Will this vertical-posts Swing set plan work?

diy_in_md

I want to build a simple 4-position swing set in my backyard,
but I want it to have vertical, single-pole supports and not A-frames.
(My reasons for this are below, but not important to the question)

I would like the swing beam to be about 9' high at the top and
about 16' long.

I talked with some friends who are in engineering and construction
and they think that I could use a 6"x6" for the cross beam,
and if I use 8"x8" posts, sunk 48" down with 24"
of concrete around them, that it will be sufficient to counter the rocking effects of the load. The ground is well compacted clay soil.

I am aware that the connection from the posts to the beam have to be
very good, and I am going to buy or have a steel connector fabricated.

Does this sound like it could work? Is it overkill? Will the swinging motion cause the posts to "loosen" eventually? (that is my biggest
worry, that they start to rock back and forth with the swing after
a few years.)

Any opinions welcome.


As for the Why of this design

My back yard is not huge, so I like to multi-task it and
not use up too much space for the swings.

My problem with the traditional style sets are the A-frame ends.
They can be 12' wide at the base, and I would rather be able to
use the swing frame as a kind of soccer goal or at least be able to not have effective 12' barriers/tripping hazards when the swings are not in use. So the kids can still play other sports in the area.

Sound reasonable?

Thanks!

oh'mike

Well--that's inherently weak---But you know that already---

It's a swing set---Try it---if the posts start to show signs of rocking---then remove the swings and shut it down until you can add more bracing.

I doubt if there will be a sudden and traumatic failure---as long as the top attachment is sound.

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ddawg16

I'm not the expert....but a few issues I see.....

48" Down but only 24" of concrete? Do you mean 24" in dia? Depth is where you get your strength....

The wood you put in the ground needs to be kept away from soil. You need to make sure that the concrete completly encases it. Even then, expect a finite life for it.

Have you considered steel? Galvanized to b exact?

One of the issues I see is that with constant swinging, and clay soil, the concrete is going to start to work loose and wobble.

Is there some side structure you could attach one end of the horz beam to....it would be easy to brace that beam to reduce the front/back moment.

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diy_in_md

Thanks for the replies!

Sorry about the confusion, I was planning on pouring concrete 24" in diameter around the pole.

I am most worried about keeping the footer from loosening up after extended time and use.

I would rather not go through the extra expense and time and then haveto use an outside stabilizer anyway, as that would make a lot of the costs a waste.

I also would prefer to stick to wood for aesthetic reasons, since I think I will be removing it before 20 years.

Is there anyone who could tell what size / depth of footer would be adequate for this application?

Any help is much appreciated!

jimmy21

i agree with mike. It may start to warp over time or even crack, but i don't think its going to fail all at once. I think you will have plenty of warning.

What you might consider doing what you plan, but add another couple of braces. Just because you make it an A frame doesn't mean it needs to be at a specific angle. The Angled braces also don't neccassarily need to go all the way to the top. If you put them at a 15 angle half way up it would still add a lot of stability to your design, i would think

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Ralph III

Hello diy_in_md

1) You might consider adding some type of T-bracing at the bottom of the vertical support poles. Otherwise an L brace (if metal) to front and back of poles. Set them in concrete just under the surface of the ground. If done properly it could significantly reduce any type of rocking effect or loosing of ground.

2) I'd have two concerns with your project.

a) A 16ft top beam, that swings will hang from, is a very long expanse. You
will definetly need to add some corner bracing as Jimmy21 suggested, as I
don't see a 6x6in supporting the weight at all otherwise, minus a center pole.

I'd opt to sandwich several 2x10's or even 2x12's together and engineer a
beam myself. There is a lot more strength in beams with varying grains
then one solid beam. In this case, you would also need to add a board or metal at the
top and bottom to help with any lateral sway and in protecting them from
the elements.

b) Because it's not an "A" frame and instead single poles, you create a shear
possibility at the base. You may want to create some rules
for swingers, such as weight limits. Otherwise limit the number of swings.

