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Lumber shortage leads to question on use

 
pollinator
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Our obstacle course, which uses telephone poles, requires 12 foot long boards. I am hoping to put up 2 swings and a climbing net. I cannot find 4x6x12 beams anywhere. I can find 4x4x12 beams. I've been looking for awhile. The swing and net are a bday present for daughter on the 26th. While it's hardly required that I put them up I'd like to.

So the question is, are the 4x4x12 beams too small to handle a swing and climbing net?
 
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Hi Elle. You could Double the 4x4x12 up. Get some good screws and predrill to avoid tearing.
If you’re sure 6x4x12 works, 8x4x12 will work.
 
elle sagenev
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I contacted my local University engineering department. Part of me is like.... eh it'll be fine. The other part is like....I JUST KILLED OUR CHILDREN.
 
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Elle,

Another, similar option would be to double up 2 2x6x12 pieces of lumber.

I, too would get several good, solid wood screws to hold the two pieces of lumber together.  Even better would be to use construction adhesive to glue the two pieces together, then hold them in place with a bunch of screws.  The screw and glue approach will make a very solid connection.

Eric
 
elle sagenev
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My concern with that is how to anchor the swing? It's a bolt that goes through the middle of the beam so having two together makes that part less stable I think
 
Eric Hanson
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Not a problem Elle,

You have a couple of options.

Option #1 is to drive screws (maybe 4-6) about 2” apart flanking the area where you want the swing screws to go.  This requires a bit of pre-measuring, but it is not too bad.

Option #2 is to glue/screw the two 2x6’s together, let dry and put in place.  Drill the swing screw holes where you want them.  Take the swing screw/anchor and put a bead of construction adhesive on the threads.  Screw the anchor screw into place and wait for it to dry.  The construction adhesive will glue it in place and it will never come out or come loose unless the wood itself rots away and I would not count that.

Either way can be a winner.  If you don’t like either of those I can think of one or two other approaches that are fairly simple.

Hope this helps,

Eric
 
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Elle, my suggestion is similar to others, make one. What I've done in making beams to support loads in home remodeling is to take two 2x10x12's for example, and in between the two sandwich in a piece of 3/4" plywood, and all three pieces get glued and screwed together to make a beam.

So I want to offer consideration of the funny forces that get involved with swinging loads, which I think is important for safety. Centripetal forces begin to happen when a load is swung like a pendulum. The load weighs a whole lot more at the bottom point of the swinging motion than the same load weighs stationary. Secondly, the beam the swing is attached to is not just supporting a vertical load, it has to handle some amount of lateral load too, and will cause some swingset beams to bow a little during use when the person swings out in front or back behind the original non-moving point (see my drawring).

Here is an equation that I found and I also don't understand, but apparently explains what is happening on a swingset: https://math.stackexchange.com/questions/215661/how-to-calculate-active-load-on-a-swingset I tried to cut and paste it here but it the formula isn't pasting legibly and looking like the original.

From what I understand, it basically says the weight of the swing seat occupant becomes tripled on the downswing.

If I were making a beam to support a swing, I would over-engineer it a tad and use two 2x8x12's with plywood in the center to make a beam as described above.

Hope this helps!
swingset-drawing.jpg
swingset drawing
swingset drawing
 
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Hi Elle, if you make your own beam there are a couple of things you need to be aware of. There are some glues and construction adhesives not made for joists or beams. Also screws have a low shear strength and I would use primarily nails for the strength, the screws to pull the wood together should be grk-rss screws.

Hope this helps
 
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Could we get a sketch or drawing of how this beam would be supported and what would be hanging from it?  It's hard for me to figure out how it's going to be used so I can't really offer any advice.  
 
elle sagenev
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Mike Haasl wrote:Could we get a sketch or drawing of how this beam would be supported and what would be hanging from it?  It's hard for me to figure out how it's going to be used so I can't really offer any advice.  

. Telephone poles attached with 1/2" bolts and the swing. We have 3 swings already on a 4x6. Can't find them anymore.
IMG_20210410_144721.jpg
[Thumbnail for IMG_20210410_144721.jpg]
 
Mike Haasl
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Ok, thanks that helps!  

Do you have extra telephone poles?  If so, use one of them as the cross beam...

Otherwise I would have the same worry James has.  Normally I'd sister two 2x6s or 2x8s together.  But with the swinging back and forth, they may bend sideways more than a solid 4x6.  Putting filler between the 2x6s will help.  The farther apart they are the better.

