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You will have two points per floor that you will need to find: center of
mass and center of rigidity.  The diaphragm will rotate about the center of
rigidity, and you can lump your mass for the level at the center of mass. 
If you are using a computer analysis program, you may need to calculate the
center of mass as well as translational/rotational mass properties to be
lumped there, which is just basic mass moments of inertia.  Your program
will calculate the location of the center of rigidity.

If you aren't using a computer program for the analysis, then you will need
to calculate the (relative at least) rigidities of all the laterally stiff
elements in both the X and Y directions.  Then just find the location of
the resultant stiffness vectors in X and Y, which will give you the center
of rigidity location.  I would suggest looking at a basic Masonry design
handbook such as Gary Hart's manual, which will likely give you some basic
examples of this procedure.

T. Eric Gillham PE
> From: Alex Gutierrez <alexgutz(--nospam--at)>
> To: SEAINT <seaint(--nospam--at)>
> Date: Monday, November 02, 1998 8:54 PM
> I have read the discussions about rigid diaphragm and master/slave
> relation. I have one question which I have not read in the thread or
> maybe, if there?s any,  I missed it. This has been a question for me
> probably for the past five years. This is with regards to the rigid
> diaphragm. In a typical rigid floor, there?s an X and Y translation and
> a rotation. My question is how can I locate this pivot from a given
> reference point? How can it be solved?
> Your response will be greatly appreciated.
> Alexander Gutierrez, CE