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4x4 Pressure treated for swing?

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Forum topic by chetrog posted 04-24-2015 12:30 AM 3034 views 0 times favorited 15 replies Add to Favorites Watch
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chetrog

84 posts in 926 days


04-24-2015 12:30 AM

I have 3 nieces and they have a cheap swing set. They are getting bigger , and I would like to make a new swing set. I have no idea how strong a 4×4 Pressure treated board would hold 3 kids. The heaviest kid is 120 lbs. I was thinking of using three 4×4x10 pressure treated boards. I would concrete two of them 2’ deep. This would make the swing 8’ tall. And I would use the last Board 10’ width. Do you guys think that this would be possible. Thanks for any info.

Here is a picture online that I kinda like to make. But I would like 3 swings

-- I had a stroke a few years back, and sometimes the words dont come out as well as I would like.


15 replies so far

View derosa's profile

derosa

1568 posts in 2297 days


#1 posted 04-24-2015 12:55 AM

That swing doesn’t show 3 4×4s but 2 with what looks to be a pair if 2×6s at the top. I wouldn’t doubt the support strength of what you want to use. The real question is how you will tie the top to the uprights. You’ll need to use metal plates or something similar. The design you show might be better.

-- --Rev. Russ in NY-- A posse ad esse

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chetrog

84 posts in 926 days


#2 posted 04-24-2015 01:25 AM

Yeah the picture was just a general picture, where I would like to use 4×4’s.

-- I had a stroke a few years back, and sometimes the words dont come out as well as I would like.

View Ghidrah's profile

Ghidrah

667 posts in 684 days


#3 posted 04-24-2015 01:42 AM

You could shoulder the verticals 3/4” both sides or you could brace the underside of the 2/2X6s. For me the issue is torque, if both swings are moving in the same direction will the 4X4s begin to compress the soil and loosen. There’s a reason for triangular posts.

-- I meant to do that!

View altendky's profile

altendky

169 posts in 1672 days


#4 posted 04-24-2015 01:44 AM

Take a look at some deflection calculators. Here’s a ‘shelf’ one:

http://www.woodbin.com/calcs/sagulator.htm

My quick calcs there show that your 4×4 would have nearly 4x the deflection of the pair of 2×6’s. I don’t know if it will fail but the increased vertical section is worth a lot. 5.5in vs 3.5 in gets you the 4x despite losing 1/2in of thickness.

Note that wood ‘beams’ are generally sized for stiffness, not strength. Often they use max deflection limits of 1in / 360in of length in houses. As derosa said, make sure you take care with the joint to keep the beam from falling off the posts.

View WillliamMSP's profile

WillliamMSP

738 posts in 1066 days


#5 posted 04-24-2015 01:48 AM

A single post on each side, sunk 2’ deep and in concrete probably isn’t going to last the season. An “A” frame on each side, sunk 2’ deep in concrete would be much better.

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

View Mykos's profile

Mykos

102 posts in 1256 days


#6 posted 04-24-2015 01:56 AM

I think that an A frame is the way to go as well. The torsion loads on a straight beam like that are going to lead to failure. A triangular frame will be under compression forces instead of shear forces for the most part. Having 3 swings increases your span as well, so I’d go for more than the 2×6’s. 2×8’s or even better 2×10s would be my choice for the top beam.

Anything worth doing is worth overdoing….

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altendky

169 posts in 1672 days


#7 posted 04-24-2015 02:07 AM

http://diy.stackexchange.com/a/246

There are a couple good points there and, in general, if you can’t find dedicated swing set info, fence info may be a good reference as well. I think fence posts often go more than 2’ deep even without the 8’ height or intentional large side loading at the very top.

View Ghidrah's profile

Ghidrah

667 posts in 684 days


#8 posted 04-24-2015 02:27 AM

A 4X6 is more than adequate for a 10’ span. I have one in the back yd. I built 23 yrs ago. for my daughters and their friends. It has/had 3 swings, it doesn’t get much use anymore except when we have a neighborhood summer bbq. You can sandwich the beam into the A frames and carriage bolt it. angle the A frames out 20° from plumb to reinforce/minimize lateral movement.

