There is a single mass system at base of which a base isolated system is attached. So this whole system is a two degree of freedom system which is subjected to rotational component of earthquake ground motion. Due to rotational motion this system is subjected to two type of motions, one rocking response (translational motion due to rotation) and other torsional motion. Base isolation part is deflected in non-linear range, while super-structure above it is deflected in linear range. So I want to write equation of motion for rocking response. In it problem is that, I can not multiply the rotation with the height directly for base-isolation part, as it is in non-linear range, while for superstructure rotation can be multiplied by height directly.