Safe Maximums for Rope Rescue

Replies to This Discussion

Permalink Reply by Ben Waller on June 24, 2009 at 11:45pm

Jeff,

1) I believe that you need to re-read my post where I mentioned the 1,200 lb load.
I was advocating against it, based on the fact that you can easily cut the load by at least half by eliminating 3 of the 4 rescuers on the litter. I also stated - repeatedly - that you shouldn't put more than one rescuer on a low-angle system if the slope is greater than 45 degrees. There is no contradiction at all in my statements. My point was that 1,200 lb loads are inherently less safe than 600 lb loads.

2) It's very possible to engineer rope systems that have a 15:1 safety factor. Double-rope systems that share the load have essentially the same strength as an unknotted rope. If knotting the rope removes 50% of of the strength, then doubling the rope adds that 50% right back.

3) I'm astounded that you would continue to advocate that a theory that doesn't account for all of the real-world variables could possibly be more accurate than a direct reading of the system force with a well-calibrated instrument. What part of "real-world measurements are more accurate than theory" is so difficult to understand?

4) You state the exact opposite of the facts on this one. The math doesn't overestimate the load, it underestimates it, sometimes by a large margin. That's why the theory can add danger by giving a false sense of security. Trig doesn't let you calculate factors like two-dimensional movement, three-dimensional movement, shock loads, friction, or other dynamic forces. Only measuring actual forces do that.

Building a stronger anchor is not always possible or practical, it tends to be time-consuming and gear-consuming, and it doesn't improve the rope part of the system. In real-world rescues, those are universally undesirable. Your other option - :"lowering the load" (if by "lower" you mean "reduce") is exactly what I've been advocating all along. I have already calculated how many people to use in the system if it's on a slope greater than 45 degrees - it's two! One patient, one rescuer. Every rescuer you add to that number unnecessarily cuts into the safety margin. No additional calculations are needed.

5) Based on my view of theoretical math, we estimate the load prior to leaving the station. One non-bariatric patient, one non-bariatric rescuer, a litter, and a little gear equals a load of 600-lbs or less.

6) Please tell Kurt and Katiethat you believe that theoretical math is more valid in the real world than actual force measurements. I believe that they would agree with me that the actual measurements are more valid. If their theoretical math doesn't take every real-world variable into account, then yes, it's flawed. The amount of the error introduced can be measured by a direct-reading instrument like a load cell.

I'm astonished that you would continue to advocate against real-world force measurements. I challenge you to keep your low-angle load to a single rescuer and the patient and measure the force on the system in a steep-angle slope evacuation, then hang four or six rescuers on the same system on the same slope and measure the forces in the same way. The single-rescuer load will weigh less, and thus be inherently safer every time, regardless of your calculations.

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Permalink Reply by TrainingTECH1 on June 25, 2009 at 8:15am

OK. This is getting us no where.

I agree, and have agreed, that cells are the most accurate way to do these measurements. However, when lacking this equipment the math is the only other way to estimate what the original question of this thread was about.

I have never had a problem with the math being too far off from the actual measurement either + or -.

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Permalink Reply by Ben Waller on June 25, 2009 at 10:44pm

Jeff,

It should be getting us somewhere. You don't need to have the load cells on hand to be able to prove that the absolute best way to reduce stress on the system is to reduce the number of people in the load to the minimum necessary to complete the rescue.

Lighter load = less risk of system failure.

Fewer people on the load = fewer rescuers at risk of system failure

Lighter load = fewer people in the load = safer operation

The original poster was - in essence - asking for a mathematical way to reduce low angle safety margins. My intent is to show that you don't need the math if you use systems that increases the safety margin.

In other words, there is a better way to do low angle rescue that doesn't require a herd of rescuers attached to the load. If you don't use a herd, you don't need math to prove how many herd members you can use.

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Permalink Reply by Joe Smith on June 25, 2009 at 11:30pm

Is there always one solution to address all situations?

It seems pretty common sense that the less the load, the safer the system!

The 'essence' of my original post was to better understand the systems we build, and the foundation behind them. Don't try to read further into things than it's worth.

FYI, it is beyond the reach of many teams to have load cells in their training rack. So if you have hard statistical data that you have gathered from your experience that would alleviate the need for me to do math I can't wait to see it. That would have been a much more valuable response to my original post; rather than the last 2 pages of nonsense.

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Permalink Reply by TrainingTECH1 on June 26, 2009 at 12:55pm

Joe,

Here is the chart that I created awhile back. I deviated from the typical loads that NFPA referenced in previous standards. (300 - 1 person, 600 - 2 person aren't referenced anymore in the standards. It's now General and Light) I used a 100 kg (220 lbs) load which is a common load estimate and it makes the math easy. I converted everything to pounds to make it more applicable.

Trying to get with a buddy of mine that has the notes for the calculations and pictogram to go with it. Mine were misplaced by someone who borrowed it. Email me at info@ technicalrc.com if you want this as well.

Attachments:

• Low-Angle calcs.pdf, 5 KB

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Permalink Reply by Joe Smith on June 26, 2009 at 8:49pm

Cool, That works for figuring out some % based averages. Mind if I ask how you had your setup when you attained these numbers?
Edge protection?
What did you use for a load?
You used a load cell for these correct?
Where was the load cell placed in the system?

Permalink Reply by TrainingTECH1 on June 26, 2009 at 9:22pm

Joe,

These numbers are 100% math based. It takes into account the angle and the effect of gravity on the load as the angle increases. What intrigues me the most is that people don't realize that the loads add up pretty quick even at low angles. They asume because the hill is relatively flat they can carry the litter with 6 people and a patient!

I don't have regular access to load cells and to be honest to come up with 4 or 5 diferent load configurations and multiple angles would be extremely tedious work. Anyone who has load cell numbers I would love to compare notes.

Permalink Reply by Ben Waller on June 27, 2009 at 2:09pm

Joe,

The rule of minimizing the load - every time - was my point.
That will give you the safest load possible - every time - without resorting to either a load cell or theoretical calculations.

I can't find my load cell data - I've moved twice since I did that testing, and the results were on paper. Suffice it to say that the intuitive - lighter loads are safer - is going to be proven, regardless.

The most important question is "How can we build the safest system", not "How close to the theoretical maximum can we load the system before it fails." I don't think focusing on asking the right questions is nonsense.

My point is that if you build the safest possible system, the theory of how to build a less-safe system doesn't matter. That's why I prefer the sled technique over a multi-rescuer carry - the load is lighter, regardless of the angle or the method used to measure or calculate it.

Heavier loads are inherently more dangerous than lighter ones.

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Permalink Reply by DUST on June 26, 2009 at 8:52am

Joe Smith,

How about a thank you for the responses to your question! You got quite a lot of info. and yeah some debate (not uncommon in a forum) from people that seem to know a lot about rope rescue.

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