How do you add up an anchors KN?

Hello,

I've searched and searched on this site, but can't seem to find my answer. When it comes to adding up the KN of an anchor, how do you add it? I thought you just added in the KN of the protection (cam, stopper, etc) but what about everything else? Do I add the biners, slings, cord, etc?

Any help would be greatly appreciated. I've read two anchor books as well, but there's nothing in there on this topic.

Chris

I'm not quite sure I understand the question. As I understand, you're asking "How do I calculate the breaking strength of my anchor?"

That is not a thing I routinely calculate, and not one that is feasible to calculate accurately due to issues of load sharing when applied to a real world system. Theoretically, though, I'm sure someone better at math than me could do it.

Given pieces of roughly equal strength and assuming all gear is placed such that it will fail at its MBS and assuming that all gear is perfectly equalized, and assuming no amplification of forces from the angle to the primary pieces (the latter two being clearly untrue in real world systems) it would be n times the MBS of the weakest piece, where n is the number of pieces sharing the load.
Strength of slings would matter only if the strength of the sling was less than given by the above calculation.

Of course, all of this is detached from a real world situation.

Nobody really knows the formula. It involves calculus, trig, statistical probability, and alcoholic consumption.

This all came about after a guided trip. The guide was required to build a 25 kn anchor at the minimum and that got me thinking about how to add that up?

johnnyrig wrote:Nobody really knows the formula. It involves calculus, trig, statistical probability, and alcoholic consumption.

alcohol consumption is the real kicker here. very important to the equation. Nobody really knows but I believe the ratio is exponential and starts off around 1 drink : -1 kn.

They likely were just referring to the kn rating of each piece added together. I.E. 2 cams at 12kn each plus a small nut at 6kn would equal 30kn. Of course the anchor probably wouldn't actually hold 30kn due to the reasons listed above, but thankfully we don't need it to! Just sounds like a general guideline.

My anchors have one of two ratings:

1. Good
2. Good enough

unless you're taking high FF falls on the anchor, it doesn't have to hold much. i could be wrong but i'm pretty sure the top piece takes 1.6x the force on the climber and the belay take .6x the force on the climber. and at around 8Kn the climbers organs start to disintegrate. so, in reality, if the belay is taking more than 5kn in a typical fall you have bigger problems (dead climber)

eli poss wrote:unless you're taking high FF falls on the anchor, it doesn't have to hold much. i could be wrong but i'm pretty sure the top piece takes 1.6x the force on the climber and the belay take .6x the force on the climber. and at around 8Kn the climbers organs start to disintegrate. so, in reality, if the belay is taking more than 5kn in a typical fall you have bigger problems (dead climber)

huh?

FrankPS wrote:My anchors have one of two ratings: 1. Good 2. Good enough

3. Just don't fall.

I wonder whether the guide's "25 kN anchor" was ever tested with a 25kN load. I bet not...

It's not as if anyone ever once said "this anchor will old X kN, and then they load it up with X kN to see if they were right. In fact, almost no climbers have ever done any kind of feedback testing to see if judgements like "good" correspond to any level of load tolerance and, indeed, if there is any consistency in what the "good" judgement represents.

Instead, people learn that certain visual cues are purportedly associated with good anchor properties, and judgements are based on the extent to which the anchor possesses a good set of visual cues. Estimation of actual load capacity is just not part of it.

What keeps this situation from producing catastrophe after catastrophe is that belay anchors are very rarely tested beyond two to four times body weight, and gear engineering is typically good enough to insure that level of load tolerance even for relatively unskilled users.

the 25 kn of the pieces wont matter much if it was equalized with the standard 6mm cord rated to around 7 kn. doubled at the master like happens most times and youll get 14 kn max.

I've seen something like this used as guideline when teaching people to build gear anchors. As this was a guide on a trip talking to a client, maybe that is where this came from?

In which case you would just add the kN ratings of all the pieces in the anchor together. The idea is to keep new trad leaders from going "I've got three equalized pieces in my anchor, so it is good", and to also consider the size of the pieces. Stops them from building an anchor with a couple of micro-nuts and a 000 C3.

