I run a social skills group for eight 14-year-old boys based on initiative tasks. Our next task is for each child to cross a small ravine on a fixed rope wearing a rock harness and chest harness and using a pulley (like in the classic "fall scene" in Cliffhanger). While I've been climbing for 15 years (so I know how to secure ropes, harnesses, etc.) I've never been instructed about how to calculate the force generated by a 250 Lb person crossing a 50-foot HORIZONTAL static rope. I plan to purchase a new static line from my local climbing shop and secure it between two very large trees, but wanted to be sure that it wouldn't BREAK with a person on it 30 feet above the ground! Any ideas? I figure that even dynamic rock ropes (standard 9 mm) should be strong enough, so a static line (much stronger) should be bombproof; but again... I simply have no experience on horizontal lines.

-- Steve Cunningham (scunningham@shawlink.ca), May 09, 2002

Answers

Do a search on "Tyrolean traverse", You need to make sure that your anchors are strong enough too. If you want the line to be tight you might finf that a cable and turnbuckle system works better.

Tom

-- Tom Dunlap (ctc@el.net), June 08, 2002.

Though, I'm too lazy at the monet to dig out the phisics books to find the formula I can tell you that the forces will be highly multiplied if you try to keep that rope straight (and you have to if you want to do this). I stop by a climbing shop and talk to some experienced climbers hopefully they can help you. If not, phone your local university. They are usualy open to helping the public. Tell them how much force might be applied to it(remember falls generate a lot of force!!! it could happen) and they should be able to give you an idea of how strong the rope has to be. Make sure you allow a margin between the rateing of the rope and the MAX force that could be applied. Personally you would never get me out there on a 9mm static. Something thicker I might think about it but still would be kinda sketched out unless I saw the math myself. Also, take a look at book on making belay anchors they usually have a part on the muliplication of forces at low angles (such as a nearly straight rope). Take care.

-- Jacob Chmielowiec (jcup@hotmail.com), July 07, 2002.

one formula for working out the tension on a span rope(tyrolean traverse ) is the load which includes the weight of the rope and pulleys x span DEVIDED BY 4 x the sag the sag should be a minimum of 5% Of the span eg load=100kg span =100FT SAG MINIMUM=5FT SAG IS THE VERTICAL DISTANCE BETWEEN AHORIZONTAL LINE BETWEEN THE TWO ANCHOR POINTS AND THE ROPE =100 X 100 =10,000 DEVIDED BY 5FT X 4 = 20 = 500KG LOAD ON THE ROPE AND ITS ANCHOR POINTS

-- terry cook (l_oneill@bigpond.com.au), August 03, 2002.

If you can get your hands on some physics formulas that'd be the best, but i'd consider looking at some rescue resources for this kind of information. I'm almost positive that on 11 mill, when tensioning a high line for rescue work, you shouldn't tension with anything more than a 6:1. I'd find out for sure beroe you do this, but i know when we rigged a 60ft tyrolean traverse we put in like 4 feet of sag. Any book on High Angle Rescue should have tables for stuff like this. But i'd say if you tension with a 5:1 across 50ft and your anchors are bomber...on 11 mill static, you should be fine. Just leave a few feet of slack in the line. In theory a perfectly horizontal line exerts infinite tension on its anchors.

-- Jim Davis (jmdavis384@aol.com), January 14, 2003.

Just a point on the original statement. Whether a rope is dynamic or static (low stretch) does not determine its strength. To state that "static line is much stronger" is misleading, because it is not so. An 11mm dynamic line rated to say 3000kg is as strong as a static line rated to 3000kg. The ability to absorb forces is different under certian conditions, but this is not a strength issue.

Many folks have killed themselves choosing a static over a dynamic and then loading it inappropriately............

-- Roger Graham (roger@ias.com.au), July 17, 2003.