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I am making a structure for a swing and I saw this design online. I have used 4x4 fence posts but I am now looking at it and testing it, and feel it is a bit flimsy.

I am going to add braces either side to aid in transfering the load. However, I think I need to beef up the post I was thinking of applying either 5X2 or 6x2 to the structure overall to encase it, gluing and fixing with hardware. Would this help do you think? It would In Effect make the post an 8 X 8. If I used 6X2

The main flex I am seeing when testing is in the main post. I weigh about 15 stone (215lb). It is for the kids but we have a hammock swing that as adults we like to use for extra seating so ideally want to support about 20 stone to be safe so if people this down in the chair it can take it.

Help appreciated.

What I Have

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    "I saw this design online" Yeah, careful with random stuff you find online! This thing is just ill-designed and needlessly under-engineered for the expected load. It basically is trying to mimic the form factor of something made in steel, directly in wood!! That's just doomed to failure most of the time from the start, but especially using meh softwood from the B&Q or wherever. For the load you'd like to be able to support I would actually recommend you ditch this design entirely, a cantilevered arm like this requires major stiffness in the structure, built into it from the get-go. – Graphus Sep 6 '20 at 7:53
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    "ideally want to support about 20 stone" Yes it's not just static loading you have to consider (which you could test out with 50kg bags of gravel or sand) but dynamic loading Even just swinging gently back and forth will put way more force into this that you might guess. And an accidental quick sit-down by someone about your mass could multiply their effective weight by way more than 1/3. – Graphus Sep 6 '20 at 7:57
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    That 20-stone (or, really, more) is going to be affecting the swing at a variety of angles, as well. So, the load is going to be pulling the structure down when at rest. But, as pointed out, those forces are going to vectored nearly 180 degrees around the fulcrum while in use. Over 20-stone of mass may be nearly perpendicular to the main beam while in use. – jdv Sep 6 '20 at 12:17
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    This is more of a DIY, builder question rather than woodworking. The joinery is, at best, going to be metal carriage bolts or lag bolts, and very little carpentry skill is necessary to make what are essentially DIY kits for assembly in your back yard. I'd consider moving this to DIY. For reference: diy.stackexchange.com/search?q=play++structure – jdv Sep 6 '20 at 12:20
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    How deep is the post underground? – Aloysius Defenestrate Sep 6 '20 at 20:21
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The fact that your main post is flexing under load isn’t surprising given how far out the horizontal beam extends. On the up side it tells me the joint between the two is very strong, so, well done with that.

Adding an additional post to your main post (to the side facing or opposite your hammock) would definitely make the design stronger and may be necessary for the structure to survive normal use over time. If you reinforce the main post, it would need to extend under ground to the depth of the current post. Otherwise you are transferring that load to the point where the post is not reinforced and you are likely to see bending there instead.

I have two more two suggestions:

  1. Consider shortening the length of the horizontal beam. This will reduce the force bending your vertical post and also put less strain on your joint. Think about holding a heavy bucket close to your body versus at arms length. At arms length the bucket exerts a more powerful force on you and takes more effort on your part to hold it up.

  2. Also consider adding a brace at a 45 degree angle from the vertical post to the horizontal beam, on the side closest to your hammock. The triangle created is incredibly strong and will make both the vertical post and horizontal beam much more rigid. This is also an old method for creating a rigid structure to hang anything from a sign to a lantern (see the image below).

Take any significant bending/flexing on this structure that supports the weight of a person as a warning sign. Ideally it should be strong enough to survive occasional rough use or even abuse, so I suggest building in a significant safety margin and testing it with a much heavier load, say 400 lbs.

Good luck and whatever you end up doing, let us know how it turns out!

lamp post

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