Myths, Cautions & Techniques of Ice Screw Placement

Transcription

1 Myths, Cautions & Techniques of Ice Screw Placement a summary of two years of research by Chris H. Harmston MSE For years manufacturers and "experts" have been telling ice climbers to place ice screws in certain ways, explaining that the holding strength will be improved in these situations. Over the two years prior to 1998 Black Diamond diverted several hundred of its batch test samples towards investigating these placements and answering some fundamental questions. What length of screw is strongest? What angle should the screw be placed? When should the screw be tied off? What is "good" ice? How Was The Research Done? To investigate these issues Black Diamond designed and built special steel cells, or buckets, that allowed us to load ice screws in our Universal Test Machine using actual ice. These ice cells are made of 1.3cm thick steel walls, 6cm deep, with a surface area 5cm high and 3.5cm wide. These cells are prepared by filling with normal tap water and allowing to freeze at 10±5 degrees below zero centigrade. Freezing takes about 72 hours during which time the ice expands, cracks, bulges, and overflows the cell. The top few inches of really bad ice is chopped off and a new layer of water is added and allowed to Freeze. A screw is placed in one half of the cell and pulled to failure at 12cm per minute. The screw generally fails by cratering the top 5-8 centimetres of ice away from the screw, bending of the screw and finally, either levering the hanger off the head of the screw, breaking the tube of the screw, or pulling the screw out of the ice. A second screw is placed in the other half of the cell near the first screw's hole and similarly pulled to failure (yes, the ice is shattered and broken from the first test). After the tests are completed any broken-off screws are removed by chopping them out with an ice axe. The cell is placed back into the freezer and water is added to the old test holes and allowed to freeze onto the remaining ice. The cells are used repeatedly in this fashion up to 20 times before the entire cell is allowed to thaw and be 'regenerated". As can be inferred, this technique of ice preparation is highly variable and unpredictable (not unlike real world ice), yet has yielded conclusive and supportable results. The trends observed in Black Diamond's static lab tests were similar to those recorded in static tests by REI Engineers in glacier ice and by Craig Luebben in Ouray and Boulder Canyon ice. This suggests that the trends observed in laboratory ice are similar to the highly variable conditions and results found in the field. What Length of Screw is Strongest? Intuition holds true in that long screws are stronger than short ones. However, the variation is relatively small for dense solid good ice. This conclusion does not hold true for "bad" ice - ice that is detached, cauliflowered, chandaliered, aerated, hollow, and/or slushy.

2 Table 1 shows the number of samples, average, standard deviation, high, and low values obtained during routine batch testing that Black Diamond has conducted in ice during the last 2 years. Length 22cm 17cm 13cm combined #samples Average (Ibf/kN) 5006/ / / /21.2 Std. Dev (Ibf/kN) 1299/ / / /6.1 High (lbf/kn) 9074/ / / /40.2 Low (Jbf/kN) 2748/ / / /8.8 Table 1: Batch test strength results for Black Diamond Ice screws placed in solid dense ice. All this data is from screws placed within 5 degrees perpendicular to the ice surface. As can be seen from Table 1 there is a very large spread in the data, reflective of the fact that not all ice is equal. Does this data mean that you should always place the longest possible screw? Mostly, but not necessarily. There are several factors to consider. Assuming that the ice is thick enough for any screw, the placement should be strongest with the longest screw. How much surface ice did you have to remove to uncover good ice and how much good ice is left? How pumped are you while trying to place the screw? Is this the only piece of protection you are likely to get for some distance? How far out from the ground or belay are you? My recommendation is to place the length that makes you most comfortable in terms of protection level and pump factor. When you are close to the ground or belay you have the potential to generate more force on your protection. Make these placements as bomber as you possibly can. In other words, use longer screws when close to the belay or even equalize two placements if you are unsure of the quality of the ice. What Angle should you Place the Screw At? Figure 1: Angle convention for testing ice screws. This question was investigated by placing screws to the hilt at various angles. Perpendicular is chosen as the reference and labeled zero degrees. The conventional "place the screw at a 10 to 15 degree angle against the direction of pull" is a negative angle. Placing the screw in the direction of loading is labelled a positive angle, see Figure 1. As can be seen in Figure 2 there is a very dramatic change with angle of placement. What we observed is that placing the screw in the direction of loading is significantly stronger. In fact, at 15 degrees from perpendicular the screws are over two times stronger when placed in the direction of load than when placed against the direction of load. The data included in Fig. 2 is a

