There was a fatal accident in Yosemite — the circumstantial evidence implies the party placed 4 pieces of gear for the belay, then the leader took a factor-2 fall[*] on the belay, all the gear of which ripped, and as a result the entire party was perished. In that particular case, clearly the belay was not strong enough despite 4 pieces of gear…

Then here is a question.
How strong is strong enough for the belay in climbing?

A simple answer is a typical one in climbing: It depends.
To be more specific, the answer is that the belay should be as strong as to withstand the load of the maximum possible force expected, and the exact value depends on each climbing situation.

An example case of the other end of the spectrum (from the accident in Yosemite) may be like this. On steep glacial ice, you could build a super-strong anchor that could hold a tank, by placing many screws or abalakov threads and well equalise them all. But it takes a lot longer time to build it than a single or two abalakov. Hence if you are to do 20-pitches abseil for an alpine descent, you may not complete the descent in a day if you build 10 abalakov threads per belay. I would rather go for less abalakov threads per pitch, which would be anyway strong enough for the purpose (abseil), to return to the safe ground in time.

As such, it is essential for climbers to understand how strong the belay must be in each climbing situation. To overestimate it is lethal, but to underestimate it can be also dangerous. Obviously we should always give a healthy dose of safety margin to avoid overestimating, but still, there is a limit. Stronger it is, better it is is not the case, well, unless some one else has already built a belay for you!

I note you may choose to considerably marginalise the strength or reliability of a belay in the case of alpine climbing, treating it as a part of many risk factors in the grand view of risk assessment. There is nothing wrong with the attitude. But in this post my focus is limited in the strong-enough belay, which poses no risk in terms of its strength even for the possible worst-case scenario.

Below I describe the rough guide for the required strength for the belay (as in the collective anchor system of the belay-station) in typical cases in climbing. Some need a lot stronger than others, hence you need to be more careful in constructing the belay for those cases. In some other cases, you don't need to construct a belay that could hang a tank. If you know and understand that, it would help you save time in building the belay and so may reduce the overall risk, as well as increase the enjoyment of climbing activity.

I should note the below is no more than a rough rule of thumb. In industry, there is the 5-times rule, that is, your safety chain must be 5 times stronger than the worst-case scenario. In climbing, the margin is a lot smaller in general or even nil. For example, the UIAA standard for the breaking strength for the cross-loading direction in karabiners is mere 7 kN, which can exceed during a severe fall; that means it is even weaker than the worst-case scenario. As such, the following guide does not follow the luxrious 5-times rule. But I do explain what is the minimum requirement in each scenario. At the end of the day it is up to you to decide!

Abseiling

Abseiling is very similar to aid-climbing in terms of the required strength. The theoretical minimum requirement for a 80-kg climber is 0.8 kN. However, some jerky moves are not uncommon during abseil, or unavoidable to some extent even with the best human effort. Every jerky move applies more force to the anchor. For that reason, 2 kN is considered to be an absolute minimum for the breaking strength of the abseil anchor. Desirably, 4 kN or above.

Note if more than one person is hanging to the same belay, such as, the case where the same (and perhaps hanging) belay is used for self-belay for the other people waiting for their turn to abseil, add 1 kN per person to the required strength.

Also I should note that in the case of abseil, the anchor is always loaded and is the single point to maintain the safety, hence the destruction of the anchor usually means a certain death, whereas in most other climbing situations the anchor may be hardly, or even not at all, loaded in the first place if the climber does not fall. Considering the seriousness of any failure of the anchor for abseiling, it is wise to give a good deal of safety margin to the anchors of abseil.

Top-roping, aka belaying a second

In top-roping, as in belaying a second from the top, namely belaying a second, stronger belay can be necessary. The exact amount depends on several factors, such as, how the second falls, what your configuration of the belay system (direct or indirect belay), how you perform the belay etc. After all, if the second does not fall, and if the belayer does not apply the force to the belay, either, zero force is applied to the belay anchor! But of course, you had better think of the worst-case scenario.

Suppose there is neither slack nor tension in the ropes between the belay device and climber with 80-kg weight (well done for your attentive belay!), and suppose the belayer with 80-kg weight is hanging in the air from the same belay anchor, then the climber falls cleanly into the air in the top-rope situation, and also suppose there is no other component like rubbing the rope with a rock edge or runner. Then the theoretical force applied to the belay anchor is 2.4 kN (= 240 [kgf] = 80 [kgf] x 3; nb., in reality there are many factors that can increase or decrease this load).

