Levelling nuts on anchor bolts - 45 ton installation

[deaner1102]

thread507-410364

Hello - I am hoping to get some help in determining the proper technique for installing levelling nuts on a large tower installation. Set up is we have a large 45 ton tower being installed. There is a 9' bolt circle on the base plate of the tower (adapter flange welded to the tower), with 36 2" diameter holes for 2" anchor rods. Rods will be imbedded 18" into a 36" foundation. I have completed smaller tower installations in the past by just installing the tower flat onto the foundation, and levelling anything connected to the tower, at the top. Current customer requirements are to have levelling nuts installed on the anchor rods, below the tower base plate (flange). I've seen this technique installed on say highway overhang signs, and such, but just not sure if this installation is efficient, correct, or even worth the effort. Not even sure how to run a proper analysis for this setup. Has anyone had experience with this kind of install?

Additionally, I read a comment by @SwinnyGG, that stated "Disclaimer: I'm not in a field that deals with "Highway Signs, Luminaires, and Traffic Signals" but I've designed and installed hundreds of machine bases with leveling bolts, supporting machines weighing a few hundred pounds on up to hundreds of tons.", in the thread referenced above. Swinny, in your install of machines weighing hundreds of tons, did you install levelling bolts like this? TY

 

[BridgeSmith]

I've designed the foundations for high mast light towers (80-120 feet, weighing just over 3 tons), but nothing on the scale of what you're doing. I hope the part about a 2" hole for a 2" bolt is a misprint. We use 2 1/8" holes for our 1 3/4" anchor rods.

We also extend those anchor rods nearly 7' into the foundation and anchor them to a 4" wide by 1" ring, in order to transfer the force to the reinforcing cage. 18" of embedment doesn't seem like nearly enough for a 2" anchor bolt, unless this tower has very little moment on it, so that there's little or no uplift on the bolts. If that's the case, my question would be why is it that big?

Anyway, an installation using leveling nuts is generally required for the precise geometric control needed for a tower. IOW, it's very difficult, if not impossible, to get the tower plumb without leveling nuts.

 

[HTURKAK]

This subject has been discussed within several threads. You may search the forum for the expression ( levelling nuts ).
One of them , thread507-473362.

Some questions , is vibration ( for wind etc) a problem? Are the bolts preloaded ? can you use PE levelling nuts to preload the foundation?
Embedment depth 18" and foundation thk . 36" seems small..just my opinion ..
Pls provide more info. to get better responds.
..

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[JoelTXCive]

ACI 318 has a whole lot of design checks for anchors in Chapter 17, which you really should run through.

For the concrete pryout failure mode, a good rule of thumb is an embedment of at least 12 times the anchor diameter. So, for your 2" anchors, you are already at 24" embed.

 

[BridgeSmith]

Not sure about your governing code for this, but the AASHTO sign spec requires the capacity of the anchorage in concrete to exceed the tensile strength of the anchor bolt. Even for low strength (36 ksi yield) 2" anchor bolt, that's over 150 kips. I don't see how you could get that with 18" embedment.

 

[deaner1102]

Thank you for the replies. We were provided a tentative site plan. Apologies, looks like I read the embedment data incorrectly. 18" for 1" bolts, and up to 36" for the 2" bolt option. There are different options that can go on the tower/pedestal, is why there is a range. Yes, there will be a moment and wind is a consideration. I was more asking about the installation process for the levelling nuts, and how to go about running an analysis for that, since the platform will be sitting on the nuts and not the foundation. I will look through the referenced code section, and previous thread that was mentioned. Thank you.

 

[dvd]

Information from another industry, but it indicates that nuts on both sides of baseplate result in a very short tensioned length.

 

[AskTooMuch]

I'm not sure if that is the proper way to level a Process Tower Vessel. It may need decent torque to do it since it's very heavy.
Are you tourquing the anchor bolts in criss cross pattern? This sounds like a bad idea to me to erect a tower.

What we normally do in our plant is put several stack of shims of different thickness located in between anchor bolts and use a survey equipment so it's at proper elevation and is level before erecting the process tower and pouring the grout.

 

[BridgeSmith]

The analysis is fairly straightforward; much simpler than for a base plate on a grout pad. All the loads go to the bolts. Neutral axes all cross through the centerline of the bolt circle. With that many bolts, it's probably simpler to use the polar moment of inertia of the anchor bolt group in the calculations.

The top of the foundation must have a tremendous amount of reinforcing in order to get adequate anchorage for a 2" anchor bolt with only 36" of embedment.

 

[DrZoidberWoop]

I'm with AskTooMuch on this one. We always supply erectors with "shim-packs," rather than relying on leveling nuts. Leveling nuts are great when everything goes as it should, but are a nightmare when things go even a little wrong. For instance, the crane operator lowers the load, it impacts an anchor and bends it. That's not fun to fix. Some loads can even buckle the anchor bolt beneath the leveling nut, that's even worse.

 

[BridgeSmith]

Leveling nuts are great when everything goes as it should, but are a nightmare when things go even a little wrong. For instance, the crane operator lowers the load, it impacts an anchor and bends it.

I would think that would be a serious problem for an installation regardless of whether leveling nuts were being used.

Some loads can even buckle the anchor bolt beneath the leveling nut, that's even worse.

The AASHTO sign spec. provisions allow bending of the anchor bolts to be ignored if the clear distance from the leveling nut to the concrete is less than one bolt diameter. We haven't had any problem meeting that requirement. I'd question the competency of the contractor doing the foundation if that's a problem. I would think you'd have to have considerably more unbraced length for buckling to be an issue. Per AASHTO, buckling does not need to be considered for a column with a kl/r < 22, which in this case would be about 9". That certainly shouldn't be a problem.

It seems to me that what's most efficient will depend greatly on the ratio of moment to axial load. With high axial load and low moment, setting the base on grout makes sense because compression on the base is much greater than the tension, and the bolts can be much smaller. For tall poles and towers, the moments are huge relative to the axial load, so the tension and compression are not much different. The bolt size required for tension will typically be sufficient for the full compression load as well.

Installation on leveling nuts would seem far simpler and require less extensive quality control. As long as the material specs for the bolts and nuts is verified, ensuring the rest of the installation is adequate is fairly easy.

 

[azcats]

TIA Code for telecom towers has the same one bolt diameter threshold that BridgeSmith notes above.

Almost all towers are installed w/ leveling nuts.

One thing to consider with AB transferring the compressive forces is that there's adequate punching shear capacity out the bottom of the footing from the end of the bolt. Or you can embed additional nut & bearing plate at/near the top of the pedestal to ensure loads enter the concrete there.

 

[AskTooMuch]

I don't think this a telecom tower. This sounds to me more like Vertical Vessel which is also called a Tower.

No, process towers are NOT typically installed using levelling nuts.