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Buckling of Anchor bolts

Buckling of Anchor bolts

Buckling of Anchor bolts

(OP)
During construction,I have come across to an event that the anchor bolts supporting the central column buckled during erection of an elevated tank. I would like to calculate if that buckling is within the acceptable limit. Any help would be much appreciated.

RE: Buckling of Anchor bolts

A photo or two would help. What caused the distortion of the anchor bolts? Just vertical load or was there a lateral component? Was the distortion caused by buckling or perhaps bending? A description of the column base would help, with size and number of bolts, etc. Anchor bolts for a column are typically not subjected to gravity loading, so would not buckle. Was there no grout under the base plate?

If you want advice, try to give us as much information as you can.

RE: Buckling of Anchor bolts

(OP)
Hi Attached is the photo of the column with buckled anchor bolts.
Column is a circular pedestal with 50mm thick base plate x diameter around 1125mm
it got 28 off 30mm anchor bolts
No grout under the base plate, column was supported on leveling nuts
It is a thickener tank, it happened during hydrotest.

RE: Buckling of Anchor bolts

You got real problems. Those bolts have zero capacity. I don't have the definition of buckling in front of me, but I'm sure it says something about uncontrolled deformation without any additional load. Structurally, unlike yield cases, we stay far away from any loads where buckling controls for that reason. I would be interested in what caused this, especially since the largest loads occur when the tank filled with water, not during construction. Maybe the lack of concrete around the bolts removed their lateral bracing and they acted like a series of little columns. Their kl/r (r being very small) became large enough where the buckling load was exceeded.
You're going to have to break some bad news to this contractor. Either remove or shore up that center column and rebuild the base from scratch or at least enough to completely reinstall the bolts. And don't let them try to get away with heat bending the bolts.

RE: Buckling of Anchor bolts

Those bolts appear to be standing, unsupported, in 6 or 8" of water. Also, why are the holes in the base plate slotted. There is another full sized hole to the right of (counter clockwise of) every bolt, what’s that all about? I doubt that that column was intended to be loaded without being fully grouted. That whole foundation situation looks screwed up. Is that what the construction drawings showed?

RE: Buckling of Anchor bolts

Screwed up, indeed. A grout pocket without any grout. What were they hydrotesting? The tank I presume, but there were unintended consequences.

Why are there so many bolts, anyway? What is their purpose?

Are there any bolts which are fractured? If the bolts are mild steel, bent but not broken, they could perhaps serve whatever purpose intended, provided you can live with the deflection which has now been built into the tank. If not, perhaps using a grout filled flat jack to jack the tank back into position is the way to go.

RE: Buckling of Anchor bolts

(OP)
Bolts were incorrectly casted in the first place and for the alignment of the columns the holes had to be slotted. I do not see, by visual any fracture on the bolts and also checked with a "hammer sound". Surprisingly one bolt stand upright as it was without any any bent. Construction drawing shows the same thing but only 230mm of pocket depth but the actual measured depth is more than 300mm.

RE: Buckling of Anchor bolts

I suspect two things - 1) - some of the bolts were already "bent" to get them into the slotted holes and 2) Someone forgot to figure out what the tank weighed when full of water instead of partly full with it's normal contents.

I wouldn't accept this in any shape or form. Loading up columns like this without the supporting grout being inserted and allowed to go off is simply negligent behaviour by the contractor and he needs now to suffer the consequences.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Buckling of Anchor bolts

Whats the purpose of having that many bolts? Is there major uplift or overturing that you need to provide them for? Do you have a picture of the rest of the tank so we can really see what the construction is.

Without all the background, its a reinstall by my book. Do it right, or dont do it at all.

RE: Buckling of Anchor bolts

At first glance, I would have to aggree with Littleinch....following the clues...bolts misalligned to begin with which would suggest an attempt to lign them up with the holes in the base pl.....if it was a case of buckling of the bolts then why did the buckling not progress to a more advanced stage and be more uniform around the perimiter, unless, only some of the bolts were bent initially to align up with the holes ....however, the unsupported length of these bolts looks dicey and untill one has the total tested load and unsupported length of the bolts one can not make an informed analysis....

RE: Buckling of Anchor bolts

I am going to agree with LittleInch's second paragraph. This is unacceptable in every way possible. Absolutely gross negligence to an extent I have not seen in quite a while.

Please correct this situation before someone gets hurt of killed. For the record, a correct solution to this problem does not involve calculation, but rather it involves cutting, welding, etc.

RE: Buckling of Anchor bolts

Did the connection experience torsional loading when water pressure was applied? is there a bend in the pipe above that would throw massive torsion when pressurized?

I see twisting plus compression failure.

RE: Buckling of Anchor bolts

T thought about torsion too, but the head of the bolts are at the opposite side of the slotted holes from what I would expect if that were the case. I suspect some alignment problem since all the bolts appear to be to the same side of the slotted holes in this picture, possibly being pried into the position by ... whomever... that bay have induced an eccentric column condition to the bolts.

Oh, and the number of bolts - was this designed as a flange connection to take hydrodynamic tension rather than compression?

Mike McCann
MMC Engineering

RE: Buckling of Anchor bolts

Until magmath answers the question of why so many bolts were required, and what are their purpose, we don't even know if the bolts are necessary. I would probably be more concerned that those gas-axed holes have decreased the bending capacity of the flange.

RE: Buckling of Anchor bolts

The tank has extensive cross bracing at the exterior, so that is where the lateral loads will be taken. If the central support has to take 2600 kN-m, you have big problems. But I doubt that is the case. I think a new analysis is in order, to define the actual loading on that central column. Is the base plate continuous under the column, or is there an opening?

