Great! I am very glad you have a clue! :)
I'm glad you mentioned that about the top storey, because I want to derive the expression for GA in the top storey. The point of contraflexure, from my observations, can be a as small as 1/4 the length of the column (going upwards) for the top storey...
I am wondering if anyone is familiar with any of Bryan Stafford-Smith and Alexander Coull's work?
I have their book Tall Building Structures, Analysis and Design.
For rigid frames they express an parameter for the racking stiffness called the shear rigidity (GA).
For intermediate storeys...