Is there an engineer in the house?!

Jared77

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Ridge? Monty? Steve? Anybody want to run it by their husband? Anybody else with a mechanical type mind?

So we're building a playscape in the spring for our kids. So this lead this lead me to want to build a small zip line for the kids to play on too. I'm looking at 20-30' long using aircraft cable with a trapeze that is connected to a proper zip line pulley.

I found 6"x6"x12' posts that I was going to use. My question is...will those be strong enough? I was going to run eye bolts through it with large washers and connect the cable to that.

Posts would be sunk into the ground at 36" and the eyebolt would be at 8' high. I had planned on building a short ladder with 2x4s to climb up, grab on and down the line you go. Incline wouldn't be too crazy but enough to let gravity work.

Was pricing the stuff out online it wouldn't be bad to build and I know the kids would have a blast with it.

Anything to keep them busy especially with another one on the way!!!

I really appreciate your help on this.
 

Smart Red

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A 6" by 6" post would hold up one side of a large porch roof with shingles and two roofers. It would even hold me up. I doubt you need worry about the girls weighing it down.
 

seedcorn

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As they grow, I would be concerned about the posts leaning. Don't think they would ever break.

Look for used electric poles that u can bury and are treated. Plus they should be considerably cheaper. I'd bury them at least 4' deep, prefer 5'. Trust me, it needs to handle your heaviest friend or family member.......
 

Ridgerunner

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I'm a retired formerly licensed civil engineer that quit paying to keep the license renewed when I retired. There were continuing education requirements I wasn't going to keep up with either. In structures it's almost always really easy to size the main members. It's the connection and support details that take the time and really determine if it will work.

The posts and cable are plenty strong enough. The question is how strong the ground is. That's going to depend on what kind of soil you have, how wet it is at the time, and how disturbed it is. How deep you bury the end is important too. One general rule of thumb is to bury a post 1/3 of its length, but how effective that is depends on the ground. You have two different forces to worry about. One is just the vertical weight going down into the ground. The weight of the post is probably going to be more than the weight of the person using it, even adding cable and rigging weight. if the bottom of that hole where the end of the post is resting is natural sand you are in good shape from that aspect. If it is clay, especially when it gets wet, you might have a problem. Sand is a great foundation as long as it is contained, but near the surface where It can move, well, be careful.

The other problem, and probably the biggest problem, is the horizontal force at the top of that post. The flatter the cable the more the weight of the person,, cable, and rigging contributes to a horizontal force on that post.

Not knowing what your soil is like or anything like that, I'd suggest the 36" depth will probably work but concrete it in. When I build a fence I generally use rocks tamped in around it to keep it steady but your talking about people's safety. It won't just be your kids. Plenty of adults will have to try it out. The important places to make sure the concrete is firm against the post is the very bottom and the very top. In the middle of the hole vertically doesn't do you as much good but the weight will help. At the uphill post, you need good support against it at the uphill side. Near the surface you need good support at the downhill side to keep it from turning over. Reverse that on the downhill post. If you drive a few nails into that post to give the concrete something to hold on to, you can really help to stop it from sinking on down.

Then put a guy line on it. I don't know how steep your slope is, but go uphill straight behind the top one and sink some type of post or anchor. Then stretch a cable pretty tightly between them. You might need a turnbuckle to get it tight or some other threaded bar connection so you can tighten it. The shallower the horizontal angle the better. If you use a post, attach that cable to the bottom of the post. You can maybe put an eyebolt through a piece of metal and concrete that metal in the ground. Then do that on the downhill post too. On your 12' posts attach that cable near the top, pretty close to your eyebolt and cable connection. You want that horizontal force going as straight from the eyebolt to the guy line as you can get it.

With certain soils you could probably just sink those 12' posts 4 feet, concrete them in, and forget the guy lines. But those guy lines are going to increase your safety factor tremendously. Without doing some pretty heavy engineering work and knowing your soils I'd be real hesitant to say you don't need them, especially if you have a clay soil and the ground is wet. And don't tell me it will only be the weight of your kids on it. I'm sure you will try it out to make sure it is safe for them before you let them on it.

And don't forget to video the first time you try It. You might make some money on America's Funniest Videos.
 

journey11

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My DH and his brother had a zipline when they were kids and they still talk about how much fun it was. Their dad built one between two trees going across a little stream.

I would think too, if you wanted to build something like a platform or playhouse tower, the high end of your line would be very stable launching off of something that had 4 posts set in the ground. One single post would be more prone to shifting unless you buried it at least half of it's length deep and even then would probably still lean with repeated use. And I think you would need a structure with two posts at the bottom end, so that when they come flying down, there is an open gap for them to dismount through and not hit a post. That might be safer, unless you trust them to jump off early.

I am not an engineer, of course. ;)
 

bobm

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With 12 ft. posts, set 3-4 ft deep into the ground with the rest of what Ridgerunner suggested, then when you install the line between posts that would be 20 -30 ft apart.... You also have to figure how much sag that line would have in the middle of the run with a grown man suspended from it. You just may have to use a 16 ' or 20' or ? post. At which point, I would install 2 posts at each end say 6 ft apart and put a cross bar at the top and use that to install a swing at each side or a swing on one and a base for a slide on the other or even a play structure or play house ? Install a thick padding on the downslope post to stop painful crashes. With this being a permanent type of structure and to eliminate wood splinters or cracking / bending, you may want to use 4" x 4" x ? tall with 1/4" wall thickness steel posts cemented into the ground and welded in place where needed. Paint with paint specific to use on steel to stop rusting. Usual disclaimer applies, of course. :cool:
 

flowerweaver

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I have been considering building two, to quickly get back and forth across the creek that divides our property in half especially when it floods and I am unable to tend my chickens or gardens. Hope you post plenty of photos as you build yours!
 

MontyJ

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Were it me Jared, I would use 12 footers and put 4 feet in the ground. Don't use 6x6 material unless you have no choice. If you must use 6x6, be sure to use a draw knife or similar tool to turn the bottom 5 feet into a round pole. A 6x6 is plenty strong enough, but it's the whole square peg in a round hole thing. Take a lesson from the power company, they use round poles for a reason (and it's not just because trees are round so round poles are easier to make). You should be able to buy round service entrance poles, most are 24' long. Cut it in half and use the heavier end for the upper anchor point. Rent a power auger and suitable bit (an 8" would probably do, but check the size of the pole first). Drill the hole, sink the pole and back fill. Use 1/4" stainless steel cable and fasteners with galvanized eye bolts. It wouldn't be a bad idea to install guy wires opposite the strain point on each pole to help relieve the stress.
And one important safety factor...either make the slope gradual enough so that the rider can touch the ground with plenty of stopping room, or pad the hell out of the lower pole.
 

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