I Shock Load on Rope: Weight Limit Analysis

[Brian82784]

TL;DR

Shock load of a rope

Hello,

I’m not sure if the proper term is shock load. If a person is using a device and rope to lower themselves from a high point and they’re descending fast and then suddenly stop does that create a shock load? The device attaches to the rope and when a person squeezes the device it allows them to descend the harder they squeeze the fast they go. If they let go of the device they come to an abrupt stop.

If a person weighs 260lbs and the rope is rated for 310lbs (6.8mm Tvac rope) when they suddenly stopped doesn’t this force increase the weight being applied to the rope? By doing this would it exceed the 310lb limit?

 

[erobz]

Thats a pretty light rope rating for someone 260 lbf!

The force of tension in the rope will be increase as the person comes to a stop. Whether or not it will exceed the rating depends on many factors (at least); the mass of the person, the elasticity of the rope, the rate they accelerate, the initial tension on the rope before they began to accelerate and the persons initial velocity.

 

[Baluncore]

Welcome to PF.
The critical parameter, determining the force needed to arrest the load, is the time it takes to stop the load falling. Longer time, less force.

A dynamic climbing rope, used to arrest a significant fall, is elastic. That will minimise the additional force on the rope needed to stop the falling load. It will take longer to stop the fall, but injuries from the harness will be reduced.
https://en.wikipedia.org/wiki/Dynamic_rope
https://en.wikipedia.org/wiki/Fall_factor

 

[Lnewqban]

Welcome, Brian!

Please, see:
https://roperescuetraining.com/physics_fall_factors.php

 

[tech99]

If my memory is correct from when I worked on radio masts, we assumed 110kg (242 lbs) for the person, including clothing and tools, and doubled that to allow for shock loading. It will depend on the elasticity of the rope. For man lifting purposes using steel wire rope we used a factor of safety of 12 to allow for all causes. The rope you quote appears to have a Safe Working Load of 310 lbs and a breaking strength of 3619 lbs, giving a FOS of 3619/310 = 11.6.
The vulnerability of Dyneema rope is being cut, and I think a second back up rope would be required if practicable.

 

[Baluncore]

The vulnerability of Dyneema rope is being cut, and I think a second back up rope would be required if practicable.

Dyneema is strong and lightweight, which is great for static lines. But it is unsuitable for dynamic ropes because it has a limited stretch, less than 5%, before it fails.