Period for multiple diaphragms in Model

Question

How does RAM calculate the period for a building that has separated diaphragms. Attached is an image of two towers that are connected on the bottom three floors and then are separate the rest of the way up.

Seth Guthrie · Answer

If you perform the eigenvalue analysis you can report or graphical animate the mode shapes. You should see some modes for the separate towers moving independently. We determine the dominant mode based on mass participation, so the program will probably use a reasonable mode shape for T is static seismic loads, but you can always over ride the T if there is another mode shape you want to be considered for the period. 
 For towers with radically different dynamic behavior, some users want one T used for one tower and another T used for the other tower. That's not currently possible short of splitting the model into two parts, or using user defined story forces.