12 Aug 10 7:49
Center of rigidity calc can be performed as kikflip's attachment shows - those calcs can be seen in many places -one particularly good source is the Masonry Designer's Guide: http://www.cement.org/bookstore/profile.asp?itemid=lT305
If you are specifically asking about the braced frame portion, you just have to determine the relative stiffnesses of the frames. You can determine the stiffness of a frame in a software analysis package (of course if you were using one of those you can find the center of rigidity without doing hand calcs as well..). If you want to determine relative stiffnesses of braced frames by hand you can do so with virtual work.
Using virtual work, ignoring the elastic shortening of the beam and columns and only looking at the braces, expressions for stiffness are as follows:
For a single diagonal:
For a chevron configuration:
k = 2*cos^2(theta)*(AE/L)
where theta is angle from horizontal to the brace, and AE/L are the brace properties.
Of course for C.O.R. calcs only the relative (not actual) stiffnesses matter so you can multiply the above by 1000 to make the numbers easier to work with if you wish.