Tower Anchor Bolts with Leveling Nuts below the Base Plate 2

[gfbotha]

Thread507-42018 has reference.

The practice of having a (tower) base plate resting on levelling nuts, and more than often, also without filling up below with grout, keeps bugging me... As pointed out in the referenced thread, one could take measures, and even do maintenance, to protect the threaded portions of the bolts against corrosion. However, my concern is the absence of tensile preload guarding against wind induced fatigue failure of the bolt at a location just below the levelling nut... Yet, everybody seems to be applying the levelling nut practice all around... Are we going to see a lot of failures say 2-3 decades from now? Or, do you think this is good practice?

 

[SlideRuleEra]

...do you think this [not grouting] is good practice?

The US Federal Highway Administration (FHWA) considers NOT grouting to be good practice:

...grouting of double-nut-moment joints is generally not recommended:

♦ It may crack, retain moisture, and then promote corrosion.
♦ It makes it impossible to inspect and retighten bottom nuts if necessary.
♦ In order to place the grout after the base plate is in place, the standoff distance between the top of concrete and the bottom of the leveling nut may exceed the recommended distance equal to the anchor rod diameter.

Where base plates are not grouted, a stainless steel wire mesh should be placed around the base plate to eliminate debris from accumulating beneath the base plate and keep animals out and protect electrical wires if present.

See pages 31 and 32 of this link for the complete explanation:

FHWA Guidelines for the Installation, Inspection, Maintenance and Repair of Structural Supports for Highway Signs, Luminaires, and Traffic Signals

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[gfbotha]

Thanks 'SlideRule' for confirming the practice. I can certainly see the benefits of not grouting but am actually more concerned about no preload and therefore the possibility of fatigue failure... Especially given the stress concentrations present in the bolt threads.
Other considerations: lower bend stiffness at base accompanied by lower bend frequency, and also, neutral axis against bending through centre compared to pivot point closer towards edge (will affect total available bolt area to work in tension).

 

[delagina]

this is also the practice for transmission poles.
the guide that I have only reduces the capacity of the bolt due to the bending of the bolt.

 

[JStephen]

Is this some new detail? Or aren't there a bunch of them already 20 or 30 years old now?

 

[SwinnyGG]

Unless you are grouting the flange before a very significant axial load is applied to the beam, how much is grout really doing to prevent fatigue loading?

When the beam is leveled, the bolts are loaded in compression. Wind or other loads applied to the column will load some (or all) of the bolts in tension, but when the grout has set it isn't going to do much for you with regard to preventing load reversal on the bolts.

This particular application seems to me to be one of those instances where you have to take the load reversal into account during the design phase, because there is little or nothing you can do to prevent reversed loads in service.

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.

 

[gfbotha]

Without levelling nuts below, one can tighten / re-tighten the nuts after the concrete/grout has set in order to generate a proper preload. It has to be noted that the modulus of concrete is say ±6 times lower than that of the bolts, allowing a significant length of the bolt to participate. From the referenced thread, I understand that levelling would typically be assisted using center shims followed by proper grouting.

I'm not involved with any particular application/project here - is just kind of getting that uneasy feeling when observing this practice all around. I guess if one is to go for the convenient levelling nuts design, then maybe one should use a lower than usual allowed design stress (don't know what the relevant codes are saying about this) and pay special attention to long term corrosion protection. Especially in windy and corrosive coastal regions.

 

[UcfSE]

The anchors can be designed for fatigue even when they are not preloaded. Are you concerned this is not being performed?

 

[BUGGAR]

I used to design these for SDG&E. We used working stress design, which I think you now call allowable stress design. Because of wind repetition effects, we kept stresses well below the endurance limit. The biggest concern was the lack of quality control during construction that would cause too much of the anchor to be exposed to bending. Our design rules were to ignore bending in the anchor if the exposed portion was less than a certain percentage of the anchor diameter. SDG&E never paid us to field check this. I would be looking for this!
Also, SDG&E used to order pre-threaded rebar (Grade 40, or what we used to call 20 ksi design stress rebar) in large quantities from local fabricators for use as owner-furnished anchor bolts on their projects. They did this after contractors repeatedly whined about their threaded rebar anchor bolts being impossible to get. The anchors were generally oversized for most projects.

 

[gfbotha]

UcfSE; Maybe...? Yes, designing/allowing for fatigue (lower allowed stress levels) is what I hinted in my previous post. Designing for fatigue brings along more uncertainties (e.g. w.r.t. loading).

As said, it just appears to be questionable or less robust practice - granted, personally I do not know what the codes/standards cover and say...

 

[SwinnyGG]

As I said in my earlier post, that can all be handled appropriately in the design phase (although in that case Buggar's concern about design assumptions being reinforced by good inspection practice is valid).

The issue I would see with grout packing as that in an exposed condition, assuming the fasteners are appropriately spec'd and sealed, there is very little to trap water and accelerate corrosion. If the assembly is grout packed, and the grout ever chips or cracks, you now have one or more locations for water to collect. Obviously that can be catastrophically bad.

 

[BUGGAR]

I have to throw this in because it is semi related: The utility industry is so concerned with entrapped water around baseplates that we used to detail those corner slots in the baseplates to drain the anchor holes. I don't know where this all came from.

 

[SwinnyGG]

My guess is that if the fasteners are designed for fatigue loading while trying to minimize cost, material loss due to corrosion is a major concern, hence the emphasis on avoiding water entrapment.