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miklaw


Jan 2, 2011, 12:13 PM
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Why are Ubolts so weak, or aren't they?
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Ubolts are stronger than rings, but not twice as strong as a ring of the same length. “Why is it so?”
In recent testing on soft sandstone of 13 rings and 10 Ubolts the average axial failure strengths were:-

Average strength (kN)
10mm x 80mm ring 28.5
80 mm Ubolt (B) 36.5
Double ring value (A) 56.9
B/A 0.64

If we multiply the ring values by 2, this is what the Ubolt should hold if there were no interaction between the holes, the Ubolt values are 64% of the double ring values for outwards loading.

There are 2 popular theories to explain this:-

The first theory is that drilling closely spaced holes shatters or damages the rock in between the holes. This may be true for brittle rock types such as limestone, but I haven’t seen any evidence of this testing on soft sandstone, in fact many outwards tests end with an apparently undamaged cone sitting between the shafts (figure 1).


Figure 1 Failure cones

The second theory is based on the observation that failure often pulls out large cones of rock, the failure load is related to the surface area of the cones. When 2 adjacent anchors are close together, the cones overlap and lose some surface area and the failure load is reduced. The actual amount of reduction can be calculated from equations based on experiments, there are a number of different equations, the one used by Hilti for glued in anchors is:-

S/S0= 0.33(s/d)+0.55
Smin=0.5d

The average anchor spacing is 35mm which is almost 0.5d, at s=35 and d=80 the factor is 0.69, given the variability of rock this is pretty close and seems to be a better explanation.

Of course 0.64 x 2 = 1.28, which is why they are stronger than a ring of the same depth.
Attachments: fig3inter.jpg (133 KB)


USnavy


Jan 2, 2011, 9:25 PM
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Re: [miklaw] Why are Ubolts so weak, or aren't they? [In reply to]
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miklaw wrote:
Ubolts are stronger than rings, but not twice as strong as a ring of the same length. “Why is it so?”
In recent testing on soft sandstone of 13 rings and 10 Ubolts the average axial failure strengths were:-

Average strength (kN)
10mm x 80mm ring 28.5
80 mm Ubolt (B) 36.5
Double ring value (A) 56.9
B/A 0.64

If we multiply the ring values by 2, this is what the Ubolt should hold if there were no interaction between the holes, the Ubolt values are 64% of the double ring values for outwards loading.

There are 2 popular theories to explain this:-

The first theory is that drilling closely spaced holes shatters or damages the rock in between the holes. This may be true for brittle rock types such as limestone, but I haven’t seen any evidence of this testing on soft sandstone, in fact many outwards tests end with an apparently undamaged cone sitting between the shafts (figure 1).


Figure 1 Failure cones

The second theory is based on the observation that failure often pulls out large cones of rock, the failure load is related to the surface area of the cones. When 2 adjacent anchors are close together, the cones overlap and lose some surface area and the failure load is reduced. The actual amount of reduction can be calculated from equations based on experiments, there are a number of different equations, the one used by Hilti for glued in anchors is:-

S/S0= 0.33(s/d)+0.55
Smin=0.5d

The average anchor spacing is 35mm which is almost 0.5d, at s=35 and d=80 the factor is 0.69, given the variability of rock this is pretty close and seems to be a better explanation.

Of course 0.64 x 2 = 1.28, which is why they are stronger than a ring of the same depth.
U bolts are not weak, there are among the strongest bolts used in climbing actually. They are even stronger than 1/2" stainless steel Power-Bolts in many cases. We use 3/8" x 2.5" titanium alloy bolts exclusively here in Hawaii. I have tested a number of them in hard basalt and found they have a MBS of about 35 kN in tension. Many held over 40 kN which is twice the CE standard and well over the UIAA standard. In every case I have tested, the bolt broke before the rock.

You even said yourself that a 10mm x 80mm bolt held almost 37 kN in sandstone. That is very impressive, that’s stronger than any 1/2" expansion bolt I have ever seen tested in sandstone.

Also the bolt you have pictured has a rather small leg spacing, one with a 3" leg spacing (O.D) would likely perform better; which is the type of bolt we use exclusively.


miklaw


Jan 2, 2011, 10:08 PM
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Re: [USnavy] Why are Ubolts so weak, or aren't they? [In reply to]
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[quote "USnavyAlso the bolt you have pictured has a rather small leg spacing, one with a 3" leg spacing (O.D) would likely perform better; which is the type of bolt we use exclusively.
This post was mainly to quash the weird info about weak Ubolts that we see a lot of...
Wider spacings tend to be slightly weaker and much uglier, in downwards load it is because the unit is relatively more flexible and the top hole gets more load and fails earlier. In outwards mode the legs pull together and crush soft rock, hard rock might be slightly better in this regard


USnavy


Jan 3, 2011, 1:21 AM
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Re: [miklaw] Why are Ubolts so weak, or aren't they? [In reply to]
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miklaw wrote:
This post was mainly to quash the weird info about weak Ubolts that we see a lot of...
Wider spacings tend to be slightly weaker and much uglier, in downwards load it is because the unit is relatively more flexible and the top hole gets more load and fails earlier. In outwards mode the legs pull together and crush soft rock, hard rock might be slightly better in this regard

I disagree about the larger spacing’s being uglier, I think they look nicer. But regardless there is something far more important. It’s a pretty safe bet to say U bolts are adequately strong in any type of rock when properly installed. In every test I have ever seen U bolts exceed UIAA strength requirements in even the softest of climbable sandstone so there is really only one other parameter that must be considered.

