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I'm planning a build of a back yard jungle gym/mini ninja warrior obstacle course and I'm in a bit of a pickle.

On the one hand, I don't mind spending more for cedar and I prefer the look, however, the availability of dimensional cedar is quite limited. Whereas PT lumber is easily available in multiple sizes.

One key consideration is whether an 8ft span of 2x6 cedar will hold up to being used as the side rails of a monkey bar since that is the largest board Home Depot and my local lumber yard carries. I expect kids and adults to be using this structure.

I've reviewed span calculators, but I'm not sure if it's applicable to a point load like this where, for example, 200lbs could be hanging on two beams at a single point in the center. I'm also not sure if 2x6 cedar is hard/strong enough for this purpose.

In another area of the structure, I will be hanging boards across it like this:

Obstacle

Will an 8ft 2x6 cedar board be sufficient to support an adult?

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For one Western Red Cedar 2x6 plank, loaded vertically, attached to bombproof supports at either end:

The estimated sag is about a quarter inch in the center, and the shear is 1.39 MpA. Materials data gives me an approximate Shear strength of 5.5 MpA, so you're well within limits.

HOWEVER, the weak point will be the attachments. These will need to be rigid in shear and in bending.

As well, you'll need to design the beams each plank is attached to to support the full load.

8 feet is an invitation to overloading.

  • "...the weak point will be the attachments. These will need to be rigid in shear and in bending." Can you expand on this a bit? – Charles Chen Apr 26 '16 at 23:13
  • I'm talking about how you attach the ends of the beams to the supporting structure. Screws will be too brittle, and unable to support the shear force at the end connections. Joist hangers can't support the bending moment at the ends. (The tendency for the top of the beam to pull away when bending into a U shape under load) – Chris Cudmore Apr 27 '16 at 12:52
  • Chris, what would you recommend in this case in terms of joining the members? – Charles Chen Apr 27 '16 at 12:55
  • Good grief. So much certainty. :) "Screws" are a wide range of things, and there are many which are suitable. "Bending moment"? I have a degree heavy in physics, and this simply doesn't apply here. Cedar is soft enough, and sheet metal is flexible enough, that rigid and complex load calculations can go out the window. Even two basic joist hangers will outlast the board itself. Finally, there's a vast range of strengths in a cedar board. It probably wouldn't be difficult to find two seemingly sound examples that have 100% strength variance. – isherwood Apr 27 '16 at 13:18
  • You're neglecting the twist as well. These are monkey bars, which people swing from. Joist hangers aren't designed for torsional loading. Bolts of a sufficient size would support the shear, but in general, screws of common types are considered to be too brittle to support a shear. As for board strength, you are correct. All one can do is take the nominal data. Unless you suggest that OP put each board through testing, and likely destroy them in the process, there's not much you can do about that. – Chris Cudmore Apr 27 '16 at 13:52
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Cedar is softer and slightly weaker than SPF, generally speaking, but is still up to the task. The key is board selection. Cedar is prone to large knots, like SPF. Look for small, tight knots and not very many of them.

When in doubt, double them up. You mention adult usage. Several adults in the middle of an 8' span is pushing the limits of two rails. Consider going with 4.

  • Definitely won't be several; at most 2, but more typically 1. – Charles Chen Apr 26 '16 at 23:12
  • One what? I'm not sure what you're saying. – isherwood Apr 27 '16 at 13:13

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