Is a 4x6x16 swing set beam capable of carrying five swinging children?

Question

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.

Answer 1

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.

Answer 2

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?

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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.

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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.

Answer 3

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.

Answer 4

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!