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How to model for flexible diaphragm analysis?? Help

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In the past I would break a complicated geometry down into smaller blocks and 
analyze each separately - accumulating the shears at the junction of each 
block. If I have a structure with skewed walls or if the roof is at a skewed 
angle from another block, I would have isolated this area and designed it for 
lateral movement normal to its angle.

The '97 code suggests that we split the design into flexible and then rigid 
(torsion) analysis and compare the results from each - increasing the shear 
as needed due to torsion. 
This poses a bit of a problem as it becomes easier to model the whole 
structure for rigid diaprhagm and shearwall distribution since it produces a 
point for Center of Mass, Rotation and where the lateral load is to be 
applied. 

Is it still valid to break apart the flexible diaphragm analysis to account 
for the difficulties of skewed sections and leave the Torsional analysis as 
the complete system?

If not, can anyone suggest how to set up the flexible model as a whole and 
how I would distribute shear to the skewed sections? Would I simply place 
gridlines in the two major axis runing through the center of skewed walls and 
taking tributary areas btween gridlines and finding the priniple vectors for 
each of the skewed walls???

I would appreciate some comments.

Thanks
Dennis

PS. Ultimately I would like to be able to relate this to a spreadsheet tool.