Moving Beyond 10:1 Static System Saftey Factor
Moving Beyond 10:1 SSSF Introducing Force Limiting Systems and Managing the Right Risk at the Right Time
ITRS 2014 By: Kirk Mauthner, British Columbia, Canada, email@example.com
Abstract: Most rope rescuers are aware or familiar with the concept of applying a safety factor – such as 10:1 – to their rigging with the aim of ensuring sufficient system strength just in case it becomes inadvertently subjected to a high load, such as a shock load. Under very specific conditions this approach has considerable merit. However, strict adherence to a specific safety factor value for all rigging applications can lead to a misguided understanding of risks and consequently this directly affects – often incorrectly – the perception of what equipment and techniques are required for particular rope rescue situations. The concept of using safety factors – such as 10:1 – has been advocated for rope rescue since the 80’s, perhaps earlier, but since then there have been numerous progressions more suited to the challenges of rope rescue. This presentation covers this progression and introduces topics such as “Force Limiting Systems”, “Mirrored Systems” as a specific subset of Two-Tensioned Rope Lowers, as well as focus areas of managing the right risk at the right time.
Bio: Kirk Mauthner is an active, internationally certified mountain guide (IFMGA) and an active member of a mountain rescue team which he joined in 1979 in the heart of the Purcell Mountains in British Columbia Canada. Through his company, Basecamp Innovations Ltd, Kirk also consults and teaches technical rope rescue throughout North America and overseas. At his facility in BC, Kirk also designs and tests equipment related to activities such as rope rescue, mountaineering, and other related disciplines. In addition to his drop tower, slow pull machine, instrumentation and 3D design infrastructure, Kirk has recently added CNC prototyping capability. Kirk is also the current Vice President of the Terrestrial Committee in the International Commission for Alpine Rescue.
Rope rescue requires practitioners to manage the right risk at the right time. Different risks emerge or fade away depending on where the load is in relation to the load started and where the end destination is.
The factors that can affect the failure of a rope rescue system can simply be divided between four major factors:
1. Human Factors
2. Environmental Factors
3. Materials/Equipment Factors
4. Method/Technique Factors.
While Human Factors tend to dominate the cause of rope rescue system ‘failures’, it is inextricably the interaction of all these factors combined that either increase or decrease the active risks which need to be managed. This presentation has relevance to each of the above factors but will start with an initial focus on Material Factors, and how our ‘thinking’ (a human factor in itself) about this topic ultimately affects all of the other factors. This presentation then provides insights on techniques to help rescuers manage the right risk at the right time. I will then attempt to explain how risks can be managed using concepts such as Mirrored Systems (whereby the tension placed on each rope is dependent on the relevant risks which need to be managed, and where each Force Limited rope system serves simultaneously as a lowering/raising line as well as a competent back-up to the other line). Depending on available time, suggestions will be given to how to better utilize high-directionals for both ropes, and how to particularly decrease human factor issues through better command & communication structures.
Rescue System Strength: A rescue system must be strong enough to neither yield nor fail from the relative worst case forces that it might be subjected to, and, under normal working conditions, the equipment must not fail from repeated use or cycling. There are a number of approaches that can be used to ensure sufficient rescue system strength. These range from concepts such as Safety Factors to Force Limiting Systems, and the following attempts to explain the progression of these concepts, up to current best thinking on the subject.
Safety Factors: One engineering concept that has been applied to rope rescue rigging to ensure that potential worst case loads do not yield or fail the equipment are the use of safety factors. For their purposes, the rope rescue community generally defines safety factors as the ratio between the breaking strength of a component to the force being applied to it. A systems safety factor is the safety factor rating applied to the entire rope system based on its lowest component safety factor – in other words, the ‘weakest link in the chain’. A static systems safety factor specifically refers to the systems safety factor when all movement of the load has stopped; in other words, it is in a static state. Rescuers often miss this subtle distinction and incorrectly try to apply safety factors when the load is ‘in motion’, such as when hauling in a pulley system, where the tension on the haul rope can be higher than if the rope is ‘static’, or not in motion. There are also “dynamic” safety factors, but they have not been applied as broadly to rope rescue like static safety factors have.
In the early 80’s, the British Columbia Council of Technical Rescue (BCCTR) advocated the use of a 10:1 SSSF for rope rescue systems. It is important to realize that the BCCTR’s primary motive behind advocating a 10:1 SSSF was to provide a simple and easy tool for rope rescue practitioners to ensure they had sufficient strength in their rigging to deal with the relative worst case event. For perspective, it is also important to note that at that time, very little research and testing had been conducted on rope rescue systems strength and performance. Drop towers and test labs with force recording instrumentation specifically for rope rescue sized loads were essentially nonexistent. This meant there were many unknowns, and for the needs of the 1980’s the application of Static Safety Factors was helpful, and easy to apply using ‘static’ loads as the starting point. The concepts of Safety Factors are essentially as old as engineering design theory itself, and lacking any other means of assessing the relative safety of a system, it is a reasonable approach. The use of Safety Factors have also been referred to as using a “crutch” in design, or a “factors of ignorance” in order to cover for unknowns.
