washer between bolt hanger and rock?

I saw this picture and it struck me as strange that someone put a washer between the hanger and the wall, I'm definitely not the most knowledgeable about this so I thought I'd see what other people think about it. Good idea? bad idea? not the best idea but not a big deal? Am I gunna die?

Seems like the reduced friction between the hanger and washer may make the hanger want to turn and loosen the nut over time.

IMHO

true, I hadn't thought about that. I think that it's going to apply more leverage on the bolt, probably not a big deal, but who knows?

Reducing the friction coefficient between the hanger and the rock will make it more likely to slip downward during a fall. This will increase the shear loading on the bolt. Bolts are made to function best in tension.
and yes yer gonna die aha h ah a ah

The washer is suppose to go between the nut and the hanger like this...

Not right but prolly nbd also. Sometimes you have to do that with a miffed cap-bolt install (Powers 5 pc.), but not with a stud bolt. Can't see any reason why it was done that way, but still not a impending disaster.

I've seen hangars with 5 washers behind them. Probably a shotty job in that case. I always see this if the hangar can't lay flat against the rock do to uneven surfaces.

John D wrote:I saw this picture and it struck me as strange that someone put a washer between the hanger and the wall, I'm definitely not the most knowledgeable about this so I thought I'd see what other people think about it. Good idea? bad idea? not the best idea but not a big deal? Am I gunna die?

Not a huge deal, but yes, it will put slightly more leverage on the bolt (not enough to cause any kind of concern), it will increase the shear loading of the bolt, it will make it so that the hanger is more likely to loosen the nut, so no it is not a good idea. That said, I wouldn't worry about falling on this bolt at all.

Tough to tell in the picture but that washer looks HUGE. Look at it's outer radius near the top of the hanger. That wouldn't fit on top of the hanger and behind the nut anyway.

Eric Krantz wrote:Reducing the friction coefficient between the hanger and the rock will make it more likely to slip downward during a fall. This will increase the shear loading on the bolt. Bolts are made to function best in tension. and yes yer gonna die aha h ah a ah

Wedge-type concrete anchors such as this one are actually stronger in shear than in tension in perfect loading conditions. Pulling in tension puts the load directly on the expansion collar, which is the point of failure in tension. Pulling along the cross section of the bolt is the safest loading condition. The hanger is shaped in a way to MAXIMIZE shear load and minimize tension and moments at the interface.

If embedded in a porous material (such as some types of rock), the tension load required to cause bolt failure drastically drops. This is primarily due to the small surface area of contact between the collar and rock. The rock can crumble, quickly leading to failure. Shear loading is not as sensitive to porous materials because of the cantilever effect.

I agree that the washer would probably contribute loosening the bolt over time. Small diameter rubber washers can be nice to reduce the chance of moisture getting inside the bolt hole.

MattL wrote: Wedge-type concrete anchors such as this one are actually stronger in shear than in tension in perfect loading conditions. Pulling in tension puts the load directly on the expansion collar, which is the point of failure in tension. Pulling along the cross section of the bolt is the safest loading condition. The hanger is shaped in a way to MAXIMIZE shear load and minimize tension and moments at the interface. If embedded in a porous material (such as some types of rock), the tension load required to cause bolt failure drastically drops. This is primarily due to the small surface area of contact between the collar and rock. The rock can crumble, quickly leading to failure. Shear loading is not as sensitive to porous materials because of the cantilever effect. I agree that the washer would probably contribute loosening the bolt over time. Small diameter rubber washers can be nice to reduce the chance of moisture getting inside the bolt hole.

The issue is actually more complicated than this. Yes a bolt is stronger in shear than in tension, but the strongest configuration is to have the shear load carried by the friction between the rock and the hanger, and that friction is provided by the tensile force in the bolt, so yes, having that washer between the rock and the bolt will decrease the friction and therefor decrease the shear strength of the bolt.

kennoyce wrote: The issue is actually more complicated than this. Yes a bolt is stronger in shear than in tension, but the strongest configuration is to have the shear load carried by the friction between the rock and the hanger, and that friction is provided by the tensile force in the bolt, so yes, having that washer between the rock and the bolt will decrease the friction and therefor decrease the shear strength of the bolt.

Ah, gotcha! That makes a lot of sense, thanks for the clarification!

kennoyce wrote: The issue is actually more complicated than this. Yes a bolt is stronger in shear than in tension, but the strongest configuration is to have the shear load carried by the friction between the rock and the hanger, and that friction is provided by the tensile force in the bolt, so yes, having that washer between the rock and the bolt will decrease the friction and therefor decrease the shear strength of the bolt.

