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Please help. I made a swing for my kids and now am wondering how safe it is. The swing beam is pressure treated yellow pine 4x6 and 16 feet long. The swing hangers are attached to the 4 inch side of the wood. At one end it sits in a deck bracket on top of a 4x4 attached to a fort. The other end sits on a horizontal beam of an A frame attached with brackets and lag screws.This A frame is placed 12 feet from the fort portion so that the other 4 ft of beam extends past the A frame. There are 3 swing stations and then a trapeze on the far end past the A frame.

My concern is that when my 60lb kid uses the swings I notice "bounce" of the beam. The center of the beam visibly moves though the attached points at fort and a frame do not seem to move at all. Is this safe? Thanks so much.

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  • The deflection or strain is not what you should be worried about. Rather, you should be worried about catastrophic failure of your structure which can occur even without visible strain. In other words, deflection is not a good indication of impending structural failure at all. – alx9r Sep 21 '14 at 21:56
  • 16' is quite the span for swings. Any chance you can add an a-frame support in the center of that to make it two 8' spans? – DA01 Feb 18 '15 at 21:58
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The usual disclaimers apply. I'm not a licensed structural engineer, I'm not providing engineering advice, I'm just reporting what standard load calculations in the literature say, etc....

Assuming no major defects, and using #2 graded southern yellow pine in wet service (the least optomistic numbers), with an actual size of 3.5" x 5.5" (typical of planed "4x6") a 415 lb load at the middle of (aka point loading) the 12 foot span should be safe.

Some deflection is normal. The above load should have a bit less than 1/2" deflection.

http://www.forestryforum.com/members/donp/beamclc_ctrpointload.htm

Uniformly loaded (not really applicable, but "the other extreme" as you interpolate 3 swing positions) 825 lbs, with a deflection a bit more than 1/2" (considered OK for a roof, too much for a floor.)

http://www.forestryforum.com/members/donp/beamclcNDS2.htm

There are some slight complications (about a 20% reduction in loading, IIRC) induced by the cantilever overhang, but none should really be of great concern for normal-szed children. On the third hand, if you wanted to bolt/screw/glue/nail a pair of 2x10 or 2x12, one to either side of the swing beam, that should stiffen it up quite a bit.

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    Those "standard load calculations" you are using are almost certainly for a static load and aren't applicable to the dynamic loading that a person on a swing would subject the beam to. Tables for dynamic load must exist somewhere for there are plenty of wooden bridges that support the dynamic load of cars crossing. The static load calculations you used, however, won't tell you much about failure under dynamic load. – alx9r Sep 21 '14 at 21:45
  • might be worth checking with UW's forest products lab. They quite literally wrote the books on engineering with wood. – keshlam Mar 20 '15 at 22:14
  • Playground equipment often have a much higher factor of safety than most engineered structures -- often 4.0 or 5.0. 1) environmental degradation 2) liability 3) single-point failure (most applicable here). A joist can fail without much harm, but in this case, not so much. – Paul May 19 '15 at 23:36
  • The other concern I have is that the beam is pressure treated. I have no idea how they grade pressure treated wood, but the typical use for pressure treated wood is for ground or masonry contact, not typically a high "bending moment" application. The rejection criteria for knot locations, size and number may be more stringent for a timber intended for use as a beam. If you are getting deflection higher than predicted, then it is likely your fail load is lower than predicted. – Paul May 19 '15 at 23:43
  • @Paul Graded treated lumber is used for deck framing, dock framing, etc. I did not mention "landscape timbers" which you seem to be confusing with graded lumber. Good luck finding a grade stamp on a landscape timber. – Ecnerwal May 20 '15 at 0:56

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