I'd probably take two 2x6s and two 2x2s and make a big wooden box beam.  A 2x6 on each side with a 2x2 spacing them apart at the top and bottom.  I'd put carriage bolts all the way through the assembly along the top and bottom every 18".  Screws or nails are maybe ok but if the pieces of wood that make up your beam can slide a bit, the whole assembly can bend side to side.  Crude photo attached.  Grey things are the carriage bolts
box-beam.png
[Thumbnail for box-beam.png]
 
elle sagenev
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Well the university replied basically saying they aren't allowed to answer questions because of liability issues. I understand that. He did say that when he was a grad student they were involved in a forensic engineering investigation of playground equipment that failed and killed a kid. He advised me to seriously consider safety. Then he basically said I could contact an engineer. So, I don't want to kill my kids, I know 4x6's are solid enough for my purposes and yeah, I'll just wait until they're available.

Though I am admittedly very much considering using the 4x4's to put up the net. Drawing attached.
net-design.jpg
[Thumbnail for net-design.jpg]
cargo-net.jpg
Cargo net we purchased
Cargo net we purchased
climbing-net.jpg
Net we purchased to use for climbing up to cargo net
Net we purchased to use for climbing up to cargo net
 
Mike Haasl
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I might be tempted to build it however you want and then put a couple bigger adults on it and see how much it wiggles.

Forensic engineering analyses will always get you worrying.  But that's just one failure out of thousands of successful builds.  I bet the cumulative hours that mechanical engineering students spent looking at the Tacoma Narrows bridge failure is in the hundreds of thousands.

I'm not saying not to worry, but there is a point of excessive over design.
 
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Is steel an option? Old railroad track I-beam comes to mind.
 
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If you have a spare telephone pole, that would be ideal for the kind of stresses you will get. If you can get 2x6 lumber, I would say for simplicity and robustness to laminate three 2x6 with glue and carriage bolts. Use an exterior grade wood glue like Titebond II; I have found that construction adhesives (the caulk gun types) don't hold up in the long term when exposed to weather. Titebond when clamped by bolts will make the wood into a single piece and be very rigid, important in the dynamic loading of a swing.

I would suggest 1/4" x 5" carriage bolts at about 12" on centers, staggered near top and bottom edges. The Titebond will hold the members together after the bolts clamp the seams tight. Drilling through the middle 2x6 for swing suspension will not reduce the strength, as the outer layers will take pretty much all of the dynamic load.
 
elle sagenev
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I don't know why I didn't think of this before but I'm going to put a 4x4 up and swing weights off it. Question about to be answered
 
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As another member who makes a living by building I will chime in as well- I definitely would not recommend a single 4x4 for this use.   I like the bits of advice from above about the double 2x8 with 3/4 plywood in between, glued and nailed, and the section of round telephone pole, or triple laminated 2x6.    Even 4x6 could (unlikely) have a big knot hidden somewhere at a critical point and break.  One could also do a backup if it works for you aesthetically- attach the swing chains also to the top of the posts with some strong rope or chain in such a way that if something broke, the whole load would not be totally dropped.  Looks like a lot of fun for the kids!
 
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I would second the excellent advice of James Freyr, especially concerning how the loads and forces happen with a swinging weight. I actually built a swing set about 5 years ago. Tried to find pics, if I do I will add them later. I used a 12’ long 4x8 as the top beam, to support 2 swings. The ends were 4x4s, set up as an ‘A’, with steel gussets plus lag bolts connecting everything at the top. A general rule of thumb for horizontal wood is that the span in feet should not exceed the width of the board in inches. So a 2x4 could span 4’, a 2x8, 8’, etc. Thus to span 12’ would require a 2x12. But, since here the thickness is doubled, a less tall beam will be adequate. A 4x6 may be enough, but I tend to over build so I went with 4x8. I also have made many beams as James described, sandwiching 3/4” plywood between 2x lumber (you can use waterproof glue and 3” deck screws. Despite screws not having much shear strength, there is not much shearing force here and the real strength is the glue). In all cases with wood, make sure everything is ‘crown up’. You also need to weatherproof the top of a sandwich beam, as the plywood will suffer rain damage over time. Even marine grade plywood is not designed to have water standing on an edge. Metal flashing works great and can be bought in a roll 6” wide, perfect to be able to bend the edges an inch over each side.
Back to the swinging forces briefly- after I built the swing, I had their kids test it out. I noticed if they swung opposite each other, the frame didn’t flex much, but if they were in sync, the frame flexed a lot more. Definitely something to consider.
 
