Tower Base Plate Anchor Bolts û should they have adjusting nuts? 2

[RHK]

Tower Base Plate Anchor Bolts – should they have adjusting nuts?

A common detail for base plate anchor bolts for an isolated tower is to have two nuts threaded onto the bolt cast into the concrete foundation. The extra nut is placed under the base plate, providing a convenient means of plumbing the tower before the base plate is grouted.

Recently there was a collapse of a tower following corrosion fatigue in the threaded portion of the anchor bolts between the two nuts. Fortunately there were no fatalities. The tower was 40 years old, and exposed to wind-blown salty water from the bay less than half a kilometre away. The hole in the base plate had been slotted during construction, allowing ingress of water into the thread under the top nut. Also, there were significant voids in the grout.

It seems to me that the extra nuts underneath are not a good idea. If the grouting is poorly placed or shrinks on curing, there is little benefit from tightening the bolts down onto the foundation. If the extra nut was not there, retightening the bolts could be used as a crude means for checking the adequacy of the grout.

The question is not why this happened, but why was the tower base designed in this way in the first place. Does anyone out there have reference material discussing the use of adjusting nuts under base plates? Especially useful would be a 40-year-old textbook on the subject.
Russell Keays

 

[cyak]

If it is US you are talking about, I would suggest you take a look at the "Manual of Steel Construction", 6th Ed. by AISC. This was used since 1963. It discusses base plates and gives suggested arrangements, but I can't find a discussion on leveling nuts right now. I would guess that this is the book they followed.

 

[evelrod]

Russell, I don't know what to say about your specific example except that it was at best improperly installed in the first place. Leveling nuts on sign posts, mono poles, light standards, etc. have been SOP for a long time. I personally have only seen one failure of a sign pole and that was more wind related--- Using a 'accepted' design for one application in an inappropriate area where it 'could' (and did) fail. Unfortunately, there was a fatality in that case (I-15 San Bernidino County California a few(?) years back). I see no problems with the method properly applied and correctly installed.

Rod

 

[Watermelon]

Hi Russell:

I've seen lots of large sign posts and similar structures with nuts above and below the base plate. The erectors seem to like the idea, because they can plumb the post, tighten the nuts, and then forget the grout. (unless the engineer does an inspection and insists on grout.)

I've seen erection crews have difficulty trying to adjust the nut after a heavy column has been placed. Some people swear by them, others swear at them.

I've heard arguments from engineers that two nuts ensures compression loads are transferred directly into the anchor bolt via the lower nut. The load then by-passes the grout and if the grout happens to spall away, it's not a concern.

I prefer not to use levelling nuts, but I've never heard someone say they had a failure because of levelling nuts. (except you now) I can see how the lower nut created a cup for the salt water. The water then ate away the bolt.

In general, for steel structures I use 4 anchor bolts. Two is too dangerous for the erection crew. The column can topple the weak way if it is not secured.

Concrete is never poured to precise elevations, so shimming and grouting is a must. The stack of shims can be shot to a precise elevation. Place the column. Plumb the structure. Grout the base. Use non-shrink grout.

Your need for a 40 year old text book is okay if you're doing forensic engineering and you need to tell someone how my great-grandfather screwed up, otherwise, buy the latest book.

Your problem seems more to do with corrosion than with the use of levelling nuts. If you're designing a repair, focus more on corrosion prevention. Coat it, pack it, cover it, protect it, use stainless. make the anchor bolts larger for a corrosion allowance.

Best wishes.

 

[kenvlach]

The main reason for this technique is that it gives some freedom in aligning the anchor bolts with the holes in the column base plate. It is difficult to perfectly place and then to maintain the positions of 4 anchor bolts during the pour and vibrating of the concrete. When time to place the column, workers can put a couple of nuts on a bolt and 'persuade' it into position with a sledgehammer.

