1616.5 Building Configuration

Buildings shall be classified as regular or irregular based on the criteria in Section 9.5.2.3 of ASCE 7.

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.

TABLE 1616.5.1.1
PLAN STRUCTURAL IRREGULARITIES

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 Torsional irregularity shall be considered to exist when the maximum story drift, computed including accidental torsion at one end of the structure transverse to an axis is more than 1.2 times average of the story drifts at the two ends of the structure.	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

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.

TABLE 1616.5.1.2
VERTICAL STRUCTURAL IRREGULARITIES

IRREGULARITY TYPE AND DESCRIPTION		REFERENCE SECTION	SEISMIC DESIGN CATEGORYa APPLICATION
1a	Stiffness Irregularity—Soft Story A soft story is one in which the lateral stiffness is less than 70 percent of that in the story above or less than 80 percent of the average stiffness of the three stories above.	9.5.2.5.1 of ASCE 7	D
1b	Stiffness Irregularity—Extreme Soft Story An extreme soft story is one in which the lateral stiffness is less than 60 percent of that in the story above or less than 70 percent of the average stiffness of the three stories above	9.5.2.5.1 of ASCE 7	D
2	Weight (Mass) Irregularity Mass irregularity shall be considered to exist where the effective mass of any story is more than 150 percent of the effective mass of an adjacent story. A roof that is lighter than the floor below need not be considered.	9.5.2.5.1 of ASCE 7	D
3	Vertical Geometric Irregularity Vertical geometric irregularity shall be considered to exist where the horizontal dimension of the lateral-force-resisting system in any story is more than 130 percent of than in an adjacent story.	9.5.2.5.1 of ASCE 7	D, E and F
4	In-plane Discontinuity in Vertical Lateral-Force-Resisting Elements An in-plane offset of the lateral-force-resisting elements greater than the length of those elements or a reduction in stiffness of the resisting element in the story below.	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 A weak story is one in which the story lateral strength is less than 80 percent of that in the story above.The story strength is the total strength of seismic-resisting elements sharing the story shear for the direction under consideration.	1620.2.3 9.5.2.5.1 of ASCE 7	B, C and D, D