Help - is my swingset overspanned?


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That's why I wrote the general conclusion to the picture in the handwriting you envy.

Basically just the mechanics of WHY his swing set will be ok.

 

you do have nice writing.  A little girly, but nice.  

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Sam - are you an engineer?  The greek you put up there sounded very convincing, I'm just not sure what it means ;-).  I did understand the compression verses tension... and that was encouraging... how confident do you feel that those 2 4x4s are sufficient, because I've gotten a lot of comments on this forum and one other one that are making me nervous about those (http://lumberjocks.com/topics/83618)

 

Also, as far as the pergola option versus the diagonal kickers from the posts to the crossbeam, why do you like the pergola idea better? or should i do both?  

 

Thanks yet again!

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Sam - sorry just realized I didn't have the diagonal kickers mentioned in my last post... so let me revise my question...

 

would you do both (1) the pergola option, AND (2) the diagonal kickers from the posts to the crossbeam (shortening the unsupported span from 20feet to about 11 feet)?

 

And if yes to #2, would it be sufficient to just do the kickers as two 2x6s going from the center of the two 4x4s to the two 2x6s that currently form the crossbeam.  (In other words, NOT also doing diagonal kickers to the pergola parts that I would be adding.

 

Make sense?

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Sam - are you an engineer?  The greek you put up there sounded very convincing, I'm just not sure what it means ;-).  I did understand the compression verses tension... and that was encouraging... how confident do you feel that those 2 4x4s are sufficient, because I've gotten a lot of comments on this forum and one other one that are making me nervous about those (http://lumberjocks.com/topics/83618)

 

Also, as far as the pergola option versus the diagonal kickers from the posts to the crossbeam, why do you like the pergola idea better? or should i do both?  

 

Thanks yet again!

 

Not yet. But the degree is in the works.

 

Basically, the diagram is to show more thought went into it other than, "yeah, I think that's good" or "no, that's too weak" - not saying that anyone here did that, but a picture always helps to describe a scenario.

 

My level of confidence for those 4x4 posts, on a scale of 1-10, is about a 9. Make it a 10 by even putting a 4x4 in the gap top to bottom. You will have a solid 4x12. I don't care if you have the cast of the biggest loser on your swing set...those posts wont break.

 

Sam - sorry just realized I didn't have the diagonal kickers mentioned in my last post... so let me revise my question...

 

would you do both (1) the pergola option, AND (2) the diagonal kickers from the posts to the crossbeam (shortening the unsupported span from 20feet to about 11 feet)?

 

And if yes to #2, would it be sufficient to just do the kickers as two 2x6s going from the center of the two 4x4s to the two 2x6s that currently form the crossbeam.  (In other words, NOT also doing diagonal kickers to the pergola parts that I would be adding.

 

Make sense?

 

I think the diagonal kickers would be just fine. But I don't think you need to sacrifice that much of the span. If I can find a table to sit at, I'll see if I can do that math on it.

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Without making any assumptions to numbers, I don't want to make the calculations. I don't have the tables to wood species data available right now. But, let me again put my house as an example.

 

6ft span, with an assumed max load of 1500 lb at the center, design called for a doubled up 2x8 beam. This is modeled in the building code of my area. 

 

Assuming you have all the kids sit on the same swing, directly in the center, and make a full pendulum swing, you might reach a 2000 lb force. What is the likeness of that happening?

 

I think you can get away with the angle braces being 3ft x 3ft. Your unsupported length will be 14ft. This gives room for 4 swings easily. By not having an odd number of swings, you keep a swing from being dead center of the span. Which will help your cause.

 

If you are going with the angle brace option, you can probably forget your pergola idea. Benefit of the angle brace is the additional lateral support.

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Sam - I talked to a family member today that is a civil engineer -- his suggestions I think overlap with yours quite nicely.  It boiled down to this:

 

1) Put a 2x10 (laid flat) on top of the cross beam. He said this will help with the front to back sway and the downward bowing

2) do the diagonal kickers from the cross beam to the vertical posts.  I'm leaning toward 4ft x 4ft, simply because that allows us to brace them on one of the support pieces between the 4x4s and with the location of the walls, we can only do three swings anyway (at 5-7ft, 9-11ft, and 13-15 feet).  

 

See if that sufficiently addresses any issues...and IF necessary,

3) put a solid 4x4 in the center of the 4x4s top to bottom (exactly as you suggested).

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Sam - I talked to a family member today that is a civil engineer -- his suggestions I think overlap with yours quite nicely.  It boiled down to this:

 

1) Put a 2x10 (laid flat) on top of the cross beam. He said this will help with the front to back sway and the downward bowing

2) do the diagonal kickers from the cross beam to the vertical posts.  I'm leaning toward 4ft x 4ft, simply because that allows us to brace them on one of the support pieces between the 4x4s and with the location of the walls, we can only do three swings anyway (at 5-7ft, 9-11ft, and 13-15 feet).  

 

See if that sufficiently addresses any issues...and IF necessary,

3) put a solid 4x4 in the center of the 4x4s top to bottom (exactly as you suggested).

 

If 3 swings was in your plans anyway, the 4ft x 4ft diagonal brace seems like the best option. Also being able to brace it on one of those support pieces helps.

 

The 2x10 on top will help with the front to back sway, but if it is available at not too much more expensive, why not go with a 2x12? This will span across the tops of all 4 4x4 posts (if looking down), and basically sit on it like a top. Aesthetically, I think it would look better. It will help with the sag as well, but nowhere near as much as those diagonal braces.

 

In the end, you should be very happy with the swing set. Watch it being used and see if you think there should be any changes made with time. Glad I could help with any information. In school to be a structural engineer now, and I want to test my knowledge in a real life situation such as this. If you have any other questions like this (this goes for anyone here), let me know, and I will try to help. The more I do, the more I learn.

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I would not recommend using a flat steel bar across tbe top of the beam. It would add almost no useful strength in the vertical direction, and not much in the horizontal direction. A better use for steel would be to sandwich it between the 2x6 cross beams. My parents house has such a laminated beam spanning over 20' in their living room, and supporting the second floor. I believe it is 2x10s with a 1/4" x 6" steel bar in the middle, but it is holding up a house, not just a few kids.

All in all, I think the most effective support would be an A-frame in the middle, as it addresses the downward sag of the top beam, as well as the horizontal / pendulum forces of swings in action. The legs don't even have to be sunk into the ground. Just make it a triangle with the bottom resting on the ground, and pin it down with a couple of pieces of rebar or pipe.

Putting them on the top and bottom surfaces is a good idea, probably more bottom than top as the steel will be better in tension than wood.  Bending loads focus on the top and bottom, hence the torsion box design.  But thin metal on the top wouldn't resist bending much as it would bend as well and even well secured to the wood I would be surprised if it added much strength.  But that is why I beams are built, they focus metal where it is most useful and don't bother to put much in-between.

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