MecaStack is capable of analyzing a elevated structure, such as a building or heater stack.  The challenge is determining the overall frequency of the stack and structure combined.  The only way to analyze the natural frequency of the combined structure accurately, is to model the structure in a general purpose software such as StaadPro or RISA.  These programs give the flexibility of modeling any structure; however, they do a very poor job of checking the stack itself.  

An “Approximate” solution to the situation can be handled in a much simpler way.  If you have  rectangular supporting structure, for discussion purposes let’s say it has a rectangular foot print of 20 ft by 50 ft.  You would take the lessor dimension, which is 20 ft in this case.  In MecaStack you would then model a cylindrical base for the structure that would be the same height as the supporting structure, and the diameter would be the 20 ft.  The thickness would be some thickness that allows the section to pass, probably ½” thick.  The thickness is not as important as the diameter, because the diameter has a much greater impact on the moment of inertia of the section.  This is not an “Exact” solution, but depending upon the situation it might give approximate results.  I have seen cases where it didn't give good results, such as the structure or frame being improperly braced, where the frame was very flexible.

The beams on top of the structure supporting the stack can also play a large role in the frequency, if they are very flexible then the above method isn't going to be very accurate.