What size and shape of steel tubing is needed for a belay bar on a slackline?

[josephd]

I am trying to complete a project and have to properly size some steel tubing. The project is a belay bar for a slackline we have in the yard.

I have 2 poles that are set in the ground 21' apart and a slackline runs between them. We want to run a steel tube beam, permanently attached to the posts, that will act as structure to attach a belay line to the kids in case they fall off.

Ive attached a sketch of the set-up. The belay line will attach around the tube and to a harness the kids wear.

I'm figuring the max load at 500lbs and max fall distance at 18". I'm not sure I've given all the info needed but I do appreciate any help.

My question is what size tubing do we need and what shape would work best?

 

[berkeman]

Thread closed temporarily for Moderation...

 

[berkeman]

Thread re-opened.

Welcome to the PF. We try to be careful about allowing discussions of potentially dangerous activities, so the Mentors needed to discuss your thread a bit before it could be re-opened.

Can you say a bit about your mechanical background? From your sketch, it looks like you at least have training in drafting, and maybe more.

How are you planning on securing and bracing the two end poles? Are you going to have a sliding pulley on the belay bar, to make it easier to keep the belay loop over the person walking on the slackline? What is the ground like under the slackline? Concrete, grass, sand?

 

[josephd]

Thanks for the reply.
So, a bit about my background: I have a Bachelor of Architecture Degree that hasn't been used for 15 years. ;)
I have experience in construction and welding, but not expertise.

The two poles are in the ground 6'-6" and are old telephone poles. They are currently not braced at all and the slackline is being used.
The ground under has a built up bed of pea gravel approx 8" deep.
A pulley sounds like a great idea. I would like to have some sort of pulley to aid the belay staying over the person.

Right now my kids and friends are using the line I'm just trying to reduce the chance of injury.