What Is Vertical Irregularity in Seismic Design?
Vertical irregularities (the ASCE 7 Table 12.3-2 — the Types 1-5) are the stiffness (the soft story), the weight (the mass), the geometry (the setback), the discontinuity (the in-plane), and the strength (the weak story). The triggers: the analysis (the modal's — the required), the systems (the special's — the required), and the prohibitions (the SDC's — the extreme's). The elevation's — the code's flags.
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The five types
The soft (the stiffness's — the 1a/1b), the mass (the weight's — the 150%; the 2), the geometric (the setback's — the 130%; the 3), the discontinuity (the in-plane's — the offset; the 4), and the weak (the strength's — the 80%/65%; the 5a/5b).
Design consequences
The modal (the analysis' — the required; the 1b/5b's), the special (the systems' — the D+; the 5a's), the overstrength (the Omega's — the 4's; the discontinuous'), the prohibitions (the 5b's — the E/F's; the 1b's), and the diaphragms (the 4's — the designed).
Avoiding and retrofit
The stacked (the walls' — the aligned; the regular), the setbacks' (the gradual — the 130%'s; the symmetric), the retrofit's (the 4's — the strengthened; the 5's), and the documentation (the plans' — the noted; the calculations'). The vertical is the elevation's — the regularity designed.
The bottom line
Check the five vertical irregularities per ASCE 7 Table 12.3-2 — weak story (5b) and extreme soft story (1b) trigger modal analysis, special systems, and SDC E/F prohibitions.
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