I am having a model with same 2 structures. Structures are "isolated" each other. "A" Structure is on left side and "B" Structure is on right side. Time history(dynamic load) is applied only at "B" Structure. After analysis, the response(displacement) from "A" structure is not what I expected. It seems like "Forced vibration" somehow.
What is the cause ?
In your model there are 2 independent structures. In STAAD the way program solves for frequency in Eigen extraction, there is no means to segregate structure in 2 parts i.e., the eigen solution to be performed as a whole. Here, the mass matrix and stiffness matrix formed to solve the eigen solution considers the whole model. Even if you have created 2 separate structures, the program will take it as a single model and hence you are getting the forced vibration in structure A. It is recommended to create 2 separate file with each structure to get the correct result.