Blog - The Raven Speaks
Summary of the 2011 International Technical Rescue Symposium
November 17, 2011
For those of you who are interested but were unable to attend we thought we would share some of the highlights from this years ITRS in Colorado. This symposium is a leading forum for up to date, state-of-the-art, technical rescue information. Rescuers representing mountain, cave, fire, industrial, and swiftwater gathered to discuss ways to improve safety and performance.
All three days of the International Technical Rescue Symposium this year were jam packed with very interesting presentations and discussions. The following is a brief synopsis of a few of the presentations we thought might interest you. To view the presenters’ research or to view more of the presentations from the symposium visit the ITRS website. Raven Rescue would like to thank all organizers and presenters for putting together a very educational symposium.
Is a pulley with a larger sheave always the better choice?
John McKently gave a very interesting presentation on pulley efficiency in rescue rigging. His research showed that in order to lift a 300llb mass a 1.5” sheave required 28lbs of extra force in comparison to a 3.75” sheave. In a 1:1 system the larger diameter sheaves were up to 7% more efficient. This result was magnified when increasing M/A. In a 9:1 system the larger diameter pulleys were as much as 17% more efficient. In the big scheme of things this difference is marginal and may not even be detectable by a haul team. Yes, larger diameter pulleys are more efficient however, is the difference significant enough to warrant the additional cost, weight and bulk? In their tests the weight difference between the smaller and larger pulleys in a 5:1 system was 6lbs. In the end I suppose it depends on your environment and resources. Also of interest was that pulley efficiency seemed to go up with heavier loads.
Their findings also seem to disprove the theory that any sheave less than 4 times the rope diameter adversely impacts rope strength and system efficiency, at least in a 1:1 system. Another interesting finding was that softer rope tended to be more efficient. The theory did hold true that in building M/A systems with a mix of sheave sizes, placing the largest sheave at the moving pulley position and the smallest at the change of direction (COD) position provides more efficiency than the other way around. I also thought you may find it interesting to know that in a typical 3:1 system the actual M/A is likely closer to 2.63:1. In a typical 5:1 the actual MA is closer to 3.95/1 and finally in a 9:1 the actual M/A closer to 7.13:1.
Carabiner Rigging for Dummies
Garin Wallace gave a great presentation about the relative breaking strengths of carabiners when loaded in various ways. We all know that carabiners need to be loaded along their major axis (spine) in order for them to be as strong as possible. The closer we keep the load to the spine the better! We know that loading the minor access (cross loading) can reduce strength by as much as 60% and tri-loading as much as 30%. What was interesting was his analysis of how we use carabiners in our systems and how that affects their strength. He found that when pulled with a single loop of rope (figure 8 on a bight) that the carabiners broke at 97% of their rated major axis breaking strength. When pulled with a double loop (double figure 8 on a bight) the carabiner broke at 74% of its major axis breaking strength. Two loops of webbing broke the carabiner at about 93% of its rated strength and 4 loops of webbing broke the carabiner at 74%.
SO WHAT DOES IT ALL MEAN? We assume when using a 40Kn carabiner in our system it will not break until we load it with 40Kn worth of force. This research suggests that even in the best of times we don’t get full strength and in most common applications we may be working with only as much as 74% of the rated strength (40Kn). Every time we clip that carabiner into a big centralized anchor point with multiple strands of webbing or rope we need to consider that we are considerably reducing the strength of that carabiner. Also interesting was that although oval carabiners are typically weaker when compared to D carabiners they are stronger when tri-loaded. If you have been taught that clipping a carabiner to another carabiner is bad you might not know that the reason is because there is a potential to put rotational forces on them. However, when pulled carabiner to carabiner along the long axis with no rotation there was no strength lost.
Empirically Derived Breaking Strengths for Basket Hitches and Wrap-3-Pull-2 Anchors
Thomas Evans and Aaron Stavens presented some great research comparing the strength of these two common types of webbing anchors. There conclusions were as follows:
1) Webbing anchors broke at a lower strength than expected. Assuming a 4000 lbs breaking strength for each strand an estimated 16000 lbs breaking strength was generated. Basket hitches broke on average at 9943 lbs and wrap 3 pull 2 anchors broke on average at 9167 lbs. The highest recorded breaking strength for the basket hitch was 11244 lbs and the lowest was 8902 lbs. The highest recorded breaking strength for the wrap 3 pull 2 was 11695 lbs and the lowest was 7445 lbs.
2) The weakest part of the anchor was not the knot but the webbing itself. In the test all anchors broke at the screw link (or carabiner in application) and not the knot.
3) Webbing anchors can break in more that one location simultaneously during failure
4) Basket hitches break on average, at a higher strength with less variability than wrap 3 pull 2 anchors (705-775lbs stronger).
5) Both anchors are stronger than 11mm rope (6000 lbs) when tied in the configurations tested so both anchors are suitable for rescue use.
If you are an emergency responder committed to professional development, mark the International Technical Rescue Symposium on your calendar for next fall. See you there.
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