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My husband just built our kids a large swing set in the back yard. We have 5 little kids (8 years to newborn). We have lots of kids over to play as well.

The original plans called for a 12’ beam and 3 swings, but we wanted to fit 4. However, my husband sprung for 16’ at the store and planned to install 5 swings.

After seeing the 16’ beam, I expressed concern over its ability to handle 5 big kids. We landed on a solution to cantilever 3’ at one end to shorten the span, but keep the 16’ length.

Well, that wasn’t how it was executed. It is a 16’ span with 5 swings. The set is very stable (just sitting on the ground) but the beam flexes quite a bit.

So my question is: what is the load capacity for a 4x6x16 beam in this situation? We were thinking we’d at least add a 2x4x16 across the top. I’d like it to be safe enough for 5 150 lb kids swinging at once. Is this realistic? I just want to be sure it is safe.

isherwood
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user148908
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    Adding an A-frame brace in the middle would definitely be a good start. Pics with drawn on dimensions of your actual build would be a good addition to your question. – FreeMan Feb 23 '22 at 15:05
  • Agree with @FreeMan .. get rid of middle swing and add additional "A" frame. – JACK Feb 23 '22 at 16:01
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    Wind does not blow down trees. Wind flexes the wood till failure point is reached. Same thing is happening to the beam. – crip659 Feb 23 '22 at 16:18
  • I cannot figure out how to add a photo, or I would. It looks just like the room2zoom link I posted in original post. I used pressure treated pine from Home Depot: 4x4x10 for each leg with 4x4 across each “A” at sides. Then the 16’ 4x6 beam with 5 swings evenly spaced. – user148908 Feb 24 '22 at 01:27
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    5 swings in 16 feet, seems a bit crowded – Jasen Feb 24 '22 at 02:20
  • Welcome. Please take the [tour] to learn how this site works. – isherwood Feb 24 '22 at 13:50
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    @crip659: FYI that isn't necessarily true, wind often does blow down trees, where the ground gives way and the whole tree tips over in one piece. – whatsisname Feb 24 '22 at 16:51
  • Even so, in quite few it is the roots that fail and break. Some cases it is the ground. – crip659 Feb 24 '22 at 16:59
  • @crip659 - wouldn't say that when the roots break it's due to flexing. More like simply being ripped apart longitudinally. – Tim Feb 26 '22 at 12:06

4 Answers4

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With 5 150-lb kids, I'm assuming we're now talking about teenage boys, all swinging together and in phase, your problems will be

  1. Anchoring. You'll need concrete anchors and to tie them to the set with brackets that are just as high quality as the ones at the top.
  2. Rot. Hoping you used treated wood. Even so, bees, ants, squirrels will all beaver away at it (assuming you don't have actual beavers). Keep a close eye on the condition of the wood, especially the beam.
  3. Dynamic Non-vertical forces. The beam may support 750 pounds statically, especially if you re-enforce it with another beam. The top brackets are well designed so that you have solid steel and wood, not screws, supporting the weight. But if the kids are all swinging together, you are now double or triple the vertical force on the beam, and you rely on the tear-out strength of screws to prevent the brackets from coming apart and on unpredictable lateral forces on the beam that might cause it to tear in unpredictable ways, especially when it's older.

IF I wanted a set that is entirely swings (no fort, ladder, etc ... all swings) and for five people (five all swinging at once) I would not do this!. I would purchase a commercial steel set and pour heavy concrete anchors for it.

Even this extremely robust one does not rely on 4 legs and a single span for five swings. And I have no doubt the main beam is strong enough. There are other reasons that this one uses multiple sets of legs and shorter spans.

enter image description here

jay613
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    Thinking ahead to the teenage years is _very_ good advice. Our kids (and their friends) continued to use their steel tube swing set through high school. By middle school, the goal had become to see how far they could tip it, requiring a constant battle of me adding new anchors to counteract their normal "kids" activities. – FreeMan Feb 23 '22 at 15:03
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    Things should be built so that for every one thing that one person can use, two 350 pound drunk idiots should not be able to break it, nor kill themselves trying to do so. – Mazura Feb 23 '22 at 22:29
  • Yes, all my kids are little, but I want it to be safe for those later years and the unexpected things kids do while playing. – user148908 Feb 24 '22 at 01:28
  • Those kids will take it as a challenge to all swing in the same direction and see how far they can bend the beam. This is an insurance claim waiting to happen. – RedSonja Feb 24 '22 at 07:57
  • Worst case may even be more challenging... put a seated 150lb kid and a standing one on the seats and get them pumping up to go for extreme swinging... and all five swings in sync... – houninym Feb 25 '22 at 12:37
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Well, that wasn’t how it was executed. It is a 16’ span with 5 swings.

That's a whole lot of faith in a 4x6x16. What species of wood: pine or cedar?

The set is very stable (just sitting on the ground) but the beam flexes quite a bit.

If you have reservations about safety now then those will really be exacerbated with bigger kids.

Flex isn't just flex, it's microscopic damage to the fibers. After enough flex and weathering that beam will snap one day, period.

Trees can flex because they're alive and will repair themselves. Dead wood flex is irreversible damage.

We were thinking we’d at least add a 2x4x16 across the top.

This could help if you mate it to the 4x6 properly.

You have to think though. Swinging is not just a vertical force, it's back and forth so that's a lot to ask of a 4 inch thickness.

MonkeyZeus
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  • Thank you for your feedback. We used pressure treated pine for all members: 4x4x10 for legs (with 4x4 across the “A” frame), and 4x6x16 beam. If the beam is supported in the center, would that cut down the back and forth pressure? I think we will just use the middle swing for the baby swing. That way we don’t have 5 big kids swinging. – user148908 Feb 24 '22 at 01:36
  • If you put another A-frame support in the center then that would dramatically improve stability on all sides – MonkeyZeus Feb 24 '22 at 12:06
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No. You've added span, which alone increases stress, and you've added load. This exceeds the design intent enough that it's clearly a bad idea.

Since this is almost a no-brainer (no offense intended), I'll leave it at that and focus on possible solutions.

  1. Revert to your original plan. While it does stress the legs and brackets at one end more than was intended, it's a reasonable risk. Just keep an eye on it as you would have even without modification.
  2. Add a third A-frame. This is the most robust of the possible solutions, but obviously involves more effort and expense. The hardware used would appear to make this a fairly easy add. You'd install it off center to allow roughly equal space for each of the five swings.
  3. Sister additional beam members. Don't stack a 2x4 on top. That's a wet noodle on a larger wet noodle. Instead, laminate another member of the same height, like a 2x6. You could even do one on each side. These could stop short of the leg brackets to prevent the need for rebuilding. The basket swing brackets may not work well, so consider using spacers between the original beam and the new member(s) to allow clearance. Pairs of bolts every 24" or so should suffice.
isherwood
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  • No offense taken. Happy to hear ideas for solutions. Is the span the biggest issue in your opinion, or the width of the beam? We’re considering adding a support between swings 2 and 3. Middle swing (#3) will just be a baby swing to cut down on my anxiety. But would adding sistered 2x6 also be a good idea? – user148908 Feb 24 '22 at 01:58
  • I haven't done any elaborate physics calculations. They both play a role. I would use just one of these solutions. A third A-frame _and_ a heavier beam would be excessive, I think. – isherwood Feb 24 '22 at 13:52
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Follow up: my husband added a 2x6 “A” frame slightly off center (2 swings on one side and 3 on the other). Then he leveled it all up and set the 6 legs in concrete. The beam no longer ripples and I feel much better about it. The center swing is reserved for the baby. Thank you for the suggestions!

user148908
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