Shims under base plates. 3

[3doorsdwn]

At the office the other day we got into an interesting discussion after a contractor told us he does not remove shims [used for leveling] underneath column base plates.

That situation sort of concerns me because even if you did pack grout underneath.......in my mind that is still partial bearing underneath (i.e. just the bottom of the base plate to the shims). I've had a few people express the opinion to me that it's not an issue as long as non shrink grout fills the voids. They say that before anything bad would happen the shims would yield and everything would reach a happy medium (i.e. it would all share the load). Indeed, I found this on AISC's web site:

Grouting Base Plates

Question:

"We were told by a steel erector that they typically do not remove shims from column base plates after grouting. They also informed us that they do not back off leveling nuts below the base plates during the grouting process. Are these practices acceptable?"

Question sent to AISC's Steel Solutions Center

Answer:

"Yes, shims are left in place underneath the base plates. From a construction standpoint, those shims or leveling nuts hold the load while the grout cures. Their presence after the grout is structural and does not reduce strength. And from an economic standpoint, removal would needlessly increase cost.

Axial compressive forces from the column will be almost evenly distributed as bearing forces on the shims and non-shrink grout. Even if the shims were to take the majority of the load, the assembly will deform in a self-limiting manner through localized yielding or crushing of concrete as the force-distribution model assumed in sizing the base plate is attained."

Bill Liddy
American Institute of Steel Construction

Posted on July 1, 2004

And I don't buy that for one second......the way I see it either the concrete or the shims are going to win that fight as to what supports the bottom of base plate. Either the shims (that are [in this case] relatively small compared to the overall plate area) will yield to the point where the grout takes over or nothing happens and the shims carry all the load on a few points (which can't be good for the bearing of the plate [as far as concrete stresses go]; or stressed in the plate, etc.). I don't believe we will all hit a happy equilibrium and it will all be shared equally. They can't be possibly be compressed at the same rate.

Then again: the paragraph above assumes a infinitely rigid plate on the bottom. You would think that the plate could deform and distribute the load like that......but the plate is a couple inches think.......so I don't know. Thoughts?

 

[ToadJones]

Pad-
I may be repeating myself needlessly, but I have always had leveling plates set on the nuts and grouted prior to steel erection.

 

[paddingtongreen]

Sorry Toad, I wasn't aiming at you. I was trying to point out that the shims or the leveling nuts only see a small part of load before the grout takes over.

I don't like leveling plates myself, the miss-match that could be caused by the tolerances on level and the out-of-squareness of the baseplate bothered me. One of those personal idiosyncrasies.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ToadJones]

I agree there can be problems there.
When done properly ahead of time, man, it can make steel erection go fast.
Our guys would usually set columns and tighten the nuts with a few soft strikes with a sledge and a wrench. After a level or two of steel was erected and roof trusses/beams/ bracing were in place, they plumb the structure, then tighten the nuts more.
I never noticed any baseplates that were not in good contact.
We did have a foundation contractor that was excellent.
He called me in panic one morning when an anchor bolt group was 1/2" off center on a 480' long industrial building.