Section 12.3.2/Vertical Structural Irregularities
ASCE 7-22 Section 12.3.2 and Table 12.3-2 define five types of vertical structural irregularities including soft story, weight, geometry, offset, and weak story conditions.
Table 12.3-2 identifies five types of vertical irregularities. Type 1a (Stiffness -- Soft Story): exists when the lateral stiffness of a story is less than 70% of the story above or less than 80% of the average stiffness of the three stories above. Type 1b (Extreme Soft Story): the ratio drops below 60% or 70% respectively. Type 2 (Weight/Mass): exists when the effective mass of any story is more than 150% of the effective mass of an adjacent story (roofs excluded). Type 3 (Geometric): exists when the horizontal dimension of the lateral force-resisting system in any story is more than 130% of that in an adjacent story (one-story penthouses excluded). Type 4 (In-Plane Discontinuity): exists when there is an in-plane offset of a lateral force- resisting element greater than the length of those elements, or a reduction in stiffness of the element below. Type 5a (Weak Story): exists when the story lateral strength is less than 80% of the story above. Type 5b (Extreme Weak Story): below 65%. Extreme soft story and extreme weak story irregularities are prohibited in SDC D through F for structures assigned to Risk Categories I, II, and III.
Why this section exists
Vertical irregularities cause seismic forces and deformations to concentrate at specific stories rather than distributing evenly over the building height. Soft story failures are the most common seismic collapse mechanism: a story with significantly less stiffness (such as a ground floor with large openings and parking) absorbs most of the lateral deformation, leading to story collapse. The 1994 Northridge and 1989 Loma Prieta earthquakes produced numerous soft-story apartment building collapses, driving the code to prohibit extreme soft stories in high seismic regions and require additional analysis for less extreme cases.
What plan reviewers look for
Plan reviewers compare the lateral system configuration between stories for offsets, changes in bracing arrangement, and changes in wall openings. They check for classic soft-story conditions: ground-floor parking with apartments above, ground-floor retail with offices above, and any story with significantly more glazing or open space than adjacent stories. They verify the structural calculations include a story stiffness comparison and mass comparison. For SDC D through F, they verify extreme soft story and extreme weak story conditions are not present. They check that horizontal irregularities are also checked, as both sets apply independently.