Section 12.3.3/Horizontal Structural Irregularities
ASCE 7-22 Section 12.3.3 and Table 12.3-1 define five types of horizontal structural irregularities that trigger additional seismic design requirements.
Table 12.3-1 identifies five types of horizontal (plan) irregularities. Type 1a (Torsional Irregularity): exists when the maximum story drift at one end of the structure exceeds 1.2 times the average drift of the two ends. Type 1b (Extreme Torsional Irregularity): the ratio exceeds 1.4. Type 2 (Re-entrant Corner): exists when both projections of the plan beyond a re-entrant corner exceed 15% of the plan dimension in that direction. Type 3 (Diaphragm Discontinuity): exists when the diaphragm has an abrupt change in stiffness, including a cutout exceeding 50% of the enclosed diaphragm area. Type 4 (Out-of-Plane Offset): exists when a lateral force-resisting element is offset from the element below. Type 5 (Nonparallel System): exists when lateral force-resisting elements are not parallel to or symmetric about the major orthogonal axes. Irregularities trigger additional requirements including three-dimensional analysis, increased forces on collectors and connections per overstrength combinations, and potentially prohibition of certain seismic systems in higher SDCs.
Why this section exists
Buildings with irregular plan configurations concentrate seismic forces at specific locations, creating stress patterns that the equivalent lateral force procedure may not capture without explicit adjustments. Torsional irregularity causes the building to twist during an earthquake, concentrating drift and forces at the corners farthest from the center of rigidity. Re-entrant corners create stress concentrations at the notch. Diaphragm discontinuities reduce the ability of floors to distribute lateral forces. Out-of-plane offsets create force transfer problems where the load path shifts between stories. The irregularity checks force designers to identify these conditions and apply appropriate analysis and detailing.
What plan reviewers look for
Plan reviewers check the floor plan geometry for re-entrant corners and diaphragm openings. They check the lateral system layout for torsional irregularity by comparing the center of rigidity to the center of mass. They check for offsets in the lateral system between floors. For vertical irregularities, they perform a parallel check. They verify the structural calculations explicitly address each applicable irregularity and apply the required additional provisions.