Ninja Slackline Fill Posts or Not 2

[CthomasAZ]

We purchased a 50 ft Ninja Slackline Obstacle Course for our kiddos for our backyard. We have 2, 9ft tall above ground, 4 inch galvanized steel pipes (40 feet apart) that are set in concrete (24 inches deep) to use as the attachment points for the course. Total pole height was 11 ft, 9 ft above, 2 t buried. Once we hung the top line of the slackline and ratcheted it to where our kids could swing like monkeys without sagging in the middle we noticed major bending in the posts. Our kids together weigh 130#. Well under the 440# maximum for the line. We're now realizing that these slackline systems require an incredible amount of tension. If we add rebar to the inside of the posts and fill entirely with concrete will that help "strengthen" the pole so it won't bend as much? I've seen other forums where people have had similar issues with the slackline systems using posts because of the immense tension. Maybe theyre really meant for 30 year old trees... Any help is appreciated. I've attached a picture from the manufacturer showing what we are trying to install.

 

[BridgeSmith]

You probably need to add guy wires with ground anchors, or you you need to loosen it up so it has some slack/sag in it. Less sag = more tension for the same load. The way to decrease the pull on the poles is to increase the sag in the line.

 

[SWComposites]

4” poles sound too small. You probably need to add guy lines from 5he top of each pole to ground. 2 lines per pole is probably better. With robust anchor in the ground for each line.

 

[IRstuff]

> You may have over-tensioned the slackline; it's shown to have sag, even with only its own accessories. Physics says it's impossible to remove all slack from a hanging line; you'd need infinite tension.
> Your anchors may be a tiny bit shallow.

https://bolderplay.com/blogs/blog/i...s are able to,posts cemented into the ground.

says 1/3 of the height of the pole

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[1503-44]

It's never going to look like that picture. Yes you need lots of tension, more and more ss the line is leveled. Problem is that 440 lbs is not an incredible amount of tension when it comes to hanging weights on level wire lines.

To keep the wire load below 440 lbs when a 130 lb load is applied, the sag angle must be at least 8 degrees. Let's say 10° That wire is definitely not going to look flat and level.

Line load is calculated by weight/2 x sin of sag angle.
Sin of the angle must not be less than Weight/2 / max wire tension.
W/2 = 130 /2 = 65 lbs
Max wire load is 440 lbs
Sin = 65/440 = 0.14
Asin = apx. 0.14 radians
0.14 radians x 2 pi = 8°
That is the Minimum sag angle. 8 degrees. Hope the wire has a safety factor, as any reduction in sag angle has a dramatic effect on wire load. Like when the wire gets cold during winter.

And I also think the 2ft embedded depth is too shallow. 3ft minimum depth and fill the hole around the pipe with cement if you are looking for minimum maintenance. 4" pipe may work, but that depends on what wall thickness the pipe has.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[EdStainless]

I put a set of these in years ago.
12' lengths of 6" pipe, set 4' into the ground, about 3' of concrete in the holes.
These have been used by teens and adults for 20 years now and are fine.
I set them pitched back about 2-3deg.


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[LittleInch]

I can't see rebar plus concrete doing much for you.

Bigger, deeper pipe, angling the poles back by 20 + degrees or guy ropes is the way to go.

Or maybe add a length of 2 x 2 angle to the outer side and bolt it or weld the angle to the pole. Or stich weld a piece of 2 inch or 3 inch plate to the edge of the pipe.

Or add a straining post.

Think of this more like a suspension bridge... or a wire fence. Look at how they tension up chain link fences.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleInch]

Like this.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

There is no advantage to angling the poles, as it tends to increase compression load. If you angled them 90° flat and level, the compressive load would equal the wire tension. Infinite. The wire load, determined by its sag angle, also determines the pole's bending moment. Once the sag angle is set, the wire tension remains constant and bending moment will not change and any change in column compression load is exactly balanced by the change in the column's shear load. Vertical pole is all shear; horizontal column all compression.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

True, but you could then hang a wire down vertically into a concrete block or screw anchor or something, so a version of the straining post picture. It's clear that the issue is bending / bouncing of the poles whilst in use, or at least that's my impression.

filling it with concrete plus a few Re bars / cage won't do any harm, but might not change the bending strength of the pole enough to make a difference compared to the other options.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Filling with cement will help, but leave the bar out.
One bar in the center of the pipe will not help bending moment very much, but would help tension or compressive load, but bending is most likely the biggest problem.

Be sure to cap the pipe.



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

A 6 inch pipe of the same wall thickness though is at least three times "stronger" though if not more. That's what going to make the biggest difference.

Maybe slide a 6" pipe over the 4" pipe and then fill the annulus with concrete.

And then watch the 4" pipe buckle at the connection with the concrete block....

I had never heard of a "ninja" obstacle course before either.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

The maximum moment will be at Ground zero, probably right after the sleeve stops.
A change in cross sectional area can also induce unwanted stress concentrations exactly where you don't want them.

I suspect a 4" std wall is probably adequate, but I have not designed the column (yet), so not totally sure.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[SwinnyGG]

Hate to say it.. but I think you need to redo your posts.

Your kids weigh 130lb together now... in 2 years that'll be 200lb, in three years 225 lb, etc etc.

6" round tube is about 3x stiffer than 4" round tube for the same wall thickness, and 2' of embedment is very shallow.

If I were doing this myself I'd probably want 6" pipe and at least 5' of embedment.

Also make sure you cap the pipe. If you don't it will trap water, quickly corrode, and fail while one of your young monkeys is swinging enthusiastically.

 

[3DDave]

Back-stays are the easiest fix - it's what is used on circus tents. Maybe even back-stay the back stays so there is less pounding to do. This is used on smaller tents in sandy soil. This will eliminate bending loads and make mowing the grass a greater challenge - perfect for ninja training. It will also make eventual removal much easier - one reason it's used at circus tents. Put two on each end at 45 degrees from the slack line to take the side loads.

 

[BridgeSmith]

I agree with the others that you need to guy it, brace it, or replace it with a larger post that's embedded deeper. The other option is to loosen up the cable, so it can sag more. It's called a "slack" line, not a tightrope, after all.