Leveling nuts under column base plate and non-shrink grout. 3

[RabitPete]

Erector wants to use leveling nuts instead of shims under the column base plate before filling 1" space with non shrink grout. If that is the case, I assume compression load will be transferred to the anchors instead of a grout and that's not how I designed it.

1. Is that a problem? What is the concrete failure mechanism of the headed anchor under axial compression? Would it be reasonable to assume punching shear originating at the depth of the embedded anchor head?

2. Anyone recommends using soft nylon washers under the base plate, so they will compress first and let the grout and concrete to take over the load?

Thanks!

 

[MIStructE_IRE]

The nuts/shims only serve a temporary purpose. Once the grout is in place the load will be spread beneath the baseplate.

 

[dik]

On a positive note... if overloaded, the anchor rods will yield and the load will be transferred.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[RabitPete]

So what stops the anchor from getting pushed deeper into the pedestal? I assume the concrete under the embedded anchor head will get crushed or punched through a lot sooner than anchor itself yields in compression. To be more specific, we are talking about 30 kips load on 1" anchor bolt with a heavy hex head (2.3 in² hex area). That is 13,000 psi

 

[BridgeSmith]

There are temporary plastic leveling nuts that break off and allow the load to be transferred to the grout pad.

As dik alluded to, the presence of regular leveling nuts will primarily load the anchor bolts until/unless they yield, which is usually an undesirable situation if it's not designed that way.

I guess option 2 would work, but it seems to me that may defeat the purpose of using the leveling nuts in the first place.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[MIStructE_IRE]

The grout is non shrink and provides full positive contact. Why would the load go into the anchors and not spread into the grout subsequently spreading it into the footing?

 

[dik]

Just because they are elastically stiffer... so the load goes into them a bit... don't think it hurts anything...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BridgeSmith]

I assume the concrete under the embedded anchor head will get crushed or punched through a lot sooner than anchor itself yields in compression.

It may punch through, if the head is close enough to the bottom face, but significant crushing of confined concrete is not a realistic assumption, even with 13 ksi pressure. Although, the pressure won't be that high since some of the load will be transferred due to bond between the concrete and the bolt along the shaft of the bolt. If some threads are embedded in the concrete there will be more transfer into the concrete near the top.

So, since calculating how the load is shared between the grout pad and the bolt is difficult to define with any accuracy, if leveling nuts are used, I would suggest assuming that the anchor bolt takes all the load up to its expected (not minimum) yield. Check the concrete and the bolt capacity in shear accordingly.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[RabitPete]

The leveling nuts are temporary, they are not intended to carry any load, as my original design assumes the load to spread over the grout. Erector wants to know if its OK to add the nuts, so its easier to level and square the structure. Steel is stiffer than concrete, and I assume as concrete/grout creeps, especially in a long term, anchors will see more more load.

 

[HTURKAK]

Mr. RabitPete (Structural),

IMHO and acc. to my best knowledge,

The use of leveling nuts instead of shims under the column base plate could be reasonable for simple architectural bldgs if the anchors are not preloaded , dynamic loading ( including wind etc) is not a concern..
My responds to your questions will be,

1- Could be problem if preloaded anchors etc.. Assuming a typical base plate with 4 anchor rods, when two anchors are in full tension, the other two will be in compression but with limited amount.. since the compressive load will be transferred to the base plate with comp. flange of the column and will be resisted mostly by the grout just under the flange...

2- My vote is for the use of nylon levelling nuts under the base plate is better than steel levelling nuts..

I would like to remind that , the stress distribution assumption ( AISC base plate design )under the base plate should be assumed a nominal distribution for design rather than the real stress distribution ..

 

[RabitPete]

My vote is for the use of nylon washers under the base plate is better than steel levelling nuts

What I meant is adding nylon washers between leveling nut and a base plate instead of a typical F436 washers. This way erector gets his nuts and I presumably don't have to worry about anchor heads punching through the bottom of the pedestal (There is only 8" of concrete under the anchor heads, pedestals are 16" x 16"). I have not heard of this practice though, as erectors we worked with in the past always used shims.

