Measuring forces at multiple locations in rigging systems
2016 | Brian Kane, PhD, University of Massachusetts Amherst and Co-Investigator Mark Reiland, University of Massachusetts Amherst
Despite the obvious danger associated with rigging trees, very little rigorous research has explored many aspects of this common arboricultural practice. Only a few studies have quantified the forces generated when shock loading a tree, but the studies have been limited by measuring forces in only one or two places in the rigging system. This makes is difficult to assess the likelihood of failure of different parts of the rigging (block, rope, sling, tree). This project will measure forces at three locations in the rigging under a variety of loading scenarios to determine friction coefficients in different blocks. In turn, these values will help us understand which parts of the rigging are more (most) likely to fail under different rigging scenarios.
We measured forces in different parts of climbing and rigging systems. For rigging systems, we measured rope tension in the fall and lead of different rigging ropes (Samson Stable Braid, Samson True Blue, Sterling Atlas, Yale Polydyne) on traditional arborist rigging blocks and X-Rigging Rings. We did the tests by dropping and slowly lifting a weight, and found that the results were different for the different test methods. This means that measuring friction in blocks or rings used for rigging has to be done with drop tests. We also found that the friction in rigging rings wasn’t always greater than on a traditional block with a rotating sheave when conducting the drop tests. This might be because the rope slides along the sheave in the block instead of turning it. When you raise or lower a weight slowly, the sheave has time to turn, which would reduce friction compared to running the rope over rings.
For climbing systems, we measured rope tension when ascending by footlocking, ascending using foot and knee ascenders, and stopping abruptly during descents. We also measured rope tension while simulating a falling climber by dropping a weight on the rope. We conducted tests using different ropes (PMI Classic, Samson ArborPlex, Samson Mercury, Samson True Blue, Sterling WorkPro, Sterling HTP), when trees were in-leaf and leafless, and on canopy-anchored and basal-anchored stationary rope systems. For the climbing tests, we measured both the amount of the load (100 lbs., 200 lbs. 1000 lbs., etc.) and the frequency of loading (how often the load is applied—once per second, twice per second, etc.). Regardless of the climber, ascent technique, rope, or whether trees were in-leaf or leafless, the amount of the load during ascents was at most about 1.8 times the weight of the climber. When a climber stopped abruptly during a descent, the rope tension could be up to almost 5 times the climber’s weight. And during simulated falls, the maximum amount of rope tension was almost 11 times the weight. And, in general, you can increase these numbers by 50% if you climb on a basal-anchored stationary rope system. So it’s less likely that a tie-in point will fail during an ascent, but preventing falls and avoiding abrupt stops while descending from or working in the tree could generate enough force to cause failure of a tie-in point.
The frequency of loading is equally important to the amount of the load because likelihood of failure could increase if the load is repeated at the same frequency as the tie-in point would sway back and forth. It’s like pushing someone in a swing: you don’t have to push them very hard, but if you push them at the same frequency that they swing back and forth, you can get them to swing really high. And with frequency, regardless of the rope, the climber, whether the tree had leaves, and whether it was a basal- or canopy-anchored stationary climbing system, footlocking had a lower load frequency than using foot and knee ascenders—about half the frequency. But it’s not that higher or lower frequency is more likely to cause the tie-in point to fail; what matters is whether the swaying frequency of the tie-in point is close to the frequency of loading. We need to do more tests to figure out typical swaying frequency of tie-in points.