Canyoneering physics
Contents
Descender heating
When descending a rope, friction between the rope and descender heats the descender. See the main article for more information.
Anchors
Shock loading
Most of the time, canyoneers slowly and carefully apply a constant load to the rope and anchor. When things go wrong, however, falling people or objects may place a sudden, dynamic load on the system when their fall is arrested.
Fall factor approximation
To a first approximation, the maximum tensile force in the system during shock loading can be determined by the rope stiffness (defined by standard elongation; often 2-5% at 330lbs for canyoneering ropes) and fall factor. This approximation is correct to the extent that the rope behaves as an ideal spring (tension is proportional to percentage elongation). Naively, this means that a 1ft fall onto 1ft of chain should be much, much worse than a 100ft fall onto 100ft of rope because they share the same fall factor but the chain is much stiffer.
Other energy absorbers
In short falls, the climbing harness and the human body also act as energy absorbers, reducing the maximum tensile force from the fall factor approximation. Some measurements that may be used to estimated the size of this effect may be found here. Peak forces measured in short falls onto tethers have been as high as 2000 pounds.[1]
Rope response to dynamic loads
The fall factor approximation assumes that the rope behaves like an ideal spring throughout the shock loading. While this is very close to true when stretching a rope slowly, the rope behaves more stiffly when it is stretched quickly.
Lanyards
Lanyards are short lengths of rope, webbing, or similar often used to secure a climber or canyoneer to an anchor. They are sometimes made of relatively stiff material like Dyneema that would lead one to predict a very high peak force even from a short fall of a few feet using the fall factor approximation. Two concerns with such a high predicted peak force are that the lanyard may break and the human may be hurt. At least one instance of the lanyard breaking has lead to a fatality[2].
Resources
Webbing on bolt hangers
Breaking strength
Typical one-inch tubular webbing has a breaking strength around 4200 pounds. Cutting through the webbing only reduces the webbing's strength by roughly the proportion that has been cut. So, if a piece of webbing would break at 4200 pounds nominally, cutting half way through it would only reduce its strength to around 2100 pounds. See the video below starting at 5:43 for a demonstration.
Typical hardware quick links are rated at their safe working load (SWL), which is roughly 1/5 of their ultimate breaking strength. So, a quick link rated at 1500 pounds is likely to endure forces up to 7500 pounds.
Mechanical advantage
Test data
See Test data
References
- ↑ http://www.geir.com/mythbuster.html Myth #5
- ↑ http://www.climbing.com/skills/anchors-away/ 2007 incident on the Grand Capucin near Chamonix, France
