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Highly-loaded anchor bolts in shear

Highly-loaded anchor bolts in shear

Highly-loaded anchor bolts in shear

(OP)
I have some very highly loaded anchor bolts ("through bolts") in shear.  The service load shear force is 32kips to the worst case bolt of an 8-bolt group.  The bolts are "through bolts" through an 8" slab with a thick steel plate at both the top and bottom of the slab.  The concrete crushing of the bolts in shear will be restrained by the top plate.  (The bolts are shear-loaded from the top of the slab.)

I imagine that ACI 318 Appendix D will be too restrictive and will not provide guidance on this condition.  Does anyone know of any resources which would provide contemporary limit states for highly-loaded anchor bolts in shear when the concrete crushing limit state is restrained by the top steel plate?

RE: Highly-loaded anchor bolts in shear

Are you trying to develop the shear load into the slab or through the slab?

RE: Highly-loaded anchor bolts in shear

It would help to provide a sketch and show the loading and edge distances at a minimum. Is the structure already built, and you are trying to add this? Or is this for something new?

RE: Highly-loaded anchor bolts in shear

See an approximate FEM model attachment for a case close to yours and included considerations.

If in the end you would be dimensioning your device by shear friction, just make the capacity of the set of bolts be higher than the factored shear, and the thickness of the plates enough to sustain quite rigidly at factored level the magnitude of shear force uniformly distributed as compression to be hold by the bolt hardpoints (implied friction factor = 1, or modify for yours).

RE: Highly-loaded anchor bolts in shear

(OP)
Thank you all for your contributions and attempts to help with this.  

My condition, as is shown in the attached pdf, is a plate with large shear loads.  Because of the confining effect of the plate below the slab, the limit state for shear in the bolts is unclear to me.  

I met with the local Hilti rep this afternoon, who believes that South Korean testing has shown that in this condition, where the top plate "confines" the concrete shear failure at the bolts, the bolts themselves will fail in shear before there will be a concrete consequence, at least for bolts up to 2" in diameter.  This includes high strength bolts.

I am now looking for support for these statements.  I thought I was looking for some kind of limit state assessment of the concrete below the confining top plate, but now I'm thinking that only testing can show this and I am now looking for this testing.

Interestingly, running these numbers through the Hilti Profis program seemed to indicate that 1 3/4" diameter 105 ksi yield bolts will work for these loads, assuming only failure of the steel.

Now my question is, does anyone know where we could find this reputed South Korean testing?

RE: Highly-loaded anchor bolts in shear

The AISC specifically recommends against doing this... should consider shear lugs...

Dik

RE: Highly-loaded anchor bolts in shear

"and edge distances at a minimum. Is the structure already built, and you are trying to add this? Or is this for something new?"

We still don't have all of the basic parameters of your problem..  

RE: Highly-loaded anchor bolts in shear

(OP)
dik, I will check AISC to see what they say; however, it's seems strange to me that a steel organization would opine on what seems to be a concrete performance issue under the steel plate.  Do you have the doc and section of the AISC reference?

a2mfk, one plate is far away from edges.  The other is say 18" plate edge to concrete edge.  The highest direction shear load is toward the free edge, of course:)  The 8" slab is not yet placed.  I intend the bolts to be threaded rod cast into to the concrete.


 

RE: Highly-loaded anchor bolts in shear

I will try to dig it up tonight...

Dik

RE: Highly-loaded anchor bolts in shear

With that info (new slab) I am with DIK, go to a shear lug and reinforce the slab as necessary. This gets you out of the dreaded App D...

RE: Highly-loaded anchor bolts in shear

(OP)
a2mfk,

In my case, I am a peer reviewer for a design that is already done.  One of my tasks as such is to determine a bolt size and capacity of this connection, as designed.  If I were convinced that the connection is inadequate, then of course I would have to recommend revision.  At this point, I do not yet believe the connection is invalid.  I do believe there is testing out there which addresses this.  

You know, theoretically-speaking, if the concrete were to crush under shear, you would have a translation of the top of the bolt, followed by a shear friction mechanism developing between the plate and concrete, assuming adequate stiffness of the plate.  (This may be why AISC does not recommend this??)  I expect that this development of the shear friction would serve to arrest the increase of shear in the bolt and concurrently increase the tension until you eventually get a shear/tension failure in the bolt, given that the bottom plate will preclude a bolt pullout failure.  

