Heavily Loaded Column on Grout Pad...? 1

[justhumm]

I'm working on a project where the contract plans have temporary steel jacking/erection towers that are supported on new concrete footings around existing concrete columns. The contract shows the steel base plates resting directly on the new concrete.

The contractor is proposing to using leveling nuts and a grout bed underneath the base plates.

Normally, I probably wouldn't give this a second thought, but the columns are supporting significant vertical loads (LL ~ 340 kips, DL ~ 170 kips) and will probably be in place for 3-4 months.

Is there any history or reason for concern with a heavily loaded column crushing the unconfined grout bed and overloading the leveling nuts? Other thoughts?

Thanks for any input!

 

[BridgeSmith]

As long as the proper nuts for the grade of the anchor bolts are used, and the anchor bolts can adequately carry the load of the column, then it should work. The grout pad should not be included in the load carrying capacity (unless it is to be pressure-grouted and a strain compatibility analysis is performed to determine the load carried by the grout at yield of the anchor bolts).

Edit: I just noticed the double nut. That shouldn't be necessary if the nuts are properly tightened. Be sure they plan on having the top nuts at least wrench-tight. Btw, for comparison, our 1 1/4" anchor bolt top nuts get torqued to 250 ft-lbs, then the leveling nuts are retightened, then the top nuts get another 1/6 of a turn. Our high mast light towers have anchor bolts about the size they're proposing (maybe slightly bigger; I don't have a set of plans handy), and we check them with a hydraulic wrench that exerts 6000 ft-lbs of torque.

 

[WARose]

Typically the plate is grouted (with non shrink grout) before any significant load is placed. (After the column has been leveled.)

But some construction guys have gotten it into their heads that the grout isn't needed and I (as a result) have had to add notes to drawings to address this.

If the grouting is done properly, I don't see the issue here. (unless the column above has some detail that prevents it from spreading the load out.)

 

[racookpe1978]

How many days after pour (for the new concrete under the new column) will it have to cure before loading?

 

[justhumm]

@racookpe1978, the concrete footing will be fully cured (say 28 days) before the steel towers / columns are placed.

The grout should hit the same compressive strength as the concrete after a day or two (according to the supplier's data sheet).

I don't think the contractor is proposing pressure-injection. And they're not planning on using a grout bed much bigger than the base plate.

So I'm wondering if there's any history of similarly loaded grout bed to have some sort of side blow-out before the load is fully transmitted into the footing.

...the leveling nuts, for sure, are not designed for the full vertical service loads of the column.

 

[WARose]

So I'm wondering if there's any history of similarly loaded grout bed to have some sort of side blow-out before the load is fully transmitted into the footing.

Nothing I've ever heard of. To be sure the adjacent grout would take over before that would happen.

I think I kind of see where you are coming from now: the grout doesn't have that same thickness under the nut....will it crush? But I doubt it.

 

[JoelTXCive]

Have you checked out 5-Star's grout guide? It has a wealth of knowledge in it.

The document is free to download:

https://www.fivestarproducts.com/resource-center/literature-downloads.html

I thought that the reason you select high strength grouts 7000psi+ is so that the underlying concrete will always crush first. I think the general rule of thumb is for your grout to have double the compressive strength of the concrete it is attached too.

Here's an image from the guide on optimal grout shoulder shape:

 

[BridgeSmith]

I'd be hesistant to count on a grout pad that is placed after the column is in place with leveling nuts. If the column is placed on the grout pad, without leveling nuts, while it's wet, and nominally loaded at that point to get rid of any air pockets, and then fully loaded after the grout has cured, that works. With leveling nuts, there may be voids in the grout under the base plate, and even if there is full contact between the base plate and grout, strain compatibility dictates the anchor bolts will carry the bulk of the compression load. If one or 2 of the anchor bolts yield, there will be eccentricity introduced at the support. Maybe it doesn't matter for your application, but the unknowns about how it would react would be a no-go for me.

I ran some rough stress calculations, and I'm confused as to why you think the leveling nuts would not be sufficient. Your 340 kips is only 27 ksi stress on the 2" bolts, assuming it's axial load only. Standard hex nuts are adequate to transfer the load from a fully stressed (36ksi) low-strength anchor bolt. To get the capacity of a higher strength bolt or rod requires a heavy hex nut, which are also readily available.

 

[SlideRuleEra]

For our most heavily loaded structures and equipment (electric generating stations), grout is sized and specified to carry 100% of the load. To ensure the bolts/ leveling nuts do not carry load, plastic leveling nuts (weak, compared to steel) are used.
A compressible fiber washer of appropriate thickness and steel leveling nut will accomplish the same thing.

http://www.SlideRuleEra.net

 

[wannabeSE]

What am I missing. Why question non-shrink grout? In buildings in the USA, steel columns almost always have non shrink grout under the base plates. The grout is placed after the column is set and before it has significant loads. The non shrink grout has higher strength than the concrete foundation to achieve the same bearing capacity. At the grout, A1/A2 =1.

 

[bones206]

Yea I agree with wannabe, this is the most commonplace method for setting columns as far as I’m aware. I’ve never heard of problems with grout pads due to the presence of leveling nuts or shim packs. This is also the first I’ve heard of plastic nuts, fiber washers or doing a strain compatibility analysis. I am intrigued by these techniques though.

With this baseplate being rather large, grouting will not be a trivial thing. I would look at putting some holes in the plate to let air escape and make sure the crew doing the work has experience grouting large plates like this. The 5 Star manual probably has more info/recommendations for large baseplates that I’m not thinking of.

