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The first iteration is used to design shear walls to determine rigidities and capacities for load and force distribution for the second, final design iteration.
Load Distribution to Shear Lines
For flexible analysis, distribution to shear lines is independent of shear wall design, and is the same for both iterations.
For rigid diaphragm analysis options, the rigidities of the shear lines are dependent on shear walls that are not yet designed. This is true whether you use the capacity-based or deflection-based design setting for distributing loads.
So for this initial distribution, the sum of the wall lengths is used in place of capacity or stiffness. That is, shear lines a greater length of shear-resisting elements have greater rigidity.
Force Distribution within Line
With shear line forces established by flexible or rigid analysis:
Since walls are not yet designed, the deflections are not known at this point, and even for deflection-based force distribution the program uses the capacity method on the first iteration, and force is distributed to each shear wall segments using an estimate of the relative capacities of the segments.
The relative capacity among the shear wall segmentss on a shear line can be estimated before the walls are designed by evaluating the Jhd factor. This provides the capacity-based distribution if all the walls are forced to have the same material specification; if not the relative rigidity of walls with different materials is estimated based on relative wall lengths.
Shear Wall Design
If all walls are forced to have the same material specification, one wall design is performed for all the walls on the line, otherwise one design is performed for each wall on the shear line. For each wall or line, the critical segment for design is based on the Jhd factor.
These walls may not be the ones that are finally designed, they are designed to determine the rigidities for distribution of loads and forces to and within shear lines for the final design.