Shims under base plates.
Shims under 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:
Quote:Grouting Base Plates
"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
"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."
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?