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Roof vs Face Bolts
Posted: Thu Jan 26, 2006 8:47 am
Here is some interesting info as per a suppliers data sheet:
Steel failure (design resistance) of M10 glue in bolts:
Tension (ie a roof placement) 21.7kN
Shear (ie vertical face placement) 13.0kN
Moral of the story is a well placed roof bolt is approximately 50% stronger than a bolt on a vertical face
Posted: Thu Jan 26, 2006 9:41 am
! bloody interesting and seems somewhat counterintuitive for me.... but then I'm not a mechanical engineer and I was hopeless at classical mechanics in physics :-)
Raises a question: how many bolters know the limits of the materials they are using, not to mention interesting facts like this one?
Posted: Thu Jan 26, 2006 10:30 am
Does anyone know the corresponding values for well-placed M10 expansion bolts?
Posted: Thu Jan 26, 2006 10:40 am
Did they mention the mode of failure? and weather it was ultimate failure or initial failure?
I also know that the higher grade stainless steels is quite \"brittle\" compaired to other steels. (this may lead to the lower shear failure) Steel also tends to be stronger in tension, if it is shock loaded. I wish someone would sponsor me to do some reserch on this. (any takers?) There are so many questions and so few answers.
What type of gluing system are you using for the ARF bolting?
Posted: Thu Jan 26, 2006 10:44 am
It seems to make logical sense if you scale it down, pull a piece of wire (no pun intended) as opposed to shearing a piece of wire, you can hang huge weight on a wire, but the shearing strength is minimal.
But isnt the weak link here the glue? On a verticle face the glue plays a far lesser role (because of the direction of the force) as opposed to a roof section where I would you are basically hanging on the glue?
I dont think the one should be considered without the other.
Posted: Thu Jan 26, 2006 10:45 am
How do you place a gluein bolt in a roof ? (Without getting covered in glue)
Posted: Thu Jan 26, 2006 11:28 am
So here are the stats for a M10 HSA-R Hilti mechanical anchor from the Hilti handbook:
Tensile (or Pullout) : 20.1kN
Shear : 24.0kN (steel failure)
Both values are the mean ultimate resistance values. The tests are performed on a bolt with a standard anchorage depth in non-cracked concrete.
The same sort of results hold true for ice-screws, where screws placed downward into the ice (i.e. placing the screw predominantly into shear) fail at lower (quite significantly lower) values than those placed upward (or predominantly in tension) in the ice.
With both bolts and screws there are also other points to consider, in screws melt out (i.e. the sun warming the screw and reducing the area of thread in contact with the ice) and with bolts, especially glue-ins it hole preperation. Glue-ins require far more time preparing the hole (cleaning the rock flour created while drilling) and the glue used also needs to be carefully measured (if necessary, some of the new systems come pre-measured) and properly mixed.
Posted: Thu Jan 26, 2006 12:07 pm
Also had a look at Hilti's adhesive system(s).
1) HVU with HAS-R rod. It's a foil pack (looks like a slender bullet) that you insert into a drilled and cleaned hole, add the STAINLESS STEEL rod and drill it into the foil pack, which shreds the pack and mixes the adhesive.
Tensile : 39.6kN (steel failure)
Shear : 23.8kN (steel Failure)
Curing time before anchor can be fully loaded:
@ 20C and above 20min
@ 10C to 20C 30min
@ 0C to 10C 60min
2)HIT-RE injection adhesive with HAS-R rod. Injection system which requires a dispensing gun that mixes the two epoxies correctly.
Tensile : 39.6kN (steel failure)
Shear : 23.8kN (steel failure)
Curing time before anchor can be fully loaded:
@ 20C and 30C 8hrs
@ 10C to 20C 12hrs
@ 0C to 10C 24hrs
These values are for M10 rods. It's clear that the shear strength of the anchor is governed by the steel (i.e. the ultimate shear value is similar for both mechanical and glue in anchors), whereas glue in anchors defnitely seem to out perform their mechanical counterparts (20kN vs 40kN).
Posted: Thu Jan 26, 2006 12:26 pm
Can anybody quote the tensile and shear strength of the glue? As mentioned earlier in the thread it is going to make the tensile strength of a roof mounted bolt irrelevant I would think. I do not know of epoxies with the same strength characteristics as steels. Other things to consider are whether the steel will fail first or the rock around an express anchor i.e. can you pull an express anchor out without breaking the express anchor (I suspect with the right tool youca) once again making the tensile strength of the steel irrelevant.
