Make it simple.Include the weight of all those components which will experience inertial force because of the movement of slab above base.In one storey building any compoment isolated from roof and columns will not experience any inertial force.
I want to put my understanding of the above said topic.We have the "n" modal shapes of our building with "n" different frequencies.Each modal shape determined by displacing one of the DOF ,or mass, and finding out the displacement of rest.
Now during earhtquake waves of different frequencies...
