Slip Critical Joint
Slip Critical Joint
(OP)
Hi, I'm working on some joints, an example of which is attached. I am looking for some calculation or formula for calculating the resistance or ability of these bolts to resist slip from a moment and shear load.
The plain shear load is fairly easy to figure out, but throw in the moment and I cannot for the life of me figure out how to relate the developed friction to the moment.
In the example the moment is centred within the tube welded to the flange. The flange is bolted to another flange which is firmly secured. Bolts are all the same size. Holes are laser cut with generous (.030 clearance). Bolts are pretensioned using a torque wrench.
The plain shear load is fairly easy to figure out, but throw in the moment and I cannot for the life of me figure out how to relate the developed friction to the moment.
In the example the moment is centred within the tube welded to the flange. The flange is bolted to another flange which is firmly secured. Bolts are all the same size. Holes are laser cut with generous (.030 clearance). Bolts are pretensioned using a torque wrench.






RE: Slip Critical Joint
A3.4
J1.10, J1.11
J3.2, J3.4, J3.6
Table J3.2
J6
Commentary J1.10
Commentary J3.4
RE: Slip Critical Joint
I also haven't any easy access to the AISC manual.
I do not believe prying or tension to be significant as the flange is quite thick. Also the plate to which it is bolted too is even thicker. Shear is also very small relative to the moment.
RE: Slip Critical Joint
The elastic method assumes that all bolts carry an equal shear, i.e. V/n where V is the applied shear and n is the number of bolts. In addition, each bolt carries a shear normal to the radius between the bolt centroid which is proportional to its distance from the centroid.
The resultant shear on any bolt is the vector sum of the two shears calculated as above.
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The other method is the method of Instantaneous Centers which is difficult to explain easily. The elastic method is easily understood and is conservative.
BA
RE: Slip Critical Joint
I am of the mind at this point that if the bolts were able to generate enough friction to keep the flanges from rotating(slipping) relative to each other, then the plate would not break.
Basically i believe as this was originally designed it is a slip critical joint. Problem being that this is a mobile peice of equipment that has serious dynamic loading present. This makes stress calculation a guess at best.
RE: Slip Critical Joint
You might be asking the wrong question. You shouldn't be missing any info. in the drawing, you posted it, it's your drawing isn't it? But, there certainly seems to be plenty of pertinent info. which has not been brought forth yet. How does the moment load the plate, does it twist the plate loading it torsionally? How long is the hidden tube and does its far end move up or down, thus twisting the plate out of plane? Is the moment induced by a torsional loading from the hidden tube, thus causing the top edge of the plate to be in tension or compression and imparting an up or down shear force in the 3-bolt group to the right? Add the various loads to the sketch, what's the plate thickness and material spec? What size and grade of bolts are you using? Show the failure line on the sketch and give some description of how and when it happens. How big is the other flange pl. and what does it look like in comparison to the one you show? On heavy mechanical equip. it might not be a good idea to count only on the bolts to take this constantly varying loading, since movement will inevitably start to occur. Why not add some strategically placed shear lugs around the flange you show, and welded to the other flange pl. to lock your detail in place rotationally. Maybe a thicker flange pl. is in order. I wonder if you have analyzed the loads which are causing the problem correctly.
RE: Slip Critical Joint
AISC and RCSC tell you how much slip capacity a bolted connection has, based on the bolt pretension and faying surface conditions. You use these values in place of the bolt bearing values. If you can solve the problem for bearing bolts, it's the exact same process with slip critical bolts, just with a different (lower) bolt value.
RE: Slip Critical Joint
BA
RE: Slip Critical Joint
The hidden tube is 85" long,
on the opposite end is an identical plate to what you see.
The moment is 27400 in-lbs generated by the force of a slurry on a large panel which is attached with structural uprights to the bottom of the cross-tube (uprights not shown).
I am considering that the moment is acting dead centre on the tube (not entirely true, but a good guess no?)
Other flange plate is a piece of 3/4 x 3 44w with 2 drilled holes and 3 tapped. bolts are grade 5 with hardened washers. Torqued to around 70% proof load.
I guarantee the loading analysis is little more than a good guess. What i'm dealing with basically defies calculation, what i'm trying to do more than anything is verify mathematically the source of failure so we can best learn going forward.
Nutte,
That is exactly what i'm looking for. Either an accurate estimation of slip capacity of this bolted connection, or a way to relate friction to the moment. Where would someone who doesn't regularly use AISC information find something like that?
