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snoboy

Something scary to think about!

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Just noticed this thread. While the digression and discussion on anchor forces is interesting, it seems to me some are losing sight of or are not clear on exactly what the situation was as far as the original accident is concerned. The 2 guys involved in the accident are both friends of mine--the leader still lives and climbs in the NW, the belayer moved away about a year after the accident. Neither are posters or readers of this board, to my knowledge.

 

The belayer was standing at the base of the third pitch, near the cave and on a ledge, facing in to the ice. He had 2 screws in for the belay anchor, and was tied in to it from the front of his harness, either the belay loop or a biner on the swami. The leader did not place a screw first thing from off right from the belay, instead placing his first screw about 15 feet up as is in the short description in 1996 ANAM. At about 30 feet up, trying to get a second screw in on the WI4 portion of the pitch, the leader pumped out and fell, pulling the only screw he had between the belayer and him. The ropes then went directly from the lead climber to the belayer's ATC. Since the belayer was facing in to the ice, once the lead climber fell below the belay and onto the rope, the belayer was quickly and violently pulled--from the point of the ATC attached to his harness--180 degrees and away from the direction of he was originally facing. Since his weight was firmly planted on his crampons on the ledge facing in, and the force of the fall turned his hips around, was the reason for the tib-fib spiral Fx.

 

ANAM's analysis seems to suggest that the belayer should have clipped the rope directly into the belay anchor--something I generally do on my bombproof anchors in rock, and admittedly and lazily have done while on ice. I also do at times put in a lead screw just above the belay (separate from the anchor), which is obviously preferred, and think after this discussion I am more likely to do so in future.

 

FWIW, here's the ANAM report:

 

FALL ON ICE, INADEQUATE BELAY POSITION, PROTECTION PULLED OUT

Alberta, Rocky Mtns, Lake Louise, Lake Louise Falls

A party of two were climbing the upper pillar (grade V) of this popular waterfall on March 20. About ten meters out from the belay, the leader fell, pulling out the one screw he had placed for protection. The resulting 20-meter fall was arrested by his belayer, but the force of the fall spun him around and he sustained a fractured fibula. The leader was not injured. The two climbers were assisted to the base of the waterfall by an ACMG guide who was working the area, and then the injured climber was slung out by helicopter to an ambulance waiting at Lake Louise. (Source: Marc Ledwidge, Banff National Park Warden Service)

 

Analysis

Belays should be set up so they transfer impacts from all likely directions to the anchor in as straight a line as possible. The bleayer then needs only to control the run of the rope. (Source: Orvel Miskiw)

 

My additional comments:

- The 2 climbers were grateful for their rescue, and the fact that of the 5 other climbers on the route below them, all helped in the rescue, and 3 of these 5 were highly trained to provide emergency and first aid assistance.

- The leader was slightly injured taking that 60-ft whipper, bruising much of one side of his body, but could gladly walk away.

 

--pindude

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I'm sorry to have missed this thread!

Back to the original nerdy physics discussion.

 

Because to keep the system static, the anchor must absorb the force of the falling leader plus the equal force of the stationary belayer (less static and dynamic friction).

 

This is the best pic I could find quickly:

 

 

Just to add to the explanation a bit here:

What is missing so far in the two explanations is the belayer's anchor against an upward pull.

If you assume the quickdraw biner has 0 friction, then you do not have a static system. Your belayer would have to suddenly exert 4.6 kN (460 daN) of force downwards to be immobile, which is impossible unless he is anchored to a separate lower anchor/piece of gear (anchored against an upward pull). However, in this case (a belayer perfectly anchored from below), you would indeed have the 2-1 pulley scenario; the belayer is fixed, the rope is free to run frictionless through the draw (imagine a really really good pulley), and the force on the draw is doubled.

 

Otherwise, if you had a frictionless quickdraw and a belayer who is not anchored against an upwards pull, the belayer would be yanked upwards (a true dynamic belay!) until he "bottomed out" against the draw - and then we're back to the static factor 2 fall scenario, identical to the "no quickdraw" scenario.

 

In sum, the only way you would have

T (falling leader) + T (force on belayer) = 2T (force on draw)

is if your belayer was anchored perfectly against an upward pull. Otherwise, the force on the belayer's end will be their own body weight (which is there to begin with, so you can ignore it in the calculation), and above that they will be pulled up (in theory, at the same rate as accelaration due to gravity) until they hit the draw and the system goes static again, with the force (T) remaining on the top draw.

 

So in a real world scenario, with friction, stretchy ropes, and belayers suddenly changing position to resist a fall, I'd bet the two methods (belay right off the anchor vs clip one of the anchor pieces as pro) are within 1 kN of each other, unless you anchor against an upward pull (which makes it worse on the top piece/quickdraw!).

 

The moral is that if the belayer is anchored tightly against an upward pull, the force on the clipped piece will be theoretically much higher, which is what we knew all along. So if you're going to clip one of the pieces of the anchor as your first piece of pro, your belayer is better off not being anchored against an upward pull in the event of a fall on to the belay (otherwise there will theoretically be a 2-1 effect on that one piece).

Edited by Geek_the_Greek

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Good point, Geek. I was wondering when you'd say something in this thread

 

So: having an overhead anchor that is built to resist a multidirectional pull is a Good Thing, and usually a better idea than anchoring the belayer tightly against an upward pull.

 

Assume the belayer will have to be pulled up into the anchor (it's overhead), thus dissipating some force with his movement and the associated friction. Then, when the system becomes static as the anchor-belayer system pulls taut, the force will increase on the leader's top piece, but by now it's a much lower force to be multiplied by the pulley effect.

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It's back. Just when we all thought it had drifted off to the thread graveyard...

 

In the moving belayer scenario, the best case is that s/he bottoms out against the draw/biner. The worst case is a full-on faceplant into the rock/ice.

 

In the worst case I suspect there is a decent chance of the belayer momentarily shirking belay duty in the name of staying pretty, checking for damage afterward, and/or losing consciousness.

 

Use judgement and be thankful for friction.

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Yeah, you got it right, Sloth.

I guess what I'm saying is that if you're building an anchor and you will be belaying the next pitch and you really want to protect against an upwards pull, you might not want the leader to clip one of the anchor points as pro. In this case, you'll obviously have to be aware that a fall right onto the belay will come from a different direction and all that (so mind yer crampons!).

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It seems clear enough to me that the force on the anchor will be twice the tension in the rope when the rope passes from belayer through a quick draw and then to the climber. This will be decreased somewhat by the fact that the rope will stretch, friction will occur on the biner and the belayer will move towards the anchor. How much of a force reduction is hard to quantitate. Personally, If I were belaying, I'd rather have the anchors take the jolt and pull me towards the rock. That's why 1) you want a totally bomber anchor of three screws equalized with a cordelette or webolette, and 2) you want your leader to set a solid screw fairly soon after leaving the belay. In this case the first and only screw pulled out, which is a very very bad thing to have happen. Falling on the belay is to be avoided if at all possible! Set that first screw right away, even if the going is easy.

 

--Brian

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