Where diaphragms are not flexible, the design shall include the inherent torsional moment (Mt) resulting from the location of the structure masses plus the accidental torsional moments (Mta) caused by assumed displacement of the center of mass each way from its actual location by a distance equal to 5% of the dimension of the structure perpendicular to the direction of the applied forces. Where earthquake forces are applied concurrently in two orthogonal directions, the required 5% displacement of the center of mass need not be applied in both of the orthogonal directions at the same time but shall be applied in the direction that produces the greater effect.
Accidental torsion shall be applied to all structures for determination if a horizontal irregularity exists as specified in Table 12.3-1. Accidental torsion moments (Mta) need not be included when determining the seismic forces E in the design of the structure and in the determination of the design story drift in Sections 12.8.6, 12.9.1.2, or Chapter 16, or limits of Section 12.12.1, except for the following structures:
Structures assigned to Seismic Category B with Type 1b horizontal structural irregularity.
Structures assigned to Seismic Category C, D, E, and F with Type 1a or Type 1b horizontal structural irregularity.
12.8.4.3 Amplification of Accidental Torsional Moment
Structures assigned to Seismic Design Category C, D, E, or F, where Type 1a or 1b torsional irregularity exists as defined in Table 12.3-1 shall have the effects accounted for by multiplying Mta at each level by a torsional amplification factor (Ax) as illustrated in Fig. 12.8-1 and determined from the following equation:
where
δmax = maximum displacement at level x computed assuming
Ax = 1 [in. (mm)], and
δavg = average of the displacements at the extreme points of the structure at level x computed assuming Ax = 1 [in. (mm)].
The torsional amplification factor (Ax) shall not be less than 1 and is not required to exceed 3.0. The more severe loading for each element shall be considered for design.
Topics covered in this section: