Building a constant tension highline system & the pros & cons of very low-stretch rope
PHOTOS LOUI McCURLEY/PMI
One student from 2005’s FireRescue Conference and Expo rope rescue class readies himself for the descent to the parking structure below. Note: If you want to use someone else’s building to perform rope rescue drills, take great care not to leave foot marks on the side of the building. |
Highlines, or tensioned track rope systems positioned between elevated anchors, provide an effective and safe method of reaching and retrieving important targets when routine methods have been analyzed and ruled less safe or less timely. At its most basic, a highline is a rescue-quality rope securely anchored to one elevated place, such as a high-rise building, and stretched across some span to another anchor. When rescuers place a pulley on this track line, it creates a riding trolley that they can use to transfer a load back and forth between anchors using separate control lines.
Like many technical rescue techniques, highlines suffer from bad public relations; there's usually a more orthodox, more practiced method of achieving a rescue, therefore rescuers aren't comfortable building them because they don't use (or practice) them often enough.
(This corollary is but a smidgen of the Hazard Curve Paradox that helps define technical rescue development — an article for another day.)
There are hundreds, maybe even thousands of different ways to build highlines, but this discussion will focus on two interesting and unique highline features: the constant tension system (CTS) and the use of very low-stretch (VLS) ropes for the track line. During the FireRescue Conference and Expo in Las Vegas in November 2005, attendees of the rope rescue practical course built CTSs and basic highlines using VLS ropes. All were impressed with the simplicity and utility of both systems. A new and exciting rope rescue course at this year's show, called the "Rope Rescue Challenge," will allow teams to rotate through a series of rope rescue scenarios.
The constant tension highline system designed during last year’s rope rescue class at the FireRescue Conference and Expo can be seen here using bright yellow ropes. |
THE CONSTANT TENSION SYSTEM
Stripped to its bare minimum, a CTS highline is built like any other highline and can be angled (from the top of a high-rise building to the ground) or nearly horizontal (from building to building). But instead of hard-anchoring one of the sides, the pulleys used to tension the track line are left to dynamically and automatically adjust to loads applied to the track line. Envision a track line tensioned by a 4:1 pulley system and pulled by three strong rescuers. If each rescuer applies 50 lbs. of force (lbf) to the rope, the theoretical force applied to the track line equals about 600 lbf (50 x 3 = 150 x 4 = 600 lbf). This is a good approximate average tension necessary to tighten up a 1/2" nylon track line.
However, if you replaced the rescuers hauling on the pulley system with 150 lbs. of dead weight, the tension of the track line would be the same, except you wouldn't have a bunch of whiny rescuers pulling on your system; they could go do something else. So instead of tensioning and then anchoring your track line, you 1) tension it, 2) leave the pulley system attached to the track line, 3) run the haul line of the pulley system to some elevated anchor so weights can be attached and 4) anchor the track line, but leave a bit of slack (18 inches or so) to allow the CTS to operate. As you place a load on the track line, the weights will rise to accommodate.
As you off-load the track line, the weights will descend to their starting point. Important: Remember to make a positive tensionless anchoring of the end of the track line. If someone knocks over the tripod or the weights melt off the haul rope due to an errant laser beam, it's best to have the track line anchored.
During the 2005 class, team members positioned an Airshore tripod on the top of the mid-level high-rise hotel. |
A digital dynamometer was attached to the track line of the CTS system. |
PHOTO GLEN ELLMAN
For the 2005 rope rescue course, students used VLS for their track line, a 100-percent nylon control line and a 2:1 vertical control line. |
Load limiters should be placed between the VLS rope and the anchor. Any shock loads that might hit the VLS track line are limited to 500 lbf at the anchor. |
CTS IN ACTION
The illustration shown below was made in support of FireRescue's FIRST DUE conference in Las Vegas in October of 2000. A group of us built a 1,200' highline from the Stratosphere Tower using the CTS to maintain tension in the highline for two days, and it worked flawlessly. To elevate the track line and the control lines, we placed them all in a triple sheave pulley anchored in the apex of an Airshore tripod. The haul line was run through a wall anchor about 20 feet from the tripod and back to the triple sheave. The weights were attached and hung down nicely between the legs of the tripod.
VLS ROPE
Most rescue-quality rope manufacturers in the states now make VLS ropes. I started using them when they first came out in the early '90s and instantly fell out of love with stretchy nylon ropes. We owe a great deal of our technical rescue know-how to the caving community, which invented the first relatively low-stretch nylon kernmantle ropes. These ropes made ascending markedly more efficient than ascending on the dynamic climbing ropes of the time; plus, they were a lot more abrasion resistant. But those same caving pioneers were slow to take the next logical step and make a rope with a lower modulus of elasticity. Something in their blood said, "Some back-up stretch sure would be nice if someone accidentally dynamically loaded the rope." And, "Most rope rescues are performed by people who aren't technical rope gurus, and are, therefore, liable to need some wiggle room when operating their systems." These two tenets are good ones, and I would never encourage beginners to use VLS ropes. However, as rescuers become more proficient at the operational and technical levels, they seek to refine their skills and their equipment. But since the heart of every rope rescue system is the rope, switching to VLS ropes in certain circumstances is like switching from a Chevy Impala to an H-1 Hummer. Note: Polyester is usually the fiber of choice; however, some manufacturers have crafted hybrids with polyester sheaths and nylon cores.
