On each level, first for flexible diaphragm analysis and then for rigid analysis, the program runs through two iterations of shear wall design. These iterations are needed because
The rigidities of shear lines for rigid diaphragm distribution to shear lines are based on either shear wall deflection or capacity, both of which require an initial design to determine rigidity, then another to design based on the loads forces thus determined.
Load distribution can be affected by the stiffness due to deflection analysis, so it is not possible to predict ahead of time which wall segment will be critical for design, and an iterative procedure is required.
