Help - is my swingset overspanned?

[Goose22j]

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!

[treeslayer]

Goose, I've built a few swing sets but none that reached 20 feet long, redwood is durable but I don't think it's that strong. I would probably put another A frame in there somewhere for support, centered would be best but 1/3 the distance I think would be ok

[pghmyn]

When I get off the bus, I can put numbers to it, but just looking at it, I'd say you're well over spanned for 2x6 material.

[Coop]

I think you have two problems:

 

1) you are overspanned

 

2) I don't think those 4x4 posts will hold up.  Every swingset I have seen uses an A-frame to deal with the lateral force you get from swinging.  It looks to me like you just have a straight post rather than an a frame.

 

I am not an engineer, just going by experience.  

 

An A-Frame in the middle might help.   I assume you want to avoid digging up your turf (looks like an artificial turf to me, or maybe you lawn is just really really green).  I really don't see a way around it though.

I agree. I've seen kids build up a good swing and I don't think either the single straight post or the vertical span would hold up.

[pghmyn]

Depending how many kids are on it, how much they weigh, a consistent 1" of deflection is easy in the center. Now imagine them all swinging and the moment all pendulums lining up perfectly and all the downward force being applied at once. Any inconsistency in the grain will just take that opportunity to spread.

As for the uppers, it could be even worse. With that design you are only resisting with bending. The A-frame style resists in compression mostly, which is a much better option.

What can you do? Well, you could find ways to incorporate metal bracing, or build up the members.

To put it into perspective, in my house I am removing a wall. The span is about 6 feet. I am required to replace the wall with a doubled up 2x8. To meet code.

[woodbutcher74]

I agree with the A frame engineering. There's going to be a lot front rear stress put on the uprights when the kids start swinging. All that stress will be at the ground level. 

 Looks like a killer trophy pole for deer hunting though.

[Goose22j]

Thanks for all the comments.

 

Yes, it is artificial turf, which is why we put it there and didn't do an A-frame.

 

A couple followup questions, I know next to nothing about any of this so I'm just trying to wrap my head around it a bit more...

K Cooper, you mentioned the possibility of the side posts not holding up.  is the idea that they would start to bend forward and backward and eventually break or...?As I mentioned, they are 4 feet in concrete (8 feet above). And there are two support pieces in the middle.  Does that help?

As far as the bowing in the middle - do the 4x4s on the side help mitigate or prevent some of the bowing, because there is only so much bowing that can take place without pulling 8 4x4s in?

One suggestion I was give was, if necessary, to place a bracing piece at a 45 degree angle on each side (about 4 feet in from the top and 4 feet down from the top), which would change a 20 foot span to a 12 foot span.

A second suggested was to place a 2x4 flat on top of the two 2x6s in order to help prevent the 2x6s from moving front to back as the kids swing. (if that description made any sense)

Any other input is appreciated!

[Goose22j]

Mike - YES! the angled 2x6s is what we are leaning towards.... the first swings start at 5 feet from the edge (cause we need enough distance from the walls), so we could easily put a support at 4 to 4 1/2 feet in.

 

Any ideas on how to support the posts WITHOUT doing an A-frame, since that isn't a feasible option with our turf and the location of the swingset?

[Beechwood Chip]

Any ideas on how to support the posts WITHOUT doing an A-frame, since that isn't a feasible option with our turf and the location of the swingset?

 

How about guy wires from the tops of the posts going front and back?  You'd still have to anchor them somewhere, but they wouldn't need a big hole in the turf, just a little hole.

[Coop]

Goose, I googled how to design a swing set, and went to the images part and this is the only one made of wood that had single vertical post. These post are wider and the span is not as great as you're proposing and there are three instead of two. There was also one that had triangle bracing at ground level, sort of outriggers if you will, so you may not be completely dead in the water yet. I hope you find a convenient, solid and safe solution. Good luck.

 

http://blog.greenplayparks.com/category/park-amenities/swing-set/

[pghmyn]

Mike - YES! the angled 2x6s is what we are leaning towards.... the first swings start at 5 feet from the edge (cause we need enough distance from the walls), so we could easily put a support at 4 to 4 1/2 feet in.