My (often controversial) opinion is that VLS ropes are superior performers when used in four places: 1) rappelling, 2) main line raises and lowers, 3) pulley systems and 4) track lines in highlines. I still encourage the use of stretchier 100 percent nylon kernmantle for belay lines and the control line component of a highline.
When rappelling on a fixed VLS rope, your descent rate and stopping distance are more predictable. And if the anchor is far from the edge, VLS eliminates that nauseating stretch as you load the rope, making for a much softer and more secure edge load.
On main line raises and lowers, VLS rope is simply more responsive. The rope moves through your anchored descent-control device more smoothly, and when you want to stop the load quickly — it stops. It does not continue to descend and bounce as if you were using its nylon cousin. On raises, when you want the load to rise, it does; there is no hesitation while the haul system sucks up the stretch. The performance difference in your systems is noticeable and predictable. You no longer have to guess when the resistance of the elasticity equals the force applied to the rope.
The greatest improvement in performance can be seen when using VLS rope in pulley systems, because you don't have to use your team's muscle to get the stretch out of the rope. If you have four people hauling on a pulley system using nylon rope, you can do the same amount of lifting with two people using VLS rope — send the other two rescuers to do something constructive.
Using VLS rope as the track line in a highline allows for much less tension to be applied to the rope, effectively increasing your safe working load ratio. If a nylon track line has 1,000 lbf of tension to create enough sag to clear obstacles, you can achieve the same amount of catenary angle with about a third of the amount of force using VLS. A 300-lbf reading on the old load cell when your load is at mid-span looks much safer to me than 1,000 lbf. Remember: Use nylon for the control lines and make sure both ends are dynamically anchored; you must be able to catch and recover the load should the stray meteor break your VLS track line.
Rope rescue icons and instructors Steve Hudson and Kyle Isenhart stand together for a rare photo opportunity. Both lent their expertise to the rope rescue class during last year’s FireRescue Conference and Expo. |
USING THE CTS & VLS ROPE TOGETHER
Once you get the hang of the CTS, you can start using VLS rope in place of nylon rope. Essentially, you'll have a dumbbell weighing 50 lbs. connected to the tensioning 5:1 pulley system of the track line supplying 250 lbf to the VLS track line. Those are impressive numbers generated by new technology and new ideas. So go have some fun and be safe.
Engineering Summary of the Stratosphere Highline, October 2000
By M. G. Brown in association with Kyle Isenhart
The event in Las Vegas was held in conjunction with FireRescue's FIRST DUE Conference on Oct. 14 and 15, 2000. The highline rope rescue system is a fairly basic tensioned rope system designed to evacuate people from elevated positions. Essentially, a 13-mm rescue-quality rope, manufactured by Sterling Rope Company using Honeywell Allied High Tenacity Polyester (HTP) Fibers, was anchored to the ground using conventional full strength anchoring techniques. This 10,000-lbf (45 kN) minimum breaking strength (MBS) rope was anchored to a secured 70,000- lb. fire service apparatus, approximately 1,200 feet away from the base of the tower.
The top of the rope was anchored to the 101st level of the Stratosphere in one of the two refuge areas immediately underneath the visitor's pod. The rope held a constant tension of 250 lbf to 550 lbf (3.3 kN-5.5 kN) using counter weights and a 5:1 mechanical advantage constant tension rigging system. The rope span from top to bottom measured 1,200 feet, with a span rope weight of 97.2 lbf. The two primary anchors for the top rigging area were AB Chance 3⁄4" x 30" all-thread, chrome-moly, eye bolts with 4" x 4" spring backing plates using double locking nuts, and rated at 18,000 lbf (80 kN) MBS, anchored into 24" thick load-bearing high-compression reinforced concrete.
The load along the length of the track line was belayed using 1⁄2" nylon rescue-quality control rope running through Rescue Systems Inc. closedend rescue racks, with internal primebar intercoolers. The control ropes were affixed top and bottom using rope clutches that allowed for limited slip in the 2,000-lbf to 2,500-lbf range (13.3 kN–15.5 kN). The control ropes were rigged in a manner to belay the remote possibility of track line failure. The control rope and hangers added 112 lbf to the track line weight. Loads of 150 lbf to 600 lbf were moved along the length of the track line. General high-end yield traffic loading on the anchors did not exceed 1,500 lbf. Unexpected dynamic loading of the system was dampened by shockabsorbing rip stop Yates zippers, preventing forces from exceeding 2,500 lbf at any point along the system. Electronic load cells and a flex beam dynamometer were built into the system as cognitive and tactically informative devices.
Michael G. Brown is a program manager in the Technical Services Division of Applied Marine Technology Inc. (www.amti.net). Mike retired in May 2003 as battalion chief for the Virginia Beach Fire Department after more than 27 years in the fire service, but remains active in the rescue field as task force leader with FEMA's Urban Search and Rescue (US&R) program on VA TF-2 and as rescue operations chief (deputy) on the IST-Blue Team. Mike supervised federal US&R rescue and recovery operations during the Oklahoma City bombing, the Sept. 11 attacks, the space shuttle Columbia incident, and he was the first federal on-scene rescue operations section chief in New Orleans during Hurricane Katrina. Mike continues to teach the technical rescue disciplines, particularly Structure Collapse Technician and Rope Rescue. Mike is the author of "Engineering Practical Rope Rescue Systems" and can be reached at mbrown@amti.net.