Alternatively, we could use nylon leveling nuts eliminating the washers and steel nuts altogether, assuming nylon can carry the load before the grout is in place and hoping they don't forget to grout before the upper level and concrete decks go up.

 

[oldrunner]

Consider expansive grout like Embeco which is cement and iron filings. Worked for me. If this is already built, most likely your anchor bolts will transfer the load to the bottom of the bolt and possibly punch out the concrete there. Make the contractor follow your drawings. He should have taken care of this with an RFI before the took the contract.

 

[HTURKAK]

Dear RabitPETE,

As far as my knowledge , if the use of steel nuts with steel washer for levelling is not an option, steel nut+ neoprene washer or PE nut+ neoprene washer are possible options or better use steel shims..

Will you pls post some descriptive sketch etc showing the size of base plate, pedestal , anchor bolts , design loading ?

 

[MIStructE_IRE]

Can we see a sketch here? Sorry.. maybe I’m stupid and missing the point here but I still don’t see a major issue. The preload effect of the temporary load (since live loads only imposed after grouting) is likely negligible.

 

[bugbus]

There is a lot of talk about the anchor bolts punching through the concrete below. I doubt this could ever realistically happen. It would only take the slightest bit of slip/yielding/crushing before the whole grout pad is engaged and then you are back to the support condition that you assumed in the first place. That initial amount of movement would have to be somewhere in the order of 0.1 mm (?), I suppose just enough to take up any tiny gap that exists at installation. Yes, the anchor bolts are provide a stiffer load path, even with full grout contact, and will still attract some additional load, but keep in mind they are also at the periphery of the baseplate in most cases and you would need to factor in the flexibility of the baseplate. I would be curious to see a bit of FEM on this problem, might even have a play around myself.

 

[MIStructE_IRE]

That’s my view gusmurr. I really don’t see an issue here.

 

[BridgeSmith]

Leveling nuts are typically used where either 1) The loading is primarily moment, and axial load relatively small, and/or 2) where precise control of the alignment of the column is required. Where neither is of those conditions exist, leveling nuts are typically not needed and not used. Where 2) applies, but not 1), temporary (nylon) leveling nuts are often used to erect the column and the grout for the pad is pumped in and cured before additional loading.

I haven't heard of the use of neoprene washers for that application, but for all of our work, both of those conditions apply (traffic signals structures, overhead sign structures, high mast poles, etc.), or neither (bridge bearings, pedestrian rail, etc.). So, we don't have a need for that kind of thing.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[JLNJ]

I’m with gusmurr. In fact, I’ll take it one step further.

It only takes a little flexibility of the base plate to redistribute the loads. In fact, I imagine most of the load goes through the stiffest load path once grouted - and that’s right under the flanges of the column. I expect the base plate does not distribute the load out all that much and the bearing stresses under the column flanges and web are five times what we traditionally think they are.

This renders concerns about the bolts and leveling washers moot.

 

[RabitPete]

Here is the original detail and the proposed change. I removed stirrups and footing reinforcements for clarity. Required design resistance is about 30kip per rod. Honestly I seriously doubt any punch through would occur, even if grout is not there and only steel leveling nuts are used. While 8" is not enough to fully develop reinforcing bars, they will surely take on some of the load. Also, how is punching shear failure of the pedestal even possible? Would not the footing restrain it any way?

 

[HTURKAK]

I do not see any punching risk..

If grout is there, and steel leveling nuts are used, be sure the design resistance is about 30kip per rod will be valid for TENSION RODS.. the compression will be transferred by column flange to the grout .. the flexibility and the distance of rods to the column face will dictate whether the compression rods are really under compression..probably tension force will develop ..

the following snap is very useful to see the concept and stress distribution..

The doc. at the following link is prepared as per EC ..suggest you to read , to see the real behavior...

https://people.fsv.cvut.cz/~wald/CESTRUCO/Texts_of_lessons/07-GB_Column_Bases.pdf