Posted: Thu Jan 26, 2006 12:27 pm
Can anybody quote the tensile and shear strength of the glue? As mentioned earlier in the thread it is going to make the tensile strength of a roof mounted bolt irrelevant I would think. I do not know of epoxies with the same strength characteristics as steels. Other things to consider are whether the steel will fail first or the rock around an express anchor i.e. can you pull an express anchor out without breaking the express anchor (I suspect with the right tool you can) once again making the tensile strength of the steel irrelevant.
Posted: Thu Jan 26, 2006 12:47 pm
MarkM quoted the tensile and shear failure stress for glue in
anchors. Read carefully dude, the strength on the epoxy is part of those figures
From the little I understand, the steel will pretty much always fail before the bond between the epoxy and the rock.
And apparently we should all be trad climbing anyway...
Posted: Thu Jan 26, 2006 2:11 pm
There are a couple of questions I'd like to know about glue-ins like how long is the lifespan of the glue? Will the strength of the bond deteriorate over time?
Also if the glue in is placed in a vertical wall (i.e. the bolt is placed hizontally) or slightly downward will you get an isolacted crushing of glue just below the point where the bolt exits the rock, so that after some time you'll get a slightly longer leaver-arm and thus progressively crush more and more glue below the bolt over time. (Same principal with an ice screw, you get a concentrated stress (compression) just at the point where the screw exits the ice). What will the effect be due to the loss of bond area?
Most (all?) of the tests quoted for bolts are once off tests to failure where as a bolt may be loaded several hundred/thousand times over its life, so it would be cool if some testing could be done in this regard.
Posted: Thu Jan 26, 2006 4:02 pm
Looking at the Hilti charts for HIT RE 500 and 304/316 SS HAS rods it apparent that max allowable Bond Capacity (the max recommended force) for tensile in 27Mpa concrete is 11.5kN while for shear it is 20.3kN for a 3/8\" (about 10mm Sorry I got the US site for Hilti) anchor set in 85mm deep. There is a neglible change in the allowable tensile value even if the anchor is set deeper (115mm), but the allowable shear force goes up dramatically to 33kN.
The ultimate Bond capacity (i.e. the point at which it will certainly fail)though is rated at nearly 4 time the allowable which I think is purely a safety factor to allow for the many unknown with this type of bonding i.e. concrete/epoxy interface, epoxy/bolt interface.
The 3/8\" 304/316 SS rod has an allowable tensile force of 16.2 kN which is higher than the allowed combined bond tensile capacity and an allowable shear force of 8.3kN which is substantially less than the allowable combined shear capacity.
The ultimate strength of the SS is rated at approximately double the allowable as there are comparitively few unknowns in the strength of a steel.
The inference I make from the above is that in tension you should be more worried about the epoxy failing (largely due to the unknown of the interface between the epoxy and the rock) than the bolt and in shear you should be more worried about the bolt failing.
It also means that there is quite a bit of safety built in on the epoxy side in shear so Mark I think the effect you mention above needs to be quite severe before the anchor is going to be unsafe in shear.
Posted: Thu Jan 26, 2006 4:19 pm
If you look at the table below all the rest above becomes academic as the rock strength varies such a lot that we need to be more careful as to what we bolt into than anything else
Strength classification Strength range (MPa) Typical rock types
Very weak 10-20 weathered and weakly-compacted sedimentary rocks
Weak 20-40 weakly-cemented sedimentary rocks, schists
Medium 40-80 competent sedimentary rocks; some low-density coarse-grained igneous rocks
Strong 80-160 competent igneous rocks; some metamorphic rocks and fine-grained sandstones
Very strong 160-320 quartzites; dense fine-grained igneous rocks
Posted: Thu Jan 26, 2006 4:54 pm
Dam, Russell you've done your homework, either that or you're a nut (ba ha ha
- Lame joke - had to say it)
So in summary what are we saying?
That the tensile strength of a bolt is more than shear strength, but after taking the glue into account, the shear setup is stronger than the tensile?
Posted: Fri Jan 27, 2006 7:51 am
In my opinion if the rock is good and the guy putting the bolts in knows what he doing we should never have to worry about the physical bolt mechanics until things like SCC have enough time to effect the strength of the bolt and as discussed before this is where glue ins really come into their own as they would be a lot less susceptible to this phenomenon. Glue ins would also be better in lower quality rock as the express anchors put a huge amount of force onto the rock/bolt interface just to set then properly thereby prestressing the rock.
My opinion in short: In granite or other high strength rock away from the sea I would think the 316 SS express anchors are great, but as the rock type becomes more suspect use glue ins and as you approach a potentially corrosive environment use titatinium glue ins if you can afford them.
