I have modeled a 40" x 40" column base plate with (4) 12" dia. pipe columns on it (equally spaced in both directions). How do I tell STAAD that the base plate will be on a concrete pedestal (f'c = 4.0 ksi)? My first guess is to assign supports at the mesh intersections:

SUPPORTS 1 TO 529 ELASTIC MAT DIRECT Y SUBGRADE 4

Any suggestions?

You can model the support as an elastic mat foundation. To do that, you first need to know the subgrade modulus of concrete. One of the methods by which the modulus can be computed is using the following equation:

Ks = Es / B ( 1 - PoissonRation * PoissonRatio )

( Reference: Foundation Analysis and Design ( Fifth Edition ) by Joseph E. Bowels Page 503 , Equation 9-6a )

In addition, if you want to make sure the concrete pedestal takes only compressive force, then specify the SPRING COMPRESSION command for those joints in the direction KFY.

An example of this is

SUPPORTS
1 TO 529 ELASTIC MAT YONLY SUBGRADE 987

SPRING COMPRESSION
1 TO 529 KFY

If you have any anchor bolts attached to the baseplate, they can be modeled as spring supports (tension only).

An example of this is

SUPPORTS
1000 TO 1004 FIXED BUT MX MY MZ KFY 5467

SPRING TENSION
1000 TO 1004 KFY