Fall Factor and Impact Force: Essential Concepts in Vertical Activities

The fall factor and impact force are two safety concepts so closely linked to each other that in climbing they are always explained together. We talked about the importance of knowing these concepts and how knowing how to interpret them correctly will make you move through the vertical with greater confidence.

Fall factor and impact force. Two concepts to know how to interpret. Photo: Beal / Kyrre Buxrud

Fall factor and impact force are two concepts widely used in activities carried out in vertical environments. Both concepts, although different, are closely related to each other, and it is advisable to know them and know how they originate in order to carry out our activities safely.

Theoretical Basis

Our bodies, attracted by the earth's gravity, are affected by potential energy, and when we climb, we compensate for that energy through our strength by holding on to the wall. If this force fails, the fall occurs and to avoid further damage it must be arrested by some element of our equipment, usually a rope, that prevents us from hitting the ground.

During a fall, the kinetic energy of our body increases with the distance traveled before braking, so a longer fall, theoretically, should be more violent at the moment of braking than a shorter one; This is not always the case, as we will see later. The severity of a fall depends not only on the length of our fall, but also on the length of the gear we’re connected to (rope, lanyard, etc.) and its ability to absorb the braking impact.

The length of the rope from the belayer is a key piece of information. Photo Beal.

Although the mass does not affect the acceleration in the fall (in the absence of friction we fall at the same speed whether we weigh 80 kilos or 50) it will affect the moment of braking, a heavy body accumulating more energy than a lighter one. This is the key point of differentiation between fall factor (relationship between the lengths of the fall and our lanyard) and shock force (energy we receive when the fall is stopped).

So, to know the theoretical danger of a fall in addition to the force of gravity of the Earth (or the planet on which we are climbing) the only thing we are going to take into account will be the length of the fall and the ability of our material to absorb the energyproduced during that fall. If we also want to know how violent that fall is going to be, we will also have to take into account the mass of the person climbing.

Fall Factor

In dynamic situations (fall) we have seen that the attraction of gravity will greatly increase the energy that our body accumulates and we will need an element that partially absorbs that energy.

The greater the amount of that element that absorbs energy we have available, logically its capacity to dissipate the energy of the fall will be greater. Thus, given the same fall length of, say, 4 meters, the potential danger of a fall will be less the more rope we have to stop it. This length of rope is measured when we talk about climbing in its section with the capacity to dissipate energy, from the belay device to our tying knot.

The same 4-meter fall causes different factors depending on the length of the rope. Petzl Chart

Why is it important to know this?

Sometimes, especially with beginner climbers, a fall close to the ground tends to give more confidence psychologically than a fall from higher up. Fear of height and other problems such as not seeing or hearing whoever belays you usually leads us to unconsciously think that the option of falling close to our climbing partner is better than doing so higher up.

The number of factors involved is large (absence of friction, dynamic belaying, absorption capacity of the rope, even the movement of the person falling reduces the suddenness of the braking), but now we are going to demonstrate that given the same length of fall, the more rope that has come out of the belay device the better it will be for our interests.

4.6 meters of fall between 2.6 meters of rope: 1.77 fall factor. Petzl Chart

The fall factor is the result of dividing the meters of fall by the meters of rope that has been fed out. By itself, the fall factor does not explain much about how we are going to feel the fall, but it does give us an idea of how we and the gear are going to feel that fall, especially the piece that stops our fall. The lower the fall factor and, other factors being taken into account, the softer the fall will be.

What other factors must be taken into account?

The fall factor is a theoretical result that, as we have already mentioned, only takes into account fall length and the length of rope fed out. However, in real situations the accumulation of factors is usually more complicated: the rope drag on carabiners and the wall or the zigzag when passing through the pieces of protection alter the result by modifying the impact absorption capacity of the rope.

In addition to that, the possibility that due to distance the person belaying us is not fully aware of us or the existence of ledges or rock protrusions are dangers that become more evident the further we fall, but regardless of this the formula of the fall factor allows us to theoretically know the rope's ability to absorb the impact.

Attention to friction when climbing. It affects fall factor and impact force. Petzl Chart

What is the highest fall factor we can receive?

