When you say the excitation is in the y axis, is that in the plane of the ground or is that vertical?
Assuming the vibration is sinusoidal:
What is the Amplitude (with units)?
What is the frequency in Hz?
The first thing you should do is run a Modal analysis on your building frame. What is the frequency of the first mode?
Look at the statistics for the Geometry where it shows you the properties. What is the Mass of the frame? Does that equal the mass of the building? If not, you need to add mass to the model to make them equal. There is a Distributed Mass object that can be added to the model.