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Will two 2x6s provide the support I need for swingset

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Forum topic by Goose22j posted 03-03-2015 05:03 PM 1153 views 0 times favorited 30 replies Add to Favorites Watch
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Goose22j

9 posts in 648 days


03-03-2015 05:03 PM

Topic tags/keywords: question

Hi – I’m hoping you can help.
My dad and I made a swingset for my three children. A picture is below, or at this link: https://www.dropbox.com/s/3esasw76g22c5en/Photo%20Feb%2028%2C%204%2055%2010%20PM.jpg?dl=0

(ignore the 2×4 on each side that is propped at an angle, that was just holding it in place while the concrete dries).

Here is that we have (all the wood is Redwood – “Construction Common”):
1. Two 4×4s on each side, 3.5 inches apart. There are a couple support pieces in between the 4×4s in the middle (16 inches long) and near the top (10 inches long) for added support. The 4×4s were about 12 feet long and nearly 4 feet of them are in concrete. I feel confident these aren’t going anywhere. :-)

2. TWO 2×6s at the top span with width of the structure.
The 2×6s are on the long edge (right term?) and are glued and screwed together, forming a piece that is 3.5 inches in depth and 5×5 inches tall.
The 2×6s are bolted in between the 4×4s at each end and are also supported by one of the above-mentioned support pieces.
—Here’s the concern

We will have 3 swings hanging from the 2×6s. We previously got advice that the 2×6s would be sufficiently strong to avoid any bowing in the wood, but a family member that works in construction has now told us he believes that we are “overspanned” at 20 feet. My two questions are

1) are we overspanned and will the wood bow in the center
2) if so, what is the best, most full-proof, and most aesthetically pleasing way to resolve it.

Thank you very much!


30 replies so far

View LiveEdge's profile

LiveEdge

486 posts in 1087 days


#1 posted 03-03-2015 08:00 PM

Google something called the “sagulator”. With the numbers I put it it said it would sag about an inch across the span for a load of 300 pounds. 20 feet is a fairly substantial span…

View SirIrb's profile

SirIrb

1239 posts in 697 days


#2 posted 03-03-2015 08:13 PM

Holy smoke that is some green grass. Do you paint it?

Bolt through and dont lag-eye-screw is my suggestion. I would think that 2 2×6 is good but like LiveEdge said, check the google machine.

-- Don't blame me, I voted for no one.

View patcollins's profile

patcollins

1420 posts in 2332 days


#3 posted 03-03-2015 08:24 PM

Hi Goose

It may sag, even from it’s own weight. Everything deforms, it is just a matter of how much.

If you wish to make the top stronger there are two things you can do. You can either split the beam up by providing a support in the middle effectively shortening your span or you can increase the height of the beam up top.

The moment of inertia is what determines beam strength and for a rectangular section the moment of inertia increases by the cube of the height (power of three). You can greatly increase the bending strength of that beam by increasing its height by an inch. You could simply lay another board on top of it and screw it together every foot or so and you will have a much stronger structure.

View Goose22j's profile

Goose22j

9 posts in 648 days


#4 posted 03-03-2015 08:35 PM

Thanks guys – what do you think of these solutions…
First -here is some more detail about the current setup, followed by an image with possible solutions

View MakerofSawdust's profile

MakerofSawdust

35 posts in 2083 days


#5 posted 03-03-2015 08:35 PM

I hate to say it, but ignoring the 2×6 issue, I think you’ll have a problem with the posts in the ground/concrete moving. I did the same thing and mine moved a lot. I even had one end attached to a building. There’s a reason virtually all swing sets have A-frame ends. I switched to an A-frame years later without any issues and no anchoring in the ground.

-- - Kevin from Cincinnati. All my work is guaranteed: Three minutes or three feet; whichever comes first.

View Goose22j's profile

Goose22j

9 posts in 648 days


#6 posted 03-03-2015 08:41 PM

MakerofSawdust – how big where your post holes? Ours are about 40 inches deep and i widened them out at the bottom.
the soil is pretty dense down there too – we rented a two man auger and the soil stopped it about 2.5 – 3 feet down (and we had dampened the ground for two days before).
Does that dampen your concern at all or…?

View MrUnix's profile

MrUnix

4244 posts in 1666 days


#7 posted 03-03-2015 08:49 PM

I had the same problem, with 3 foot deep holes in basically solid coral rock (South Florida). Wound up going A-frame to solve the problem and still had to sink them pretty deep.

