Static and dynamic load
A dynamic load or shock load is an unconstant load, that can rapidly change in time.
A static load is a constant load on a system or an anchor.
The differences between static and dynamoc loads can be easily demonstrated with a simple experiment:
- Gently and slowly place a load (a pack of sugar, for example) on a kitchen scale. What is the highest reading? What was the nature of the scale pointer movement? what was the reading a few seconds later?
- Drop the same load onto the scale. What is the highest reading in this case? What was the nature of the scale pointer movement ? what was the reading a few seconds later?
In rope systems a true static load is rare, even though in most cases it is near static. Mopst people think that the load is dynamic only when breaking a lead fall. Let us consider a few cases:
- While free climbing, for most of the time, all parts of the belay system (anchors, rope/ropes, harnesses, belay device etc.) are not loaded. In case of a fall there is a dynamic loading of the whole system.
- While aid climbing, there is an almost static load on the leader's harness, on his/her cow's tail or daizy chain and on the last piece. Every time the leader pulls or steps up on an anchor, the load becomes more dynamic.
- While rappeling, the rope, harness and ancors are constantly loaded. It seems like a static load, but the load changes with rappeling speed. In a very jerky rappel, where the person alternately free falls and then abruptly stops, the load changes radically; when falling, there is no load, when stopping, the load rises quickly, and is actually just like breaking a fall. Furthermore, bouncy rappels repeatedly puts a dynamic load on the anchors. It is especially important when using just one anchor with no back up.
- When toproping, there might also be a dynamic load. When the climber climbs, there is no loading, and when he falls, there is, i.e. the load changes. of coursae, the load in this case is not as high as in a lead fall.
- A failure of a piece of protection within a complex anchor of a belay might, depending on system configuration, cause the remaining pieces to be shock loaded. This applies especially to a sliding x anchor, as it allows for a long extention. This is more serious in rope rescue systems with pulling systems where loads are high. This is a major consideration on long trad routes and in the mountains, when marginal anchors are not rare. usually, it is preferable to use a cordelette rather than a sliding x.
