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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:

  1. 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.
  2. 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.

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
  1. Seismic design category is determined in accordance with Section 1616.

Related Code Sections

Section 1616 Structural Design, Earthquake Loads — Criteria Selection
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 ...