Cobra Anchors -- Reviews?

Very well stated Francis-

My testing done to date was relegated to essentially pure tensile pull-out with a calibrated hollow core ram/yoke and custom standoff, where other considered combination shear loads may be calculated or derived given the substrate compressive strength, geometry/ stiffness of the attaching fixture, and angle of load application among other factors. The Allowable Strength Design (ASD)  used in engineering assessment of concrete anchors are claimed to be predictive of these complex interactions. Unfortunately, our climbing certification conditions are limited to UIAA 123 20KN minimum axial capacity which seems to be devoid of these additional details and way overly simplistic. Are there other climbing specific standards that give prerequisite loading conditions, importance factors and peak load durations you're referring to?

If so, I'm very interested in setting up comparative test protocols in the same rock compositions to really understand the underlying failure modes and limitations of various anchor types.

Thanks,

M. Hanna

Right, I´ve had a lot to do with concrete screws in the past as I worked with a manufacturer in Germany on the problems with stainless ones in hard concrete and stone. Back then they were only certified (for construction) for soft concrete like C20 and in harder concrete they ripped the threads off, as they make my wedge bolts we had discussed problems with hard stone before so they asked me to do some research.
For those that don´t know the original concrete screws appeared in the 1970´s and were plated HARDENED steel to cut into the concrete, the building codes in Europe changed and for many applications stainless steel was required which gives problems. You can only work-harden austenitic stainless steel and to a limited extent which isn´t enough for hard concrete or rock, the threads which are supposed to cut into the rock collapse and normally the anchor jams half way in as the metal bonds to the rock (and snaps off). The German Alpine Club (DAV) and the Australians did some testing a few years back with limited enthusiasm for the end results.
The two common solutions are either tungsten carbide on the first few threads or making the anchor from two pieces, the initial cutting part from hardened steel and the rest stainless.
The first difficulty is climbers being climbers there are those who will buy the expensive option which will give adequate results, the others who will buy the cheapo versions and work out how to install them by overdrilling with no idea of testing or how reliable they are.
The second problem is they are hard to install without an impact driver so climbers tend to overdrill the holes.
The third problem is the rock is critical, some types of rock seem to take them well and others they are truly appalling. As they are not torque controlled anchors it´s anyones guess how good they are.
Another difficulty reported (apart from them cutting down through soft rock such as sandstone) is keeping hangers tight, the thread pitch on the anchors is such that it is hard to get sufficient clamping force to keep the hangers from moving and the maximum allowed torque for concrete screws is generally far below that allowed for a wedge bolt
As to the standards;- the sizing system for concrete screws is generally the hole you install them into and for Euro´s the only usable size is 8mm, the 6mm ones are physically too weak and the 10mm ones require a larger hanger than is commercially available. (An 8mm screw requires a 12mm hanger hole).
The standard requires all parts of the anchor are from the same material, one could argue the hardened steel tip is a "tool" and not part of the anchor, whether the lab would bother anyway is questionable!
When I tested concrete screws the results were too erratic to meet my internal standards, they would probably pass the test for eN959 since this is performed in the material they were designed for i.e concrete but in the five or so sorts of rock we tried they weren´t good enough for us to put them on the market.
The actual tests for the standard are completely adequate for our purposes so long as they are also used in conjunction with the other requirements of the standard, that is for "normal" rock the anchor will be strong enough for rock climbing purposes BUT the end user is also required to ensure the anchor is suitable for their local conditions, that is if you are bolting in solidified mud/chalk or whatever it´s YOUR responsibility to do the pull tests in your rock. The two test load directions (axial and radial) cover the best and worst cases and one could interpolate intermediate values, the reality is generally if they pass the worst case then they will pass the best case with ease and with wedge bolts the normal failure values are identical no matter the direction of force. The bolt is rarely the failure point anyway, for a 10mm wedge bolt it should be around 38kN so normally the hanger will have failed first. The duration of the maximum load is irrelevant for wedge bolts (and most others), the maximum certification load is usually exceeded by torqueing them tight and the maximum failure load is a transient one, the ram pulls at 100mm/min until the bolt breaks (the manufacturer is free to publish this value if they wish but this is not mandatory).
Personally I would prefer not to see concrete screws come into general use, wedge bolts are far better and idiot proof, an important consideration in modern climbing.

I just wanted to thank Jim Titt and M. Hanna for their posts, both of which were very thorough and helpful. When I first read on the this post that folks were using concrete anchors my reaction was "you've gotta be f*cking kidding me". But after reading Mr. Hanna's post concerning all the testing he has done (which clearly demonstrates his significant knowledge), I am less worried. And given the circumstances that those bolts are being used (GU in the alpine), it seems totally reasonable. That said, I certainly don't trust most installers to be conducting the due diligence that Mr. Hanna has done and thus I completely agree with Jim's sentiment, which I have repasted below because I think anyone reading this should consider it sage advice.

