Structures are classified as having horizontal or vertical structural irregularity in accordance with ASCE 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.
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1a. Torsional Irregularity: The case where Torsional Irregularity Factor, which is defined for any of the two orthogonal earthquake directions as the ratio of the maximum story drift at any story to the average relative story drifts at the same story in the same direction, is greater than 1.2. Maximum story drift, computed including accidental torsion with Ax =1.0. Torsional irregularity requirements in the reference sections apply only to structures in which the diaphragms are rigid or semirigid. |
1b. Extreme Torsional Irregularity: The case where Torsional Irregularity Factor, which is defined for any of the two orthogonal earthquake directions as the ratio of the maximum story drift at any story to the average relative story drifts at the same story in the same direction, is greater than 1.2. Maximum story drift, computed including accidental torsion with Ax =1.0. Extreme Torsional irregularity requirements in the reference sections apply only to structures in which the diaphragms are rigid or semirigid. |
2. Reentrant Corner Irregularity: The cases where dimensions of projections in both of the two perpendicular directions in plan exceed the total plan dimensions of that story of the building in the respective directions by more than 15% |
3. Diaphragm Discontinuity Irregularity: The cases where there is a diaphragm with an abrupt discontinuity or variation in stiffness, including those with a cut-off or open area greater than 50% of the gross closed diaphragm area, more than 50% effective diaphragm stiffness change from one story to the next. |
4. Out-of-Plane Offset Irregularity: The case where there is a lateral force-resistance path discontinuity, such as an out-of-plane offset of the vertical elements. |
5. Nonparallel System Irregularity: The case where lateral force-resisting elements are not parallel to the major orthogonal axes of the seismic force-resisting system. |
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.
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1a. Stiffness–Soft Story Irregularity: The case existing where a story has a lateral stiffness less than 70% of the above story or less than 80% of the average stiffness of the three floors above. |
1b. Stiffness–Extreme Soft Story Irregularity: The case existing where a story has a lateral stiffness less than 60% of the above story or less than 70% of the average stiffness of the three floors above. |
2. Weight (Mass) Irregularity: The case where effective story mass is more than 150% of the effective mass of above or below story. Light roof story need not be considered. |
3. Vertical Geometric Irregularity: The case where seismic force-resisting system dimension is more than 130% of that in above or below story. |
4. In-Plane Discontinuity in Vertical Lateral Force-Resisting Element Irregularity: The case where there is a discontinuity in a lateral force-resistance path such as columns or structural walls are removed at some stories, or the structural walls of upper stories are supported by columns or beams underneath. |
5a. Discontinuity in Lateral Strength–Weak Story Irregularity: The case where in each of the orthogonal earthquake directions the story lateral strength is less than 80% of that in the story above. |
5b. Discontinuity in Lateral Strength–Extreme Weak Story Irregularity: The case where in each of the orthogonal earthquake directions the story lateral strength is less than 65% of that in the story above. |