Picket Test, Back of Envelope Calculations?

[korup]

(Repost from picket thread).

 

Question- any physics geeky types want to guesstimate some force numbers? Coeffecient friction for snow ranges from 0.05 to 0.6, Ptex is very low, I weigh 90 kilos, and ??

 

At Alpental a friend and I did some on-snow picket testing with skis. All the tests were done with a 24" MSR Coyote (no cable, just runners), a 10.5mm rope, and the rope was (unrealistically) tied directly to the anchor. Snow conditions were 3-4" of wet corn/slush, and then more consolidated snow below that, on a slope (Sessel) of 15-20 degrees. Speed at impact with anchor felt like ~25 mph(?).

 

Test 1: Vertical placement, top clip, hammered straight into the snow. Skied 30m of rope. Slight tug on harness, followed by explosive anchor failure, with picket landing ~10 feet from where I stopped downslope, pulling the entire coiled rope with it. FAIL.

 

Test 2: Vertical placement, top clip, ~15 degrees back from horizontal, placed below slush level. Skied 15m of rope, with the same results as above. FAIL.

 

Test 3: Vertical placement, middle ("Sierra" style) clip, ~15 degrees from horizontal, below slush level. A slot for the runner was excavated, and then snow was replaced and compacted. Skied 15m of rope. FAIL.

 

Test 4: Horizontal "deadman" placement, ~2' down, with a trench for runner. 15m of rope skied. Anchor INTACT.

 

I don't have any way to easily estimate the forces involved, but might sacrifice a Screamer if I do it again. The forces were all directed down, and should have pulled the anchor deeper in the snowpack, but they simply ripped straight out through the slope.

 

 

Edited June 3, 2008 by korup

[bstach]

This method doesn't sound very safe. I'm glad you didn't get impailed by a flying picket.

[hafilax]

The difficulty with modeling impact forces is in determining the nature of the force as a function of time as the rope slows the falling body through elongation and dissipation. Apparently Hook's law works well for the elongation but the model for dissipation is more complex.

 

It's easy enough to estimate the kinetic energy just prior to impacting the anchor but what happens after that can't be analyzed analytically.

 

It appears that there is a gradient in the strength of the snow. Theoretically angling the picket should cause it to go deeper on impact but if the upper snow is weaker it will torque and pull out. My guess is that clipping the middle is supposed to be a compromise between a picket a deadman but you're weakening the upper part of the snow by digging the groove for the cable. I wonder if clipping further down would start to even out the tendency to torque out and would encourage digging deeper instead of pulling up.

 

A screamer won't tell you much other than that you exceeded the activation force. Too bad force gauges are expensive. You could do a series of test where you start off small and increase the speed at impact until failure. That will give you more information.

[Ishmael]

This method doesn't sound very safe. I'm glad you didn't get impailed by a flying picket.

 

Pickets have very low mass... should not be able to cut you up too much...

[mattp]

I used to do these kinds of tests with groups of beginner students. It gives you a sense of what works and what doesn't, kind of like doing some aid climbing to learn what kinds of placements of rock gear are likely to actually hold. Since most of us don't carry calculators and x-ray glasses a gut sense analysis based on prior experience, even if the testing method is flawed, is worthwhile. It is not a bad idea to consider the information presented in Freedom of the Hills, or the latest analysis from engineer-geek-climbers.com but these kinds of field tests are very helpful.

 

 

[Gary_Yngve]

let's call 25 mph 10 m/s and say that g = 10 m/s^2.

 

If an object were in freefall, that would mean it took a full second to reach 10 m/s, during which it traveled an elapsed distance of 5m.

 

30m of rope total before stretch and a 5m fall would be a fall factor of 0.167, which seems pretty damn small. Your extra mass (standard is to use 70kg) would up the fall factor to about 0.233.