Jim Titt wrote: Personally I would prefer not to see concrete screws come into general use, wedge bolts are far better and idiot proof, an important consideration in modern climbing.

yeah, i didn't agree with that statement (ie wedges being idiot proof) either.  particularly for wedge anchors in soft sandstone.

i think i would be nervous installing the screw anchors ground up.  a part of me would be constantly worried that the screw would bind up, snap off, or the impact driver would run out of juice.
mark, have you noticed if using the impact driver to drive them in drains the battery pretty quick?

definitely pretty interesting though.

Wedge anchors ARE idiot proof (depending on your criteria of idiot of course), if you can tighten them to the correct torque they are good, if not they are bad. It doesn´t get simpler than that.

Jim-
Thanks for taking the time to explain your past research efforts on the topic. I always find it interesting to hear how these products evolve through iteration and research. I also appreciate your knowledge on test protocol and your innate ability to navigate these sometimes-messy waters.

Your thoughts on developers just buying whatever concrete screw and modifying hole sizes or installation procedures is indeed a valid and important caveat. Unfortunately, with the “what should be” idiot proof wedge anchors, there are a huge number of manufacturers and easy cheap availability, and very few are suitable for climbing applications. Also, 316 stainless is not common here at all. In the go-to 3/8” size, many are very weak having cut threads and poorly conceived wedge angles and clip configurations. In recent years and with better education efforts we have hopefully moved away from these in climbing applications in favor of easier to replace options. In trying to mentor to younger developers, I have been discouraging the use of wedge anchors for many years, as have others.
 I’m sure your wedgies are strong (you clearly do a lot of testing and have a vested interest in quality, and possibly Fixes’ as well (unsure, I can never seem to obtain any testing reports….) If I’m to clip wedge anchors, I would hope they would be one of yours or Hilti’s Kwikbolt which have a proven track record. Sadly, I can think of several thousand in western Washington that are of lesser quality that will not see replacement anytime soon. A few areas are within 5-10 miles of saltwater too, which will eventually be a difficult situation not unlike other seaside crags.

Even with the recent good work in spinning and removal techniques, I’m hoping we can move past wedge anchors entirely for climbing.
It seems that all valid anchor types (sleeves, pound ins, wedge, etc) have a full range of the best to really terrible quality, and all readily available for purchase where cash drives the boat. I think the only way out of that one is through education of those coming up.

The use of the impact wrench is not a downside, my Milwaukee weighs in at something like 2 pounds, and in some cases you can ditch the hammer or take a lighter hammer for sounding the stone. Even with the small battery I have installed 20 of these in 2.5” length on a single charge. The shorty Titens (1.75”) can be easily installed with a 6” box wrench, then removed and replaced quickly with the longer SS version on the way down. The 3/8” version does need a 1/2 “ hanger, which is not a problem. In hard rock, the key is the addition of water to act as a cutting fluid, it really changes the force needed to cut the thread. They are not scary at all to lead with, and I have never broken a screw. If you start them right, they go in without thread gall. As far as loosening in soft stone, I'm aware of a crag in Eastern Washington (Mission Creek), where the developer used the Powers version some 20 years ago. Never seen a spinner out there, and the stone is the very softest sandstone I've ever seen. I think he used the 3" model.

So to sum up, this particular product has seen major improvement over the family of concrete screws in predictability, installation ease, clamping force issues, thread cutting tip, and material composition. I share your trepidation with the possible use of other brands that are inferior, but this is a problem with every type of bolt family. I’m not sure if the Titens are available in Europe but you should try a few for fun and see how they work, they really are different than the predecessors. The Hilti's may be suitable as well, though at 9$ US, they are well... Hiltis :) I’ll continue testing in all sorts of rock and report findings as they are generated for those who might be interested.

Thanks,

M. Hanna

Simson aren´t represented over here but getting them would be no problems through my US distributor. However while I´m always good for a project testing concrete screws has a drawback or two. (I rarely use bolt-ins anyway so personally I´ve no real interest in changing.)
There are at least half a dozen manufacturers of "hybrid" screws with either tungsten carbide or hardened tips (Hilti, Heco, Fischer, Würth, and some others I can´t remember at the moment) and to get reasonable results I want to see five pulls in each direction just to start with so that´s going to be 60 per rock type and normally we would use a variety, concrete, soft and hard sandstone, soft limestone, marble, maybe coarse and fine grained granite and slate. So thats 360 to 400 test pulls.
Even at trade price that´s going to be a thousand bucks in bolts, another couple of hundred in hangers and then about a hundred hours labour. Climbers being what they are I would publish the results and they would buy them elsewhere so I´d be out by $5,000 bucks and earn nothing!
Either one of the bolt funds/national organisations is going to have to do the testing or someone will have to organise a GoFundMe  

So true Jim

Keep up the great work. I have a ton of respect for your contributions to this site and to climbing safety. Really appreciate this discussion-

Mh 

You would earn eternal gratefulness, Jim. That must be worth $5000.