Another method allowing some lateral freedom for the upper ends of the anchor bolts is to place a polyethylene sleeve over the upper bolt prior to the pour, giving a 4-6" deep cylindrical cavity about the bolt. The sleeve is later pulled, the bolt aligned and the cavity grouted.

The more freedom available to bend the bolts, the finer the tolerance can be for the baseplate holes. No need for slotted holes.

The nuts under the column baseplate are there to support the column, whether or not grout is used. I think the grout is there to reduce vibration, not as primary support.

Re trapping corrosive media in the baseplate holes: Either a) don't grout under the plate (let the rain wash through), or b) grout the holes and seal with an epoxy sealer.

 

[boo1]

Having installed hundreds of towers (before I went back to college), one item might have been missed. Before the top nuts are installed, the clearance between the baseplate and the anchor bolt should be filled with a sealant.

Sorry, dont remember what was specified, the 70s are a little foggy. Cheers

 

[Ron]

RHK...you noted a couple of important points in the failure...the tower was 40 years old and the failure was a result of section loss from corrosion.

It would seem that this tower was in an aggressive environment and should have had appropriate maintenance over the years, though I doubt that was the case. We cannot expect structures, particularly those exposed, to last ad infinitum without appropriate maintenance.

A bit of well placed axle grease each year would have likely prevented the problem. At the least, an appropriate inspection would have tipped someone to start corrective action rather than being surprised by the failure.

The leveling nut concept is not inherently flawed in my opinion. It is another of those design decisions that comes with repercussion/limitation (i.e.....needs maintenance) and as long as the designer knows and conveys such, I see nothing wrong with utilizing the concept. It is common here in the US for monopole structures and generally performs well. I have seen as many corrosion problems with grouted base plates as non-grouted.

 

[evelrod]

All have covered the question well. I especially like Watermelons approach on smaller columns as it make plumbing them much easier. That is the way I did it whenever possible. Obviously neither method works on BIG base plates, jack bolts on those. My biggest was 14 tons with a 19 ton column added later.
A couple of notes---currently 2 bolt base plates are out and 'leveling nuts' are just that, for plumbing and NOT for support. That IS the job for the grout. As a matter of fact, in several gov't jobs I have had to remove leveling nuts and/or jacking bolts post grout(difficult at best)!

Rod

 

[dugal]

What is that old saying "What goes up , must come down".
I think how you install a base plate for any vertical structure depends on a lot of issues. Like:
How much money do you have to waste, i.e. make it all of stainless and 10 times the required strength?
What is the life you want out of the structure i.e 5yrs, 20 yrs, 100 yrs?
What is the worse case scenario for it's environment i.e constant salt water spray with 120 degree fluctuating temperatures and average 20 mph winds? I have seen electric power towers crumple after being coated with 2" of ice.
What is the worse thing that can happen if it falls over ?How critical is the temporary loss of the structure i.e. national defense radar vs light standard on a lonesome highway ?
Do you want to do inspections and maintenance ?
There is a 100 ways to do a job, but only one way is the best. Through a process of elimination you will find the best way, or close to it.
They only thing keeping the Eifel tower standing, the Golden Gate bridge, the Statue of Liberty is the MAINTENANCE. Left to their own elements they would have all come down by now.

 

[Polecat]

RHK:

Having lived in Florida all my life, I have always had a problem with base plates and anchor bolts. It seems that no matter what you do to them, when subjected to salt spray, they just love to corrode!

I would have to agree that double nuts are a necessity because you'll never get the drilled pier or anchor bolt placement perfectly aligned for plumbing the shaft.

Given that, it boils down to how you protect the assembly and still be able to have access to the nuts later. I see that several of the forum's structural heavyweights have already commented here, and have collectively offered some sage advice. Corrosion protection, regular maintenance, axle grease, sealants, grout, etc. all have to play an important part in a structure's longevity (I've even seen grease cups and styrofoam placed over the upper nuts; and in some cases, cathodic protection).

