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I am building an a-frame swing set for my kids using a 4x6 and 4x4 beams. We are using pre-built brackets to attach everything.

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What I am wondering is, is there a max length that the 4x4 can safely be? I want the swing to be as high as possible for the kids, but don't want to compromise safety.

  • Did you mean that some of those are posts? 4x4 aren't suitable for beams carrying children. Use 4x6 oriented vertically for that. Please revise to clarify your design. – isherwood Apr 23 at 16:44
  • @isherwood I have updated the post with the bracket image. Are you saying that 4x4 are not suitable for the down beams? – Nic Hubbard Apr 23 at 16:48
  • So are you asking how tall the posts can be as well as how long the beam can span? Are you seeking to maximize size, or can we just examine a particular design? – isherwood Apr 23 at 16:49
  • Sorry, I don't know the terminology. Beam is about 16'. Hoping to make the posts high for a nice swing arc. – Nic Hubbard Apr 23 at 16:51
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    Also, at some point the concern becomes the height from which the children might fall. I personally wouldn't go past 12' 4x4s in this application even if longer 4x4s could support them. Guidance for the little ones neola.com/lincoln-wv/search/ag/ag7410.htm – UnhandledExcepSean Apr 23 at 17:21
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Yes, a 4x4 that is 20’ long will work. A 4x4 that is 12’ long will support about 4,000 lbs. each (depending on the species and grade). A 16’ 4x4 will support about half that and a 20’ 4x4 will support almost 1,000 each, if its horizontally braced about half way up. (Make the end look like the letter “A”.)

If you get over 12’, I’d worry about the swing “racking” if a couple of 300 lbs. guys start swinging sideways. The metal braces look strong enough, (those are fillet welds which will hold about 1,000 lbs. per inch), but the entire swing set will want to “parallelogram” on you (with enough force). And that “enough force” could come from a couple of drunk buddies celebrating your birthday.

When you exceed the “norm”, you had better be extra safe. I think attorneys call that “an attractive nuisance”.

I’m glad you added that last sentence, “you don’t want to compromise safety”.

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There are two primary points of wood failure in this design: the beam span and post lift. Since the posts are primarily in either tension or compression, they're unlikely to be a failure concern.

However, as they get longer, past say 12', they'll start to impose more diagonal stress on the brackets you've shown than they're intended to tolerate. That's my primary concern. If they fail laterally by weld tears, you're now relying on whatever penetration into the ground you've implemented to hold the thing up.

If you go more than 12 feet above the ground I'd look for a more robust design.

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