Equivalent Mass and Stiffness Distribution

Hello everyone,

I was preparing a 2 dimensional model of a gravity dam monolith. The section is not uniform/regular along the direction normal to the plane, i.e. the dimensions change owing to the presence of chambers, conduits and piers. These irregularities in the geometry need to be addressed. I read a report prepared fro USACE about Folsom Dam which has the hint of this. This reoprt has used a equivalent distribution of mass and stiffness. The monoliths are separated into zones(mass concrete body, chambers, conduits and piers) assigning different material properties (Unit weight and modulus of elasticity). This method is really convincing. However I didn't find any explanation as to how were the values arrived upon for different zones for the equivalent mass and stiffness values. 

Any ideas about this...?

Thanks for your time

Deepti