ABAQUS/CAE - How can I model a two-storey shear frame with rigid floors and LUMPED masses?

This would seem like a simple task. Anyhow, I am having some trouble.

 Problem description: I want to model a two-storey shear frame with rigid floors and lumped masses? I.e., in all its simplicity, a two-DOF system ignoring the mass of the columns and lumping the mass at the two rigid floor beams. A typical example in all books on structural dynamics.

 The columns are modelled with B23-elements while the floors are chosen as analytically rigid parts. I have used 'special - intertia - point masses' to simulate the lumped mass at the reference point of the floors.

By applying a static load, the results seem exact as compared to my hand calculations. Anyhow, when I am doing a frequency analysis, the results get all wrong indiciating that the problem lies here.

When I use a typical density for the columns and neglect the lumped masses of the floors, everything seems correct. As soon as I add the point masses, the eigenfrequencies are way too low. As far as I have understood, when I used SI-units length in [m], force in [N] etc., the point mass is supposed to be input in [kg], right?

Please help me somebody. It seems as if I have missed a basic principle somwhere.