I hope you are doing this as an approximation to get a rough, ballpark idea of the behavior. Not sure where you are located, but the proper thing to do is perform a full blown structural analysis.
In any case, you could do the following:
1. Calculate the moment of inertia of a cross section (plan) of the building based on the column size and location. Then simply calculate the deflection of the building as if it were a beam cantilevered at the base. (The beam consisting of all of the columns)
2. Similar to 1 above, but neglect contribution of the columns. Look at it as if the braced bays form a vertical truss and calculate the deflection at the tip of the cantilevered truss.
3. Combine the effects of 1 and 2 above

Note that this is a very crude method and more of a classroom exercise.

Yes, and the I will vary as you proceed down the building as the column sizes change. Very rough indeed...

Mike McCann, PE, SE (WA)

I realize this is a bit late, as the question is a few weeks old, but do you have any 3D Analysis tools available? I work at a company that has tools that could help with this.

Under lateral loads, both flexural and axial stiffness's of the elements intervene. Also please note that the stiffness cannot be calculated as a property. We talk about the stiffness in a point of the bar, not just about the "stiffness". Because it varies along the height of the building.

Kamal Safa, PhD
Ecole Polytechnique