Finally got a chance this week to do some preliminary testing of bolted hanger brackets. The primary goal was to check out the testing equipment and make sure it worked properly. It did.
Secondly, we wanted to test a few bolts to see how violently the apparatus would react when a bolt let go. (I don't relish dodging sandstone shrapnel.) Fortunately, all the failures were gentle -- the anchors died with a whimper, not a bang.
Although very preliminary, some of the results were interesting, and in some cases surprising. If you are a climber who doesn’t really care about that hardware from which you are hanging your precious butt, then this report is not for you. You can stop here and move on to other threads.
But, if you are somewhat curious about the integrity of your lifeline to the rock, this may be of some interest.
In the future, we plan a lot of testing with many variables and types of gear. The results will be posted on the muirvalley.com website as we collect, analyze and sort it all out.
Okay, on to the tests. A bolted hanger bracket, installed with 35 foot-pounds of torque in a test piece of Corbin sandstone, was attached to a hydraulic test apparatus that applied gradually increasing force to the anchor (via a hand-operated pump) until something started to fail. The hydraulic pressure gauge was closely observed, and at the first sign of failure, the pressure was recorded. This pressure was then converted into pound-force values. This type of testing has been done at other climbing venues worldwide with a variety of anchors. We believe this is the first known failure testing of anchors in Corbin sandstone done in the Red River Gorge region.
For now, here are some interesting -- and in some cases, surprising -- observations. Please remember that this is all very preliminary and may not be generally characteristic of typical failures.
Hanger brackets were bolted to test rocks in generally the same strata orientation as would be found on actual climbs. Later, bolts will be pulled out of vertical walls, rather than boulders broken away from walls.
The test rocks were chosen by test drilling to find harder samples. This is very subjective, of course, and we will try to find a better way to measure the compressive and tensile strengths of the test samples. Anyone who has climbed in the Red knows how the rock quality can be significantly different from place to place. It would be virtually impossible to generalize bolting solutions. In one location a hanger could hold up a truck. Sixteen inches away, another anchor could be installed worthlessly in powdery choss.
The test apparatus was attached via a 1/2-inch quick link to a Metolius hanger bracket that was bolted to the rock with a 3.75-inch long, 1/2-inch dia. Dynabolt Gold (plated steel) sleeve bolt. The bolt was oriented perpendicular to the rock face.
Gradually, pressure was applied to a 3.5-inch hydraulic cylinder, which pulled on the hanger bracket. The pressure gauge registered rising pressure as the hand pump was operated.
In the tests conducted, failure occurred when the bolt started to be withdrawn from its hole. For the first half-inch or so, the force necessary to extract the bolt was roughly the same. At a point approximately half way out, the rock usually broke away in a non-explosive manner.
On the final test rock, the extraction force reached 3570 pounds before the bolt started to move. At this point the Metolius hanger bracket was deformed but completely intact. It is UIAA rated at about 5800 pounds. The bolt was also bent about 20 degrees near the head.
As pumping continued, the bolt continued to be extracted with about the same amount of force applied until the bolt head was about 1.3 inches from the rock face, at which point the rock fractured and the bolt was free from the rock.
It is important to note that the sleeve sections of the anchor bolt remained in their installed position in the rock. As the bolt pulled free, it dragged the cone-shaped nut into the lower sleeve, further expanding it and creating an ever-tightening bond with the rock. After extraction, the threads showed no sign of failure, and in fact, were easily threaded in and out of the cone.
Later, when we test adhesive anchors, it is anticipated that, upon failure of the adhesive bond with the rock, the bolt will instantly pop free. The sleeve bolt, on the other hand, fails gradually, possibly providing more warning.
It is also important to note that this bolted hanger bracket was tested in its “worst case” scenario – a straight out (perpendicular to the rock) pull. Bolts installed in roofs would experience this type of pull.
Later, when we test bolted hanger brackets with a force applied substantially at a right angle to the longitudinal axis of the bolt, we anticipate failure of the metal hardware will occur before the bolt will be extracted from its hole. This was the case in Australia and S. Africa tests of anchors in sandstone.
Now for the most important observation – one of which many route developers in the Red are well aware. Very few bolted hanger brackets installed down here in the Red River Gorge region of Kentucky could pass the UIAA standard test for toprope anchors, at least in a direct pull-out situation. To pass, the anchor must withstand a direct pull of approximately 4500 pounds (UIAA 123.2.2.1.3). This is based on the assumption that most hanger brackets here are installed with 1/2-inch sleeve bolts, such as Dynabolts and Rawl 5-piece bolts.
The final test I conducted would have FAILED the UIAA test for toprope anchors. 3570 pounds falls significantly short of 4500 pounds.
Would you be comfortable with taking a leader fall on a roof anchor that could withstand a 4500 pound pull? How about a 3500 pound pull?
So, how could one meet the UIAA 123 standard here in the Red, if one wished to do so?
In our tests with 1/2-inch Dynabolt Golds, it was pretty clear that additional length (greater than 3.75 inches) would not increase the pull-out resistance of sleeve bolts. Corbin sandstone is soft enough that the bolt will merely drag the cone out of the hole, extruding rock as it goes.
Of course we don’t yet know until we test, but my guess is that the only way to install a hanger bracket to a Corbin sandstone wall so that it would meet UIAA 123 is to install a threaded or notched metal rod with an adhesive deeply enough to create a bond with the rock sufficient to withstand a pull-out force of 4500 pounds. Again, I’m guessing, but I think a 3/8-16 threaded rod and certified hanger bracket installed with epoxy in a 6-inch deep hole would pass UIAA 123. Please understand that this is provided only as information. We are not advocating this or any other kind of anchor installation at this time. There is much more testing ahead, before we start drawing conclusions.
That’s more than enough for now. We’ll let you know on this forum when more test info is available.
I suppose if there is a bottom line, it is this old reminder you’ve heard a thousand times. Potentially, climbing is damned dangerous, and maybe even more so here in the Red. Most of us are more than willing to accept the risk as small payment for the rewards. But, if you decide to take a 30-foot whipper on a roof anchor, and it blows, that was YOUR choice and YOUR responsibility.
Rick