 

Any ideas on how to support the posts WITHOUT doing an A-frame, since that isn't a feasible option with our turf and the location of the swingset?

Honestly, I can't think of anything right now that is going to make it stronger without some type of outrigger framing... i.e A-frame type.

 

Reason why is because of how your structure is made. Think of your vertical posts as cantilever beams. Your ONLY method of securing the to structure is the direct connection to concrete. While as a foundation base this may be more than sufficient to resist rotational force in the concrete, you have another story with with the wood. You have a moment/lever arm in that cantilever. You are applying the force at one extreme (the top) and it is being resisted at the other extreme (the bottom). No matter HOW thick you make that post now, you only have a connection of 12.25sq.in. between you and the foundation. And unfortunately, being this is the place where the most rotational force is applied, it is the weakest link.

 

By adding some sort of outrigger framing, you turn the rotational force into compression force. Much more allowable resistance in compression as compared to bending.

 

If it is a matter of not wanting to dig up your turf, think about an a-frame where the legs only make contact with the surface. Tie them together with a turnbuckle and you are set. If you need to remove them, would take a few minutes.

[Eric.]

Whatever you end up with, tell your three fattest neighbors the pizza's on you if they give it a test run before you put the kids on it.

[pghmyn]

Whatever you end up with, tell your three fattest neighbors the pizza's on you if they give it a test run before you put the kids on it.

Beer is included if you can get them all to sit on the middle swing at the same time.

[Beechwood Chip]

If you want to reinforce the horizontal beam without sacrificing any play area underneath it, you could use a king truss.

[wtnhighlander]

Beechwood beat me to it, I was going to suggest a truss. As for the post construction, my FIL built one in his backyard, using two 4x4 posts on each side, bolted together and set in concrete, very similar to yours. It only had two swings, but the posts never budged during the 5 or six years they lived in that house.

[wdwerker]

Truss is an awesome idea ! Still need to address the swinging stress on single posts somehow.

[Goose22j]

You guys are awesome - thank you for all the comments and suggestions.

Unfortunately, due to homeowner restrictions, a king truss is not an option.  (Can't go higher than 10 feet and i'm already at just over 8.)

 

If I (1) added support diagonal beams that went out and down about 4'3", which would reduce the unsupported span to 11.5 feet, and

     (2) added a two by four (flat) on top of the 2x6 crossbeams (glued and screwed in, or glued and nailed if screws would compromise the 2x6 strength too much), would that sufficiently address both:

• the potential for downward bowing on the crossbeam, AND
• the potential for forward/backward swaying on the crossbeam

??

 

As far as the support posts, what if I added sort of a mini support piece like what K Cooper mentioned -- sort of little outriggers.  

 

See the pic for illustration: https://www.dropbox.com/s/i7vk9i0ij0r0kta/Options.jpg?dl=0

EDIT: I tried to paste it in body, but it wouldn't work

 

 

 

Again - thank you all so much!

[pghmyn]

Adding that 2x4 flat on top of the 2x6's will increase the strength, but not as much as you would hope. You are increasing the cross-sectional area, but now you have three separate pieces secured by fasteners, instead of one solid section. Glue will fail, and so will the nails OR screws eventually. It is only a matter of time until they are loose enough where they don't help at all.

 

For deflection, the only thing I can think of that would help you exponentially is the introduction of some sort of metal. Or, some sort of truss structure on top. You said that you are at 8 feet, and you can not exceed 10 feet. Are you opposed to going to that 10 feet, or somewhere close with a truss? If designed properly, could be a aesthetic feature as well.

 

For the lateral sway, go with the outrigger option that we mentioned before.  If you want, I can try to draw up an idea of what I was thinking.

[Goose22j]

Sam,

Thanks for the quick response. 

Dumb question, but when you say "deflection" - are you referring to the front to back sway of the crossbeam?  And as far as introducing metal, would a steal plate along the top work for that (i.e., 1/8 or 1/4 inch, 2 inches wide, running along the 20 ft board?  or even separate 3 or 4 foot steal plates centered above the swings).  Or an angled piece of steal that was on top and back?