BA, I guess that's a reference to the above, i will look for it. We don't have anything like that where I'm at.
RE: Slip Critical Joint
BA
RE: Slip Critical Joint
RE: Slip Critical Joint
RE: Slip Critical Joint
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Slip Critical Joint
The 27.4"-k is the torsion imparted to the flange plate from the tube? What are the fixed end moments and shears about the x & y axes at the joint btwn. the tube and the flange plate? How much does the tube deflect in the x & y directions under load? I suspect it's a fatigue problem too, and I'm betting on a line through the middle bolt and the change in direction on the bottom edge of the flange. Show us a jagged line on the sketch representing the crack line. Are the holes beat up from bolt bearing when you take things apart? I think slip-critical bolted connections on a piece of mechanical equipment which sees hundreds of dynamic loadings per day or hour may be different, and act differently, than slip-critical joints on less dynamically loaded joints such as those on bridges and buildings. I don't think I would depend only on 3 bolts in tapped holes the way these are being loaded in that joint.
RE: Slip Critical Joint
My understanding of slip critical connections is that when properly designed and assembled act to limit fatigue stresses by spreading the loading across a large clamped surface. This may be a misplaced understanding but hey, i am new.
Shear is simply the product of 3500lbs broken down into vector components. Also this weight is carried by 2 equal plates.
Remember that if the bolts are tightened and strong enough to resist slip between this connection then the strength of the plate is equal to the sum of both plates together as they are firmly fixed to each other. In order for one plate to break and not the other, slip must occur.
RE: Slip Critical Joint
To what tension? Maybe the bolts were not tightened enough to get a slip-critical connection.
BA
RE: Slip Critical Joint
The torque i believe relates to around 70% proof load.
It is in fact 200 ft-lbs. Its been a long time since i did those calculations, and i know i relaxed the torque just abit to prevent our guys from stripping the threaded holes.
We have tried to get them to lube the threads everytime with wd-40. Where i am ultimately going with this is that the 2 cases of plate failure i am aware of, both had loose bolts. I want some evidence that suggests if we maintain proper torque and faying surface prep that we can achieve satisfactory results. Also i believe there were assembly errors on these failed parts. It is odd that during both failures, the bolts are loose, and not the same bolts either, but not all bolts. On one the 3 bolts to the right in drawing were lose, and the other the two to the left were lose. Both failed in the same manner. At the same time I am redesigning part to accomodate 6 bolts and adding 1/4" to the thickness of the thinner plate. This all adds cost, but frankly the old addage is correct, an ounce of prevention is worth a pound of cure, every time. The added thickness also helps resist tearing the threads out of the plate.
RE: Slip Critical Joint
With hardened plate washers. The washers are stamped F436, and are assembled with chamfer facing bolt head.
Bolts are torqued using a 4' long torque wrench, so there is not jerking going on, its very smooth.
RE: Slip Critical Joint
With the (your product) history, it does seem that very tight control during assembly is real important. Torque is not always a good indicator of bolt tension without this absolute consistent control of all the important factors associated with the bolting process. This can vary from batch to batch of bolts, finish on threads, thread tolerances, etc. Clean the treads with a wire brush first, and then lite lub. Maybe you should be using something like locktight. The exact torque is your best guess, then keep records of results for long term success, since most installations do seem to be working. These kinds of problems are usually one big, long term, experiment; lots of good educated judgement finally solving the problem. And, it seems you're thinking and heading in the right direction.
But, you still aren't admitting to, at least, the fixed end moment at the tube/flange pl. joint, due to tube bending and deflection in the horiz. plane, which causes prying in the flange pl. which shows up at the middle bolt and crack. These stresses are added to the torsionally induced stresses and tensile stress from the 3.5/2k shear. The cost of the .25" thicker flange plate is a pittance in comparison to one failure, but do a couple other things too. Make the change in direction (transition) at the bottom edge of the flange pl. a generous arc with tangent points an inch or two away from the current sharp change. Clean this edge up and even grind a small radius on the pl. corners in this area. Ream or grind the burrs (sharp corners) off that middle bolt hole too. If I could, I would make the flange pl. wider in this region, by making the bottom edge a straight line from the left corner to the lower right corner. Maybe move the top edge transition point further right also to improve plate strength (stiffness) in this region. Sounds like the two failures looked about the same, where did the crack start? I don't know if an extra bolt will solve the problem, but I don't know where you're going to put the bolt either. I think you thinking about slip critical joints is about right, re: clamping force, faying surfaces, and bearing area distribution. The big problem with our type of dynamically loaded equip. is keeping the bolts tight under these kinds of loading, and that's why I don't like to count on them alone. You can't pry on this type bolted joint hundreds of times a day without a few bolts eventually loosening.
