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We made a swing set out of 6x6 posts. The horizontal is two 6x6s, bolted together one on top of the other spanning 12'.

Is this strong enough for 3 large kids to swing at same time?

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Assuming you bolted them together correctly, I would trust the swing to hold a Chevy Cavalier during a windstorm. –  maple_shaft Oct 17 '12 at 14:04
    
Hope you glued and bolted, not just bolted! :D –  The Evil Greebo Oct 17 '12 at 14:58
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3 Answers

I'm sure that will be fine.

The American Wood Council has a free load table table on their website.

If you assume a conservative 1400psi allowable bending stress, a single 6x6 with 12' span can support about 2000 lbs. So your double 6x6 can support at least 4000 lbs (possibly more if you do a good job bonding the two members together so that there's no slip between the faces).

If you have 3 swings, you should assume that each one is packed. Let's say each individual swing has 3 kids at 150 lbs each, for a total of 1350 lbs. [You want to design for the absolute worst case. I bet 3 kids can pile onto one swing.] So you're not that close to the swing limit. You could hang your car from those posts and still have a safe structure.

If you take a look at a swing set or jungle gym at a local park, I'm sure you'll find much skimpier designs.

While I'm quite confidant that your double 6x6s will properly support the bending caused by the kids, you should make sure the rest of the structure is adequate. You didn't ask about this in your question so maybe that means you have it all figured out, but you should be careful not to overlook:

  1. The vertical supports. As Michael Karas says, there will be considerable lateral momentum from 3 kids swinging.

  2. The connection between the horizontal piece and its supports. Especially since you seem to be designing this for use by active kids and not just toddlers, you want to make sure that the beam doesn't fall off the supports with a lot of synchronized swinging. Consider metal brackets (e.g. "hurricane ties") to strengthen that joint.

  3. With 3 swings there will be a lot of momentum and I think you're going to want the whole swing set anchored to the ground. You definitely don't want the whole thing tipping over. Consider that a single 6x6x12' post weighs about 100 lbs. You don't want that falling on a kid.

EDIT: As The Evil Greebo points out in the comments below, the swinging motion will create additional force on the beam—as much as 3x the weight, if the kids can get up to 90˚. I'm still more than comfortable with the structure; while it's conceivable that three chubby kids could pile on a single swing, I seriously doubt they could all swing that high in unison.

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3 swings, 3 kids, 150lbs each does add up to 1,350, but that is not the load to consider, because that's a passive, sitting still load. Actively swinging - descending, decelerating and being deflected forward and up - that increases the effective load weight on the board. I don't know how to calculate the effective weight - that's probably a question for Mathematics.stackexchange.com ... –  The Evil Greebo Oct 17 '12 at 14:15
    
Posted a related question here: math.stackexchange.com/questions/215661/… –  The Evil Greebo Oct 17 '12 at 14:19
    
Ok per the related question the approximation for maximum load to use is three times the sitting load on the swing. So take the max load you anticipate (ie: your 450 lbs) * 3 swings * the swinging factor of 3 = a peak swinging load in the vicinity of 4,050 lbs. (with all 3 really fat kids swinging in synch) –  The Evil Greebo Oct 17 '12 at 14:53
    
@TheEvilGreebo: Good point about the swinging. –  Henry Jackson Oct 17 '12 at 15:01
    
I think that should be 2mg, not 3. The tension in the rope is 2mg, and that's the force transferred to the beam. –  Chris Cudmore Oct 17 '12 at 15:54
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The pair of 6x6 timbers spanning the twelve foot distance should be plenty strong to hold up the swings and swingers. I do see a problem with just using a single post for the verticals on each end. Swings can expose a lot of sideways force and so the posts need more support than you have described. You should add diagonal braces as in the picture below where 4x4s may be a suitable size. Another alternative is to use two 6x6 posts that are arranged as an A type frame with a cross bar about a third of the way up from the bottom.

enter image description here enter image description here

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Could you add details on what the maximum load would be? How does the type of wood change the max load? Perhaps there is a link to a table somewhere? –  Pigrew Oct 17 '12 at 4:48
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Yes.

Based on the load charts posted by Henry, a 6x12 beam on a 12' span is at least capable of supporting just over 6,000 pounds.

With all three swings going in synch with a 450 lb person on each, the load is 450*3 per swing, *3 for 3 swings, so: 450*3*3 = 4,050.

Your 6x12 beam over 12' will have 2,000 pounds of force minimum, to spare.

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