r is set to 1.0 for Seismic Design Categories A, B, and C, to comply with 12.3.4.1
r is set to 1.3 for Seismic Design Categories D, E, and F to comply with 12.3.4.2, but only after the program checks the following conditions that allow it to be set to 1.0:
12.3.4.2 a: For stories resisting more than 35% of the structural base shear, it determines whether it complies with Table 12.3-3 by finding the shear wall segments (not whole shear walls) with height to width ratio of 1.0 or more (a square segment or thinner) and comparing the capacity with these segments removed with the story capacity and if it has been reduced by 33% or more, r remains 1.3. Furthermore, for rigid diaphragm design, it checks if removal of such elements results in an extreme torsional irregularity 1b, and if so, r = 1.3.Otherwise, r = 1.0.
12.3.4.2 b - For those stories resisting more than 35% of the accumulated base shear, the number of bays as defined in this clause for light framed construction is at least 2 on each side. For this calculation, Shearwalls uses all exterior walls on a building face, and compares twice the length of these walls to the story height to determine the number of bays.
That the regularity conditions are met as described in Irregularities are met.
Shear Wall Design
Since some of the above calculations are based on shear wall capacity and shear wall design can be unknown at the start, the program iteratively distributes loads through the structure, designs, assigns a r value, and then designs again.
r is calculated independently for each force direction, but the same r is used for all levels of the structure if it is found to require it on any level.
In the Site information dialog, it is possible to circumvent the automatic calculation of r and set the value to 1.0 or 1.3 based on your own calculations.
