Buildings designed using the simplified analysis procedure in Section 1617.5 shall be classified as regular or irregular based on the criteria in this section. Such classification shall be based on the plan and vertical configuration. Buildings shall not exceed the limitations of Section 1616.6.1.
Buildings having one or more of the features listed in Table 1616.5.1.1 shall be designated as having plan structural irregularity and shall comply with the requirements in the sections referenced in that table.
| IRREGULARITY TYPE AND DESCRIPTION | REFERENCE SECTION |
SEISMIC DESIGN CATEGORYa APPLICATION |
|
|---|---|---|---|
| 1a | Torsional Irregularity - to be considered when diaphragms are not flexible as determined in section 1602.1.1 |
9.5.5.5.2 of ASCE 7 1620.4.1 9.5.2.1 of ASCE 7 9.5.5.7.1 of ASCE 7 |
C and D, D, D, C and D |
| 1b | Extreme Torsional Irregularity — to be considered when diaphragms are not flexible as determined in Section 1602.1. Extreme torsional irregularity shall be considered to exist when the maximum story drift. |
9.5.5.5.2 of ASCE 7 1620.4.1 9.5.5.5.1 of ASCE 7 9.5.5.7.1 of ASCE 7 |
C and D, D, D, C and D |
| 2 | Reentrant Corners Plan, configurations of a structure and its lateral-force-resisting system contain reentrant corners where both projections of the structure beyond a reentrant corner are greater than 15 percent of the plan dimension of the structure in the given direction. |
1620.4.1 | D |
| 3 | Diaphragm Discontinuity Diaphragms with abrupt discontinuities or variations in stiffness including those having cutout or open areas greater than 50 percent of the gross enclosed diaphragm area or changes in effective diaphragm stiffness of more than 50 percent from one story to the next. |
1620.4.1 | D |
| 4 | Out-of-Plane Offsets Discontinuities in a lateral-force-resistance path, such as out-of-plane offsets of the vertical elements. |
1620.4.1 9.5.2.5.1 of ASCE 7 1620.2.9 |
D, D, B, C and D |
| 5 | Nonparallel Systems The vertical lateral-force-resisting elements are not parallel to or symmetric about the major orthogonal axes of the lateral-force-resisting system. |
1620.3.2 | C and D |
- Seismic design category is determined in accordance with Section 1616.
Buildings having one or more of the features listed in Table 1616.5.1.2 shall be designated as having vertical irregularity and shall comply with the requirements in the sections referenced in that table.
Exceptions:
- Structural irregularities of Type 1a, 1b or 2 in Table 1616.5.1.2 do not apply where no story drift ratio under design lateral load is greater than 130 percent of the story drift ratio of the next story above. Torsional effects need not be considered in the calculation of story drifts for the purpose of this determination. The story drift ratio relationship for the top two stories of the building is not required to be evaluated.
- Irregularities of Types 1a, 1b and 2 of Table 1616.5.1.2 are not required to be considered for one-story buildings in any seismic design category or for two-story buildings in Seismic Design Category B, C or D.
| IRREGULARITY TYPE AND DESCRIPTION | REFERENCE SECTION |
SEISMIC DESIGN CATEGORYa APPLICATION |
|
|---|---|---|---|
| 1a | Stiffness Irregularity—Soft Story |
9.5.2.5.1 of ASCE 7 | D |
| 1b | Stiffness Irregularity—Extreme Soft Story |
9.5.2.5.1 of ASCE 7 | D |
| 2 | Weight (Mass) Irregularity |
9.5.2.5.1 of ASCE 7 | D |
| 3 | Vertical Geometric Irregularity |
9.5.2.5.1 of ASCE 7 | D, E and F |
| 4 | In-plane Discontinuity in Vertical Lateral-Force-Resisting Elements |
1620.4.1 9.5.2.5.1 of ASCE 7 1620.2.9 |
D, D, B, C and D |
| 5 | Discontinuity in Capacity—Weak Story |
1620.2.3 9.5.2.5.1 of ASCE 7 |
B, C and D, D |
- Seismic design category is determined in accordance with Section 1616.