An Analysis of Traditional Un-tensioned Belays and Two-tensioned Rope Systems in Rope Rescue
This year I had the opportunity to speak to a group of 150 rescue professionals at the International Technical Rescue Symposium(ITRS), in Portland, Oregon. In light of some recent testing, my fellow instructors and I wanted to dig into the issue of Two-tensioned Rope Systems TTRS. These are systems where rather than having a two-rope system with a traditional un-tensioned belay, both lines share part(ideally half) of the load.
Over the years- the rescue community has spent a good deal of time researching and discussing belays. The term belay was an old sailing term used to describe a rope catch or hold-fast, when sailors on big ships were hoisting the sails. Rock climbers and mountaineers have been using an un-tensioned safety rope, called a belay for many years. People understand the need for a rope safety, that allows for freedom of movement while climbing at the gym or crag. The rescue belay has been a subject of passionate debates, if not knock-down, drag-out, fights. In the 1980s the belay became popular as many looked to improve on the safety and professionalism of fire & rescue services. Early belays came straight off the climbers’ rack. These were belay tubes, belay plates, figure-8 plates, body belays, and Munter hitches. Professional rescuers borrowed extensively from their alpine counterparts.
The late 80s saw testing and the dawn of the great debate: the Tandem Prusik belay(TPB) versus all other rescue belays. The much misunderstood and misapplied British Columbia Council on Technical Rescue Belay Competency Test (BCCTR B.C.T.), though a very well thought-out and scientific way to test belay devices, the BCT lacked the human element and the existence of an edge component. Purveyors of the TPB and the BCT would duke it out with the rest for two decades. The late 90s brought us the Traverse 540 and the Petzl I’D. The 2000s brought the MPD and other engineered devices that signaled an age of enlightenment in the rescue belay realm. No longer were rescuers dependent on rock climbing methods alone, rather industries had provided solutions specific to the rescue problem. The only thing that remained the same, would be rescuers’ desire to leave one line slack or un-tensioned.
The reason for a slack belay has been traced to the desire for unimpeded movement, that slack ropes are more resistant to cutting, and that the belay system is an unused and un-stressed “insurance policy” for the failure of the mainline. The obvious problems seem to be potential elongation of the belay line upon loading. But are there others?
In 2014 Kirk Mauthner presented on this topic at the ITRS in Golden, CO. He discussed several topics, but most noteworthy was his drop testing analysis comparing TTRS and traditional un-tensioned belays. The crux of the BCT is that the worst fall condition would occur at the edge with limited rope in service. This failure would be something like a litter attendant and patient falling around 1 meter distance onto the edge while maybe only 3 meters of rope are in service. The other more modern possibility is the artificial high directional, like an AZ Vortex, toppling or failing at the edge. These produce high forces and there is limited rope in service to absorb them.
Mauthner demonstrated that when this happens, the outcome is more favorable in a TTRS than a mainline with an un-tensioned belay. His belay line had a scant 15 cm of slack difference from the main. His testing can be seen here:
This research really has some implications on edge considerations, pendulum load movement, and certainly the merits of TTRS.
Our staff wanted to see this for ourselves and take the research a step forward. Mauthner pre-measured the slack between lines, and the height of the drop, then let the load freefall onto the once-loose ropes. We performed these tests as well as a series of tests where the ropes were measured, but they were also pre-loaded as they would be in field use. Our drops were compared similar conditions between TTRS and un-tensioned belays when released from an AHD and also when the litter falls on the edge.
We used two tests masses. One was a rigid, steel, and concrete 200 kg test mass. The other was a “flexible” litter with 450 lbs of sand bags lashed within. The ropes were 250 cm in length and the drop heights ranged from less than half a meter to just over one meter. The ropes would fall over a 90-degree edge. The edge was a piece of sharp angle iron that was welded into place. The edge was protected by three plys of commercial edge canvas for most tests. PMI and CMC rescue donated a variety of unused ropes for the testing. Over the course of two days, we performed 26 drops. Ten of the drops had issues or anomalies and are not shown in this data. Their results can be provided upon request. Sixteen paired comparisons and their respective data can be viewed in the attached graphs.
The film footage corresponds to the test data. The goal of the first series of tests was to reproduce the conditions that Kirk Mauthner tested in 2014. The entire playlist can be viewed below.
Here’s what we learned: the research that Kirk Mauthner presented in 2014 was totally validated. In no instance did the un-tensioned belay fair better than the TTRS. The TTRS was more robust and resilient in every instance. Additionally, the same held true for the systems when they were released from a state of tension in the AHD or from a skate-block. Another byproduct of the testing was that impact forces on the anchors were reduced by 25% on average when TTRS were employed versus the un-tensioned belay.
There seems to be ample reason to give TTRS a second critical look. The tests seem to be busting allot of preconceived myths out there. Literary evidence and talks to other rescue instructors will offer additional benefits. The slack and subsequent stretch in the belay is a serious concern. Many un-tensioned belay beliefs-once held dear, do not deliver 100% of the time. In TTRS forces are shared across two systems and in a one meter fall, forces are reduced by 25%. The TTRS clearly has a certain toughness when a load crashed at or on to the edge.
Other reasons to consider TTRS that are not as safety driven include: the ability to perform loaded changeovers or knot passes with devices like the MPD and I’D. The TTRS is a hard target for rockfall and un-tensioned belays have a tendency to induce additional rock fall. In TTRS- rigging is the same for both sides and sudden movement is mitigated or absorbed by the other system.
The last area of benefit includes the fact that each side of the TTRS operation are receiving on-going feedback from the tensioned rope on the load. They have reasonable assurance as to how effective they are at their job by feeling the load and the rope speed relative to the other line. The person running the un-tensioned belay rarely has the satisfaction of knowing whether they did a good job or not.
But hold on. . . it’s not all TTRS sunshine and rainbows. We are not even mandating that you have to change what you are doing. There are layers to this issue as well. The TTRS does not fit every rescue profile. There are plenty of rescue models that fit into single rope technique (SRT), and rope access style rope movement. There are also tensioned rope systems or highlines that will not all fit nicely into this technique.
As with most things, however, the biggest problems are the people. The reason TTRS seems great is the widespread use of devices like the Petzl I’D and CMC MPD in hauls and lowers. Both devices seem to work really well for this application. Both pass the NFPA certifications for belay devices. Both pass the BCCTR BCT. So what’s the problem? The problem occurs in the lowering configuration. When operating each device for a lower, the operator is “defeating” the cam or the belay potential for the device. If the TPB debate taught us anything, it’s that people’s panic instinct is to grab and hold rather than let go or release. There-in lies the problem- similar to the TPB when paying out rope, the operators must let got of the MPD and I’D when the other line fails. A delayed reaction for a fraction of a second can cause a load to deck out. We are seeing this in field tests.
Solution: The next logical phase will require human testing and training with these types of devices in TTRS. Many of the benefits of this method of doing business are apparent. The human element always acts as a wildcard. Perhaps the future will incorporate engineered devices that cannot be defeated into these systems. One such device is the Petzl ASAP that is activated by centrifugal energy and resists human interference. With a little staffing, these types of tests and training evolutions are not difficult to replicate. We use a Wichard 2674 Quick Release Snap Shackle, but there are many ways to make a release mechanism.
The merits of the TTRS make it worth learning, testing, and employing in places where you might normally use an un-tensioned belay. This is especially true when the consequence for rope stretch is severe. It’s not a fix-all. It doesn’t replace good belay training and technique. And the human limitations should be explored. But, TTRSs are a very valuable tool for the modern rescuer.