Establish Factors and Design Walls

The following iterations are made for the whole structure, in order to evaluate and apply parameters that can only be determined by an analysis of multiple levels, or which are applied to multiple levels. These parameters are

• the redundancy factor r from ASCE 7 12.3.4, which is applied to all levels if the specified conditions exist on any level
• whether the structure is torsionally irregular according to Table 12.3-1 on any level, so that accidental eccentricity is applied to each level in torsional analysis of rigid diaphragms
• the torsional amplification factor A_x from 12.8.4.3, which is determined from deflections from several levels and applied to accidental eccentricity on each level

Initial Iteration (Iteration 1)

For the initial iteration, if r is set in the Site Information dialog box to be calculated by the program, the program sets r to 1.0, otherwise it is set to either 1.0 or 1.3 according to the input. For rigid diaphragm analysis it also sets the A_x on each level to 1.0. It then distributes forces and performs a shear wall design for the whole structure. It then determines whether there is a torsional irregularity on any level for each direction, and calculates r and A_x. on each level and in each direction.

Iteration for Redundancy Factor (Iteration 2)

If the r value for the designed walls is determined to be 1.3 when it had been set to 1.0 on the initial iteration, another design iteration is performed with shear line forces factored by r = 1.3. For this iteration, if the structure had been determined not to be torsionally irregular on the first iteration, then no accidental eccentricities are applied to torsional analysis. If torsionally irregular, the A_x.values calculated on iteration 1 are used.

After the walls are designed, A_x r and torsional irregularities are re-evaluated using the new walls.

It is possible that the walls that are designed on the second iteration with r = 1.3 only require a r of 1.0 when re-evaluated. Walls on the next iteration will be designed with 1.0, possibly leading to same or similar walls to those that required a r = 1.3. When checked on the final design check, the program will apply the 1.3 factor, leading to a failed design for these walls. This is unavoidable, some buildings can oscillate indefinitely between r = 1.0 and r = 1.3 solutions. This situation is extremely rare, but the program warns you if it occurs and recommends manual input of r = 1.3.

For the great majority of buildings, r = 1.0 and iteration 2 will not be necessary.

Iteration for Torsional Sensitivity (Iteration 3)

If you have indicated in the Structure Input that rigid diaphragm analysis is being done, the program performs an iteration for torsional sensitivity. If the structure had been determined not to be torsionally irregular on the first or second iteration, then no accidental eccentricities are applied to torsional analysis. If torsionally irregular, the A_x.values calculated on iteration 1 or 2 are applied to the accidental eccentricities.

A new set of walls is then designed, which are the final design walls. The torsional irregularity status, r, and A_x values are recalculated using these walls, and the forces that arise from these parameters are compared to the final design in the design check iteration (iteration 4).

Summary of Iterations

The following table shows how these iterations proceed -