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I'm planning a swing set for 4 children. I'm considering using a 4x6 beam (with the 6" dimension placed vertically) to support the swings over a span of about 15 feet with a center support. What is the breaking point and should I opt for 6x6 or bigger?

isherwood
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Nick
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    It's not as simple as its dead-weight carrying capacity. Swings introduce a twisting and bending motion. How are the swings to be attached? A large hole drilled through the beam? How will the beam be fixed at the ends? – spuck Mar 22 '23 at 15:34
  • For things that don't have to fly, you design for typically 1/6 of the "breaking point" or less (safety factor - arguably important for things your kids will use) and acceptable deflection may limit the loading further than that. – Ecnerwal Mar 22 '23 at 15:47
  • I've rethought my design and am now looking at 2 4x6's laminated together. Maybe have an eye bolt sandwiched between them with a carriage bolt going all the through both pieces and the eye bolt? – Nick Mar 22 '23 at 17:26
  • The bolt I'm picturing being sandwiched doesn't seem to exist. Hold that thought – Nick Mar 22 '23 at 17:28
  • Note that when carrying weight, it's more the height of the beam than the width (though they're usually proportional) that's important. Having an 8x6" beam with the 6" vertical won't carry more weight than a 4x6" beam. It may well resist the twisting & flex better than a 4x6" though. – FreeMan Mar 22 '23 at 17:37
  • @FreeMan noted. But I don't think I want to change the design on account of that. Finding a hardware solution to mount the swings that I'm comfortable with might do it though – Nick Mar 22 '23 at 18:52
  • @FreeMan, your claim about the 8x6 being equivalent to the 4x6 is false. It's capacity won't be double, but it'll be a lot more. As a thought experiment, compare a 1x6 to the aforementioned 4x6. The situation becomes much more clear. – isherwood Mar 22 '23 at 21:07

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4x6 is commonly provided with swing set kits for residential use (and you should be referencing them for your design--they're tested and approved). As long as:

  • You keep to two swings per span
  • Your spans don't get much longer than you're planning
  • It's a quality specimen with small knots and no major checks
  • You use it in the proper upright orientation
  • It's fastened well to the leg assembly
  • You don't expect drunk men to try it out in vigorous fashion

... it'll be fine.

isherwood
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