Should have stated more clearly above that the prelim runs are for assesing the plate stresses and you need to be sure the mass/m is the same as the elipse.....this is a rough way of obtaining a sensible start size only.
I've been designing steel chimneys for a number of years now and have always done my best to guide the client away from eliptical chimneys due to a lack of information on the subject, it's not always possible though. I have not designed an eliptical stack to CICIND as yet only to the now...