RE: Slip Critical Joint
That is exactly my plan of attack for this going forward. I will take some time in the future to try and compare this to the AISC specified abilities for the connection. We do like many things about building this unit in this manner so I want to ensure that going forward with new designs I can make some more "scientific" evaluation of how the joint will perform before we build it.
Obviously the most efficient method is to ensure that the bolts do not enter bearing and remain non-slip, as that is the only way to ensure fully spreading the load across all bolts.
RE: Slip Critical Joint
Having looked at your sketches I would ask if your twisting the square 85" tube then isn't that the twisting centre for the screws?
I have calculated the maximum load on the bolt assuming the above centre of rotation and I get the resultant load on the screw at 6.328" to be 980lbf this includes the 700lbf shear load.
Basically the two extreme righthand screws are doing all the work, the two inside the square tube do very little in terms of resisting the moment and the screw in the middle of the plate falls somewhere in between.
I think you need to get as many screws as possible, as far as possible away from the centre of the tube to try and even out the screw loads.
I also noticed that your torquing them to 200lbs-ft but that is usually a figure without screw lubrication and you mentioned wd-40, if your torquing to the above figure with wd-40 I would guess your over stressing the screw and tightening it to above yield, if that is the case you will lose preload on the joint and might also be the cause of the screw loosening.
Any chance of a picture of a broken plate?
desertfox
RE: Slip Critical Joint
I get a value for an 3/4 UNC SAE grade 5 bolt, assuming a lubricated coefficient of friction at 0.18 of 223.5 ft-lbs, and using a preload of 0.7 proof(source Bowman Distribution, from Mechanical Engineering Design, 7th ed, by Joseph Shigley). From this i expect it is unlikely the bolts are being over-stressed.
I was a little perplexed at this exact situation, the moment is being applied to the plate at the centre of the tube, however the moment resisting that motion is being generated at the centroid of the bolt pattern. So long as the bolts dont slip. If they slip it becomes a bearing load on (murphy's law) the two farthest bolts, about the centre between them. This starts bending the plate, back and forth, and we have a fatigue failure at that nice sharp radius at the bottom.
At least that is what my best guess is :)
RE: Slip Critical Joint
RE: Slip Critical Joint
I would normally agree about the two plates one with an offset load and connected with bolts resisting movement about the bolt pattern centroid but I think in your case it is different, imagine gripping the square box with your hands and twisting it, it would want to rotate about the tubes centre line, however the bolts would prevent that.
I can't see if you twist the tube about its centre, why the plates would want to rotate about a theorectical bolt centre
which is a distance away from the tube centre.
desertfox
RE: Slip Critical Joint
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Slip Critical Joint
its the same as a bolt group on the end of a cantelever, why would they "rotate" about the bolt centroid, you would think they would pivot about the bolt closest the load, but that is not the case. Its an egg before the chicken scenario i think. Which is applying the load to which? It may seem that the load is obviously being applied by the tube, but an equal reaction is generated by an arm and a bolted plate.
paddingtongreen,
so long as the bolts dont slip, resultant forces should be about the bolt pattern centroid.
This is my understanding of how this system works, i'm definately open to references suggesting otherwise. In the meantime i'll wait patiently for my Steel Construction Manual. :)
RE: Slip Critical Joint
@Travis, My worry is that the plate does not have sufficient stiffness. The torque is applied at the two bolt group and for them to work together, the plate must be stiff enough to carry the load to the three bolt group without bending. In structural work, we usually have the connection more central and with a much meatier plate.
Just check the deflection of the plate under the load from the three bolt group. I could be wrong in practice, it might be negligible, but it exists.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Slip Critical Joint
We have two plates bolted together,
one is 1/2" 50w and the other is 3/4" 44w. My thoughts being that if the friction generated is sufficient to prevent slip, that both plates can work together as one. When that doesnt happen, and slip occurs, obviously the 1/2" plate fails.
I expect the deflection is negligable, especially if the 2 plates are considered together. The geometry here lends itself to high bending stresses and very little deflection. I havnt the time today to work through that but it may be worth it when time permits.