Seismic Analysis - Tower on a Base?

[bmich]

I have an existing building with a tower. The tower is small with respect to the base. The equivalent lateral force method doesn't really consider this. By using the ELF method, I get the same force at the top story of the tower that I would if the tower were the same size as the building. I found a great paper on this, but it was written in Japanese. The paper appeared to be saying you could make a vertical 'cut' in the building some distance from the tower and model them as separate buildings, but that was just what I got from the diagrams in english.
Does anyone have any good references on a way to model this condition or know of a method? Thanks.

 

[ishvaaag]

The response spectrum method with a complete 3D model will reveal the forces to be taken unto account for the design for both tower and building.

Another way of looking to the problem is determine the main modes of the building alone, then the main mode for the tower, get an amplification factor of these for the ground acceleration and use this amplified ground peak acceleration for the design of the tower alone.

You may use also the simplified methods in UBC 94 Section 1630 and 1632.