We've been taught this about falls while ice climbing: DON'T. Not bad advice, but what's it based on? Yes, there are all of those sharp objects involved with our choice of climbing medium, any one of which could do severe damage. But just how much can that screw hold? Beverly and Attaway put real screws to the test in vertical waterfall ice. (Climbing Nerd Level 2)
They tested three hypotheses:
- The force determines whether the ice screw fails or not
- Short screws in good ice don't resist high forces
- Falls on ice protection generate the same forces as falls on rock protection, and placed correctly in good ice may surpass rock gear
Previous drop testing on screws in aerated ice by Luebben found lots of failure, but the study did lead to a change in the orientation of ice screws: counterintuitively, 10 to 20 degrees downward instead of the previous practice of 10 to 20 degrees upward. (Why? Because a hollow tube resists a force better because of skin friction (and threads) than taking a force from the side.)
Details: GriGri with a stopper knot to reduce rope slippage, Rescue Randy (172 lbs.), ice type typical of most water ice characteristics, fall factors 1/2/1.7, both lead climbing and anchored set ups. Screws tested: Grivel 360, Petzl Laser, Petzl Laser Sonic; all (13-cm?) stubbies.
Meaty tidbits of knowledge:
- Ice screws failed when the force was between 4 and 14.5 kN (message: ice screws can fail under pretty low load when placed in suspect ice as was the 4-kN test event)
- Ice screws held forces between 5 and 15 kN (message: ice screws can hold even under pretty high load)
- 11 out of 61 tests showed screw failure, which means 50 out of 60 held the fall
Huh? The screws that failed tended to face a higher load than those that were successful, but clearly there are other factors at play besides how much force is placed on the screw. Turns out it's selecting good ice for placing a screw. Anecdotally, they mention that they found more failures in the morning than in the afternoon, possibly because of morning/afternoon temperature changes. Brittle conditions appear more prone to failure.
Recommendations:
- The fall factor 2 drops generated a maximum force of 8.6 kN with no failures, but this is not a good reason to place your first screw way above your last anchor
- Place screws in a zone of compression (read this as a concave depression, as opposed to a convex zone of tension)
- Small air bubbles doesn't necessarily mean it's poor quality ice
- Ice hooks did not hold any falls
Source file (sorry for the ads) DynamicShockLoadEvaluationofIceScrews_Final.pdf: http://www.sendmefile.com/00547701
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