Suggestions to improve my swing set frame design

I am going to build a swing set for the kids. I want 4 spots for swings with ample room. I am working with 2 different books on swing set design.
My frame will be 2 vertical 4x4 posts that sandwich my main beam. Then 2 4x4 posts will be at 30 degree angles to create an A frame on the ends.
The center beam will be 18 foot long. It will be 3 2x6's bolted together (to make a 6x6 beam) This beam will be close to 10 foot off the ground
I am really not sure about the vertical 4x4's and the angled ends.
I am not really sure how to do a beefy a frame design
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I don't quite follow. You have two books on building swing sets, what do you not like about their designs? "I am really not sure..." doesn't give people much information to work with - what aren't you sure about?
R
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"theedudenator" wrote:

You're comments above indicate a lack of understanding of the design fundamentals of beam design.
I don't want to burst you're bubble, but you have some serious engineering study ahead of you before attempting to design a structure that could possibly inflict serious bodily harm to the users if it fails.
Lew
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What am I missing on beam design? sandwiching 2x6's to make a beam is not correct???
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"theedudenator" wrote:
What am I missing on beam design? sandwiching 2x6's to make a beam is not correct???
Not really, it is a waste of material resulting in a heavy and weak beam.
Find a Strength of Materials engineering text and learn an understanding of the term "Section Modulus" and how it applies to beam design as a starter.
Lew
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Here's a link for the convenience of the brave reader:
http://en.wikipedia.org/wiki/Section_modulus
Challenging looking stuff to say the least... I hope this doesn't keep the kids from getting a swing set! ;)
Bill
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I understand those terms. Why would a book on swing set design say to use 3 2x6's or a single 4x4 Thats why I asked, seemed odd to me also.
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Lew's point is that a beam gets _much_ stiffer with increasing depth/ height of the beam as compared to increasing the width/thickness.
You definitely want a stiff beam. You definitely want a shorter continuous span, so either have an intermediate A-frame support, or build two swing sets and put them together. I'd think that having two separate swing sets would be a little more work, but not much, and would be more interesting for the kids to play on. They could more easily see each other and have competitions and such.
R
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The book I have is suggesting what I posted, that is why I wanted suggestions. It didn't not seem correct to me, but I never designed swing sets before.
I am an engineer....
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"theedudenator" wrote:
===================================The book I have is suggesting what I posted, that is why I wanted suggestions. It didn't not seem correct to me, but I never designed swing sets before.
I am an engineer.... ==================================== Then it's time to dig our your Strength Of Materials text and review.
Sounds like whoever wrote the book you are reading may not understand beam design.
Does:
Z = I/d I = (b)(h^3)/12 + 1/2(A)(D^2)
where: Z = Section Modulus I = Moment of Inertia b = base dimension of a rectangle h = height dimension of a rectangle d = distance from center of rotation to the outermost fiber (usually h/2) D = 0 for a rectangle when the axis of rotation is about the centroid.
Ring any bells?
Lew
Lew
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The biggest issue you will have with such a long beam is that it will want to bend laterally when multiple people start swinging on it, particularly when they are all swinging in the same direction. I design playsets for my website, www.arciplay.com, and though I do not have a swing beam that meets your exact requirements, you could lengthen the playset 1.2 swing beam a few feet if you replace the beam I have shown with an 18' long 6"x6" or better yet 8"x6" (the base design shows a 4"x6") (a beam like this will be very expensive) the cables in the design will support the beam laterally. I do not recommend using 3 2x6" together as you mention unless they are pre- engineered and fabricated as 3 already combined 2"x6"s.
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SFWIW, if you truly need an 18 ft open span beam, consider the following:
1) Build an "I" beam construction consisting of 20 ft long 2x6 top and bottom flange with a 2x12 web.
2) Since the web is basically dead weight and contributes "Zip" towards strength, lighten it along the lines of a bar truss design for a flat roof industrial building.
Start with a 2x12x10 ft timber and cut it into 5 pcs, 2x12x24" long.
Locate a 2x12 at each end of the 2x6x20 pieces, then space 30" to the next 2x12x24 piece, then repeat.
What you end up with is a 20 ft long structure with 5-24" wood pieces and 4-30" open spaces alternately spaced.
Assemble with some 3" galvanized deck screws with hex heads if you can find them.
Drill pilot holes for the deck screws.
You end up with a beam that is "strong like bull" yet light enough that two people can lift it into place.
3) Attachments.
I would use loops around the beam rather than drilling holes thru the beam for bolts for the swing chains.
Have fun.
Lew
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Deck screws are too light, and 3" is too short - only 1.5" of penetration? And how many is "some"? Most engineering designs fail in the connections and the lack of detailing.
If I'm understanding you, you're trying to give the OP a design for a full length truss with no intermediate support, but it's not really a truss. A truss joist has diagonals that are opposed. Where one diagonal is pushing or pulling, the meeting one is resisting that force - there is no distance between where those forces are applied. In your design you are relying on the 2x6s on flat to resist those forces, there is a distance between where the forces are applied, and the screws at the ends of the 2x12 blocks will be required to carry more load than 1.5" of penetration will allow. This will tend to sag over time. I didn't run the numbers, but it will happen.
A swing set should be designed with a large factor of safety and it should be designed for the occasional show off adults competition really going at it. There's also the cyclical nature of the applied loads, the combined tension and shear load on the screws with the 2x12 design, etc. A #12 wood screw has something like 200 lbs of pullout resistance in SYP (PT wood) per inch of embedment - 1.5" =~300 lbs. I would think you'd need a minimum of seven or eight hundred pounds per fastener for the end conditions with the 2x12 design instead of three hundred.
It would be better if the OP skipped the 2x12, which has problems with shrinkage and twisting, and used some 3/4" pressure treated plywood as the web. Even better would be to use the 3/4" plywood and PT 2x6s (might be able to get by with smaller 2xs) and build a box beam joined with construction adhesive and 2.5" epoxy-coated deck screws at 4" - 6" on center.
The top of the beam should be covered so there's no standing water on top of the beam and water won't enter the edge of the plywood. The plywood joints should be staggered and - belt and suspenders - screw and glue plywood scabs to join the ends of the plywood pieces to each other.

