Based on the structural configuration, each structure shall be designed as a regular, or irregular structure as defined below: Regular Structure: Regular structures have no significant physical discontinuities in plan or vertical configuration or in their lateral force resisting systems. Irregular Structures: Irregular structures have significant physical discontinuities in configuration or in their lateral force resisting systems. Irregular structures have either vertical irregularity or plan irregularity or both in their structural configurations.

a)     Vertical Irregularity: Structures having one or more of the irregular features listed in the Table 1 shall be designed as having a vertical irregularity.

Exceptions:

When no storey drift ratio under lateral force is greater than 1.3 times the storey drift ratio of the storey above, the structure may be deemed not to have irregularities of type I or II in the Table 2. For this case, the storey drifts may be calculated neglecting torsional effect and the storey drift ratio for the top two storey’s need to be considered.

b)     Plan Irregularity: Structures having one or more of the irregular features listed in the Table 1.2 shall be designated as having a plan irregularity

Plan Irregularity of Structures