What Is Torsional Irregularity in Seismic Design?
Torsional irregularity (the ASCE 7 Table 12.3-1 — the Type 1a/1b) exists where the maximum story drift (the one side's) exceeds 1.2x the average (the 1a) or 1.4x (the 1b — the extreme). The triggers: the accidental torsion (the 5% — the amplified; the Ax's), the analysis (the 3D's — the required), and the SDC's (the prohibitions — the extreme's). The plan's asymmetry — the code's flag.
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How irregularity is determined
The drift (the max vs. the average — the 1.2/1.4; the 1a/1b), the accidental (the 5% — the mass's; the Ax — the amplification's; the 1.0-3.0), the 3D (the analysis' — the required; the 1b's), and the diaphragm (the rigid/semi-rigid's — the only).
Design consequences
The Ax (the amplification — the torsion's; the 1.0-3.0; the ELF's), the 3D (the modal — the required; the 1b's), the SDC E/F (the 1b's — the prohibited; the design's), and the elements (the perimeter's — the designed; the torsion's).
Avoiding and detailing
The symmetry (the plan's — the balanced; the stiffness'), the perimeter (the frames' — the torsional; the resistance), the diaphragms (the rigid's — the distribution), and the retrofit (the existing's — the 1b's; the strengthened). The torsion is the plan's — the symmetry designed, the Ax applied.
The bottom line
Check torsional irregularity per ASCE 7 Table 12.3-1 (1a: 1.2x, 1b: 1.4x drift ratios) — apply accidental torsion with Ax amplification (1.0-3.0); extreme (1b) prohibited in SDC E/F.
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