I'll try to iterate what eli was trying to say:

Humans are filled with bags of meat (organs), bones, and other stuff. All these are at different densities, therefore, when subjected to very fast deceleration (falling while on lead) the said bags of meat will decelerate at different rates, bumping into each each other and such. If you generate a fall of 8kn, you will suffer massive internal damage (you will be killed instantly - hey no suffering).

Here's a cool article:
fallpro.com/fall-protection…

Looks like you can break your spine at around 4 kn.
And that is why we climb on dynamic ropes - they absorb the energy of the fall so your bags of meat don't melt, and your spine stays in one piece.

Here you go climbing friend.

ropebook.com/information/ve…

Consider for a second that a newton (minus the 'kilo') is the amount to force needed to accelerate one kg by one meter per second.

Climbing gear is pretty damn impressive!

rgold wrote:I wonder whether the guide's "25 kN anchor" was ever tested with a 25kN load. I bet not...

exactly my thought.
every guide I've been out with just used common sense.

I don't recall ever hearing of an anchor failing because the placements in it (nuts/cams/etc) all broke.

I have heard of anchors failing because the placements were not good (i.e., they pulled out, the rock broke, etc.). But there would not be any way to calculate the point at which that happens based on the strength ratings of the gear.

csproul wrote: huh?

when you fall, three thing take force: the falling climber, the belayer, who is tied into the anchor, and the top piece. because the top piece acts like a pulley, it takes the force on the climber + the force on the belayer, which adds up to be about, IIRC, an average of 1.6x the force on the climber. this means that the belayer takes about .6x the force of the fall on the climber. this info is just stuff off the top of my head, coming from the results of some testing, probably done by john long (i think thats the right name- the anchor guy who came up with the cordallete) or jim titt or somebody else. i'm sure bearbreeder or jim titt or somebody else could easily post those test results but i don't really remember where exactly that info came from. i also recall reading somewhere that when the human body is subject to around 8kn force, your organs start to disintegrate. so if your belayer is subject to ~4.8kn the climber is subject to ~8kn, which is lethal. and since this doesn't happen in reality, we can conclude that the belay anchor isn't subject to this force (4.8kn) unless you're taking high fall factor falls on the belay anchor, which will drastically increase impact force. somebody who is more knowledgeable, please step in and explain this better than i can, or correct me if i'm wrong

Schalk wrote:In which case you would just add the kN ratings of all the pieces in the anchor together.

This is wrong. assuming all the pieces share the load perfectly equally (basically never happens in reality) the strength of the anchor would be the strength of the weakest piece x the number of pieces. for example, if your weakest piece was a small nut rated to 5kn and you had 3 pieces, your theoretical strength would be 5x3= 15kn. in reality, though, 1 piece will take most of the load and pieces will usually rip before they reach their load limit so all of the theoretical stuff doesn't really matter. what does matter is that you get at least two, preferably 3 or more, very strong, reliable pieces in for anchor and try to do a decent job of equalizing them and, most importantly, don't fall directly onto the anchor.

Well, gee if the top piece holds, I guess you don't need the anchor to hold anything, right?

Is that how you construct anchors? Assume the top piece will hold, and that the belayer will only be subject to 0.6x the force of the falling climber (in an upward direction)?

That's not how I construct anchors. I construct them for a worst case scenario, which pretty much assumes a high factor fall straight onto the anchor. Of course I do everything to avoid this, but you need to construct your anchors with the worst case in mind.

of course you build your anchors to strong enough to hold a factor 2 fall. if you don't even attempt to do this, you are a dumbass and you really should stick to sport climbing. but, in reality, anchors are very rarely subject to high factor falls as we also take measures to attempt to prevent this, such as leading past the anchor to get the "jesus piece" in before even starting the next pitch. i was just trying to explain why the OP's concern with calculating the total strength of an anchor is, for the most part, not useful, even if it was possible.