Having the swings hung from the top of the beam is definitely preferable with a truss or box beam. I have no idea what sort of hardware is available for a swing set, so I can't recommend any specific method of attachment. Another poster mentioned the hardware is expensive, I'd price that stuff before finalizing a design.

The most important part!
R
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"RicodJour" wrote:

In this application, fasteners are strictly in shear, so penetration is not an issue.
Don't like 3", use 3-1/2" or 4".
Deck screws that size are about the same as 16d common nails which would do the job, if your arm is in shape to handle a 24 OZ framing hammer<grin>.

How about 3"-4" spacing or whatever looks good in the shower.

This is about as classic a design as it gets. The fasteners are in shear, supports are in compression. I wouldn't expect these joints to weaken the design. ==================================> If I'm understanding you, you're trying to give the OP a design for

truss. A truss joist has diagonals that are opposed. Where one diagonal is pushing or pulling, the meeting one is resisting that force - there is no distance between where those forces are applied. In your design you are relying on the 2x6s on flat to resist those forces, there is a distance between where the forces are applied, and the screws at the ends of the 2x12 blocks will be required to carry more load than 1.5" of penetration will allow. This will tend to sag over time. I didn't run the numbers, but it will happen. ================================Not quite.
The bottom 2x6 is in tension, the top 2x6 is in compression, and the 2x12 web provides the diagonal support.
No details have been specified for the end terminations; however, the beam must rest on an adequate support.
Fasteners will not carry the load by themselves.
I'm lost about your fastener concern?
===================================>A swing set should be designed with a large factor of safety and itshould be designed for the occasional show off adults competition really going at it. ===================================Yep.
===================================>There's also the cyclical nature of the appliedloads, the combined tension and shear load on the screws with the 2x12 design, etc. ==================================You lost me.
===================================> A #12 wood screw has something like 200 lbs of pulloutresistance in SYP (PT wood) per inch of embedment - 1.5" =~300 lbs. I would think you'd need a minimum of seven or eight hundred pounds per fastener for the end conditions with the 2x12 design instead of three hundred. ===================================You lost me. The beam must rest on an adequate support.
Fasteners in tensile will fail.
===================================> It would be better if the OP skipped the 2x12, which has problems

the web. Even better would be to use the 3/4" plywood and PT 2x6s (might be able to get by with smaller 2xs) and build a box beam joined with construction adhesive and 2.5" epoxy-coated deck screws at 4" - 6" on center. ===================================Building a box truss with plywood is certainly an approach; however, using pressure treated lumber for anything other than a ground contact member is way down on my list of acceptable materials.
Exposed exterior construction would require considerable preventative maintenance, thus wasn't considered.
===================================> The top of the beam should be covered so there's no standing water

plywood joints should be staggered and - belt and suspenders - screw and glue plywood scabs to join the ends of the plywood pieces to each other. ============================

hardware is available for a swing set, so I can't recommend any specific method of attachment. Another poster mentioned the hardware is expensive, I'd price that stuff before finalizing a design. =================================3/4", 3 strand nylon, replaced every other year.
Works for swings and dock line for my boat.
Have fun.
Lew
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theedudenator wrote:

I just want to be clear on what you're proposing--you're talking two 4x4 uprights equally spaced and on each end an A-frame made of 4x4s so that the beam is supported in four places and the longest span is about 6 feet?
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I just built one for my princesses. A couple of thoughts...
18' is too long if you plan on having no center support. Even 3 2x6s will flex and wobble with a swing in the middle and a 60lb kid swinging on it. And they keep growing...
My 'beam' is 2 2x6s screwed together from both sides with 2 3/4" deck screws. This barely flexes with an 8' span. The point is that it does flex at only 8'.
I think the vertical posts are overkill and won't do much. On the A- frame side of mine I used 2x6s at an angle with a plumb cut to hold the 'beam' vertical. Then I bolted a horizontal 2x6 on either side of the angled 2x6s to support the 'beam'. Similar to the collar tie here:
http://qcfocus.com/files/Collar%20Tie.JPG
If you are planning on only having a swingset (no fort or platform) you'll also need to address the side to side forces. For example this appears to be a death trap:
http://www.justoutdoortoys.co.uk/store/image/1iv0k/Domestic_Maxplay_Surveyor_Double_Swing_Set.jpg
The swings, bolts, chains, screw hooks, etc. are *very* expensive. You are probably better off buying a diy kit complete with hardware and plans. It will probably end up costing the same as if you design it yourself and buy all of the bolts, swings and so forth you are going to need.
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I think, but not sure that the center vertical 4x4's are supposed to address that.
I am going to work up a 3 leg tripod type design for each end. Then a 2 leg tripod to place in the center.
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There isn't all that much side to side force on a swing set. The tripod ends are overkill, the intermediate support legs are necessary.. I hope you are planning on burying the legs in concrete as the uplift force and fore/aft force can be substantial, particularly with kids swinging in unison.
If you're an engineer, designing this thing should be trivial for you, unless you're an electrical engineer which aren't real engineers. :)
R
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