Thank you for  your contributions to this thread.  I will post what I learn and my conclusions.  

RE: Highly-loaded anchor bolts in shear

I have been trying the case in the sketch in FEM (I still am) and in the case that surfaces contact, no bond and no friction a solution is found but I think not surprisingly the crushing stresses in concrete are too much; then I add friction and I don't manage to get a solution without warning error (yet) (1.25 inch diameter bolts) what makes me think that the stresses imparted by the solicitations to just 8 inserts may be found too much. Whilst the computer crunches numbers I am thinking that a simple evaluation of the force at each bolt can show this, on a linear rotation of the plate, and doing it such way I get that the worst inset rod would have to pass 13.18 metric tonnes, what would make 40 MPa crushing stress on an optimistic half depth of the concrete inset (projection) (actual bearing should be less). The model that closed did at around 70 MPa. The other is still crunching.

So I start to think that 8 1 1/4" bolts are not going to work for the forces in the sketch.

RE: Highly-loaded anchor bolts in shear

If this is the case after so becoming convinced, I would introduce a modification that would ensure positive introduction of the forces. The bolts inserts would disappear and something far more substantial would take their function within the concrete.

RE: Highly-loaded anchor bolts in shear

(OP)
Ishvaaag,

I ran the problem through Hilti's Profis program.  Based on steel strength alone, it indicated that 1 3/4" bolts at 105 ksi yield would work.  We ignored concrete effects, just to see how strong the bolts would have to be.

Thanks for presenting these numbers.  What program are you using to do this analysis?

 

RE: Highly-loaded anchor bolts in shear

I discover than in my worksheet estimate I have made an error in a final vectorial combination, that I made as if the two vectors were at 90 deg when are not; angle yet to be determined; anyway the forces to be passed stand above 12.84 tonnes (and might near 16 tonnes) and so the comments stand.

RE: Highly-loaded anchor bolts in shear

The bolts also passed in the contact without friction evaluation that closed. Was under 40 MPa (that I remember, for I have been overwriting the model). It is the concrete that it seems to me is failing.

RE: Highly-loaded anchor bolts in shear

Grouting or using a rigid adhesive/filler around the bolts would reduce problems associated with local crushing where the bolt diameter is smaller than the hole or where the holes are not perfectly aligned.  I do not see anything wrong with the proposed solution, and it will likely be stronger than anything involving embedded, headed studs.

As large as the plate is, I agree that something to stiffen it from out-of-plane bending would be useful.  If pre-installed plates or cutting slots into the slab was an option, attaching stiffeners to the back of the plate would also provide lugs to transfer the load into the concrete.

RE: Highly-loaded anchor bolts in shear

The program is the current reencarnation of Algor, that its now buyer-owner-developer Autodesk calls now Autodesk Simulation Multiphysics 2012. The solution with friction has failed again but maybe on its no convergence to a solution. I am not precisely the most reliant nor reliable user of FEM yet, but I like to use it to learn, someday I may be more. Also, for this particular case, I find the "hand" check (really a worksheet on instantaneous center of rotation for bolts on shear and 1 eccentricity -so I have yet to properly add the transversal shear) made with Mathcad quite consistent and promising in that some day I can use these things with more confidence.

RE: Highly-loaded anchor bolts in shear

I could concurr with TXStructural but I frankly wouldn't feel cozy with evaluated crushing stresses at 40 MPa or over level on 35 MPa concrete, even with allowance for 2D confinement; the tests may pass marginally because would be facing actual limit strength, but I don't see these are the levels to be imparted to concrete as per a calculation, I mean, you need to leave some allowance for crushing stress even at factored level.

RE: Highly-loaded anchor bolts in shear

If you want, I would admit the level of stress permitted for struts in strut and tie schemes multiplied by a factor derived from some evaluation of bidimensional confinement effect, I think to remember ACI 318 has one. Noting that there maybe close to that point ugly tensile stresses trying to spall the concrete that seems enough.

There may be more to exact from concrete, and anchor vendors may be making a life from it; but then, they test and warrant. And if some particular is not in their catalog, no warrant. So the designer/reviewer decides.

RE: Highly-loaded anchor bolts in shear

(OP)
I think what we are seeing is that this is not a problem that can be addressed with a finite element model because it is not elastic.  Even a plastic model of some kind may not be able to adequately predict the ultimate strength and deformation of this connection because we could probably not quantify the effect of the confined and crushed concrete under the plate.  

