How to determine the rotational mass for a rocking block

[xieyazhou]

Hi all,

I was trying to model the behavior of a free standing rectangular rocking block with a concentrated mass. Since rocking is equilibriumed in the rotational direction, the determination of the rotational mass is critical. I wonder how OpenSees considers this, which value should I use for the rotational mass? I.e. if we consider the equilibrium to the toe at the block bottom, mass inertial = 4/3 m R^2, where R is the distance between the mass point to the toe. While is this the correct value I should use? I tried this and the solutions is not charming...

Thanks in advance,

Yazhou