Posted: Fri Jan 27, 2006 12:20 pm
To answer Dave D, there is no way of avoiding getting the glue everywhere when using a dispenser system such as for the RE500 hilti glue! My pants and bolting pack are covered in the stuff! Be sure to use gloves when using these ceramide epoxies as its not great for the skin.
However having used the glue I would not recommend any other (except perhaps the capsule/sachet glue which is neater to use but requires turning the bolt at least thirty revolutions to guarantee a good mix, not much of an option if you have to do this by hannd as whith an eye bolt. Threaded rod can be fitted to an electric screwdiver to turn it but I would recommend cutting them down to 150mm in length as drilling 200mm+ into quartzite is blerry impossible!) The RE500 has superior bonding characteristics and is less affected by moisture or dust in the hole. It also lasts a very long time, way in excess of the lifespan of the bolt itself. Again I must put out a plea not to use mortars, whilst they are used for fixtures in building, they do not bond to the rock or the steel at all and can easily be picked off with your fingertip! A damn scary scenario! These have been used on numerous anchors in the past and some of the placements are looking very dodgey, noteably the u-bolts on the easy routes at the playground in Montagu where the mortar has parted from the bolt showing an obvious crack or gap all around the studs of the u-bolts. I strongly recommend that all these placements be checked and redone with a proper adhesive.
Again we should avoid placing any more mechanical anchors at peninsula crags, SCC is just too much of a danger. Glue ins are a mission to place taking at least three times as long to place and they are very expensive but what value do you place on your life??
Posted: Sun Jan 29, 2006 10:08 pm
Russel, you may like to note that the Tortuga titanium 'p' bolts are not presently available or in production. In Thailand they have been forced to change to American made titanium staples (a type of oversized 'u' bolt). These are apparently hideously expensive and because of their size not aesthetically pleasing at all. Its a shame the Tortugas are no longer being produced as they looked like the bomb! Made by an aerospace company and super high quality right down to the logo lazer etched on the eye of the bolt. Mmmm! nice!
Liberty Mountain works (who own Ushba, who made the bolts) have promised to inform us if the bolts come back on line, but dont hold your breath. Their suggestion to me was that we try and get them made in SA, so, if anyone out there knows anyone with enough experience to machine, bend, weld and finish 10mm grade 5 titanium rod (a highly specialised and difficult task) let us know, we may have some business for them. In the meantime it looks as though the only way to go is with the (also excellent looking) SS 'p' bolts produced by companies such as Raumer or Fixxe.
Posted: Mon Jan 30, 2006 8:22 am
I'm glad to see a small snippet of useless info can create so much interest.
Please be aware that the current policy is for climbers to only use M10 Fischer or Hilti Express Anchors. Only trained re-bolters should be using glue-ins. Although the glue-in system is great, if it is not installed correctly it will fail at a very low load. Examples: Somebody placed some anchors in the Eastern Cape which started to pull out because the glue mixer didn't work properly, and an Austrian gentleman told me that you can mix the glue capsules by just turning the anchor by hand - I tried that on a non-safeguard application and it did not work - and the suppliers state you must spin it with a drill.
Keep it simple - keep it safe.
Posted: Wed Feb 01, 2006 9:47 am
You are right Andy, my wish to see only glue-ins at Peninsula crags is unrealistic, ditto that glue in placements are easy to mess up (trapped air bubbles, bolts moving whilst the glue sets upsetting the integrity of the glue matrix, misallignment of the shaft in the hole and more) It would however be nice to see the crags on the False bay coast one day equipped with only glue ins.
I think the confusion re: spinning the bolt with the sachet glues stems from the Petzl catalogue recommending sachets for use with their eye bolts. The glues have been improved recently aiding easier mixing but last time I checked Hilti still prescribed using a drill to spin the shaft.
Ps if you are ordering any more eye bolts from Raumer or similar at any stage please give me a shout I would like to piggy back on the order and get some myself (missed you this round) Thanks.
Posted: Thu Feb 02, 2006 8:55 am
So what happens to glue-ins that are at a crag which is swept by a velt or forest fire? Will the glue be damaged? Aparently glue-ins can be removed using heat.
Take special care with glue. On 2 occasions in EC top bolts did not set(mine). Make sure, if using a nozzle system, that enough glue is squirted(wasted) through, before using on an actule anchor. Try to carry a packet to squirt this waste into... glue squirted all over is crap for the enviroment & gear. Check the expiry date before using any glue. This is important. Loverly fellows in the west love to send expired stuff in Africa. I recomend not using glue in top anchors.
Posted: Fri Feb 10, 2006 10:21 pm
Sometimes glue-ins are essential. Perhaps the standards could be extended include the placement of these anchors by trained people when the situation dictates their use.