Zipline post trouble

**Harleylady** · 15th April 2017, 04:52 AM

Well hello everyone I'm brand new to this forum community but I am an old school DIYer. I'm installing my son a Zipline. There's no slope to the area that I'm doing this and however I'm building a platform so the beginning of the zipline will be plenty high enough for creating a slight angle. So the end of the zipline I have one 12 foot pressure treated 4x4 post that I'm going to cement in 4 foot down. My question is can I attach 2 scrap pieces of pressure treated 4x4 posts to the existing post.? My thought is I want it sturdier than just the one post and I'm looking to get rid of some scrap versus have to go out and get a whole nother solid 12 foot 4x4 post. if the integrity is compromised because it's in half then it defeats the purpose so I'm not sure if this is something worth doing. Anyone out there know if I attached 2 6ft 4 x 4 posts to the existing solid 12 ft post if it will still reinforce the first post? Or does it have to be a solid post attached to a solid post to maintain the strength? Hopefully this isn't too silly front some of you pros. I just have tons of scrap wood to include 2 x 4's but not at 12 ft length. Any ideas as to how I can reinforce the original 12 foot 4x4 and make it stronger use pieces less than 12 ft? Thanks for anyone who can help this learning mom.

**labr@** · 15th April 2017, 12:45 PM

Howdy Ma'am, welcome to the forum

.

I'm not an engineer but I know that cables under tension can put huge forces on beams and the height of your post would give the zip line a lot of leverage. I suspect it will need substantial bracing and adding smaller pieces could end up with a rather complex structure. If I was doing it I would look at a cable based bracing system as in the diagram below. The diagram may look small - just click on it to enlarge. Hopefully it makes sense but if it doesn't then feel free to ask for more explanation. It could even pay to have 2 more braces angled out about 45 degrees sideways to help with lateral stability.

Zip line forum post.JPG

By the way a zip line was always called a "flying fox" in Australia when I was young.

**old1955** · 15th April 2017, 01:17 PM

Welcome to the forum.

**GeoffW1** · 16th April 2017, 03:13 PM

Hi,

I strongly recommend you brace it as Bob has said above. I think one brace will do, exactly in line with the main line, as then there is little sideways force and the post is carrying all compressive load.

It would be ideal to arrange the toppling moment (turning force) at the top of the post to be equal both ways so that the net toppling-over forces are zero. I would do that by attaching a small block to the side of the post near the top, then dangling a plumb-bob from it to a mark on the ground. Tighten both cables (or ropes) so that the plumb-bob remains on the ground mark.

Cheers

rrich · 17th April 2017, 04:22 PM

Without getting too technical, the force applied on the posts by the zip line is multiplied by the ratio the length of half the zip line to the vertical deviation of the zip line times the weight of the zip line rider.

If a 100 Kg rider causes a 1.5 M vertical deviation on a 100 M zip line the force on each post will be about 1.5 tonnes.
(Assumes a perfect system and a perfectly straight zip line.) In the real world, I would estimate the forces to be 500 to 600 Kg.)

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