Arboriculture & Urban Forestry 35(2): March 2009 Arboriculture & Urban Forestry 2009. 35(2): 75–79 75 Residual Strength of Carabiners Used by Tree Climbers Brian Kane and H. Dennis Ryan Abstract. Tree climbers increasingly use carabiners and apply them in situations for which they are not designed. Be- cause failure of carabiners can result in serious injury or death, the following study tested how well carabiners endure the stress to which climbers subject them. This study distributed carabiners of four types (all manufactured by Petzl) to climb- ers in Massachusetts and New York, USA, and collected them a year later. Then, carabiners were broken in a universal test- ing machine and measured the maximum load, as well as surface roughness. No carabiners broke below their rated strength; and used carabiners were, with one exception, as strong as new carabiners. Surface roughness was a weak, but significant, predictor of strength. Findings are discussed in light of climber safety and the importance of conducting long-term studies. Key Words. Carabiner; Strength; Tree Climbing. In the past two decades, professional tree climbers have ad- opted and adapted climbing techniques and gear from other high-angle rope disciplines. These techniques and gear may improve efficiency and/or safety. However, without proper understanding of a new technique or piece of equipment, par- ticularly the limitations on it, the consequences can be dire, as a recent injury demonstrates (Laver 2008). One piece of gear that has become more popular among climbers is the carabi- ner, which is used to attach ropes, split-tails, and lanyards to a saddle. While the carabiner has gained support among climbers and is frequently described in trade publications (Oxman 2001, Adams 2004, Dunlap 2004) it was not ini- tially designed for use in the tree care industry (Blair 1995). Carabiners have been used for many years in various high- angle rope disciplines such as caving, rock climbing, and moun- taineering (Blair 1995). Having undergone changes in shape and material, most carabiners used by rock climbers are made of aluminum alloy to maximize the strength-to-weight ratio. One advantage of carabiners is that they can easily accept rope by opening the gate, which allows the use of eye-and-eye spliced split-tails for tying one of the newer friction hitches (Adams 2004). The American National Standard for Arboricultural Opera- tions—Pruning, Repairing, Maintaining, and Removing Trees, and Cutting Brush—Safety Requirements [ANSI Z.133 (Anony- mous 2006)] permits the use of carabiners for climbing, as long as they meet two requirements. First, carabiners used in tree climbing must be positive locking, which means that a climber must perform two separate actions prior to opening the gate. Second, carabiners must meet the minimum tensile strength for traditional steel rope snaps [22.24 kN (5,000 lbf)] (Anonymous 2006). Even with these two conditions, there are important limi- tations to use of carabiners for climbing. For example, cara- biners only meet the ANSI Z.133 minimum strength require- ments when loaded along the spine and when the gate is closed. In light of their adoption into a discipline for which they were not designed, as well as the taxing conditions to which climbers subject their gear, our objectives were to determine the durability of cara- biners used in professional tree climbing, and assess any visual and operational cues that could indicate diminished performance. METHODS In the spring 2006, we distributed aluminum carabiners of four types to climbers from five tree care companies in Massachu- setts and New York, USA. Three or four additional carabiners of each type were stored in a cabinet as controls. All carabin- ers were manufactured by Petzl (Clearfield, UT, USA), and were categorized by shape (Am’D and William) and gate mechanism (Ball-Lock and Tri-Act). We asked climbers to provide the following information: type of saddle, type of at- tachment knot for climbing line and split-tail, type of climb- ing line, and type of split-tail. We also asked climbers to re- cord the number of hours climbed each day and to consistently use the same carabiners for their climbing line and split-tail. Approximately one year later, climbers returned the carabin- ers, which we visually inspected for damage and tested for normal gate operation. Too few climbers recorded the number of hours climbed, so we measured surface roughness as a surrogate, assum- ing that it reflected the amount and/or intensity of use. We mea- sured surface roughness with a profilometer (Model 3800, Starrett Machine Co., Athol, MA), taking five measurements on the same spot of each carabiner, and using the mean value in the analysis. We placed carabiners in a universal testing machine [133 kN (30,000 lbf) capacity, accurate to 0.1%; MTS, Eden Prairie, MN], and tested them roughly in accordance with the F1774 standard (Anonymous 1999). In contrast to the F1774 standard, we used galvanized steel shackles [19.8 mm (0.78 in) diameter] instead of steel dowels [10 ± 0.1 mm (0.0039 in) diameter] to attach carabi- ners to the testing machine. We used the shackles primarily out of convenience given the existing set-up of the testing machine. All carabiners were loaded along the spine, as required by the standard, although the larger diameter shackle would cause a greater bending moment to be applied to the carabiners. This would, if anything, reduce the breaking strength of the carabiner, and the bias ap- plied equally to all carabiners. The larger diameter shackles more likely mimicked the loading during tree climbing since climbing lines can be 14 mm (0.55 in) in diameter, and often girth-hitched, which doubles the rope diameter where it attaches to the carabi- ner. When tied in a friction hitch, eye-and-eye spliced split-tails extend 16–20 mm (0.63–0.79 in) from the spine of the carabiner. ©2009 International Society of Arboriculture
March 2009
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