2x6s - but for the minor difference in thickness, yes. Two 2x8s would be stronger. You don't need to glue and bolt them together unless you anticipate lateral loads as well as vertical. If the tube is a slide (i.e. it slopes down to the ground) there will be a lateral load. Swings will generate a lateral load as well. In that case, you'd have to provide the correct number of evenly spaced nails/bolts to enable the two to act effectively as one. The number required depends on the lateral load. If you use glue (epoxy if this is an outdoor playset) you may be able to skip the nails/bolts.
Mike
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(Ringo) wrote:

I am NOT an engineer and am usually clueless in these matters, but it seems to me that if you include a piece of 1/2" plywood sandwiched between those 2x6s (I assume epoxy glued and nailed) you will get a resulting beam that is stronger than the 4x6 and the proper thickness which will be a benefit if you are trying to match a plan.
Dave Hall
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snipped-for-privacy@nhsd.k2.pa.us (David Hall) wrote:

From a structural point of view, I suspect you're correct. But, from a practical point of view, you would then have a piece of plywood exposed edge-up to the elements. Unless you live someplace where it never rains, I would think the plywood would not last very long before it delaminates. Even exterior plywood.
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(Ringo) wrote:

4x6
would
(i.e.
will
the
to
It can't delaminate while sandwiched in between two 2x6's. A little something to seal the edge and you'd be off and running. I'd think the lamination might be a bit of overkill for the project as I understand it though.
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Yes. The strength of the two beams oriented that (long dimension vertical) way are additive. You don't need to glue unless you want to.
RB
Ringo wrote:

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Ringo writes:

> Would 2 2x6s (or 2x8s) glued and bolted together be as strong as 1 4x6

The section modulus of a beam determines it's ability to handle a bending load such as you describe.
The math for computing section modulus is as follows:
Z = ((b*h^3)/12)/h/2, where:
b = base dimension of cross section h = height dimension of cross section.
As you can see, increasing the height dimension pays big strength dividends.
For your application, I'd use a pair of common construction grade 2x10 timbers bolted together, glue optional unless it is resorcinol.
Forget the 2x6's and the 2x8's are marginal.
HTH
--
Lew

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A single 2x8 will be 18% stronger (less bending) than a 4x6, or 2-2x6s. Remember that bending varies directly with the width of the member and as the cube of the height of the member.
RB
Ringo wrote:

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Assuming vertical loads only. A 2x8 will be somewhat weaker than a 4x6 or two 2x6 if lateral loads are involved.
Mike
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