Cheers,
Brad

-- Brad in FL - To be old and wise, you must first be young and stupid

View WillliamMSP's profile

WillliamMSP

757 posts in 1071 days


#8 posted 03-03-2015 08:53 PM

I don’t have the knowledge set to calculate what would be appropriate for that span, but I sure as heck would err generously on the side of over-building it.

A) when thinking about who would be using it, I wouldn’t even want to imagine it failing, but on the non-emotional side…
B) when thinking about the loads that it’s going to see, consider that we’re not talking about static loads, we’re talking about absorbing and redirecting the kinetic energy of multiple children dropping from a height of 4 or 5 feet once every second or so.

-- Practice makes less sucky. (Bill, Minneapolis, MN)

View patcollins's profile

patcollins

1420 posts in 2332 days


#9 posted 03-03-2015 08:54 PM

Your corner braces will do a little bit for the flexing of the top beam, but not a lot. Where they will help is in the racking motion side to side.

I am afraid the other guys are right about the postholes. The swing set at my grandfathers house did not have this problem but its uprights were telephone poles (he was a pole man for the local power company and acquired them some how along with the truck auger to put them in …..)

View Goose22j's profile

Goose22j

9 posts in 648 days


#10 posted 03-03-2015 09:00 PM

MrUnix and Maker of Sawdust… were your holes traditional post holes or…? As I mentioned, mine are about 40 inches down and pretty wide…i used a total of 23 bags of 50-pound post hole cement.

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MakerofSawdust

35 posts in 2083 days


#11 posted 03-03-2015 09:00 PM

My post holes were 36-40” deep in SOLID clay. They were 12” in diameter. I seriously questioned if the concrete we put in the holes was going to be harder or softer than the clay. I suggest keeping your posts and adding A-frames to them, all the way to the top. It appears you have some room to at least get a decent A-frame. Your posts probably won’t move right away, but over time.

-- - Kevin from Cincinnati. All my work is guaranteed: Three minutes or three feet; whichever comes first.

View knotscott's profile

knotscott

7224 posts in 2842 days


#12 posted 03-03-2015 09:01 PM

I’d definitely try to shore that beam up some more. We had a 4×6 main beam with a 2×4 screwed in as a top plate (like what you show as figure “I”) holding 3 swings across an 11’ span. I also added diagonal supports like you show as “H”. Little kids are fine, but when the kids got bigger, the beam flexed enough to concern my wife, though I was fairly sure it wasn’t gonna break. In hind sight, I wonder if a third 2×6 along the side of the first two might actually be more effective than addimg a 2×4 as a top plate. Bigger kids will swing as high as they can possible go. Even when your kids aren’t huge, some of the neighborhood kids might be. Ours stood for about 13 years without a problem, but it always drove my wife crazy watching it….I told her to stop watching.
;-)

-- Happiness is like wetting your pants...everyone can see it, but only you can feel the warmth....

View Goose22j's profile

Goose22j

9 posts in 648 days


#13 posted 03-03-2015 09:05 PM

MakerofSawdust—good to know…mine are about 24 inches in diameter at the widest and probably closer to 16-18 at the narrowest (widest is front to back). Hoping that is enough of an increase to make a difference…
as far as the A frame, there is room in front of the swing but only 2-3 feet behind the swing. unfortunately an A-frame would defeat the aesthetic purpose of building it how we did…and also force us to build (or possibly anchor) on the artificial turf, which is why i’m hoping to avoid it. what do you think of the added footing support in the picture I added (item J)

KnotScott – thanks for the feedback, good to know!

View bondogaposis's profile

bondogaposis

4036 posts in 1818 days


#14 posted 03-03-2015 09:08 PM

A swing set is a dynamic load. I think the span is way too long for any of your solutions. You really need to beef up the top I’m not sure if 2×12’s would be adequate, but I know that 2×6”s will be way over stressed even w/ a 2×4 on top. I also think you will have to guy wire the uprights to front and back sides to achieve any kind of stability.

-- Bondo Gaposis

View Goose22j's profile

Goose22j

9 posts in 648 days


#15 posted 03-03-2015 09:09 PM

KnotScott—what happened after 13 years :-). I’m assuming you moved or took it down? what was your vertical support? A-Frame or Vertical Posts like mine?
were your diagonals at a 45 degree angle, and how long were they (i.e., how long was the unsupported span?)

Thanks!

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