Structures are classified as having horizontal or vertical structural irregularity in accordance with Section 12.3.2. Horizontal Irregularity

Structural irregularities should be avoided owing to the negative structural behavior under earthquake, the situations that cause irregularities in the plan are given in Table 12.3-1.

Diaphragms of concrete slabs or concrete-filled metal deck with span-to-depth ratios of 3 or less in structures that have no horizontal irregularities are permitted to be modelled as rigid. If the structure has horizontal irregularities, it should be modelled semi-rigid. Vertical Irregularity

The situations that cause irregularities in the vertical direction are given in Table 12.3-2. Structures that have one or more of the irregularity types listed in Table 12.3-2 should be designated as having a vertical structural irregularity.