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Tuesday, March 11, 2014

Q&A: Strength Impact on Webbing

Q&A: Strength Impact on WebbingREADER QUESTION:
Regarding the Roco video/technique for shortening webbing... what is the strength impact on the webbing with this technique? When going from the looping double/basket method to a choker configuration changes the strength quite a bit because of all sorts of twists and bends... would the strength impact be 50%?

After reviewing your question, we decided to do some “in-house” testing. Of course, it’s unofficial, but here’s what we did and the results are shown below.

Using new 1-inch tubular webbing anchored to a fixed anchor 2 inches in diameter, a “webbing shortening technique” knot was placed in the webbing. The webbing was marked with a felt tip marker on both sides of the knot to measure slippage. A carabiner was placed in the test webbing and attached to the dynamometer by a short piece of 1-inch tubular webbing that was looped, doubled and ran through the attachment opening on the dynamometer. The dynamometer was then anchored to the load.

Unofficial In-House Test Results:

  1. Unloaded 20-ft of 1-ft piece of tubular webbing:
    Force applied 3,340 lbf with 2½ inches of slippage in the webbing.
  2. Loaded 30-ft webbing of 1-ft piece of tubular webbing:
    Force applied 3,560 lbf with ¾ inch slippage.
  3. Loaded 30-ft webbing of 1-ft piece of tubular webbing:
    Force applied 5,080 lbf with 1 inch slippage.
  4. Loaded 30-ft webbing of 1-ft piece of tubular webbing:
    Force applied 5,460 lbf with 1 inch slippage.
  5. Loaded 40-ft webbing of 1-ft piece of tubular webbing:
    Force applied 5,620 lbf with ¾ inch slippage.
  6. Loaded 40-ft webbing of 1-ft piece of tubular webbing:
    Force applied 6,230 lbf with 1 inch slippage.

Tests #1 & #2 were slow tension pulls on the knot. Tests #3 & #6 were dynamic shock loading. Tests #5 & #6 both had failures of the 1-inch tubular webbing anchor at the attachment to the dynamometer. The failed anchor webbing was in a basket looped and doubled configuration and failed at the sharp angle connection on the device.
The test webbing did not fail in any of the tests, but on tests #5 & #6 it showed slight glazing on the inside of the knot when inspected.

Due to the failure of the anchor webbings, we were not able to generate more than the 6,230 lbf force on the system. Based on these unofficial tests, I would feel comfortable using the technique for NFPA General Use loads and would not expect it to weaken the webbing any more than the efficiency loss of any other knot that would be tied into the webbing, including a water knot.

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