Hi There
Does any one one know much about the new u bolts under the over hangs at Sphinx Rock?  They are pretty ugly and over kill in my opinion.
Who ever is making them Im more than happy to let them use one of my jigs (with a way tighter bend radius) for bending them.  Or I can show you how I make some others.
I write this as it would be a shame to see  these hit the pipes .

Regards

Garry
 

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10 Comments

  1. garry phillips AUTHOR

    Oh and my tel number is 0438087477. As I don't want to start a debate off.

  2. Jon Nermut

    HI garry, Dave and I placed them, and used 10mm instead of our normal 8mm Us as the rock is quite soft, and certainly the bigger the leg distance the stronger the bolt in soft sandstone. Wouldn't use them in hard rock.

    by the way Garry do you know what all the projects are on that stretch of rock - one left of Phoenix then near Pharos (has this been rebolted?)

  3. garry phillips AUTHOR

    Hi John
    The project left of Phoenix is Jake's and Callum's.
    I have 2 projects near Pharos with one already done and Cam veal did one there as well.

    As for Us I don't agree. They don't need to be that big at all in every dimension.  

    The leg lengths can be  longer if need be.

    Take it easy 

     

  4. Simon Young

    i agree they dont need to be massive, 8mm is still strong enough! 20mm reo bar will be much stronger still, but when the bolts stronger than anything attached to it i think its adequate!

    1. Jon Nermut

      Thats kind of the point. Pull out strength of U bolts in soft sandstone is determined by the size of the cone of rock that gets pulled out, which is improved by the leg spacing - see Steve Hawkshaw's thesis and Mike Law's work - http://www.chockstone.org/TechTips/SandstoneBolting.pdf  and http://www.chockstone.org/TechTips/SoftRockBolting.pdf

      I dont disagree that these bolts look pretty big and ugly compared to 8mm bolts with narrow spacing, but it was a conscious decision of balancing the aesthetics (I went to the effort of spray painting them grey to match the rock) versus making sure they are safe in the quite soft rock.

      Maybe we should test some in some of the blocks at the base of the cliff compared to 8mm narrow spaced ones and I will eat my words. Need a testing rig that can do > 40kn though.

  5. Roger Parkyn

    Re U-anchor strength in Tas sandstone:

    I agree with Garry, you can make an adequately strong U with 8mm and without having a big distance between the holes,

    Jon, your "cone of rock" model is relevant here.  What counts is the surface area of the cone, as that it the area that the critical shear forces apply over.  Try doing the calcs on two cases:
    1) Your dimensions (leg length and separation distance) vs
    2) Closer separation distance but with longer leg-length

    You will find that extra leg-length gives a massive increase in cone-surface-area whereas either increased leg-separation, or increased steel diameter, will only give a modest increase in SA

    There are other failure mods to consider as well; such as glue-rock contact area but I don't think there is any evidence that 8mm would be problematic there either 

     

    1. Dave Humphries

      To explain the reasoning behind the anchors:

      • The legs are widely separated to decrease the amount of overlap on the cones formed at each leg. This increases the area of the cone and the strength of the anchor. Increasing the length of the legs only provides small increases in strength due to this overlap.
      • The size of the material. The bigger Us bend less and can transfer more of the load to the rock (and further along the leg with out deformation). 
      • The leg length after a certain length (around 100mm)  doesn't seem to increase load carrying. That is, most pullout failures have a cone of rock and then a length of anchor protruding below it (the leg length that wasn't able to transfer load to the rock).

      While the above explains why we placed and made the bolts the way they are, having no load testing or any other data to suggest smaller was OK we went with the above. In all likelihood 8mm Us at 45mm legs would be fine. But the area we were in was the softest rock at Sphinx.

      As for the aesthetics, there is so much steel work and paraphernalia at sphinx it takes a discerning eye to pick larger than usual hangers as the major aesthetic issue.

  6. garry phillips AUTHOR

    The link I'm sure is full of very useful information.  Yet I still think these Us are way way over the top (and many others will agree).  
    If the rock is so poor i actually wonder why you bothered to equip it!  The hand holds must be no better.  Its the same at Neika if its the white sand stone don't even bother.

    The other routes on the aretes look nice additions for the community though so well done.  

    Also a 8mm u in a flat sand stone wall (i.e. not in a flake) is not coming out.  The leg lengths on some of the other Us at Sphinx are 70mm long and they are straight up in a horizontal roof.  They are bomber.

     

     

  7. Mark Polinski

    Thanks for posting the honors thesis link Jon, as a scientist I like seeing actual data rather than just hearsay and personal experience. I found it interesting that a single machine bolt with hanger performed equally as well (if not better) in virtually all aspects compared to the U's. It would also appear that although leg distance and steel diameter both contribute to the strength of the anchor, a properly placed 8mm U with 35mm leg spacing embedded 70mm is still going to hold above 20 kN (15 kN is the recommended Euro standard) and that the most important aspects with regard to failure are providing adequate notching in the legs for glue adhesion and using correctly mixed glue. 

    I have no idea of what diameter and leg distances you guys are all using (we stick pretty much exclusively to single shafts with hangers in the states), but any benefits for using more than 10mm diameter stainless steel, or 45mm leg distance appears to be minimal or superfluous as judged by those tests in Sydney sandstone. 

  8. Roger Parkyn

    Jon & Dave, This is a useful discussion; I hope it doesn't feel like we're getting at you as my only intention is for us to all learn from each other.

    There are two issues being discussed: 1) 8 vs 10 mm steel; and 2) leg spacing (not "legs" or "leg spacing"!) ...

    10mm U-bolts
    I don't have any problem with this and have placed some myself.

    Dave has a valid point that where there is doubt then upsizing gives us the benefit of the doubt.

    If you bend them with a radius to give 45mm leg spacing then they look fine.  To the casual observer these look just like the 10mm P-bolts that you will see at the Hoff and at Paradiso - no problem - the extra 2mm of steel showing isn't a big deal!

    Leg Spacing:

    The reason we have two legs is principally to contain the biner not because we can't make a single embedded rod strong enough: you can make a single-rod placement stronger by embedding it deeper (although at some point this becomes pointless as it is already strong-enough after a certain depth).

    Another point about going deeper is that you not only make the anchor stronger you also make it more reliable as it effectively spans across more rock so localised weaknesses may be bridged

    Yes; making the spacing really wide will be stronger but the point is that it isn't the only option for making an anchor strong enough.  The first article quoted above by Jon says that for their 45mm spacing there is a 75% reduction in strength but it also points out that across the two legs this amounts to 150% of a single-embedded rod.  This is the point to remember: when you add a second leg, even if closely spaced, you increase the strength relative to a single rod.  If you want to be really sure about strength then design the embedment leg as if it was a single-rod anchor (like a P-bolt).  If that is somehow impractical and you feel the only way of squeezing out the required strength is by wide leg spacing then perhaps you should stop, because you must be bolting some really woeful rock. 

    Now, I haven't seen the U's Garry is talking about.  If the leg spacing is of the order of 50 mm (centres) then I think the aesthetics are fine.  If the rock is really bad then the legs maybe need to be over 100mm. 

    Here's a suggestion: bring one of them in to the BBQ tomorrow as I am curious to see what we are talking about!