24 takes FF1 on Dyneema

video: 24 takes with the fall (~FF1) on Dyneema rope without shock absorbing devices
Source: BTS for Megafon commercial
calc: Calc for the fall from the train
Related:Impact force in a fall

Let’s calculate the approximate load for the fall from the train:

t (time of impact) on Dyneema: 0.0175 sec
(no shock absorbing devices were used in the system)
g (gravity) is constant: 9.8 m/sec
m (mass): let’s say 70 kg
h (height of the fall): 1 m

F = $\frac{m*\sqrt{2g*h}}{t}$
t (Time of impact on Dyneema) =
g (Gravity is constant) =
h (Height of the fall) =
m (Mass) =
Load and repeats in the train fall

We have calculated the load of 17.7kN here. Let’s deduct the shock absorbed by the pendulum, broken stitching/fabric and the shock absorbed by body and muscles, nylon harness.

I would say webbing in the attachment point in this case received ~8kN under not the best angle (stress point was on the edge of the webbing). And it was repeated 24 times (24 takes were made to get this shot done). After 24 takes performing stuntman asked coordinator to change himself because it was too difficult to endure. During changing fabric damage in the stressed part of the seam on side point was discovered by stunt coordinator.

Case analysis

Top side point was used for attaching the rope. After 24 takes on the video from above performer said that he need to be changed because he got too much shock on the body. During changing the performer the head rigger found that fabric on top side point of the vest got damaged. Webbing didn’t have visible damages.

If you need a fall like this you should never go with ultra static system made of Dyneema without any shock absorbing devices.

Anatomy of the body tells us that top side point is definitely bad attachment to absorb the shock by the back. Helmet is adding some load to the neck as well. Luckily the stunt performer in this shot is a good athlete and didn’t get any serious injuries.

Do understand how big is the difference in loads between dynamic ropes (sport climbing nylon ropes) and ultra static ropes (Dyneema, Technora) please follow this link: Time of the impact

Do see the tests of materials with different shock absorbing properties check this: Dyneema and nylon slings

What can we do to avoid such accidents? Except choosing the other rope type which is made to fall arresting purposes, you can also use Brake Bar (aka goldtail), absorbers and other techniques to prevent performer and the system from overload.

Summary from manufacturer

By the year of 2023 we have received 3 feedbacks about vests damage. All 3 cases were about tearing the fabric in the stitching area of side points after shock load. Sometimes shock load was repeated many times in a row. The webbing was always fine.

The main thing to prevent yourself from accidents is to use high quality equipment and using it in proper way with understanding of the loads in the system and human anatomy.

After such cases we always improve vests not only to prevent from fall by absorbing shock by stitching or fabric breakage but also making whole construction stronger to increase longevity of the product. But what is most important is to avoid improper usage and loads which overcome WLL.



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