 

I'm not opposed to going to 10 feet but with 3 small children, I am honestly fairly limited in time capital...not to mention funds :-)

 

I'd be curious to see what the outrigger option would look like.  one problem is that I have a minimal amount of space going directly behind the support posts (2-3 feet at most)  it also would cause some aesthetic and functional issues in the yard. is the thought that this is the ONLY way to provide support for 3 children to swing on this?  Again, I know very little about this subject, but it just is so surprising to me that FOUR 4x4s, buried in 3 1/2 feet of concrete, can't provide sufficient strength

 

Thanks again,

[Guest]

--> Again, I know very little about this subject, but it just is so surprising to me that FOUR 4x4s, buried in 3 1/2 feet of concrete, can't provide sufficient strength

 

 

It is really a matter of leverage.  No, your posts won't come out of the ground and the whole thing won't tip over.  But all the force of the swinging is concentrated on the one spot where the posts come out of the concrete, so your posts could actually (eventually) snap right there.  You will certainly see the entire post sway a bit when the kids are swinging hard.  

[Tpt life]

Let's just say, I felt steel pyramid frame swings move when I was a kid. Now, I hope your family is nothing like the terror I was...

[Randy Thurman-Keup]

I added a truss to the swing beam that I built.  It also spanned about 20', but in my case it was 2 beams in parallel covered with netting and used as a bridge.

So far it has worked really well.  Even once the kids got significantly bigger.

[wtnhighlander]

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.

[Goose22j]

1) I'm considering the A-frame in the middle, but the problem is I am not able/willing to sink it in or pin it to the ground because it is artificial turf and it will ruin it (not cheap stuff).

Would an A-frame in the middle be effective it it just rested on the ground, not pinned down at all?

 

2) 2nd alternative, what if I just added an additional 4x4 in the middle that went straight down and rested on the ground (i.e., not an a-frame).  This would certainly address the downward sag of the top beam, would it also help with the forces of swing, simply by virtue of being more weight and another source that could take some pressure?  Or useless?

 

3) 3rd alternative... what if I add sort of a half-size, half A frame.  in other words, if i put a diagonal 4x4 from halfway up the back 4x4 on each side to the ground about 3 feet behind.  Here's an image: https://www.dropbox.com/s/qldthwomv30xfct/alternativefix.jpg.  Thought is I would anchor it in shallow concrete, about 12-18 inches, since when kids were swinging back, the force would be pushing on it, and when kids were swinging forward, the pull force wouldn't be sufficient to pull it out of the ground because it was held in place by the vertical 4x4 (and the minimal movement of that post would not exert an inordinate amount of pressure.) did that make any sense whatsoever, and would that be a big help?

 

 

4) 4th idea, what if instead of (or in addition to) adding diagonal kickers (item H in the above-linked pictures), I added two more 2x6s to the cross beam. basically a mini pergola  This is what I'm thinking: https://www.dropbox.com/s/hyqu1fmidcqe1ga/pergola.jpg?dl=0.  I would imagine that would go a long way to addressing the crossbeam sag, and also the forces of swing on the crossbeam, though I'm guessing that wouldn't do much if anything to help relieve pressure off the vertical posts.  Thoughts on this?  What would make a bigger difference, this or the diagonal kickers running from the vertical posts to the crossbeam?

 

thanks again guys!

[pghmyn]

If I had to pick out of any of those, I'd go for option number 4.

 

I misread your initial posts, and I thought you only had one 4x4 posts sunk into the concrete. Having two, in the configuration that you have, you really shouldn't have anything to worry about. I plotted a diagram of the forces for you to see physically what is happening.

 

Because you have those two 4x4 posts spread, you are much less likely to encounter the snapping like you would if you just had one like I thought at first.

 

Adding those additional 2x6's on the top will help a great deal. Especially if you bolt them, and not screw them. By bolting, you are transferring the load all the way through the section, instead a 3" screw possibly moving inside the wood and losing the effectiveness. With those provisions, I think you should have no problem with the design.

 

Let's be honest here. You WILL see sag. You WILL see forward and backward sway of the top beam if all people swinging happen to sync up at the exact moment. Should you be worried at this point...I'd say no. Test it. Determine if you need to change something, and go from there.

 

Only option that I can think of to help with the sway is to add a 2x12 vertically along the 4x4 posts like was mentioned before.