washer between bolt hanger and rock?

Jim Titt wrote:... so the hanger starts slipping at 10kN. Once you get over this point the load comes on the bolt itself ...

thank you, that's just over a ton, now how much force is generated in most falls?

Jim Titt wrote:However this is only true if you stay in the working limits for the bolted joint which we don´t.

You don't, but out in the real world we do. The OP's bolt will never have a hydraulic puller attached to it, and will likely never see the 10 kN that makes the hanger slip more than a few times. Pull testing to ultimate failure is interesting and fun, but engineers design things to be used within the working limits. So, why do you torque the nuts to 20 kN? To give the bolt a working load of 10 kN.

Jim Titt wrote:The additional factor that no engineering book can help you with is that the clamped object is considered to be rigid and bolt hangers are far from that..... clearly friction between the hanger and the rock isn´t a major interest!

Just like your house is considered to be rigid, until it has an encounter with a bulldozer. However, houses weren't designed to have encounters with bulldozers, they are made to withstand expected winds, with some safety factor. Not fair to level it with a bulldozer then say "see, clearly friction between a nail and a stud isn't a major interest!"

For what its worth, the coefficient of friction between steel and steel is ~0.8, and according to Jim the coefficient of friction between steel and rock is ~0.5...

Eric Krantz wrote: thank you, that's just over a ton, now how much force is generated in most falls? You don't, but out in the real world we do. The OP's bolt will never have a hydraulic puller attached to it, and will likely never see the 10 kN that makes the hanger slip more than a few times. Pull testing to ultimate failure is interesting and fun, but engineers design things to be used within the working limits. So, why do you torque the nuts to 20 kN? To give the bolt a working load of 10 kN. Just like your house is considered to be rigid, until it has an encounter with a bulldozer. However, houses weren't designed to have encounters with bulldozers, they are made to withstand expected winds, with some safety factor. Not fair to level it with a bulldozer then say "see, clearly friction between a nail and a stud isn't a major interest!"

Well you wrote "However, Kennoyce is correct about shear being carried by the friction between the rock and the hanger, which is provided by the tension in the bolt.

It's not intuitive. Engineered bolted joints are always preloaded for several reasons, one being that the bolt itself won't carry the shear, it's carried by the joint through friction."

And to pass the CE and UIAA requirements for rock anchors this is not the case, the load is carried by the stud itself.
That you think 10kN is strong enough is your opinion but myself and a great many other people would rather see a rather standard for bolts which would cover all falls, not just most. Probably why the CE standard has become more widely accepted than the Krantz standard.

The preload (torque applied) has nothing to do do with whatever you are fastening to the rock or building. The installation torque is merely to test that the bolt has achieved a satisfactory pull-out resistance and post-installation the nut may be removed and a suitable torque for the installed object applied such as timber or hollow metal sections. They aren´t called torque-controlled anchors for nothing.

Tom Grummon wrote:For what its worth, the coefficient of friction between steel and steel is ~0.8, and according to Jim the coefficient of friction between steel and rock is ~0.5...

Not sure where you're getting .8 from, steel on steel is generally considered to have a coefficient of friction of around .3 if I'm remembering correctly.

kennoyce wrote: Not sure where you're getting .8 from, steel on steel is generally considered to have a coefficient of friction of around .3 if I'm remembering correctly.

http://www.engineeringtoolbox.com/friction-coefficients-d_778.html

probably not the best source, but several other results online corroborated this. This was just an internet search, if you have the information in a text book please correct me.

Edited to add: It is much lower for oxidized steel, but one can hope that is not a problem with stainless and/or plated steel climbing hardware.

Tom Grummon wrote: engineeringtoolbox.com/fric… probably not the best source, but several other results online corroborated this. This was just an internet search, if you have the information in a text book please correct me. Edited to add: It is much lower for oxidized steel, but one can hope that is not a problem with stainless and/or plated steel climbing hardware.

Looks like a good source. Like I said, I was just remembering .3 for some reason. Regardless, I have a hard time believing that there is more friction between a hanger and a washer than a hanger and rock.

kennoyce wrote: Looks like a good source. Like I said, I was just remembering .3 for some reason. Regardless, I have a hard time believing that there is more friction between a hanger and a washer than a hanger and rock.

It´s one of those things with friction, it doesn´t obey the rules very well!
The engineering tables I use have various coefficients depending on what the application is, for nuts/threaded holes 0.12-0.0.18, for a nut against a clamped object 0.1-0.18 and for clamped surfaces 0.15-0.25 but they also give values up to 0.75 with other tests and applications. I use 0.2 as a rough figure, 0.8 would make doing nuts up too difficult!

Jim Titt wrote: It´s one of those things with friction, it doesn´t obey the rules very well! The engineering tables I use have various coefficients depending on what the application is, for nuts/threaded holes 0.12-0.0.18, for a nut against a clamped object 0.1-0.18 and for clamped surfaces 0.15-0.25 but they also give values up to 0.75 with other tests and applications. I use 0.2 as a rough figure, 0.8 would make doing nuts up too difficult!

Thanks, Jim. I didn't realize it was that variable.

Tom Grummon wrote: Thanks, Jim. I didn't realize it was that variable.

It´s the tests that are so variable which is why one is always cautioned to atually test the specific application. For nylon the usual values obtained by testing and used for climbing ropes on aluminium are in the 0.2-0.3 range but experimentally values of 0 and 2 have been observed. Some of the "Laws of Friction" are merely more or less useful approximations!

Eric Krantz wrote: thank you, that's just over a ton, now how much force is generated in most falls?

That is a difficult subject and one I have been testing a bit lately. There are a few different calculators out there that can predict the impact force subjected on the top piece in a fall, but they assume a rigid mass, a completely static belay, and a perfectly perpendicular and linear fall pattern - none of which is realistic to climbing in most cases. In the field, I have been able to generate 2.5 - 4kN on the top piece with a lead fall. Those values represent legitimate lead falls in typical sport climbing scenarios. The falls ranged from 10 - 25 feet depending on the test. Some of my earlier testing focused on generating the highest possible impact forces that are realistic to a single pitch sport climbing scenario. I tested the impact force on the second bolt of a route when I fell clipping the third and the belayer had to execute a full-on running belay. With a locking belay device, a rope with a impact force rating of 10.4kN and my weight of 155 lbs, I got around 4 kN. I have not been able to break 4kN in a single pitch environment so far, but I still have a lot more testing to do.

fair weather climber wrote:I think we are all going to die from washers put on the wrong side of the hanger. BTW, I think this thread has been hijacked^^^^ I think someone needs to test a washer behind a hanger a few hundred times just to be sure.

Not only that, Jim has once again shown himself to be quite the resource for things like this. Much as I love armchair engineering, there's no arguing with facts.

It was probably just a gym rat trying to get out into the wild.