I really believe we need testing, followed up with some kind of anaylysis procedure which is developed based on the empirical testing results.

From a design perspective, I like the idea of added shear lugs to assist the bolts.  But again, he first mission here is come up a with a capacity of the connection as designed.

RE: Highly-loaded anchor bolts in shear

I don't understand all the heavy lifting you are doing as a peer reviewer, I probably don't understand your role. Usually you'd be just reviewing calcs and drawings and making comments...

RE: Highly-loaded anchor bolts in shear

(OP)
a2mfk,

Good question and good point, but my preference is not to demand that the designer redesign if what they are proposing works.  I don't know that it doesn't and I suspect that it does, given the right bolt size and plate thickness.

The difficulty with this problema and the reason I brought it to this forum is not to have it analyzed for me but to find the testing and possibly the resulting design approach which I believe is out there.  I was pretty much told this by my Hilti rep.  

I also believe that the results of this testing would be valuable for all of us in the future, given the limitations of ACE 318 Appendix D on our designs.

RE: Highly-loaded anchor bolts in shear

I think I'd do two things if a shear lug is not possible:
1) Design/check it in Hilti's program
2) Assume an edge failure with something like a conservative X degree failure plane off each bolt closest to the edge, and then reinforce the slab using strut-tie methodology.

RE: Highly-loaded anchor bolts in shear

I misquoted the tensile stress in the studs in the model that closed at 40 MPa... it was under 400 MPa. 3900 something kgf/cm2.

Certainly this thing is not elastic etc but give you insight of what happens. The steel constitutive law as elastic-plastic can easily be input to the program,and the analysis be made elastoplastic. On the other hand, if the steel stresses don't attain the elastic recovery point, notional or actual, and whilst the concrete is not showing cracks nor the concrete stresses attain -again- the limit stress for elastic recovery, notional or real, the model can be quite good... if closed; and not to forget that this should be thought more as a simulation that an analysis, once the friction or behaviour of the surfaces kept sliding one over the other is mandated. Useful but never as definitive as test.

The observation of a2mfk on peer review may be correct in what expected by average practice, yet might not be always what expected of the reviewer, or asked of. On my part, I use the questions here posted as much to share anything I may know as to exercise skills and learn.

RE: Highly-loaded anchor bolts in shear

So do you have the EOR's calcs? Seems like the first place to start from if you are checking out his design...

This is not your average run of the mill connection, which is probably why you are having difficulties locating research or testing on it. 8" slab is comparable to a tilt wall panel, but thru bolting with sandwich plates is pretty uncommon with tilt. What I am getting at is I'd be surprised if you can find research or testing on something close to this situation. You will be left with bolt manufacturer's tables and programs like you have done, ACI Appendix D, or another approach like I mentioned before. My recent experience with the first two leads me to other approaches once the load gets above a certain threshold...

RE: Highly-loaded anchor bolts in shear

I make a necessary correction to my comments in p. 12 of the above file, that I upload corrected, since the maximum tensile stresses are not, as could be expected, in the same range in the second model than in the first; they are about ten times higher. I also make some minor cosmetic or spelling corrections.

One professor quoted the adage of that an architect should have more eraser than pencil. I am fortunate in that if I was a native american my name would be "I always correct"

RE: Highly-loaded anchor bolts in shear

at p. 12 also it must say...

"... since compressive strength is 35 MPa"

RE: Highly-loaded anchor bolts in shear

The AISC recommendation which dik attached suggests using shear lugs rather than depending on bolts because column base plates normally have oversized holes and grout under the plate.  If neither of these apply, I see no reason why through bolts wouldn't be appropriate.  As to the confinement by the plates on both sides, I would use an analogy to the increase in bearing stress based on ratio of loaded area to total area.

RE: Highly-loaded anchor bolts in shear

For large shears, I used to use welded plate washers... now I use shear lugs.

Dik

RE: Highly-loaded anchor bolts in shear

(OP)
Thank you all for your contributions.

Ishvaaag,  I like your native American name, "I always correct," but when practicing engineering as a business, you're better off with the native American name, "You always correct!"

Thank you for the FEM outputs and your observations and comments on them.  I noticed that the plate shear friction modeling would not close on solution and unfortunately, it appears from my limited review of a couple documents yesterday that that is where the ultimate practical solution to this problem lies:  shear friction and confinement with the plate.