The GoFundMe actually sounds like a good idea. I bet it would work.

M Sprague wrote: You would earn eternal gratefulness, Jim. That must be worth $5000.

The GoFundMe actually sounds like a good idea. I bet it would work.

Buying the wife some roses would probably be cheaper for better returns  

First, I think that natural stones, at the opposite of concrete, is a whole world where you can find a large number of different behaviors and it's hard to speak about one solution that would be fit for all.  

About the installation issue of the concrete screw, we are using all the time Milwaukee impact M18 and it makes the concrete screw a much easy and faster solution than any other mechanical anchors, especially since it requires 2 actions (drill + screw) while the wedge anchor requires 3 (drill hammer screw).

I haven't made a complete study for all the existing natural stones but in about 10 various local sites (in Israel), we have never faced troubles with concrete screws.  Following your remarks, it looks like some stones won't be fit for it...  That's why I think it's wise to make a pull-out test before approving an anchor in natural stones site.

Here is by exemple a pull-out test made on natural stone with a THDEX 16x160 (Spanish concrete screw) for 80.5kN

For the sensibility to the diameter of the drill, for me this is valid for all mechanical anchor without any exception.  The extra sensibility of the concrete screw anchor are, at my best knowledge never been studied or prouved.For wedge anchors, I got personally 2 other problems : 1st when the stone is weak, crumbled or cracked inside, I have absolutely no way to know what is the capacity of the anchor.  With concrete screw, it's usually a black/white result : or it works, or it doesn't, 0% or close to 100%.  So, I found it safer to use concrete screw.2nd : since with wedge anchors most of the stress is concentrated in a specific areas and doesn't spread along the thread like in the concrete screw, when you are unlucky with a crack at the wrong area, or with a vacuum, the wedge anchor is behaviing badly following my own experience.Last, with wedge anchors, sometimes climbers close them to much (much higher torque than required) and are starting the pull out failure of the anchors without even knowing it, which also weaken the wedge anchor that will be installed at a smaller depth than required.  Let's say that you need to check closer the knowledge of the one installing the anchor with wedge anchor...
Another option, much more complicate and uncumfortable to install but extremely efficient it's obviously chemical anchors...  But that's already another subject...

Jim Titt wrote:

Buying the wife some roses would probably be cheaper for better returns  

And remember Jim already has 1 Ferrari     

Jim Titt · Mar 21, 2018 · Germany · Joined Nov 2009 · Points: 490
Anyway I think wearing out top anchors is great, how else can I afford my second Ferrari?

I also recently had the opportunity to install several of the Hilti HUS style in 3500 Psi concrete and they worked really well. Thanks for the info Yves. The Milwaukee M18 is what I use too, drives them right in even in hard stone.

What stones are you testing in Israel?

MH

Yves De Lathouwer wrote: 

I haven't made a complete study for all the existing natural stones but in about 10 various local sites (in Israel), we have never faced troubles with concrete screws.

Is that in the Israeli limestone or the southern desert sandstone? How do you think it would perform in softer rock like, for example, the soft sandstone Wadi Rum?

Thought I would add this link to a paper to the discussion.  I haven't seen it pop up on any of the other bolting threads around (here, UKC etc).  It's relevant as it tests both studs and threaded screws (confusingly named "WedgeBolts by powers) in stone substrates (limestone and sandstone).  Good info I thought.
​MECHANICAL ANCHOR STRENGTH IN STONE MASONRY​​​

Thanks Matt!

mattm wrote: Thought I would add this link to a paper to the discussion.  I haven't seen it pop up on any of the other bolting threads around (here, UKC etc).  It's relevant as it tests both studs and threaded screws (confusingly named "WedgeBolts by powers) in stone substrates (limestone and sandstone).  Good info I thought.
MECHANICAL ANCHOR STRENGTH IN STONE MASONRY

Interesting read. If you look at Appendix B, shear strengths, you can see that the maximum yields at which the bolts sheared were all over the place. They experienced bolt failure down to 3000 lbf, and as high as 8000 lbf. That seems wildly inconsistent to me. Im also not sure what the authors mean by "Approximate Yield" and "Maximum".