Extension and Shock Loads for Highlines

Extension and ShockLoad

Extension and shockloading are two of the most important concepts to understand when constructing robust highline anchors. They are important because they are two things you DO NOT want to have happen within your anchor.

Example of main anchor built using static rope and BFK configuration. The yellow webbing is one of 3 backups going from each bolt to the hardpoint within each weblock. In case of a main anchor failure, these backups will prevent extension of the system, therefore limiting shockloads to the anchor system.

Example of main anchor built using static rope and BFK configuration. The yellow webbing is one of 3 backups going from each bolt to the hardpoint within each weblock. In case of a main anchor failure, these backups will prevent extension of the system, therefore limiting shockloads to the anchor system.

When applied to highline anchors, "extension" occurs when something fails within the system, then rapid movement occurs as a result of the failure. If you think about it, a rigged highline contains a high degree of potential energy. The slackline can be compared to a rubber band. If a component in the anchor system were to fail, let’s say a highline anchor bolt, the anchor system would extend because the highline (rubber band) is contracting. Due to the failure, all of a sudden there is slack in the anchor system and the slackline wants to remove that. The slack will be removed by the slackline and the masterpoint will extend out. This extension of the masterpoint directly relates to shockloads because eventually the extension will stop. When it stops, it creates a high peak force on the remaining bolts; a shockload.

Shockloading

Shockloading is the instantaneous loading of components within the slackline system. We DO NOT want this to happen. Shockloading is a form of dynamic loading, but under relatively static conditions. It has the tendency to break things by creating very high instantaneous forces. To expand on the example of a bolt failure; if a bolt were to fail the masterpoint will extend and eventually be “caught” by the remaining bolts. Once caught, the load is transferred to the remaining bolts. These remaining bolts can experience a sudden dynamic load 3 or more times their original load due to the dynamics and distance of extension. This effect changes depending on how long the slackline is, what type of webbing used (nylon is stretchier and contains more potential energy), standing tension and how much extension takes place. To protect against both extension and shockloading we create backup anchors that limit how far an anchor can extend as a result of failure, thus reducing the effect of shockloading.


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