I believe this problem is essentially a baseplate design problem.  Here is what I've found so far.  I've only skimmed through them so far and am not sure if they will be useful for this problem, but I wanted to share these with all of you:

--  AISC Steel Design Guide #1 - Base Plate and Anchor Rod Design
--  Shear Transfer in Exposed Columnn Base Plates, Report to AISC March 2009

A third document which does not necessarily address this problem, but has some useful insight on large diameter bolt performance is:

--  Shear Headed Anchors with large Diameter and Deep Embedment in Concrete, by Lee, Park and Suh, from transactions... Toronto August 2007, Paper #H07/2.

This weekend I have billable work to do and will have to continue this study next week.

RE: Highly-loaded anchor bolts in shear

Certainly, as other says, the expert knows. Those that have not the fortune of true expert advice or endowment are left to the path of progressive perfectionment.

Respect the question, I have not tried more but may, I still think the model not closing being precisely a matter of not enough section. On the friction matter, I also have the doubt that it can be as effective as to show the 1.25 inch diameter valid along the lines of this model, whether the materials and analysis are upgraded to nonlinear or not.

I may have also some info on related questions of anchors etc that may also give insight to the behaviour and search for it.

RE: Highly-loaded anchor bolts in shear

See the evaluation of the loading at the worse bolt by one instantaneous center of rotation scheme, per the letter or former steel EA 95 code (I have dispensed myself for now of establishing the forces in rational way and have just modified one worksheet that I had for the case where the eccentric shear was parallel to the columns of bolts). This workheet should do the same when it is inclined, and if not a correction by rational assumptions would be required. It places the load at the most solicited bolt at 14.8 tonnes or 32.62 kips.

RE: Highly-loaded anchor bolts in shear

"I believe this problem is essentially a baseplate design problem."

Yes, and that is why several folks have tried to steer you toward the AISC's recommendation that you not rely on the anchor rods, or thru bolts in this case, for shear resistance.  It is my understanding that AISC's recomendation is intended to address, or rather avoid, the unknown distribution of shear to the bolts that is a result of oversized holes in the baseplate and high construction tolerances for setting anchor rods in cast-in-place concrete foundations.  Some use welded plate washers to essentially "lock in" the shear distribution, but apparently AISC is not a fan of this method.  As hokie66 alludes to, if you can be sure of the shear distribution to the thru bolts, then AISC's recommendations don't necessarily apply to this situation.

Crushing of the concrete is an entirely different design criteria which will have to be satisfied, but has nothing to do with the AISC recommendation.

If this is a peer review, why not just request the designer's calcualations to review as a start?

RE: Highly-loaded anchor bolts in shear

(OP)
gte,

In answer to your question, the designer's calculations were incorrect regarding both load distribution and capacity of the connection.  This got me wondering what capacity could be justified for this connection and why.

Assuming no oversize (standard) holes, which would be easy to do using templates, I believe that the confinement provided by a thick enough plate would result in a capacity greater than that provided for by ACI 318 Appendix D, given that the shear capacities given by Appendix D do not consider this confinement.

My search for this testing only turned up base plate design guidance, of which some of it is useful, but none of it addresses my initial question reqarding the impact of confinement of the lateral capacity of this connection.  The testing I did find assume oversize holes with welded plate washers and grout and only conclude that standard holes may be different.

There is, however support for the shear friction concept using a fiction coefficient of 0.45.  If shear friction were the mechanism, the concrete crushing, by definition is considered.  Normal column baseplates have significant normal force from the column loads, which this connection does not have.  I guess I would worry about the deflections in this case.

Regarding the peer reivew project, it is, and has been for about 5 days in the designer's court to correct her loads to the bolts.  Once she does this (and thank you Ishvaaag for supporting the 32 kips per bolt.  That's what I got too.) we will move on the actual capacity of the connection.  All these posts are my journey preparing for this capacity discussion.

Thank you all for your contributions.  I will let you all know where this goes from here.