To back up a little bit, the ultimate aim of achieving sufficient strength in rigging is to ensure that the relative worst-case loading does not yield or fail the material or equipment. For life safety applications, a good rule of thumb in engineering is to have a system strength which is about 1.5 to 2 times stronger than the worst-case expected load. In the early 80’s the BCCTR essentially accomplished that rule of thumb by applying a 10:1 SSSF to their rigging. The logic flows in that if the worst case event is an edge transition gone wrong (i.e. represented in testing as a 1m drop of a 200 kg mass onto 3m of rope), then the peak force of such a drop (say, less than 12 kN or so), would be at or below the yield of the equipment if was at least 10 times stronger than the original static load (i.e. 2 kN x 10 = 20 kN). In this example you can see that a minimum system strength of 20 kN falls within the rule of thumb requirement of being about 1.5 – 2 times stronger than the worst peak force of about 12 kN.
The Problem With Safety Factors:
While the application of a 10:1 SSSF has undoubtedly helped guide the rope rescue community in building appropriate and sufficient levels of strength in their rigging, particularly as it applies to the relative worst case event, strict adherence to using 10:1 SSSF in all rope rescue applications is overkill, and has resulted in more than its fair share of misguided thinking and rigging.
A 10:1 SSSF works well for the above described scenario because it adequately protects against the relative worst-case event. Paradoxically, the problem with broadly applying a 10:1 SSSF to all rope rescue situations is that there are many situations where the initial static load may be greater than proverbial 2 kN load – such as a moderate angle lower of a multi-attendant load – but because of the terrain, conditions and circumstances, it would be impossible to have a worst-case event peak force higher than that of a 1m drop on 3m of rope with a 200 kg mass, and therefore more strength is not at all required despite a higher initial static load. The paradox is that strict adherence to a 10:1 SSSF will have you build a stronger system with an initially higher static load, even though the worst-case maximum peak force would be lower than what would occur on an edge transition gone wrong with a 200 kg mass. As such, rescuers have and will purchase and rig with stronger equipment for these situations, despite lower potential peak forces, because of rote application of static safety factors. Rote application of 10:1 SSSF leads to misguided thinking and rigging.
There’s ‘Less’ to it than the Worst-Case Event: The key for rope rescuers to realize is that the worst-case event in rope rescue literally is an abrupt edge transition with a rescue-sized load gone wrong. It is the one situation where you might have little rope in service, a large load (i.e. more than one person), and most importantly, an opportunity to gain kinetic energy due to a free-fall. In essentially all other applications, there is either more rope in service or there is no true free-fall; in other words, the load has a top-rope belay and any failure of the supporting system would only result in a sudden ‘settling-in’ to the remaining system.
Unfamiliar to most rescuers is that a sudden ‘settling-in’ to a rope system without any free-fall, essentially follows another engineering principle – that of spring constants. To oversimplify, basic spring theory would tell us that a sudden ‘settling-in’ to a spring-like rope would result in a peak force of about double the static load. In reality, testing of rope rescue systems shows this to be closer to about 2.5 times the static load (this is due to the non-linear behavior of rescue rope elongation as well as the effect of knots tightening up). However, what this means is that even if you have a 3 person load (e.g. 280 kg or about 2.8 kN) suspended vertically, and the supporting line fails and the load suddenly settles into the back-up rope, then the peak force would only be about 7 kN (2.8 kN times a factor of 2.5), which is substantially lower than the 12 kN peak force which a 200 kg mass could produce on a 1 m drop on 3 m of rope. Therefore, a by-rote application of a 10:1 SSSF would have this system with a static tension of 2.8 kN be rigged with a strength of 28 kN (2.8 kN x 10:1), even though, unwittingly, the peak force would only be about 7 kN if something went wrong. Even though the initial static force is higher, in these situations, it is misguided to rig with greater system strength because the highest potential force is comparatively low.
In hindsight, the application of a 10:1 SSSF in the 80’s has served the purpose well for establishing a means for rescuers to easily determine the minimum strength requirement for equipment which might be subjected to the relative worst-case event, but unfortunately it has misguided rescuers into applying this thinking to all other rigging. One could argue that if simplicity and ease of application for system strength were the primary objectives, then rather than using a 10:1 SSSF approach, we could have simply required a minimum strength of 20 kN, which could easily be accomplished primarily by purchasing decisions. You may ask, “What about the multiplication of forces due to a redirect or change of direction through a high directional?” What about aerial rope systems such as Guiding Lines and High Lines? Would a minimum strength requirement of 20 kN meet the objective of ensuring there is sufficient strength in the rigging so as not to yield or fail the equipment? The short answer is yes, and the cautious answer is yes, but there are some additional considerations, and there are other ways of looking at the problem of ensuring there is sufficient strength in the rigging. Rather than concentrating on system strength, it requires a basic paradigm shift into focusing more on ensuring that the peak force of the relative worst-case event – no matter what it is – does not, or more specifically ‘cannot’ exceed a particular value. With this approach, system strength becomes more of a guideline and a constant, as does the maximum peak force should something go wrong, even if there are variations in the initial static force.
