What’s a Tension Back Tie?
In the rescue world, we love a solid anchor but sometimes anchors just aren’t in the right place and you have to engineer what you need. We use a tension back tie element to focus a super good anchor that’s not in a great location to a, not-so-good anchor that’s in a great location. A great location means a spot that’s safe to work and lines up with the spot where we want to go over the edge. In rescue, we are dealing with bigger two-person loads so we want the anchor to be redundant and stout.
We can use any rope for the tension back tie but let’s face it, 11mm or 13mm rope is heavy and stretchy. We have been using Sterling Power cord for rigging anchors for at least five years and its very low stretch (1.5% with 600 lbs), its 5.9mm with a MBS of 19.7 kN, and it only weighs about a pound and a half per 100 feet. Compare that with 11mm static rope at six and a half pounds per 100 feet. But is Powercord strong enough? Tom Pendley from Desert Rescue Research came to Hownot2 to find out.
The test sample is a 5-year-old, moderately used length of PowerCord from Sterling Rope. PowerCord is a Technora core with Nylon mantle. Since this sample is 5 years old we started out on the bollards to get a base line on strength and it came in at 19.3 kN which was close to full strength. Then we tested figure 8 to figure 8. The average of 3 pulls was 7.8 kN which was about 60% strength loss. Did you just say 60%. Yup, Aramid fiber cord is known for pronounced weakening from knots. However, the PowerCord makes up for this in its baseline strength, lightweight and very low stretch.
When we moved to the 2:1 tension back tie we got an average break strength of 15.56 kN (3,498 lbf). On the 3:1 tension back tie, the average break strength was 24.05kN (5,406 lbf).
In all of the tests, the PowerCord broke at the knot and I asked Tom if he had a concern about the significant strength loss in the Aramid cord at the knot. He said “In my opinion, it's old school to apply a 10:1 static safety factor requirement with a broad brush to your entire system. With dual main systems, we just don’t have that worst-case shock load potential in our systems. That’s especially true if we are using devices that reliably force limit.” He also said, “Look at these forces, it’s incredibly difficult to generate failure forces in these anchor components. If we choose sound anchors and make a clean system, it's even less likely.” “This is why we test, to understand the material performance. Yes, you can go big, it's fine but it's not necessary and it’s not efficient. There is no body count from blowing up anchors.” “Spend your time and energy preventing edge trauma events to your ropes”.
The big advantage of tension back ties is twofold. First, you can introduce a strong, low-stretch tension element between anchors and second, you can easily get equal tension on the components of a master point in your anchor system.
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How NOT 2 use the Ronin by a guy who repairs them when you do it wrong.