Relative to what exactly Mick. What fixed point did NIST have them moving relative to ?It's irrelevant, because it's the relative motion that's important.
Do you understand what I mean by relative motion? For two points A and B it's the change in A-B. (i.e. (A'-B') - (A-B) )
So if the column is A and the end of the girder is B, then any absolute movement of A and/or B is incorporated into the relative motion of A-B