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.

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

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.

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.

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!