 

[Ingenuity]

With the OP's large base plate of dimension of 3'4" I would use SCC as a grout material. With a grout thickness of 3" a SCC mix (with 3/8" aggregate) will result in reduced shrinkage compared to pre-bagged sanded-grouts, and better flowability (thanks to viscosity modifiers and water-reducing admixtures in SCC) .

I have recently grouted 100+ steel column base plates to a quarry crusher/mansand plant. Largest base plate was 5' x 6' for a radial conveyor. We developed a SCC mix using field-mixed components (course and fine sand, 3/8" aggregate, cement, water and admixtures) in a gasoline mixer. 3 day compressive strength of 8,000 + psi, 10 day of 10,000 psi and 1 year strength of 13,000 psi. Flow slump diameter of 24". Raw material costs totaled approx 20% the cost of a pre-bagged product.

Use steel angle forms, magnetic steel chamfer strip, and a big 'grout box' to 'deliver' the grout to the forms.

These have been in service for 1+ year and no signs of any cracking or distress.

 

[Settingsun]

AASHTO recommends you don't grout under highway sign bases when using levelling nuts, and the design has to ignore the grout. The commentary isn't too clear but I understand corrosion (improper grout construction) and/or fatigue are the issues. Neither is a worry on your temporary structure.

Re strain compatibility: why worry more than a reinforced column? Column reinf is often near/at yield under service loads due to creep.

 

[wannabeSE]

AISC recommends having a grout hole in the base plate when the least dimension exceeds 24”. More information is in AISCs base plate design guide. Typically, I see a bag mix used for non shrink grout.

 

[BridgeSmith]

"Re strain compatibility: why worry more than a reinforced column? Column reinf is often near/at yield under service loads due to creep."

Our bridge columns usually aren't that highly stressed, but fully bonded and confined reinforcing steel in a column, with a typically large L/D ratio, is far different than a base plate where "L" is 3" and "D" is over 3'.

 

[justhumm]

Just following up on this...

The detail that the contractor sent over in their field change proposal showed a generic base plate cross-section (4 anchor bolts).

When I remembered that the actual base detail plate for the contract had 12 anchor bolts, I stopped worrying about it.

Thanks for all the input and discussion!

 

[Celt83]

not to cry wolf but a change from 12 bolts to 4 bolts would throw off a lot of red flags to me.

Edit: think I misread your post and they didn't actually change the number of anchors just sent you a representative detail of the leveling nut option.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[oldrunner]

If the load is transferred anytime to the leveling nuts, then you must check the anchor bolts punching through the bottom of the footing. I usually do not use grout. But for load transfer, I have used Embeco grout - which has iron filings and expands. First used it on the Cobble Terminal at Mangla Dam. The terminal was sinking and tilting because it was over an old river bed instead of sandstone. Got it fixed. I have also used an additional nut that is level with the new concrete, then have the leveling nut above that. This then will have the compression at the top of the footing and that might work. Saves having to make a footing thicker.

 

[Tmoose]

I know lots of light posts etc use leveling nuts under base plates. For sure they are "convenient."

A least in cases where rotating machinery is involved, the intended function of anchor bolts is to be tightened, and stretched 1% or so after taking up all the slop and strain in the J-hooks or washers or whatever the interfaces are down in the foundation, and thus secure the base plate against the grout with a clamping force and so provide a substantial "load path" for the machinery's varying and steady force shenanigans into the foundation or structure beneath.
I see leveling nuts as unavoidably severely interfering with that function. When the top side nuts are tightened, immediately the leveling nuts get into the act, bump against the underside of the base plate, and block any tension from being applied to the anchor bolt below.

To compound that most undesirable effect, a look at many if not most "non-shrink" grout curing curves will show, the volume may end up near the original, but there often is some expansion before returning to near zero at final cure. The effect of that expansion can be to tend to squeeze the grout laterally out of the space between the base plate and foundation like the jelly filling out of a donut, so when the final cure kicks in there will be a small gap or at best only gentle support of the base plate. As mentioned above, with the leveling nuts on guard, "tightening" the anchor bolts simply jams the base plate against the leveling nuts no matter how hard the millwrights lean on the wrenches.

Later on, when the out-of-spec vibration measurements on the bearing housings cause a flurry of e-mails and phone calls, the question of whether the anchor bolts are "tight." will likely arise. With good intentions the anchor bolt nuts will be tested, and found not to move even under extra torque. Hopefully sooner rather than later, someone will request vibration measurements all around the anchor bolts, the foundation, and the base plate which will reveal differential motion there despite the "tight" anchor bolts ( nuts ).

Plastic leveling nuts sound to me like a positively genius idea to allow convenient leveling but not be able to interfere with proper and effective anchor tightening for structurally grouted baseplates.

 

[BridgeSmith]

Tmoose, we use exclusively leveling nuts on everything from small luminaire poles up to the 120' high mast light towers with 2" anchor bolts. They are subjected to literally millions of cycles of wind loading and the design loading stresses the 105 ksi bolts near their design capacity. We don't use grout at all, just an expanded metal screen around the base plate to keep the critters out. We snug the top nuts, snug the leveling nuts, and then torque the top nuts using a turn-of-the-nut method, and haven't had any problems with that part of the system.

We check them occasionally with our hydraulic wrench, and only a few that didn't get torqued at installation show movement. To my knowledge, we've never had any loosen up.