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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 resisting element using an estimate of the relative capacities of the elements.
The relative capacity among the shear resisting elements on a shear line can be estimated before the walls are designed by evaluating the perforation and aspect ratio factors. 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.
Aspect Ratio Factors
Note that on the first iteration, the relative capacity of the interior and exterior sheathing is not known, so the precise effect of the aspect ratio factor applied to just one side of the wall is not known. The program applies the height-to-width factor to the whole wall. This results in a load distribution that can put slightly excess load on the more heavily loaded walls that do not have a h/w factor. In rare cases, this can result in an initially conservative shear wall design.
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 element for design is based on the perforation and aspect ratio factors .
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.