Anchor Strength?

Hello, just recently found this forum and i was hoping someone might be able to give me some insight in regards to building anchors. Specifically high line anchors but general climbing knowledge should pertain directly as well.

Does anyone have any knowledge in regards to how strong one bolt is. Not how strong the bolt itself is. but how strong it is when it is set into lets say granite vs concrete vs a very soft type of stone. I understand that the bolt itself has a KN Rating on it but how do people calculate that the rock itself is strong enough to hold the force?

Are these numbers non existent because the rock is not what needs to be worried about in any situation?

But what if you wanted to anchor into a really soft type of rock? if that scenario were to come up is there any way of calculating how manny or how large of anchors you would need in order to be bomber?

Also assuming that the general consensus is just that the rock is not going to break off before the bolt does then is there a way to calculate how much force multiple bolts in lets says sliding x anchor can hold?

Is it as simple as
1 Bolt + 2 Bolts + 3 Bolts = 75KN (Random number)
75KN Multiplied by the Angle Of each anchor sling = The total force loadable?

I might be over thinking this entire process and just by typing it all out i think iv answered my own questions but please feel free to comment any experiences anyone has had in regards to climbing/ high line anchors?

Thanks

The "rating" on a bolt is the force it is required to withstand in a standard concrete test block, it may well hold considerably more but the 25kN is the certification load. I manufacture bolts which will hold over 110kN but whether you can install them so they hold that much or whether the rock can withstand the load is another matter.
If the installer is working with rock weaker than the certification concrete or with another resin then the installer should test the application but they rarely do, basically you just guess and use longer bolts.
Bolt hangers don´t have usually have much to spare over the 25kN, 27 to 30kN is a fairly normal failure level. We make a rod hanger which holds over 60kN but getting the actual bolt to hold this is a challenge!

The "ratings" for climbing equipment are not transferable to slackline/highline applications, the load duration is different and untested. The certification (EN959/UIAA) is anyway ONLY for recreational mountaineering and so void in a highline. You should be using the construction industry ratings for the bolts themselves, not the climbing ones and there is plenty of information about multiple anchor strength calculation available for these.

Incidentally for a three-piece equalised (or equalising) anchor you can normally expect one of the anchors to be taking at least 50% of the load, this has been tested often enough in climbing research.

theuaeboltfund.com/uiaa-123/
theuiaa.org/upload_area/Saf…

Highlining will typically generate a lot more force than one would find normally in climbing. I've never built an anchor for highlining (because I don't do it) but have seen it frequently. If building the anchor with bolts, I'd suggest at least three in each anchor and not very soft rock. There's not really a good way to accurately test the strength of the rock without destroying it so be conservative if you have to use soft rock.

The most common anchors I've seen are a couple of well rooted trees or one really big well rooted tree and this is probably the most bomber anchor you can get.

Finally, keep your equipment separate for climbing and highlining as highlining can cause fatigue that can significantly weaken the metal.

Thanks everyone for the super fast and insightful responses. So What I'm gathering is that High-lining hasn't really had any extensive tests done like climbing has and for that reason, precise calculations can't really be made, because climbing ratings just don't pertain to loads that a high line anchor would be under. A rig is either Bomber or its just un-suffice.

although, you could assume that because 99% of high liners rig 4 bolt anchors, that 100KN of climbing bolts set properly, In equal to concrete strength rock and that are Equalized properly is Bomber.

And if one of those things is less then perfect i.e softer then concrete rock, Not great anchor equalization, Etc. Then as a rule of thumb you practice redundancy and install additional Bolts/ Longer Bolts/ ETC.

Masafumi Nagasaki wrote:Thanks everyone for the super fast and insightful responses. So What I'm gathering is that High-lining hasn't really had any extensive tests done like climbing has and for that reason, precise calculations can't really be made, because climbing ratings just don't pertain to loads that a high line anchor would be under. A rig is either Bomber or its just un-suffice. although, you could assume that because 99% of high liners rig 4 bolt anchors, that 100KN of climbing bolts set properly, In equal to concrete strength rock and that are Equalized properly is Bomber. And if one of those things is less then perfect i.e softer then concrete rock, Not great anchor equalization, Etc. Then as a rule of thumb you practice redundancy and install additional Bolts/ Longer Bolts/ ETC.

The calculations are easy enough and there´s a reasonable amount of strainmeasurement been done on the forces involved but there are other issues than simple math.
The problem is climbing equipment "ratings" are for a one-off event NOT repetitive or continuous loading. That hanger with 25kN stamped on it or that karabiner with 20kN is expected to survive ONE short-lived impact and then be retired, not continuously loaded to anywhere near this. Certified climbing bolts have a repetitive load test but it is only 8kN and deformation is permitted, in other words they bend like hell. Mechanical bolts shift and come loose as well as bending under these loads as well. Climbing this doesn´t matter but for a highline it may well be important.
I make hangers especially for highliners in Europe and have also made custom bolts for some huge highlines and we advise the users work on a safety factor of at least 10 as I (or the instllers) can´t test the rock or the installation. The anchors we supplied for the N face of the Eiger had a total rated stength of 219kN and the "normal" anchors are 3 x 12mm SS wedge bolts which should total 157kN if the rock holds, the hangers are stronger than this.
There´s absolutely no guarantee though, rock is always good for a suprise!