Eric Anholt (anholt) wrote,
Eric Anholt

Backyard slackline limits reached

Two weeks ago we built a slackline setup in our back yard. The issue we had was that we don't have any trees back there to tie up to. Common solutions in this case involve building an A frame and using whatever sort of anchor you can come up with, with plenty of options available.

We wanted better. The yard could only go to about 40 ft of line, and we didn't want to sacrifice precious length between our anchors and the A frame.

The first plan we were working with was to put a pipe in some cement, then slide a smaller pipe into it, and use that as our fake tree to anchor to: Now there's a solid anchor, but it's removable if I decide to sell the house or something some day. I found some numbers for guidelines for building railings, though, that indicated that you'd need massive steel pipe to support the loads we're talking about.

What we went with in the end was a wooden 4x4. We'd heard that slackliners were successfully using those in home setups. But we were a little wary of trusting a wood 4x4 more than a steel pipe. So what we buried in the cement was a post sleeve so that we could just slide our 4x4 into the cement hole after it was set. The cement was 3 feet deep and just over 1 foot across (if you decide to go this route: post hole diggers are *awesome*). This let us put an 8 foot 4x4 in each and be able to set a line at heights up to around 4 feet off the ground. But just in case, we also dropped some heavy chains into the cement as well in case we want anchors for A frames if this posts thing doesn't work out.

We first used the system last Sunday with great success. It's a typical 4-carabiner primitive system but we used a double pulley system behind that to get enough tension from a single person tightening that you'd stay off the ground in the middle. There was a disturbing amount of bending and some creaking in the 4x4s, but they held.

Today Scott was setting up the line again, and said "I got it nice and tight, look at that!", and I hopped on. I made it about 1/3 of the way, when there was a snapping sound and suddenly I was on the ground. Luckily failure wasn't as catastrophic as we feared. The post had just bent over, and not detached and gone flying.

Our next plan was to use steel I-beams: the backup plan that justified the 4x4 sleeves. I'm still concerned though -- a beam stress calculator program says that for what we're thinking is like up to 1600lbs of force at 4 feet from the support point, we end up with a maximum bending stress at the support point of 164 ksi on a S3x7.5 I-beam (the biggest that will fit in our sleeves as far as I can see). If I'm supposed to compare this number to the yield stress of the steel the beam would be made of, that number is only 22 ksi.

The plan for the moment is to throw together some A frames (actually, X frames -- Scott built and used some of those successfully this week, and it sounds easy enough) and use that unless we can figure out that I was wrong and steel will hold.

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