The terms “self-equalizing” and “load distributing” have often been used to describe anchor systems which are rigged in such a way that they have some ability to adjust laterally under load and align themselves with the direction of the force applied to them, purportedly maintaining an equal distribution of forces on each anchor leg. Hence the term “self-equalizing”.
The most common of these methods is arguably the Sliding-X anchor. A webbing loop sling is attached to two anchor points and the strands of the loop are crossed at the carabiner clip in point. By applying a lateral force to the clip in point you can see the system tracking back and forth while maintaining some tension in both anchor legs.
There are several other methods of rigging anchor systems with these properties, and the CMC paper referenced below has some good depictions of some of these, including three point systems.
The Sliding-X anchor has historically been a popular anchor in the climbing community as well as seeing limited use in the rescue community. Lately there has been a decline in its popularity primarily due to the recognition of the hazards of anchor extension, discussed below. What is not often discussed however, is the inability of these anchor systems to perform as intended under load.
Any anchor system that has material available to allow itself to adjust laterally under tension will therefore have material available to extend in the event of a failure in any one of the anchor legs. This sudden extension of the anchor creates the potential for delivering a shock load to the system when the carabiner clip in point reaches the end of the anchor leg.
Note that the potential for a “shock load” is dependant upon the properties of the rope system as a whole. The presence of a dynamic rope connecting the load to the anchor system will mitigate the peak force, while a static rope will transmit the total force to the anchor system in a shorter timeframe, resulting in a higher peak force. The amount of rope in service and fall factor will also have an effect on both the amount of force generated and how it is transmitted to the anchor system.
The introduction of limiting knots (overhand knots tied in one or more anchor legs to reduce the amount of extension and therefore lesson the magnitude of a resulting shock load), is the recommended method of mitigating the hazard of anchor leg extension. Limiting knots also reduce the ability of the anchor to adjust laterally.
The potential extension distance in the event of an anchor point failure is directly related to the lengths of the anchor legs, a two-point sliding-x will extend to the full length of the sling used to create it. Self-equalizing systems using more than two points will redistribute the anchor material between the remaining legs, the extension distance is a function of the length of anchor material and the length of the individual legs.
It has been shown through testing that friction from material moving within the system prevents the system from adjusting laterally with the load, therefore preventing equalization, and typically focuses the preponderance of any applied force to the shortest anchor leg.
Richard Delaney has shown that when subjected to a load of 1kN the “Sliding-X does not slide for a 5-degree lateral shift”, and concluded “The ‘Sliding-X’ certainly does no achieve anchor equalisation during load movement.”
Testing has repeatedly confirmed that these anchor systems are incapable of sharing the forces of an applied load equitably amongst anchor legs, other than when perfectly aligned symmetrically, and that multi-point anchor systems that have been rigged to be non-self-equalizing (ie. multi-point anchor with a tied focal point) are typically as good or better at sharing load forces between anchor points.
Owen & Naguran tested a non-self-equalizing anchor system vs. a “self-equalizing” anchor system and fond a highly significant difference in applied load between the limbs of these systems, indicating that a non-self-equalizing anchor system resulted in more equitable distribution of the load than a “self-equalizing” anchor system. Their conclusion: “…a non-self-equalizing anchor system distributes the load more equitably between its component anchors than a self-equalizing anchor system.”
McKently, Parker, and Smith, of CMC Rescue, tested several different configurations, including how they performed when one of the three anchor points was failed. In failing one of three anchor points, “Load Distributing Anchors developed high Maximum Arrest Forces with poor
equalization.” They also found that “The Load Sharing anchor (non-self-equalising) had a much lower MAF (Maximum Arrest Force) but 87% of the load ended up on one anchor point.”
Thomas Evans, in his 2016 paper, verified that LS (non self-equalising) anchors catch loads after a limb failure with lower forces than LD (self-equalizing) anchors. He also found that “LD anchors, on average, do a better job of preventing an anchor point failure by more evenly distributing the load between anchor points during dynamic events.” This finding was produced in a lab setting, on two point anchor systems that provided for symmetrical loading.
Self-equalising anchors are more likely to be used in a climbing setting, where the use of a dynamic rope mitigates the shock load potential and extension can be assessed and limited with careful construction of the system. Their use in rescue systems, where both larger loads and static rope connections are common, is less likely and not recommended. The reality is such anchors are unable to shift laterally under load, negating their primary purpose, and present potential extension hazards of shock loading and lateral as well as vertical movement of the focal point in the event of an anchor point failure. These hazards can be eliminated by using non self-equalising or load sharing systems.
Author: Kevin Ristau
References:
1. Delaney, R. “Sliding-X Anchors”. 2018, Ropelab.com
2. Evans, Thomas. “Peak Fall Arrest Force During Simulated 2-Point Multipoint Anchor Limb Failures”. 2016, SAR3
1. Mauthner, K. “Self-Equalizing Anchors Don’t”. Rigging for Rescue, 2008
2. McKentley, Parker, Smith. “Load Distributing and Load Sharing Anchor Systems”. ITRS, 2007
3. Owen, R & Naguran, S. “Self Equalising Anchors: A Myth”. Technical Rescue Magazine Issue 37