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 4×6 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 4×4 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.
Best Answer
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.