RE: Buckling of Anchor bolts

If the bolting is for pressure/piping as msquared48 mentioned, it may be possible that there is a mismatch in the flange types. This is a common problem with large diameter flanges as there is a difference between API/AWWA/ASME B16.47 Series B and ASME B16.47 Series A flanges. Some of the sizes have the same number of holes, but different bolt diameter and bolt circle diameter. That might lead a crew to try to bend the bolts to match the mating flange.

This does not change the fact that a repair is necessary.

RE: Buckling of Anchor bolts

Maybe this "column" saw vertical loads far beyond what was anticipated. Just looking at the photo, it seems like the forest of smaller columns should have born most of the load. Possibly a strain compatibility or foundation settlement issue?

RE: Buckling of Anchor bolts

I noticed the tank bottom sheets have deflected between the bents. There is no secondary support here.

Bolt displacement appears post loading as compared to pre-installation bending....no marks on the bolts indicative of mechanical bending and some are on the wrong side of the slot to have been done for that purpose.

This is a screwy design....why would the central column be loaded so heavily when you have a substantial structure otherwise? Granted, all of the interior columns would be loaded heavier than the exterior ones; however, that could be easily handled without passing high loads to the central column. Is there a discontinuity between the end of the bents and the central column? If so, you have a whacky load path.

Is there a chance that the central column was intended for mechanical purposes and not structural? Seems not with so many bolts, but I suppose it could be. Maybe the bolts were only sized to take the self weight of the column and maybe a small amount of sheet/liquid above and it is taking more than anticipated because of the screwy load path.

If foundations for the first row of supplementary columns settled, it would put more load on the central column.

No answers, just more questions.

RE: Buckling of Anchor bolts

OK, then it is structural, not mechanical, or in any event not just mechanical. The column should have been grouted before loading, but that didn't happen, so the bolts had to take the load, or some of it probably found an alternate load path. The question remains as to whether all those bolts are indeed necessary in the completed structure. I suspect not, but that will require reanalysis. If the bolts are not doing much, then jacking the column to the proper level and grouting may be all that is required, provided the design can live with the loss of bending capacity in the base plate due to the slotted holes.

RE: Buckling of Anchor bolts

Tanks like this are commonly built with a scalloped bottom, that is intentional, and allows relatively thin bottom plate spanning over a much wider area.

Magmath, once upon a time, I got involved on some projects similar to this. We "inherited" a design method, and that method consisted of designing the radial beams for all the vertical loads involved and basically neglecting all other loads. When I began looking at it in more detail, it presented some real problems. Specifically, the scallop plates develop membrane tension, and much of that tension results in a large vertical load, for which the beams are designed. But there is also a net inward load on each beam from that tension, and if you start trying to find the leg loads, that inward component also produces a large moment that affects distribution of the loads on the legs, and shifts loading more to the inner legs. That kind of thing may or may not be part of the problem here, just be aware that the load at the center may have been miscalculated due to unwarranted simplifications in the design. An additional issue is that some amount of load from the scallop plates must be transferred to the shell, but you don't know how much. An additional issue is that if the scallop plates are detailed as cylindrical sections, you get sort of an indeterminate shape out of them in the assembled structure.

Where I ran into this, was we had circular ringwalls under those legs, and were getting differential settlement of one ring relative to the others- with the effect that when the tank was unloaded, it would pop anchor bolts out of the concrete on the inner legs, and base plates would be suspended a couple of inches above the concrete.

RE: Buckling of Anchor bolts

if alignment of the bolts & base pl is anticipated beforehand, then, that might explain the extra length of exposed bolts to enable field alignment of the bolts....as JS pointed out this is indeterminate structure and sensitive to the relative deflection of each radial steel frame and it's col/fdn which could put unintended loads on the center col..also, if it is subject to seismic loads, I can not see how all this load is reacted by the perimiter bracing and that the centre col could see a proportion of this seismic load based on it's relative stiffeness....I would rather see a pinned base at the center col...back to the problem at hand, I can not definitively say what caused the existing problem, but, would hazard a guess that the length of the exposed bolts was a major factor in the result...

RE: Buckling of Anchor bolts

I see there is a visible rotation which indicates twisting or torsion. Is it possible the outside support envelope had swollen up with ground water causing the center to raise? Is it possible it could retract under dry loading conditions?

RE: Buckling of Anchor bolts

When I see this many anchor rods in a base plate it makes me wonder if the connection was designed by an ME who was familiar with bolting steel-to-steel flanges typical in process design but not so familiar with base plate-to-concrete foundation embeds. This part of the design where the steel process interfaces the concrete foundation should involve a structural engineer.

I am also wondering why the center column should be designed to attract so much lateral load. It is unlikely that the connection between the radial beams and the center column can generate the kind of moment for the lateral resistance that is anticipated by the shear key and anchor rods. In an indeterminant structure like this I guess it is not surprising that center column bolts appear to have failed due to some unexpected lateral translation and racking under compression.

Ideally the lateral load should be taken care of with diagonal braces in the column array. The center column would then be modeled for bearing only with perhaps a horizontal slider connection to avoid attracting lateral load. A pinned connection could work if the center column is tied into the diagonal brace system.

Just a knee-jerk response based on two photographs :) Sorry to be a debbie downer. Maybe the design is fine and they just should have grouted the anchor rods before loading.

RE: Buckling of Anchor bolts

It looks like a tailings thickener tank to me, which typically have very high vertical and shear loads and moments on the centre post due to the scraper etc.
The bolts as shown are not adequate for any reasonable level of tension.

RE: Buckling of Anchor bolts

Sure am curious to know how this situation turned out. Doesn't look like the OP is very active though.

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