model a semi-fixed base plate?? 1

Apart from your question I think to have seen somewhere some statement about typical rotational stiffness at footings. If I find will post.

Respect your precise question you may try the rational approach, which, for noncyclic situations, and some daring assumptions (relative mainly to the footing itself being fixed, and that you may find the relevant deformations with the tool) you may deal with with a mere sectional analysis tool in compatibility of deformations; the tool would be accurate if your plate was on short fixed concrete stub (St. Venant local effects mercifully forgotten).

So the process would be: you state a moment, find the corresponding elongation (with the proper strain-stress relationship, of course) at faces and from them get the corresponding curvature. Repeat this for some given fractions of the moment capacity and you can chart the moment-curvature relationship.

Then you need to identify your acting moment, be it service level or limit strength. Whatever the case you can linearize the non-linear moment-curvature relationship for your particular case at the standing level of moment (you won't be using it in an elastic solution, a non-linear relationship) this meaning more or less that you substitute your non-linear Moment-Rotation relationship with a linear one going from zer.zero to your Moment·Rotation of interest.

Once understood this process you can better it and get rid of some of its limitation with the use of a finite element program. With brick elements you can state the moment in whatever the way in the finite area of the plate (or the pair C-T in akin surface and point loads) and even account the compression spring support on some elastic half-space of the footing, and then get again, for the displacement of the nodes, the rotations corresponding to applied moments, where the rest of the procedure as above stated follows.

google or ie warned not to go this taiwanese site but my antivirus showed no distress, nor the article when checked for virus

ftp://ftp.usc.edu/pub/todorovs/transfer/Proc/Contributions/Gazetas.pdf