RE: Highly-loaded anchor bolts in shear

Your intuition and gut feeling about this connection is well warrented.
Using bolts/anchor bolts for the transfer of large shear loads in concrete is not advisable because of their poor performance.
Same goes for depending on friction.
As others have suggested and is my practice, also, to use shear lugs for any significant shear load.
Here are the questions I would ask:
1....what length of the bolt is actually effective in
     transferring this shear..I would say 2 to 3" and based on
     that it would result in conc. crushing.
2....what is the profile of the bearing stress on the bolt
     around the circumference...would have a peak stress at 90deg
     tapering to zero at 0 & 180deg. One of the reasons flat pl
     shear lugs are more effective is because of their almost
     uniform bearing stress distribution.
2a...can a bolt "travel" thru a chrushed conc zone due to it's
     profile?...resulting is larger than normal displacements.
3....how much bending in the bolt?...difficult to quantify.
4....given that there is conc chrushing, is the confinement
     by the plates sufficient?...again difficult to quantify.
5....what is the overall affect on the slab that now has a
     localized zone of pockets of chrushed conc ...I would have
     to check this weakened area?
6....are the loads reversible..if so, that could result
     eventually, in unaccepable displacements.
7....are the plates rigid enough to distribute the loads, as
     assumed?.

The mounting number of questions/unknowns usually signal that there is got to be a better alternative out there...trying to justify this, as it stands,is like trying to put lipstick on a pig.
      

RE: Highly-loaded anchor bolts in shear

(OP)
SAIL,

Yes, I agree.  These are the questions and I believe they could only be answered adequately by a testing program designed to separate out the variables.  

This is what started this whole thread.  I was hoping to find this testing, but it probably doesn't exist.  For my current problem I will have to rely on ACI 318 Appendix D and use the capacity limitations for concrete breakout and steel strength and reinforce the concrete in the vicinity of the anchors.

Thanks for your contribution.

 

RE: Highly-loaded anchor bolts in shear

You have stated that you are waiting for the design engineer to provide correct calculations.  If they can provide calcs that satisfy the various failure modes you are concerned about, so be it, end of story.  If not, then why not abandon this detail in favor of an embed plate.  The design citeria for an embed are perfiectly well documented and spelled out.

The choice to use post-installed anchors like this in new construction seems an odd one to me in the first place.

RE: Highly-loaded anchor bolts in shear

(OP)
gte,

The current design envisions threaded high-strength rod cast into the slab.  The anchors themsleves would be cast-in-place through rods.

Regarding the likely resolution of this peer review, I agree.  I expect that she will go back and use ACI 318 Appendix D to get the right bolt size and we will reinforce the concrete in the vicinity of the connection.  

The advantage of the connection as envisioned is that you can't have pullout of the anchors.  The bottom plate prevents that failure mode.  A more conventional embed plate would have this pullout as a risk.

 

RE: Highly-loaded anchor bolts in shear

You could use that same detail if you want, replace the bolts with a couple of HSS members (shear lugs) welded to both plates. You can as a peer reviewer simply suggest an alternative detail, some details are just not very good (have done it on my own dwgs).... Working way to hard on a bad detail IMO.

RE: Highly-loaded anchor bolts in shear

I see... I didn't realize the rods were cast in place.  I thought they were post-installed through drilled holes.

In your case, the construction sequence is analogous to setting of anchor rods for a typical column baseplate.  The use of templates can't be relied upon for accurate results.  Templates are always used, and anchor rods are routinely misplaced.  Therefore, in my opinion you should avoid loading the anchor rods with high shear loads, as suggested by AISC.  Use a shear lug instead.

RE: Highly-loaded anchor bolts in shear

(OP)
a2mfk,

The tube shear lugs are a good idea, should the bolt sizing become unworkable.

RE: Highly-loaded anchor bolts in shear

I disagree with gte447f that this detail is similar to column anchor bolts in the difficulty of location.  I have used a similar detail frequently, and the workers in the field love it because they can precisely locate the fitment on the formwork and nail the bottom plate to the form.  The key is to get the assembly in place before the reinforcement.

RE: Highly-loaded anchor bolts in shear

hokie66, note that in this case, the intent is to cast the anchor rods in the concrete, then attach the bottom and top plate/base plate during steel erection.  Now, if the bottom plate was welded to the anchor rods as part of an embed assembly, I would probably agree with you, but I would probably still specify oversized baseplate holes just in case.

RE: Highly-loaded anchor bolts in shear

Flat pl shear lugs might work out better than a tube section in this application as it would require much less space. Anyway, if tubes are used..check the local buckling of tube walls for the bearing stress.

RE: Highly-loaded anchor bolts in shear

gte447f,
I went back and looked at his sketch.  Hard to figure why or how you would cast those bolts in as shown.  Double nutted to formwork, maybe.  Won't make him popular with the formworkers.

RE: Highly-loaded anchor bolts in shear

hokie66, I agree.

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