rope stretch when falling on MRS vs. SRS

climbstihl

climbstihl

Branched out member
Location
Germany
My boss and I were having a disagreement today about the difference when taking a fall into a climbing system, he was of the opinion that a MRS system would be better to take a fall on, because you would get more stretch because of the amount of rope in the system, while I think you would get less or the same stretch as in SRS, because the load on each leg is only half of your weight.
The scenario we are talking about is tying in to a nearby beech tree while blocking down a dead spruce. He would be climbing above the level of his TIP here.
He is someone who only uses SRT for ascent and as a lifeline when spurring, everything else is MRS, which is standard practice here in Germany. I myself SRT almost everything.
Could anyone please tell me who is correct here? A good explanation of why would also be great.
 
My two cents without getting into fall factors etc: Generally more rope in the system is probably better but arborist ropes are generally considered to be static ropes, pretty much period (at least compared to alpine ropes that elongate 15% or more). Also as you go up in rope diameter from 11 mm, you usually go up in terms of breaking strength (ABS) and the amount of load it takes to get a certain amount of elongation (measured @~10% ABS say). So with the same weight, you expect to get even less elongation on a larger rope. Ouch. Somewhere in the X files here are figures for how much force is needed to do spinal or hip damage - but it's not a lot. Generally on any kind of arborist rope system - don't fall. What's a fall - this is one of the crux's of the UK 2 rope arborist fandango - if you can fall more than I think it was four inches or so, you're outta the work positioning world (current arb harnesses and lanyards and such) and into fall protection (fall arrest gear, full body harnesses usually industrially etc. etc.). The US Forest Service outlines (maybe old now) a system of slings and biner belays and dynamic rope for use in seed collection in conifers - sort of an alpine climbing system in conifers - but this is frowned upon by many in the arb industry. So perhaps we don't call them falls, but slips? Bottom line on the gear we use - don't fall. Don't ever climb above your TIP. ANSI has guidance. Static equipment will give you a heck of a yank, easily doing damage. This doesn't even consider the damage possible from a swing into another tree. Whoa!
 
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The last time I had a suuuuper sketchy dead tree to climb, I used srt from a nearby tree. Srt does allow for more elongation, as the one line takes all your weight vs splitting it in mrs. A base anchor gives even more rope in the system (in a substantial union, of course). In my scenario, I intentionally had my line (anchor side of basal system) deflected away from the trunk by several limbs, for extra cushion in the event of spar failure or fall.
 
My thoughts on this:
If the ratio between load and stretch is linear and the system is perfectly frictionless the stretch should be the same; the rope has half the load, so it stretches half the amount, but at the same time the rope is double the length, which means it stretches double the amount. These factors cancel each other out, so in that case the fall factor is the same.

if you fall on two fixed lengths of rope (TRT) you'll have half the stretch since the length of the rope isn't doubled.

In real life you have a substantial amount of friction on the anchor. This means that the actual outcome will be a mix of those two cases, the effective stretch will therefore be smaller on an MRS system, and the force on your body will be greater.
 
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We're all in agreement that we shouldn't take any sort of fall, it was more of a hypothetical point in our ongoing discussion of the benefits of MRS vs. SRS. The spruce is also nothing we wouldn't climb without a nearby tree.
 
A single length of line will stretch more than one that is double the length but has been doubled over. within lower peak loads the stretch is activated faster, Samson have data on this shown in their Rigging Peak Force Calculator thingy that looks like a wheel, it shows that thinner rope (less material) reduces peak forces.
Conversely a single leg has less capability to absorb force due to its shorter length.
a trunk anchor run through a pulley will be most effective, then a single leg tied at the top, and then a MRS line doubled over at the top.
 
Also need to factor slip factor of climbing system. Ascenders - none. Hitches some. Mechanical friction devices maybe some more.

Bending branch TIP best case scenario. Pendulum swings also. Vertical drop worst case.

Maybe tailor climbing style to take care advantage of these than relying on internal padding of harness to save you?
 
Maybe couple more cents without getting into fall factors etc: F = m X a. In a fall, you are depending on the rope you choose to absorb the energy of your fall (a static system will break your back) over a period of time (the "a" is in m/sec, which has a time dimension in it). This last bit is important. The rope type (dynamic not static) and amount you use spreads this fall energy out over time - in rigging, esterlon for example stretches, but it is still not a dynamic rope in the climbers sense: i.e. one that stretches a great amount, to spread the force of the fall out over time (like the fenders and hood crumple zone in a car). You're using the rope like a spring thing. In climbing, dynamic twin ropes, designed to be used doubled up, are engineered to still spread the force of a fall over time but they mustn't be used as single ropes (not good things will happen in a high fall factor fall).
In a much earlier post in TreeBuzz some years ago I posted a link to ACMG break tests with prussiks on rope - while the prussiks did slip, they also did rope damage as well. That thread also contained a comment that climbers who go through the Alpine Club of Canada fall arrest training always walk away amazed at the forces involved in catching a fall ("boot the bag off the cliff") - it's an eye opener.
For falls, dynamic ropes, more rope, and harnesses designed for fall protection (full body with ventral and dorsal attachment points) are your friends. Static arb ropes regardless how much rope is in the system - not so much. Stay safe everybody - climbers and your families in these weird times.
 