3 compilation of all three lengths of Black Diamond ice screws using the highly variable test conditions described above. It is amazing that such a strong trend exists in such a variable experimental setup. A general trend shows that screws are stronger when placed in the direction of loading. Figure 2: Failure load versus angle of placement. See Figure 1 for a definition of the placement angle. So, What Do These Placement Angle Results Mean? Should you place your screws 10 to 20 degrees in the direction of pull? The answer depends on several factors: First, is that these results are only for Black Diamond Tubular Ice Screws. In other words, for these results to hold true the screw must have well spaced, external, high-relief threads that resist pullout. (See Table 1.) Second, what is the ice quality? Is it detached, hollow, slushy, and/or rotten? If so, and this is the only option for protection, it may well be better to rely on the potential hooking/lever resistance ability of the screw rather than the holding power of the threads (an ice hook may be better in these conditions). If the ice is shaded, thick, dense, and 'good' the placement will be stronger if placed in the direction of loading. Third, if there is any possibility that the screw will melt out during the time you will need it to support a load, you should not angle it in the direction of the load regardless of the quality of the placement or of the ice. I have personally seen screws melt out so much that, in the time it took to lead and follow a pitch, a screw at half height on the pitch could be removed without unscrewing. This occurred on Experts' Choice Center W16 in Canada's Waterton Park which was south facing (full sun), with the ambient temperatures below freezing. Due to the variables of ice condition, aspect, and temperature it is important to do what is needed for the specific route, conditions, and protection options you have at hand. Simply stating that all ice screws should be placed at one particular angle is not sound advice and could actually lead to failure of that protection. Every leader must carefully consider every placement and variable they can to optimize their protection system.

4 Figure 3. Effect of exposed leverage on the strength of a placement. The data points represent averages of tests conducted over several years with no record of what the angle of placement was (approximately 10 samples per data point). The line represented by "Tie Off" is the average of four tie off tests (7-8 cm of leverage) that ranged from 2354 to 3299 lbf ( kN). This suggests that if the screw sticks out more than 5cm it should be tied off to increase the strength. When Should You Tie Off A Screw That Is Not Placed To The Hilt? In the limited testing we have conducted with tie offs and various extensions clipped into the hanger, our data suggests that it is best to clip in directly to the hanger if the screw sticks out less than 5 cm. If the screw sticks out more than 5 centimetres, tie it off as you normally would. Figure 3 shows the effect of increased leverage on the screw versus strength. In the few tests conducted with tie offs the average strength is near 2700 lbf/12 kn. The typical failure mode of tied off screws is that, after about 1500 lbf/6.7 kn, the screw flexes and bends causing the sling to slip to the head of the screw, resulting in high leverage. Failure is generally caused by the sling being cut by the edge of the hanger in the 2000 to 3500 pound (8.9 to 15.6 kn) range. For the majority of placements this strength range is enough to hold typical falls. However, with a high impact force rope and a high fall factor, failure of the tie off could result. The best option is to simply place a screw of the appropriate length and not have to worry about tie offs altogether. A word of caution relative to the tie off conclusions. This data is very limited. There is a very large variation in any given test setup. It is possible that the trends stated above lead us to the wrong conclusion. What Is "Good" Ice? In our experience most ice is bad to varying degrees. Good ice is relatively difficult to find on modern vertical or overhanging routes. Good ice generally is found on lower angle slabs where the ice forms thick solid flows (blue, green, or clear ice). If the ice is hollow, layered, slushy, aerated (white appearance with obvious air pockets), chandeliered, cauliflowered, etc. screw strength should be suspect. What is the surface ice like? Does it dinnerplate easily? If so you must clear this surface ice off. My experience has indicated that dinnerplate conditions occur most often right after or during rapid temperature changes and usually at colder temperatures. Does the ice have running water under or on it? If so, you may have to worry about the screw melting out due to the water. In general I have found that warmer ice shatters less and is in general capable of holding higher forces. From talking with climbers who have fallen (including myself, unfortunately) protection that held was more often in warmer

