Irregular and Regular Classification per ASCE 7-16
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 loads. The situations that cause irregularities in the plan are given in Table 12.3-1.
Description |
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1a. Torsional Irregularity: The case where the Torsional Irregularity Factor, which is defined for any of the two orthogonal earthquake directions as the ratio of the maximum story drifts 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 semi-rigid. |
1b. Extreme Torsional Irregularity: The case where the Torsional Irregularity Factor, which is defined for any of the two orthogonal earthquake directions as the ratio of the maximum story drifts at any story to the average relative story drifts at the same story in the same direction, is greater than 1.4. 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 semi-rigid. |
2. Reentrant Corner Irregularity: The cases where the dimensions of projections in both of the two perpendicular directions in the 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 involves 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 decks with span-to-depth ratios of 3 or less in structures that have no horizontal irregularities are permitted to be modeled as rigid. If the structure has horizontal irregularities, it should be modeled semi-rigid.
Vertical Irregularity
The situations that cause irregularities in the vertical direction are given in Table 12.3-2. Structures with one or more irregularity types listed in Table 12.3-2 should be designated as having a vertical structural irregularity.
Description |
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1a. Stiffness–Soft Story Irregularity: The case exists where a story has a lateral stiffness is 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 exists where a story has a lateral stiffness is 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 the effective story mass is more than 150% of the effective mass of the above or below the 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 the 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. |