You may notice this theoretical value is a couple of times higher than the theoretical minimum of the abseiling situation. If there is any slack in the rope, it can be a lot higher. Or, if the belayer shock-loads the anchor, as a result of being pulled towards the climber irregistably, it can be even higher. Even worse, if the belayer is connected to the anchor with non-stretchy material, such as a dyneema sling, it shockloads the anchor.

As a rough-guide in the top-rope case, providing the belayer keeps the rope taut all the time and never develops a slack in the rope between the belayer and climber, 5 kN is the absolute minimum requirement, and preferably 8 kN, for the breaking strength of the anchor. Falls in top-rope is the most gentle form of climbing falls in terms of the applied load to the anchor, if done appropriately, because the potential fall factor is small. Nevertheless a fall is a fall. Plus, chances are two people, that is, both a climber and belayer, may hang from the same anchor. Therefore the required strength for the belay anchor is much higher than the abseil.

There are two notes in the top-roping. If the route involves the traverse, treat it as similar to a lead climb, as the fall factor can be a lot higher (see the section below about traverse in multi-pitching). Or, if you use the direct-belay system in top-roping, refer to the bottom-roping as described below, that is, higher than 6-8 kN.

Bottom-roping (often referred to as top-rope)

In bottom-roping and lowering-down, as in belaying a climber from the ground, where the rope is threaded through the anchor at the top of the route, the amount of the force applied to the anchor can be significantly higher than the top-roping situation, up to doubled due to the pulley effect, while the longer rope (than the top-rope situation) does contribute to absorb the momentum of a fall, reducing the required strength.

6 kN is the absolute minimum requirement for the anchor, and preferably 8 kN or higher.

Multi-pitching

Finally, the situation that needs by far the highest strength is the belay anchor in multi-pitch climbing. In principle it has to withstand a force with a fall with the fall factor of 2. Indeed, that is the whole purpose, apart from belaying the second to the belay as in top-rope, of the belay in the multi-pitch climbing. In short, the minimum requirement is 10 kN, and desirably 12 kN.

Having said that, if there is no possibility for the leader of the next pitch to take a factor-2 fall, the requirement is different accordingly. I describe some example cases below.

Case 1: Multi single-pitch climbing

In some routes, especially in many easy-grade routes, the belay ledge may be massive, where in some cases you could even pitch a tent, and the leader of the next pitch might deck but would never fall farther. In that case a belay as strong as for the top-roping is adequate.

Case 2: Traverse pitch

The next pitch is a horizontal traverse. In that case, the highest potential fall factor is 1. As a rough guide, two-thirds of the required strength of the belay anchor for the normal multi-pitch climbing are adequate, that is, 7-9 kN. But make sure the belay can cope with the multi-directional pulls, particularly horizontal pull, which is not always easy! Also, if the direction of the line changes upwards during the next pitch, the highest possible fall-factor can be (a lot) higher than 1 — treat it accordingly.

Case 3: Jesus Nut is placed

Jesus Nut, that is, the first solid runner in the next pitch, is already placed. In that case, the factor-2 fall would not happen, hence the belay can be marginally weaker than the ideal case for general multi-pitching. Providing the Jesus Nut is absolutely bombproof, the belay as strong as top-roping situations is adequate, because the belay is not used to protect the leader. In reality, you may often not be 100 per cent sure of the soundness of the Jesus Nut. Then, adjust the acceptable level of the strength of the belay accordingly. The bottom line is, if the leader takes a factor-2 fall as a result of the failure of Jesus Nut, the belay has to withstand the force, that is, no different from the standard multi-pitching belay.

Final note

I should note the required strength for the anchor is the function of total mass of the climbers who are using the anchor at the same time, and the dependency is proportional approximately. If your climbing partner is massively heavier than the standard, you had better take it into account. For example, if your mate weighs 120 kg, that is 50 per cent higher than the standard 80 kg, adjust all the values quoted above multiplied by 1.5, such as, the abseiling anchor should withstand at least 3-6 kN. If you climb with kids or some one as light as kids, you can get away with somewhat weaker belays. But remember the belays have to be strong enough to cope with not only the lighest person but all the party members. For example, if you belay a kid in the top-rope, the belay must cope with not only the force of the kid's falls but also any potential load from you the belayer, which can actually be higher than the kid's fall.

Similarly, the peak force applied to the anchor is a function of elasticity (strechness) of the main rope(s). If you are using stretchy (generally, thin) ropes, again you may get away with somewhat waeker belays than the above-quoted values.

Summary Table

Next post

will be about the principle in rigging belay anchors.