5 conditions than in colder conditions and the protection that failed was almost always in cold brittle ice or horrible thin or slushy ice. A final word of caution relative to equalizing two screws. Ice generally fails horizontally. Placing two screws horizontally also increases the force on the screws due to the triangle force multiplier (American Triangle). Place the screws vertically with one screw above the other. This will give the best possible chance for both screws to hold. Below are excerpts from a letter sent by Alex Lowe to Chris Harmston relative to the above information. Alex was formerly the Quality Assurance Manager of Black Diamond. Some of the research reported here was from early work that Alex conducted while he worked for Black Diamond: Comments on Ice Quality by Alex Lowe "Good work on the screw research. Having read it and thought about the mechanics of pulls along the axis of the screw as opposed to loading such that shearing through the ice plays a role, it makes intuitive sense that a screw placed at a positive angle should indeed hold better, but only in ideal ice conditions - that's the big qualifier. Of course determining what constitutes "ideal ice conditions" is the art and essence of placing ice gear. I felt you made this clear in your article. My personal conclusion from your tests is to place screws at a positive angle when I feel the ice is 'very solid'. Obviously some rather ill defined terms in that statement! 'Very solid' will remain an intuitive assessment. But here are some attributes I associate with 'solid' ice. 1. Appearance: Ice that is clear (glassy) in appearance usually contains less air, thus having greater density. Grayish, opaque ice is often shot through with air bubbles and thus is less dense and has less ability to support a screw placed at a positive angle. (At what declining density though does positive angle yield the advantage to zero or negative angle?) The effect your ice tools have had on appearance as you've climbed toward the screw placement is also informative. Pick holes will reveal brittleness, plating and layering and other indicators that affect my assessment of ice quality. 2. Sound: Low density ice often receives picks without creating much surface deformation. High density ice, because the dense ice must be displaced upon pick entry, often reveals a conical pit and other surface distortion where the pick enters. I would be more inclined to place screws at a positive angle in the latter situation. 3. Torque required to turn the screw: How hard is it to turn the screw once you've made your initial assessment and begun to twist it in? If I don't need the mechanical lever-arm advantage of the hanger to turn it in, I would not feel comfortable placing at positive angles. I would remove the screw and place at zero or negative angle. 4. Consistency of force required to turn the screw: As I turn the screw I feel for consistency of resistance to turning. If I feel the screw break through into less dense layers I would remove and place at a negative angle. Again, these are general and intuitive indicators I use. In the end, there are absolutely no absolutes in assessing ice quality. I find this to be one of the appeals of ice climbing, wherein the art of the sport lies. Protection on a typical ice lead will still run the gambit of Spectres, tied off ice sickles, pins, positive angled screws, negative angled screws and most importantly the confidence and experience to climb through unprotectable sections sans pro. Prudence, trust in gut feeling and experience are the requisite essentials for leading and protecting ice safely. The research you've done is great and becomes additional weaponry in my arsenal of criteria and options available in the back of my mind as I seek to make an ice lead as safe as I can. To apply the positive angle technique across the board would be folly but to add it to ones repertoire of savvy techniques adds certain advantage."

6 Also worth reading are some Canadian tests on ice screw placements on the Raven Rescue Website. And finally a video on How Not To Climb Ice. The link is posted as one really needs to read Will Gadd's comments. "Winter climbing is a constant battle with one's equipment!" Unknown Copyright October, 1997 & 2009 Black Diamond Equipment, Ltd. Alex Lowe Reproduced by kind permission of Black Diamond Equipment Ltd