If you want to limit the length and height of the swings arch, limit your beam height to 8’, girls can be as competitive as boys.

-- I meant to do that!

View chetrog's profile

chetrog

84 posts in 926 days


#9 posted 04-25-2015 01:53 AM

Thanks for the suggestions , and the info. I had a stroke a few years, and It’s hard for me to understand some stuff.

-- I had a stroke a few years back, and sometimes the words dont come out as well as I would like.

View altendky's profile

altendky

169 posts in 1672 days


#10 posted 04-25-2015 02:15 AM

Sure thing. If you do a more standard A-frame style swing you will reduce the stresses on the uprights drastically (10x or more? I haven’t calculated but it’s a big deal). While I wouldn’t really worry about a single vertical 4×4 breaking, you will have to sink it really deep and do a really good job to avoid it wiggling loose.

That said, I can understand the interest in the simple, different, and sleek look. You just have to be careful to do it right. There’s often good reason it’s different. :] Best of luck and I’m sure the kids will love it.

View BurlyBob's profile

BurlyBob

3663 posts in 1727 days


#11 posted 04-25-2015 03:02 AM

I’m assuming that your in the Valley, judging by the vegetation in the photo. I’ve found that any post, P.T. or otherwise will rot off in concrete due to getting water soaked. Concrete will hold the water. May I suggest that you dig your hole a few inches deeper, pour a few inches of concrete in each hole. Let it dry for a few days, then pour in 3-4 inches of round gravel, set the post and pour concrete around it. This technique will give the post an ability to drain water in a manner it wouldn’t have encased in concrete. Another technique to use with concrete and wood posts is to dig the hole in a cone shape, upside down. Wide at the bottom, tapering to the top. This will reduce tipping from torque at the top of the post.

View altendky's profile

altendky

169 posts in 1672 days


#12 posted 04-25-2015 03:17 AM

I saw something suggesting that the wide base concrete cone would also help avoid issues caused by the ground freezing and thawing. When it freezes having it tapered that way it would push it down rather than up and out of the ground.

View dhazelton's profile

dhazelton

2324 posts in 1758 days


#13 posted 04-25-2015 11:36 AM

If you do the concrete keep it all below the surface of the soil, don’t mound it up to shed rainwater. If you want to remove it ten years from now you can just cut the posts off and not have to bust up concrete.

View fuigb's profile

fuigb

403 posts in 2419 days


#14 posted 04-25-2015 08:08 PM

Additional perspective: my neighbor sunk a pressure-treated 4×4 at 45 into concrete to complete his hammock stand (other end was an eyebolt in a tree). A few little kids horsing around in the hammock caused the 4x to break at a knot. I’m not an engineer but I believe that the posts above about the power of triangular frame are right on. And whatever your final dimension I’d make it by laminating 2x stock to control for what happened to the bonehead across the street.

-- - Crud. Go tell your mother that I need a Band-aid.

View Ghidrah's profile

Ghidrah

667 posts in 684 days


#15 posted 04-25-2015 09:38 PM

Seeing how this is still open I figured I take pics of the one I made, this thing has been put through the ringer. At 1st it included a double swing then removed for a Tarzan swing then just 3 regular swings. When they got older there was a hammock between the 2 cross braces. I occasionally use it to hang freshly painted items during the summer, can still see the strings from last fall.

I lied, the hdr is 11’6 and from grade to the top of the hdr is 8’7”.

Here’s a close up of the joinery locking the hdr into the A frame, the A frame is 45° from the hdr.

The A frame angles out
20° from the hdr. Everything interlocks with carriage bolts, the 4X4s go 36” deep mostly for the frost line/heaving in our area. I had to dig an elongated hole to get the concrete in correctly. If I remember correctly the bottoms are in about 12” of sakreet. If you have issue with termites/borers/moisture you can coat the concrete and 4X4 with foundation tar to grade. We even hang the Xmas tree from it in a bucket of water till a week before then fin cut and bring the tree in.

Altendky’s suggestion for the BF is only valid if the footing is at or below the frost line. If it is in the frost line it will heave with the rest of the soil.

-- I meant to do that!

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