Unfortunately, maintenance seems to be at the bottom of a typical owner's list so I should approach the design as if it were never going to be done.

It has always struck me as being rather strange that when a design analysis comes up with huge groundline reactions (especially for monopoles), we seem to pay little attention to the fact that those loads are usually based on a recurrance factor of 50 years or so. Well, if we're going to wait that long for the big blow to happen (or the killer ice storm for our Northern friends), why then shouldn't the most critical parts of the structure be designed to resist those loads 50 years hence?

Answer is --- they should. Unfortunately, if one approaches it that way, you must convince the owner that it will be well worth the added up-front costs to do so. Like Dugal pointed out, Gustav Eiffel's surperb monuments (including the Statue of Liberty) wouldn't have lasted half this long without proper maintenance. However, I doubt seriously that a power company would view a 230 kv lattice tower out in the boondocks with the same TLC as we would place on Eiffel's masterpieces.

The safest bet is to assume that whatever is done to that tower on the very day that it's installed will be the last thing ever done to it.

http://www.spiraleng.com

 

[RHK]

Thankyou for all your comments. I have been exploring other avenues, and have a few myself.

I have located two references on base plates, but haven’t had the opportunity to peruse them as yet.
#1 DeWolf,J.T., “Column Base Plates”, American Institute of Steel Construction, Design Guide Series No.1, 1990.
#2 Concrete Society/British Constructional Steel Association/Constructional Steel Research and development Organization, “Holding Down Systems for Steel Stanchions”, 1980.

I have discussed adjusting/levelling nuts with an experienced rigging foreman. He thinks they are useless, and the easiest way to plumb a column is to:-
1. Place packing/shims in the middle of the base to the correct RL.
2. Erect the column.
3. Add 4 nuts, and snug them down, before removing the slings.
4. Adjust the plumb by tightening/loosening the nuts. Because the packing is in the middle, the column will easily rock from side to side when the bolts are loose.
5. Grout under with a slurry (not dry pack), making sure the grout fills the gap between the anchor shaft and the hole in the base plate, right up to the underside of the nut.
6. Oversize holes have the advantage that they allow a greater tolerance on anchor bolt position, and probably ensure the grouter can see that he has filled the void.
7. If bolts are required to resist uplift, make sure there is a substantial washer/plate covering the hole in the base plate. Adding this after grouting is an option.

It seems to me that galvanized bolts plus proper grouting is better protection for the anchor bolts than grease or inspection. I don’t like the idea of leaving the joint open to allow water to flow through – that relies on the gardener being aware that he has to clean the area around the baseplate.

I am not convinced that leaving a cavity around the top 6” of the anchor bolt shaft is worth the trouble. The crew will still need to fix the position of the tops of the anchor bolts accurately, and do so in a way that the bolts are held in position while the concrete is poured.

Another problem with the detail that failed was that there was insufficient thread exposed above the base plate. This meant it was not possible to add washers to cover the slotted holes.

Once again, thanks for all your input. Perhaps this is one of the entries for “Eng-Tips – The Book”.

Russell Keays

 

[evelrod]

Russell, your rigging foreman is right on up to a point. Columns with base plates of, say, one to four square feet shim packs work well, quick and easy to plumb corner to corner. Larger than that and things get a bit 'iffy'. I don't know where you are located but in California you are required to have enough thread projection regardless of what it takes to get it. Welding the anchor bolt to the plate is an accepted alternative in certain cases but is frowned upon in most circles. Strength must remain equal to or greater than the correct nut/washer (obvious!).

Since last week, I have seen several very large transmission monopoles with large circular base plates using multiple leveling nuts (20 or so) exposed, galvanized with no grout. New to me, I had not seen that done before. I know I haven't been retired that long so I must not know everything, eh? LOL

Rod

 

[StephenEd]

I see ungrouted pole base plates around here (Ohio) all the time. Each one I come across I think to myself: Hmmm. I'd like to see the structural calculations on that base plate and those anchor bolts.