An "A" frame is much stronger in this case, no matter the spread, and limits any
possibility of shear. If you opt for the single pole, I'd go overkill
due to safety concerns.

good luck

jimmy21

I didnt even think of that, 6x6 seems too small on top for a 16' span. Use 8x8 for everything, But again, I don't think it would have a catastrophic failure. youd probably have warning. That it isn't safe. The top triangles I showed, I figure for the reason of the whole thing being able to twist side to side, they probably wouldn't be needed if your corner brackets were a foot or more and thick steel

Ron6519

You mention 4 posts. How is this thing shaped?

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Msradell

Another problem you will probably have is that if you embed the wood posts in concrete they will start rotting quite rapidly. While you need the strength to resist swaying every time you bury the wood in the concrete instead of attaching it to the top with brackets you create the potential for rapid failure even when using pressure treated wood.

Ralph III

Msradell, so concreted wood will rot significantly faster? Is it a chemical issue or just from accumulating water? It would seem a code issue with decks and such if that were the case, I don't know? I mean you see substantial stand-alone carports being built with heavy corner posts embedded in concret all the time, seems fine?



Jimmy, I'd still be concerned with an 8in horizontal beam or any solid beam for that matter, in this case. Even headers call for more support than that over much shorter distances, not to mention the lateral force the swing will undergo. I think he really should consider building a beam, or buying one, as described.

I'd be worried if four 60-80lbs kids got to swinging in unison of indeed a possible catastrophic failure. Either one of the poles shearing at the base or the horizontal beam breaking in the middle or at a weak spot, such as a knot. That is the problem with solid beams whereas a manufactured beam with the varying grains, offer much greater strength.

IMHO

He definetly should consider adding the corner bracing as you advised but I'd build the support beam......

God Bless

Msradell

In many places building codes will not even allow wooden structural members be imbedded in concrete. They must be attached to the top using a bracket. The rotting is due to the fact that water accumulates in cannot escape from the wood.

jimmy21

the problem with using 2x10's or something like that, is those are only for downward pressure. This gets a twisting motion. Also, comparing this to a header isn't really accurate. A header has to support a heavy roof during an earthquake, high winds, hurricanes, all while staying perfectly flat and not bowing or flexing at all. If a swingset ends up with a 1/2" bow in it after a couple years, who cares? An 8x8 is pretty darn stout at 16'. which is higher than living space. I do hear what your saying about it being a solid piece of wood that could crack, but i think if id developed a crack, youd see it




This is why you first put pea gravel in the hole, then the post, then concrete. That way it can drain on the bottom. If you fully encase the post it will rot much faster. Also if it were me, i would put a 50/50 mixture of diesel and used motor oil in a bucket and soak the bottom of the PT post in the bucket for a few days. Shouldn't rot for a LONG time. This isn't an approved method because of contamination of the earth, but i really doubt its going to do any damage.

Msradell

The EPA certainly isn't going to like you!

jimmy21

Just being honest on what I would do. I'm not telling anyone else to do it. I don't think its much different then spraying the same mixture on form boards. A bunch gets on the ground.

Ralph III

1) Yes as noted, if he fabricated a beam from sandwiched 2x10's or such, he'd have to put metal or wood on top/bottom of the beam to control any lateral issues. I think the 8 x 8 beam will be sufficient though so agree with you, see #4.

2) I wasn't trying to make a perfect comparison with headers, only to draw attention. Headers must be strong for load weight but they do not undergo the lateral and/or twisting the swing beam will, thus the required strength could be comparable or at least a reference point.

3) You're right, he should be able to monitor any cracks and such in a solid beam. My concern would be with knots as those represent weak spots. He should be careful to avoid timber with large knots, especially at the center.

4) I just checked our swing and we have an original 4 x 6 beam that covers a 12ft span, with three swings (previous owner built). I put some
2 x 6's on each side when we moved here, as it was aging and had a little more lateral play than I cared for. I think your right in that a 8 x 8in beam should be sufficient for his 16ft expanse. He could always add 2x8's on each side with any future concerns.

Take care, Ralph