This is the way section properties of a cylinder are evaluated when there is a round breach opening in the stack. We break up the stack into beam elements, and each beam element has one set of section properties.
If it's a round opening things are a little more complicated because don't know what opening width to consider for each stack element since it is always changing. We evaluated the options and we chose to take the approach that we would calculate the opening width at the "Top" of the stack element (Top of Circular Opening). That means for a round opening above the centerline we assume NO opening exists. This is because at the very top of the round opening there is no removal of material in the cylinder. Then at the opening centerline we assume the full opening width from there to the bottom of the circular opening. We argued that this is more conservative because the larger opening is lower in the stack. We could have assumed the full opening width for the distance from the top of the opening to the bottom of the opening; however, that might be too conservative and it artificially decreases the stiffness and changes the natural frequency.
