wood
I'm going to make a swing as a project with my kids. I'm constrained by the timber available and wondering if doing the whole thing with 4 by 4's will be ok rather than a thicker top bar. Or if I could double up on the top bar?
Ten foot high and close to that wide is my plan so far, all legs concreted into the ground 50cm, but I want 2 swings on it, rather than just one.
Will an a-frame bolted together with 4 by 4's be safe? Would I need to double up the top bar?
Unsupported 1x12 over a 5' span will sag (even under its own weight). As an alternative you could get some of these shelf tracks (~$2.25/track)
and some 12" brackets (~$1.30/brace)
and then lay your 1x12's on top to make the shelves. This option also allows you to easily adjust the shelves up and down, to suit your needs.
If you want to make the shelves completely from wood (as you suggest), you'll need to beef up the design and add additional supports.
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.
Best Answer
Yes, two 4x4’s would work fine to support two swings, provided 1) lumber is “Select Structural” grade and 2) fastened so they don’t rotate.
1) Make sure there are no large knot holes so that it is of the highest grade of lumber.
2) The “beam” is strongest when the 4x4’s are stacked on top of each other.
To test my calculations, I’d try testing them out on some short blocks with a couple of children standing/sitting on them.