Engineered Shims Under Column Base Plates 2

[Canucks1010]

Hello,

I am currently working on an oil and gas project, and the contractor wants to use engineered shims under the steel column base plates prior to grouting. My question is: is there a maximum area of shim allowed, as a percentage of base plate area, beneath base plates? My concern is that is too large of shims will not leave adequate room for grouting.

Many thanks.

 

[klaus.]

What kind of forces are in the connection ?
Pressure force only or also Moment and shear ?

 

[Canucks1010]

Column bases range from pin connection to moment connection. All bases are anchored with anchor bolts embedded into concrete pedestals. Due to some of the large forces, we predict large area of shims required for concrete bearing. Is there any maximum allowable shim area? Or is it up to the discretion of the engineer?

 

[structSU10]

The loads are large prior to grouting? I would imagine you would just need dead load of the structure itself, plus a 300 lb load eccentric to the column for this analysis.

 

[Canucks1010]

The Contractor wishes to leave the structure ungrouted throughout the winter months. Therefore, for reaction forces, we must combine dead load with wind load. Due to large height of structure and small footprint, the overturning moment causes high compression on the foundation.

 

[structSU10]

how much of the structure is erected? will it have any cladding or a roof installed? Maybe some form of temporary cable bracing will help relieve the excessive moments.

 

[Canucks1010]

The structure is large open steel structure supporting numerous large bore piping and mechanical equipment. There really is no other means to reduce the forces at the column base. I'm really just looking for guidance on maximum shim area. If we limit the allowable shim area to say <50% of the base plate area, is that reasonable? Or is there no real prescribed limit, only provided the gaps between shims can be fully grouted?

 

[CANPRO]

By the sounds of it, the grout is going to more or less cosmetic...as you noted, the shims need to take the full design load. With that said, I don't think there is any maximum shim size, as long as you leave enough room to get the manufacture's minimum thickness of grout. You can always oversize your grout as well - if your baseplate is 10"x10" for example, maybe your grout covers a 14"x14" area...that way you can bring your shims to the edge of the baseplate if needed.

If the shims are doing all of the work, I'd be inclined to make them as large as possible. For the baseplates with moment, if you make your shims too small, you might induce additional prying on the anchor bolts.

Instead of shims, could you provide enough of a gap between the underside of baseplate and top of concrete to install leveling nuts? The anchor bolts would have to be designed for compression and to carry the base shear from column to concrete, but could be a more reliable (or at least more predictable) solution than shims.

 

[TLHS]

The grout won't quite be consmetic, but it's likely going to be more of a protective coating against corrosion. Make sure you leave enough gap to get grout consistently in. If they need to, they can seal it up and pressure grout the thing later.

What's you lateral load transfer system for this? Anchor bay forces on a large bore pipe support system could be significant and you'll want to take a serious look at the implications of this decision on transfer of lateral forces. If you don't have shear lugs or other transfer systems, I personally might start bolting angles or something similar into the foundation and welding the vertical leg to the elevated plate to ensure a good load path.

For something like this, even if I could make the numbers work, I'd stay far away from permanent levelling nuts. The ruggedness you'd normally want on an industrial system isn't really there with that kind of install.

 

[BridgeSmith]

"...I'd stay far away from permanent levelling nuts. The ruggedness you'd normally want on an industrial system isn't really there with that kind of install."

I'm curious as to what you mean by this. We use leveling nuts on our most demanding installs, for overhead sign structures, high mast light towers, and traffic signal poles (including those with 55' mast arms).

 

[WARose]

I like leveling nuts too.....but I've learned you have to communicate to construction that it's not permanent. You've eventually got to put the non-shrink grout under there.

 

[BridgeSmith]

We do grout on some installations with leveling nuts, but on our high mast light towers we use expanding foam to keep the dust out and expanded metal mesh to keep the critters out. Of course, those are 2 1/2" anchor bolts torqued to over 4000 ft-lbs, so they're not going to be loosening up.

 

[WARose]

For me the worry is: it's not designed to bear a lot of load on those points. So if they just leave the nuts there and don't grout.....and then they start loading up the column.....the risk is a yielded plate and localized yielding of the column. (It's not practical to design such columns (in my experience) for heavy industrial loads coming down to 4 small points.)

I usually refer them to a note on the drawing telling them at what stage I expect the grout to be under there.

 

[Agent666]

What benefit to the project is there in leaving the grouting until later? If there is none, then you have to question whether its a good thing to leave it as it creates a lot of potential issues which you as the engineer will ultimately have to deal with.

All I've ever seen is issues when contractors don't grout until far too late, usually by ignorance of assumptions that go into the design.

I've seen things like:-
- open section columns twisting,
- Columns moving/kicking out to take up hole tolerances (especially when nudged/hit by plant, sometimes bending bolts in the process)
- Trouble maintaining plumbness of the framing and erection stability as construction progresses.
- Localised yielding/bending of the baseplates due to insufficient or inappropriate shimming as load comes onto the system.
- ...and generally mud and crap finding its way into small gaps under the baseplate that's sometimes impossible to remove (or at least impossible to verify its been removed) which affects long term grout performance as you loose the bond and load transfer mechanisms in localised regions.

Most of the time I see it not being done because the contractor doesn't appreciate the importance of it and can move onto something else onsite having already been paid for erecting the thing, why spend more time on it if you can make money elsewhere.

In my opinion shims are for an erection aid only, I would not be relying on them for longer term support. Once the self weight dead load of the frame is supported and plumbed, grout the baseplates.

 

[CANPRO]

Virtually all of the large road signs where I live have structural nuts top and bottom of the base plate and no grout. But those types of structures wouldn't have the large vertical forces that WARose noted above. For large road signs, grout or no grout probably doesn't make a huge difference in the base plate design and grout probably just becomes more of a maintenance issue over time. I don't think you can make a blanket statement on leveling nuts vs grout.

WARose, I don't think I've ever seen a note on structural drawings indicating when the grout should be installed, but it sounds like a good idea.

 

[WARose]

WARose, I don't think I've ever seen a note on structural drawings indicating when the grout should be installed, but it sounds like a good idea.

I didn't get it until I was out at a job site one time and saw about 300 kips coming down on a column base I designed......with just nuts underneath.

I had green-lighted the leveling nuts over the phone (only for a limited time).....but the idiot superintendent thought that was a substitute for grout.

So I'm not sure who wins the Gomer award there (me for trusting him or him for doing it)......but I cover that now.

 

[BridgeSmith]

We specify snug tight for the top nuts, then wrench tight the leveling nuts, and then fully torque the top nuts (usually by turn of the nut). Later, our guys check them with a hydraulic torque wrench.

All our installations with significant load where steel bears on concrete have a fabric (cotton duck) or neoprene bearing pad, since we never count on the concrete finishing to be level and even enough to provide an adequate bearing surface. In a few non-critical instances (pedestrian railing/fence posts, etc.), we specify that the base plate is set on uncured high-strength grout, so that we have a solid bearing surface.