Logic leads us to think that the maximum fall factor that we can experience will be a factor of 2, that is, falling twice the length of the rope. These are situations that we usually expose ourselves to on multi-pitch routes when leaving an anchor until we clip into the first piece of pro. If we were unlucky enough to fall before finishing, we would fall twice the distance we had climbed. Thus, in climbing and mountaineering we can reach a maximum fall factor of 2.

However, a situation may arise in which the feared fall factor 2 is greatly exceeded and it is in those cases in which we are anchored with a rope to another element of progression. In canyoning or caving, the existence of handrails that force you to advance anchored to them is quite common. If the handrail were diagonal or even vertical, in the event of a fall of several meters, the moment of stopping the fall on a short rope would generate fall factors dangerously above 2.

In the case of via ferratas, where the progression occurs from start to finish using cables, it is necessary that these ropes use energy dissipators to avoid the high shock forces that would be produced when arresting a fall.

Ferrata dissipator kit, essential for safety in vertical sections. Photo Edelrid

If you want to know more about these activities, we recommend reading the articles about esssential gear for canyoning and essential gear for via ferratas, which will surely be very useful to understand the importance of using good-quality equipment.

Impact Force

Impact force is the energy received when our fall is arrested. It is related to several elements, the most important being the fall factor, the weight of the person falling and the absorption capacity of the rope and other elements of the system.

It must also be taken into account that laboratory data usually show data that is apparently more exaggerated than what is actually the case because the rigidity of the metal mass used in the tests does not absorb any energy, compared to with a human body and that, above all, the ability of whoever belays us to mitigate the impact is of great importance. About how to correctly belay dynamically< /a>, we wrote this article that we invite you to read.

Petzl / F. Kretschmann
Dynamic belay: the impact forces are very low. Photo Petzl / F. Kretschmann

What is the greatest impact force we can receive?

The greatest impact force that we can receive in outdoor activities will depend on the element that is stopping our fall. They are usually measured in falls of factor 2 and with a mass of 80 kilograms and, although the regulations impose many conditions, we are going to summarize the main ones very briefly. Here are some examples:

  • Climbing with single rope (sport climbing) the maximum impact we can receive is 12 kN.
  • Climbing with double ropes (traditional climbing or mountaineering) the maximum impact we can receive is 8 kN on a single rope (measured with 55 kg.).
  • In UIAA-109 approved lanyards, the maximum impact we can receive is 10 kN.
  • On via ferratas the maximum impact we can receive is 6 kN.

What weight is our shock absorption material designed for?

We have commented that, in addition to the fall factor, the absorption capacity of the rope or lanyard and the mass of the object (our weight) are the other elements that must be considered to calculate the impact force. In the regulations and approvals of material it has been protocolized that the mass with which the tests are carried out is 80 kilos.

However, there are some exceptions in specific material where the impact force transmission is required to be lower. In double ropes the mass for each rope individually is 55 kilograms or in dissipators for via ferratas where, in addition to the standard mass of 80 kilos, tests are also carried out at 40 and 120 kilograms, we can find the difference from the norm.

Climbing Technology
Maximum impact forces according to regulations for via ferratas. Climbing Technology Chart

Other Elements for Shock Absorption

Not only ropes and lanyards are designed to absorb impacts. There are also dissipative tapes used as a quickdraw that begin to dissipate at tremendously low values of 1 kN so that the impact on the anchor that is stopping the fall is as gentle as possible. They are used above all in ice climbing, although they are also very useful in trad climbing or in all those situations in which we have to rely on precarious gear placements.

Also crash pads for bouldering or bouldering, although in a different way, have their approvals to absorb the impacts of the fall of the person who is climbing. Starting in 2018, the UIAA-161 standard established the HIC (head injury criterion) value as the test standard; For crash pads it is already considered a critical drop if the HIC is greater than 400. By way of comparison, in the automobile industry a HIC value of 700 is already considered the maximum acceptable and a value of 1000 has been proven to cause injuries in 90% of the people affected.

This is the end of the explanation of the theoretical concepts of fall factor and impact force. We hope that it has been useful to you and has helped you better understand the data offered by technical climbing, mountaineering and via ferrata products. For any additional questions and to learn about the products, our technical advice teams are waiting for you on our website and in our physical stores.

Web store: www.barrabes.com

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