An Introduction to Force Limiting Systems: If a rope system is reliably ‘force limited’, then even if the initial static load is 2 kN or 3 kN, it will not make a difference in the peak force outcome of a dynamic event because the peak force will be ‘limited’ or ‘governed’ by a device capable of doing that once system force reaches a specific target value. However, if a strict safety factor approach is used, then a static 2 kN load would require a 20 kN breaking strength and respectively, a 3 kN static load would require a 30 kN breaking strength in order to attain a 10:1 SSSF. Conversely, if a Force Limiting system were used where the peak force is reliably limited to no more than, say 12 kN, then from an engineering design standpoint, the system strength would only need to be about 1.5-2 times stronger than 12 kN; in other words, somewhere around 18-24 kN, in order not to yield or fail the system. In these two scenarios, a rescuer following the strict adherence of a 10:1 SSSF would seek stronger equipment to meet the demands of a static 3 kN force (i.e. a breaking strength of 30 kN would be required) whereas a rescuer using a reliable Force Limiting system would not be concerned whether the static load were 2 or 3 kN since the peak force outcome would be the same.
Limiting the peak force of systems to a target value is certainly not new; simply look at many fall arrest systems to understand how widespread this concept is. However, in addition to ensuring the peak force is limited, it is equally imperative that a minimum resistive force is provided in such systems. For example, if a system began to slip (or force limit) at 2 kN, then this would immediately result in a ‘runaway’ load once that level of tension has been reached. This minimum slip force is relatively easy to calculate for rope rescue systems. One just needs to look at what the potential force is if the rope system received a sudden ‘jolt’, such as the sudden transfer of tension of the load from one rope to the other (e.g. one rope fails and the load gets caught by the other rope as it settles into it). Using the ‘spring constant’ theory described earlier, a sudden jolt of a load on a rope can result in a 2.5 fold increase in tension on that rope. For example, a sudden jolt of a 3-person (280 kg…about 2.8 kN) load will spike the rope tension momentarily to about 7 kN, or less. Therefore, if the minimum slip force of the system was at or greater than 7 kN, then ‘inertial runaway’ would not occur.
Rope rescue systems which have built-in force limiting within the range of 7-12 kN, and components are at least about 20 kN strong, can therefore be applied to not only the ‘relative worst-case even’ type situations as well as moderate angle slopes with multiple attendants. A poorly executed edge transition with little rope in service and a large load might approach the upper limit of this force limiting range whereas the failure of a rope system with a multi-attendant operation on a moderately angled slope may not even cause any slippage whatsoever, even though the static load is much greater than the edge transition example. As such, force limiting systems can be broadly applied to rope rescue systems, including aerial ropeway systems such as Highlines and Guiding Lines. Force limiting systems do not result in a rescuer trying to calculate how much strength each component requires to meet a static systems safety factor approach, which for the vast majority of rope rescue situations – as demonstrated earlier – leads to misguided thinking as to what is really required in terms of system strength.
Where to from here? The use of reliable Force-Limiting System opens the doors to fully re-examine all of our rigging systems and we can continue to question why we do things the way we do. For example, is it acceptable (i.e. defensible) to lower or raise 2 rescuers and a patient on an 80 degree slope with 11 mm rope systems? How can we better utilize both ropes to improve our management of risks? How can this thinking be used to develop very lightweight systems for applications where conventional rigging is heavy and cumbersome – such as certain mountain rescue applications, mountain guiding or military needs? How can we improve our Command & Communication structure to better reflect managing the right risk at the right time? A good understanding of force limiting systems allows us to more easily explore answers to these questions rather than using the rigid structure of a strength-only based approach. Some examples of this will be provided in the presentation.
Kirk is the owner and president of Basecamp Innovations Ltd and is an active internationally certified IFMGA/UIAGM mountain guide based out of British Columbia Canada. His combination of strong guiding and technical rescue skills, his sciences background and a keen sense for problem solving has allowed him to make numerous noteworthy advancements and contributions to both the guiding and technical rescue communities.
As such, Kirk’s expertise in technical rope rescue training, consulting and product design is internationally sought after.
On behalf of Parks Canada, Kirk also represents Canada in the Terrestrial Rescue Commission at the annual International Commission for Alpine Rescue (IKAR/ICAR) symposiums and is also a member of the Technical Committee for the Association of Canadian Mountain Guides.
ACMG/UIAGM Mountain Guide
P.O Box 399
Invermere, British Columbia