With wedge bolts it makes no difference at all to the strength whether the hanger is pressed against the rock or left slack. Correctly torqued, over-torqued or not tightened at all the hanger moves away from the rock long before bolt failure. In both tension and shear with a normal hanger the failure is the same;- bending shear (or more normally the hanger itself fails).
But as shown the hanger will invitably become loose and a pain in the ass to clip.

kennoyce wrote: The issue is actually more complicated than this. Yes a bolt is stronger in shear than in tension, but the strongest configuration is to have the shear load carried by the friction between the rock and the hanger, and that friction is provided by the tensile force in the bolt, so yes, having that washer between the rock and the bolt will decrease the friction and therefor decrease the shear strength of the bolt.

Source? Friction between the rock and the hanger seems insignificant at best.

Its likely that the placer of the bolt in question just didn't like how the hanger sat on the rock without the washer.

barnaclebob wrote: Source? Friction between the rock and the hanger seems insignificant at best. Its likely that the placer of the bolt in question just didn't like how the hanger sat on the rock without the washer.

I don't have the book with me right now to give you a page or anything, but the source is Shigley's Mechanical Engineering Design 9th edition.

20 kN wrote: Maybe in an improperly installed bolt scenario. If you install the bolt in material you should not install it in, such as soft rock or pocked rock, then your statement would hold true in with some placements. But, if the bolt is correctly installed in good rock, the strongest configuration is tension. I have pulled a number of these bolts in a few different types of material and I don't think I have ever been able to get one of these bolts to support a greater load in tension than in shear in the same material. Once you step up to the 1/2" size the strength differences level out and the bolt is pretty much equally strong in shear as tension. On sizes 5/8" and above, the bolt is pretty much always strongest in shear. The downside to installing the bolt in tension is that you are possibly more likely to get a spinner which is more of a critical issue in tension.

wow, you might want to read my post again, and then read your post again, because I think that we're in agreement but can't quite tell since you seem to be contradicting yourself a couple of times.

barnaclebob wrote: Source? Friction between the rock and the hanger seems insignificant at best. Its likely that the placer of the bolt in question just didn't like how the hanger sat on the rock without the washer.

You may be right about the driller, maybe they didn't like how the hanger set crooked?

However, Kennoyce is correct about shear being carried by the friction between the rock and the hanger, which is provided by the tension in the bolt.

It's not intuitive. Engineered bolted joints are always preloaded for several reasons, one being that the bolt itself won't carry the shear, it's carried by the joint through friction.

Or google "bolt preload": bit.ly/RyJbJs

People read Jim's post right?

kennoyce wrote: wow, you might want to read my post again, and then read your post again, because I think that we're in agreement but can't quite tell since you seem to be contradicting yourself a couple of times.

Well, it was late so that might explain the poor writing. But, you and MattL said wedge bolts are stronger in shear than tension. I am saying they are not, at least not when used in hard rock like they should be. For the most part, these bolts are really only stronger in shear in softer rock. But, you can look at the charts below and create your own examples. What I was trying to say before was that some wedge bolts are stronger in shear than tension once you reach the diameter of 1/2", but normally not in smaller diameters. Lastly, no blanket statements can be made about wedge bolts. Although the bolts are normally stronger in shear than tension, there are times when they are not. In the end if you want to know the specifications you need to look at the manufacturer's technical data.

MattL wrote: Wedge-type concrete anchors such as this one are actually stronger in shear than in tension in perfect loading conditions.

Actually, the exact opposite is true, at least in the case of perfect loading conditions. But, you can look at the charts below to see all of the examples.

However this is only true if you stay in the working limits for the bolted joint which we don´t.
A typical (say Hilti) 10mm wedge bolt has a recommended tightening torque of 25Nm which gives a clamping force of near enough 20kN. The coefficient of friction of the hanger to the rock is around 0.5 so the hanger starts slipping at 10kN. Once you get over this point the load comes on the bolt itself which then sets further into the cone clip (and the clip into the rock).
This allows the bolt to move outwards and the hanger has no clamping force on the rock, Hilti give the figures for this for various scenarios and with a 20kN load this is in the order of 2 to 3mm and testing shows that in granite at failure load this is around6 to 10mm.
The additional factor that no engineering book can help you with is that the clamped object is considered to be rigid and bolt hangers are far from that, they distort considerably around the nut and bend the end of the bolt where the hanger contacts to about 45° causing the threaded part to start tearing at this point.

I´ve tested hundreds of wedge bolts as part of QC and this is what an M10 stainless wedge bolt in coarse granite looks like just before radial test failure and clearly friction between the hanger and the rock isn´t a major interest! (The hanger is 8mm stainless rod as no commercial hangers are strong enough to break the bolt anyway).

I think we are all going to die from washers put on the wrong side of the hanger.

BTW, I think this thread has been hijacked^^^^

I think someone needs to test a washer behind a hanger a few hundred times just to be sure.