U bolts are plagued for their tendency to enable a quickdraw to unclip itself if the gate on the bolt side biner is facing the direction in which the climber is climbing. Although I have never seen a U bolt come unclipped while someone was climbing, I have been able to unclip one myself through controlled flicks of the rope, thus I know it’s possible for them to come unclipped. Well the wider bolt spacing helps alleviate this issue. The smaller leg spacing makes it easier for the draw to come unclipped.


(This post was edited by USnavy on Jan 3, 2011, 1:23 AM)


miklaw


Jan 3, 2011, 4:17 AM
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Re: [USnavy] Why are Ubolts so weak, or aren't they? [In reply to]
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USnavy wrote:

U bolts are plagued for their tendency to enable a quickdraw to unclip itself if the gate on the bolt side biner is facing the direction in which the climber is climbing. .

....So why would anyone clip them (or more importantly a bolt hanger) this way?
Or,as we say in Oz, "there are 2 ways to clip, the corrct way and the common way"
No, we haven't seen this either.


(This post was edited by miklaw on Jan 3, 2011, 4:23 AM)


USnavy


Jan 3, 2011, 4:49 AM
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Re: [miklaw] Why are Ubolts so weak, or aren't they? [In reply to]
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miklaw wrote:
USnavy wrote:

U bolts are plagued for their tendency to enable a quickdraw to unclip itself if the gate on the bolt side biner is facing the direction in which the climber is climbing. .

....So why would anyone clip them (or more importantly a bolt hanger) this way?
Or,as we say in Oz, "there are 2 ways to clip, the corrct way and the common way"
No, we haven't seen this either.

Laziness? Lack of knowledge on how to properly clip them? There are a number of reasons, all I know is I have seen climbers who climb 5.8 clip them wrong and I have seen climbers who climb 5.14+ clip them wrong.

I will admit I have clipped them with the gates facing the wrong direction in a few instances because it was in the middle of a hard ass move and it was easier to clip them with the gates facing in that direction.

Also many climbers climb with gates opposed. With gates opposed its generally best to face the bottom biner away from the direction of travel so if you fall the rope stays away from the gate. But doing so means you have to clip the top biner the wrong way.


Kinobi


Jan 3, 2011, 12:57 PM
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Re: [miklaw] Why are Ubolts so weak, or aren't they? [In reply to]
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Most biners unclip not because of shape of bolts (or U bolts), but because they are "stiff". Rubber pieces now fashionable, add a significant "stiffness" to any sling.

Long soft slings make life easier.
Ciao,
E


kennoyce


Jan 3, 2011, 1:19 PM
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Re: [Kinobi] Why are Ubolts so weak, or aren't they? [In reply to]
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Kinobi wrote:
Most biners unclip not because of shape of bolts (or U bolts), but because they are "stiff". Rubber pieces now fashionable, add a significant "stiffness" to any sling.

Long soft slings make life easier.
Ciao,
E

On many hangers a biner won't unclip itself no matter how stiff the sling is. Given th right shape of hanger, a stiff sling will make it easier to unclip itself. Unfortunately, a stiff sling will also make it easier to clip the draw which is why they are used for sport climbing. I guarentee that long soft slings do not make life easier when climbing hard sport.


Kinobi


Jan 3, 2011, 10:01 PM
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Re: [kennoyce] Why are Ubolts so weak, or aren't they? [In reply to]
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kennoyce wrote:
On many hangers a biner won't unclip itself no matter how stiff the sling is. Given th right shape of hanger, a stiff sling will make it easier to unclip itself. Unfortunately, a stiff sling will also make it easier to clip the draw which is why they are used for sport climbing. I guarentee that long soft slings do not make life easier when climbing hard sport.

I disagree. And I have a certain feeling of what hard sport climbing is.
E


mattm


Jan 29, 2011, 4:16 PM
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Re: [USnavy] Why are Ubolts so weak, or aren't they? [In reply to]
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Navy,

I assume the bolts you speak of are similar (if not the same) to the ones mentioned recently in one of the Mags. There was a blurb about the ASCA doing a bulk order of 1000 U-Bolts in Titanium for Thailand, Cayman Brac and HI. Do you have any more info on these? The article was sorely lacking in the tech specs a gear/lab geek would want!

cheers!


USnavy


Jan 29, 2011, 8:37 PM
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Re: [mattm] Why are Ubolts so weak, or aren't they? [In reply to]
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mattm wrote:
Navy,

I assume the bolts you speak of are similar (if not the same) to the ones mentioned recently in one of the Mags. There was a blurb about the ASCA doing a bulk order of 1000 U-Bolts in Titanium for Thailand, Cayman Brac and HI. Do you have any more info on these? The article was sorely lacking in the tech specs a gear/lab geek would want!

cheers!
I dont know what article you read, but the bulk order from ASCA was not for U-bolts, they were for a bolt similar in nature to the Ushba Tortuga bolt. Here is a pic of the bolts you are referencing:



They are completely custom in nature, and are not rated or certified (as far as I know). But I do intend to pull test a few if my boss will let me have a couple. Although I am confident they will meet CE strength requirements and possibly UIAA requirements as well, I strongly doubt they will be as strong as the u-bolts we were using before. The u-bolts we were using were pegging out at 35 - 40 kN in tension!


(This post was edited by USnavy on Jan 29, 2011, 8:43 PM)


mattm


Jan 30, 2011, 5:48 AM
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Re: [USnavy] Why are Ubolts so weak, or aren't they? [In reply to]
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Ahh. Yep, re-read the article and no mention of a U-Bolt. I think remembered seeing custom Ti U-Bolts on here recently and somehow put 2 and 2 together in my head.

Those new ones look good though. The grooves look like they'll provide a better glue interface than the tortugas.

Hopefully they work for you (and the other guys) and it becomes an option that's more accessible to others as well!


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