Building a swing set

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Old 01-16-14, 08:23 AM
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Building a swing set

I am attempting to build a swing set from wood I have collected, so I have had to make some modifications from the standard swing set. For a cross-member, I took a 2x4x10' oak board and scabbed on a true 1" x4x10' oak board on either side. I will have a 2' overhang on one side for a baby swing, leaving an approx. 7.5' span, which allows for 3 swings. Will this type of cross-member be sufficient? I figured it's something I should know before I attach it! Thanks!
 
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Old 01-16-14, 12:36 PM
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Welcome to the forum.

First things first--unless done properly, I think it could be a mistake, trying to attach 3 separate boards to make a single unit. Both the effects of weather and constant swing torque reactions will always be working, trying to separate the 3 members. And once you start drilling vertical eye-bolt holes (for the swing chains) in the center 2 x 4, you will certainly weaken it, possibly to the point of failure.

The last thing you need is total collapse while your kids are using it.
 
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Old 01-16-14, 01:07 PM
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I did think through the fact that I couldn't drill through this for an eye bolt. The alternative I was thinking would encompass the entire piece, distributing the weight and not threatening the integrity of the boards. The previous posts I read explained the importance of using 2-2x6 boards, but I'm assuming that is pine. I didn't know if the oak would compensate for the 2 inch board loss.
 
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Old 01-16-14, 02:26 PM
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Perhaps you could be more forthcoming on how you plan to attach the chains, and what you will do to mitigate weathering on the assembly.
 
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Old 01-16-14, 03:02 PM
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My thought was to wrap a separate piece of chain around the beam and then attach the swing to the established chain with an s-hook. I wouldn't want the chain to rub or move, so I was thinking of using some heavy grade staples to keep it in place.
As to the weathering, this is really my first time using oak outdoors, but I assumed due to it's density it would weather well without much shrinkage. I was planning on staining to protect from the elements. I do understand the issues with water getting in and then possibly freezing between the boards, etc. But I didn't think that would be an issue to its strength any more than it would the rest of the structure. Feel free to let me know if you think this is the worst idea ever, that's why I'm here!
 
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Old 01-16-14, 03:38 PM
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I would not use anything smaller than a Double PT 2x6 and optimally not smaller than a Double PT 2x8 bolted together. Your oak is not going to hold up in the long run and the forces on the main beam will be too great for a 2x4. Main "V" posts should be 4x4 and secured into the ground with metal garden stakes Driven almost completely underground that you can screw through to stabilize. I had one built to the above specs and it still scared me when a larger kid got to swinging on it.

Easiest way to gauge what you should have is to read the lumber requirements on the kits they sell at the box stores. They sell you the hardware and brackets and you supply the lumber.
 
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Old 01-16-14, 04:45 PM
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I wouldn't build something outside unless it was PT wood or one of the rot resistant species like Redwood or Cedar. Oak is not going to hold up outside.
 
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Old 01-16-14, 04:46 PM
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A member's Section Modulus is a good measure of its ability to withstand bending stresses. A double 2 x 6 will have a Section Modulus of 15.13 C.I., while a double 2 x 8 will have a Section Modulus of 26.28 C.I. Interestingly enough, any member's S.M. is purely related to the geometric configuration of the member, and not what it's composed of. Oak, pine, steel--they will all have the exact same S.M. if the shapes are exactly the same. The difference in service load resistance comes from the inherent allowable bending stresses each material can handle.

The OP's configuration will have a Section Modulus of just 6.13 C.I. Hopefully, the differences between the foregoing numbers should tell him something.
 
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Old 01-16-14, 07:02 PM
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Thanks for the input all!
Sorry, I'm going to need a little more explanation on definitions. What is C.I. and S.M. and how is it found? I would like to better understand this for the next time I'm dealing with load-bearing structures.
I did find that oak's bending strength perpendicular to the grain is almost twice that of white pine...for whatever that's worth
I am using 4x4 posts for the legs, untreated. Again, this is lumber I have collected and am looking to build with what I have. I know PT is usually the way to go, but will a good stain/sealer protect untreated oak as well as untreated pine?
Lastly, is there a formula or rule-of-thumb regarding the distance spanned? I figured there would be something out there that would allow me to throw in the span, length of pendulum (swing), and weight, to show me what type of structural strength I would need based on the force exerted.

Obviously if there's a safety risk, I won't build it. I'm just trying to see how I can determine what is safe and what isn't...
 
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Old 01-16-14, 08:06 PM
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S.M. = Section Modulus. It's the result of dividing the Moment of Inertia (I.) by the distance between the neutral axis and exterior bending fibers (c.).

C.I. = Cubic Inches. The measure of units of Section Modulus.

I suspect you already know that what you're planning is foolish and potentially dangerous. And, no, there is no magic formula to make you a practicing engineer. Four years at a good engineering school is where you need to go for that, followed by four more years of active practice working under a P.E., at which time you're eligible to take an 8 hour P.E. exam.
 
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Old 01-17-14, 05:23 AM
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will a good stain/sealer protect untreated oak as well as untreated pine?
How well untreated wood holds up to the elements depends a lot on it's exposure. Paint/stain helps but it can only do so much. Siding fairs well because it doesn't have ground contact and sheds water easily. Your posts will wick up moisture from the ground, the top of the beam will also hold water. This will be detrimental to both the wood and the paint job. Years ago I built an above ground pool deck with untreated wood because it was free. I thought a good oil stain would protect it but within a year there were boards that needed replacing.
 
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