 

As for how much force a fall of a certain fall factor generates on a rope, that's a harder question. Certainly the rope provides an upper bound of 7-9 kN, and when you're hanging in space from a rope (fall factor 0), you're 0.9 kN. In either case, the top piece could have an x2 multiplier from the pulley effect (e.g. belayed from beneath), but not in this case because you're tied straight in.

 

(Repost from picket thread).

 

Question- any physics geeky types want to guesstimate some force numbers? Coeffecient friction for snow ranges from 0.05 to 0.6, Ptex is very low, I weigh 90 kilos, and ??

 

At Alpental a friend and I did some on-snow picket testing with skis. All the tests were done with a 24" MSR Coyote (no cable, just runners), a 10.5mm rope, and the rope was (unrealistically) tied directly to the anchor. Snow conditions were 3-4" of wet corn/slush, and then more consolidated snow below that, on a slope (Sessel) of 15-20 degrees. Speed at impact with anchor felt like ~25 mph(?).

[Gary_Yngve]

http://www.getbeta.com/fall_factor.asp

 

says that 2.5m above the last piece w/ 30m rope out would generate 5.5kN for a 90kg climber.

 

That's a lot of force! (1200 lbs of force!)

[Gary_Yngve]

although that form seems to be bogus because it says that falling a distance of 0 would generate 5kN of force for a 90kg climber (1100 lbs of force).

[korup]

Petzl has one; maybe more accurate?

 

http://en.petzl.com/petzl/SportConseils?Conseil=56&Activite=14

[Gary_Yngve]

yeah, the petzl one is better. with no fall distance, just a shockload, it's saying 3kN w/ pulley effect, which would be 1.5kN tied straight in to the piece.

 

with the stats that you gave, it's saying 5kN w/ pulley effect. so straight to the piece would be 2.5 kN.

 

so it seems that you had roughly 500 lbs of force on it, and in all cases but the deadman, it blew. dunno how much less force you'd need for it to hold, but the point is that 500 lbs of force is very easy to generate.

[NateF]

Most of the online calculators are incorrect in some way or another, although I believe the 5kN answer is still in the ballpark. The loads generated are relatively high unfortunately.

 

With 30m of rope out, a fall without climbing above the last point of protection will still generate a shock load in excess of body weight, because rope stretch will allow the falling climber to accelerate downward. 5kN is not the right answer though, it should be lower.

 

If the climber yells take, and the belayer pulls rope until the climber is supported, the dynamics are mostly eliminated and it's a different story.

 

Try this calculator:

http://en.petzl.com/petzl/SportConseils?Conseil=56

 

[NateF]

Oops, Korup beat me to the Petzl link.

 

Gary, I'm a little confused by what you're saying about the pulley effect. The 3kN (or 5kN as the case may be) calculated IS the shock load on the last piece. It is not half that value. Scary, but real.

[korup]

Thanks for the numbers and confirmation that the system wasn't too far from real world scenarios.

 

Take home message- vertical pickets in spring snow = totally psychological pro.

[Gary_Yngve]

Oops, Korup beat me to the Petzl link.

 

Gary, I'm a little confused by what you're saying about the pulley effect. The 3kN (or 5kN as the case may be) calculated IS the shock load on the last piece. It is not half that value. Scary, but real.

 

Korup's setup is a load attached to a rope attached to a picket. The force on the picket from the load is F. Since the picket stands still, there is an opposite but equal force on the picket resisting the movement.

 

However, for a top piece with the rope running through with no friction/drag (a pulley), both the load and the belayer are exerting force F. Since the top piece stands still, the opposite force on the top piece is F+F = 2F.

 

This is the pulley effect, which is modeled in the Petzl simulator.

[NateF]

Gotcha, I see what you're saying. I wasn't thinking in context with Korup's setup

[selkirk]

 

There was some good picket work previously done by Kiwi's.

follow the linky linky.

http://www.alpineclub.org.nz/documents/activities/instruction/snow%20anchor%20report.pdf

 

As memory serves the conclusion was that all snow anchors are very suspect.