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Its the width that gives strength
Example a 2x6 will span 6 to 8 ft
But you need a 2x8 to span further
Doubling up supports more weight  but not increase the span
Search for dimensions for headers
 
Julie Reed
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Doubling up supports more weight  but not increase the span



Doubling up supports more weight for the same span, or in the case of a swing set, the same weight over a greater span. Reason being you are not hanging one swing in the middle and putting twice the weight there. A beam transfers weight to the end supports, and the closer the weight is to the ends, the less is carried in the center of the beam (where it will be weakest).
The other factor is that with 2 boards you are combining strengths, so if one has a weak spot, the other compensates. Adding the plywood in the middle makes a very strong header.
 
pollinator
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Why not use steel.
Its much stronger, is often easier to get than timber and if used correctly will not rot.

I have used it in place of large timbers because the timber is not easy to get and the steel does not burn in a bushfire which is an issue for us,
4 inch pipes would be ideal.
You can usebotled joiners as well, or screw plates between them.
 
Julie Reed
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Wellll.... it is the woodworking forum, but otherwise I agree, schedule 80 pipe would work great!
 
John C Daley
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I see the forum as a starting place, I often see timber structures exposed to wether and in poor condition.
I use both materials a lot and use them in the best location for each material.
 
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elle sagenev wrote:Our obstacle course, which uses telephone poles, requires 12 foot long boards. I am hoping to put up 2 swings and a climbing net. I cannot find 4x6x12 beams anywhere. I can find 4x4x12 beams. I've been looking for awhile. The swing and net are a bday present for daughter on the 26th. While it's hardly required that I put them up I'd like to.

So the question is, are the 4x4x12 beams too small to handle a swing and climbing net?



As someone has IMPORTANTLY noted, the issue is not a vertical dead/live weight/load from human swingers. The design tables for headers and the like are for situations which always stay the same [except large snowfalls] and always with loads perfectly vertical. Span tables for construction are ONLY for what can be carried vertically in any given structure due to the vertical loads placed upon them, [dead loads like shingles, plywood etc and live loads eg SNOW. The design parameters for these members do not include and are NOT subject to horizontal loads, ie, swinging, swinglng with ever increasing speed and therefore with ever increasing horizontal loads thruout the entire swing arc/zone. Anyone remember from their youth just how high kids can get a swing going?

Think of it this way. You could support twenty kids on a swing, [maybe even adults] on a double 2x6 header, 12', straight vertically, with no movement. But start to include back and forth swing motions and the bearing capacity of the 2x6 has changed dramatically. The loads are now being moved to the horizontal. And the 2 by members structural strength in this plane is not great, OR EVEN GOOD. Especially being that it is 12 foot O.C.

To my mind, a double 2x12 with 3 - 2x6s and the whole thing made into a box beam would be better, but I HONESTLY can't say that that would resist the tremendous  horizontal loads being placed on the box beam and the post/box beam connections.

The connection point of the "horizontal beam" to the vertical post is also VERY IMPORTANT. The forces exerted on them is HUGE.

Angle braces from the vertical posts to the ground would reduce strain on the horizontal box beam and the box beam/vertical post connections.

With those kinds of vertical and horizontal swinging loads being placed on the entire structure it's ENGINEER TIME!


 
elle sagenev
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So I put up 4x4s for the net. My hubs decided to hang on one, which happened to have a big knot in it and you could hear it cracking. None of the others had an issue but yeah, not even going to try swinging weights at this point. I did tell hubs we can install the net and whenever 4x6s are back in stock we can put them underneath the 4x4s as support.
IMG_20210425_192421.jpg
[Thumbnail for IMG_20210425_192421.jpg]
IMG_20210425_192355.jpg
[Thumbnail for IMG_20210425_192355.jpg]
 
Terry Byrne
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elle sagenev wrote:So I put up 4x4s for the net. My hubs decided to hang on one, which happened to have a
big knot in it and you could hear it cracking.



Did he hang on the one in the pic, Elle? I don't mean to be unkind in any way, shape or form but that oughta have
been clear before you put it up. If you reverse the 4x4 and hang lightly on it and fill the crack with construction adhesive
it will likely be strong enough for your hubby to hang on BUT NOT FOR SWINGING ON OR FOR ANY LARGE HORIZONTAL MOVEMENTS!!

elle sagenev wrote: ... not even going to try swinging weights at this point.



Totally agree. I watched a video of some Wyoming people who lived in the same type of land area as your place seems to be. The video was about them getting beetle killed pine logs for firewood. How far are you from the mountains? A large log, living, could be made to work for your swing.

 
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