There is much higher potential for energy dissipation in an SRS than an MRS. It will however, depend on how you set it up.

Canopy anchor will have the least and will probably not be significantly better than an MRS. A base anchor, over several carefully chosen redirect points, maximizes system potential for energy dissipation.
 
DSMc said:
There is much higher potential for energy dissipation in an SRS than an MRS. It will however, depend on how you set it up.

Canopy anchor will have the least and will probably not be significantly better than an MRS. A base anchor, over several carefully chosen redirect points, maximizes system potential for energy dissipation.
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This is the clear, succinct answer.
 
ghostice said:
Maybe couple more cents without getting into fall factors etc: F = m X a. In a fall, you are depending on the rope you choose to absorb the energy of your fall (a static system will break your back) over a period of time (the "a" is in m/sec, which has a time dimension in it). This last bit is important. The rope type (dynamic not static) and amount you use spreads this fall energy out over time - in rigging, esterlon for example stretches, but it is still not a dynamic rope in the climbers sense: i.e. one that stretches a great amount, to spread the force of the fall out over time (like the fenders and hood crumple zone in a car). You're using the rope like a spring thing. In climbing, dynamic twin ropes, designed to be used doubled up, are engineered to still spread the force of a fall over time but they mustn't be used as single ropes (not good things will happen in a high fall factor fall).
In a much earlier post in TreeBuzz some years ago I posted a link to ACMG break tests with prussiks on rope - while the prussiks did slip, they also did rope damage as well. That thread also contained a comment that climbers who go through the Alpine Club of Canada fall arrest training always walk away amazed at the forces involved in catching a fall ("boot the bag off the cliff") - it's an eye opener.
For falls, dynamic ropes, more rope, and harnesses designed for fall protection (full body with ventral and dorsal attachment points) are your friends. Static arb ropes regardless how much rope is in the system - not so much. Stay safe everybody - climbers and your families in these weird times.
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This is a clear and long, but very good answer
 
It would certainly be nice if there was a simple answer, but it's a complex problem... so, no bueno on that. For example... for any given rope...

If you perform drop tests with the same rope (in both doubled and single config) to determine how far a given weight can be dropped and absorb all of the energy above a certain point (say, 900 lbs. so you don't die) they will be radically different for both single and doubled configurations. No rope elongates with a linear curve, although there will be a section of the curve that is fairly linear and would allow some fairly accurate predictions for a narrow range of weights/fall distances.

You still end up trying not to fall any distance. But, I think you'd find that most arborist ropes would absorb the bulk of the drop forces for the kind of drop distances that common sense tells you might be survivable, assuming your hitch/device grab, etc. and that the rope OEMs really do try to engineer rope performance such that short falls are not fatal.

I wonder if a more pressing concern isn't the point at which we are pushing our luck and should give serious thought to adding a fall arrest device to the system. The anchor point might be the best/easiest place to implement one.

Most of our multicender and hitch climber devices will act, within reason, as one... but if we need to climb above our TIP... well, I think anything short of a true fall arrest device isn't going to prevent serious injury.

Just some thoughts.
 
On a side note, to be filed under "what kind of crazy shit was that?" yesterday I used 12-strand rope in a Rig 'N Wrench to allow my helper to ease me back to the trunk on a long and scary horizontal limb walk, that would have been a nasty big swing if I made the slightest mistake. Worked like a charm. Yet another unexpected use for that setup. We were using it to lower the end of the limb (about 15') to the roof, so it was already anchored where it needed to be. I'll need it there for tomorrow, so left it up there. I don't know that I'd go to the effort to set it up as ground retrievable just for that use, but it got me thinking that the RnW could be anchored somewhere near the ground and used for this purpose. Climber could just pull the end of the rope up with a haul line.

I probably need to cut back on the coffee. I don't want to be up half the night thinking about this.
 
JeffGu said:
I wonder if a more pressing concern isn't the point at which we are pushing our luck and should give serious thought to adding a fall arrest device to the system. The anchor point might be the best/easiest place to implement one.
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I previously used some shock absorbers attached to my harnesses. Never needed them, but gave a nice smooth stop when I was testing them
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Brocky said:
...smooth stop when I was testing them...
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I think having them around isn't a bad idea at all... if you can anticipate their need when you plan your climbs, especially. Doesn't look like those ones would be much of a burden to carry up with you, if you thought they might be needed.
 
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