No the CoS can change in a building storey by storey.
As I said to Mick, we both know that Dr Hulsey was referring to what we are calling the CoS.
Yes, it also changes over time as the structure heats, cools and loses integrity.
And Hulsey says "
thermal centroid", and then goes on to imply that it's the same as stiffness centroid. Which is nonsense. You said yourself that they are different.
Hulsey is very confused here. "
Thermal centroid" is not even a thing. "
Center of thermal expansion" is. And if anything, that is what his model can potentially find when he heats it and it expands in response to thermal loading. What you do not, and cannot, find in that modelling is the Center of Stiffness. Why? Because the Center of Stiffness is the the point that a plane (diaphragm, floor)
rotates about when subjected to a uniform (or perhaps put better: centered on its Center of Mass) lateral force, such as seismic force or wind. Hulsey does not at all analyse torsion, and so his model
cannot find the Center of Stiffness.
So all this talk of CoS, in Hulsey's presentation or on the last several pages here, is
nonsense, as that is simply not what his model is basing its coordinate system on.
If anything, his initial term "thermal centroid" might be a realistic candidate, if you correct the term to "center of thermal expansion", because that's what the model actually computes.
But in reality, I would bet a 20-pack of beer that his slide actually shows movement relative to the ground. This bet is based on the reasonable assumption that Hulsey models the lower end of columns as fixed to the ground. If he doesn't, and has them floating freely, then all bets are off, literally, as we would then be dealing with a really inept model.