What Is Torsional Irregularity in Seismic Design?
Torsional (the 1a's — the 1b's; the 12.3.3's) is the irregularity (the 1.2's — the 1.4's; the 1a's — the 1b's) that triggers (the Ax's — the 1.0-3.0's; the 12.8.4's) via (the 1.2's — the 1a's; the 1.4's — the 1b's). The irregularity: the 1a's (the 1.2's — the 12.3.3's; the Ax's), the 1b's (the 1.4's — the 12.3.3's; the 3.0's), the 1.2's (the 1a's — the 12.3.3's; the 1.4's — the 1b's), and the 1.4's (the 1b's — the 12.3.3's; the 1.2's — the 1a's). The plan’s — the 1a twisted.
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The 1a/1b test
The 1.2 (the 1a's — the 12.3.3's; the Ax's), the 1.4 (the 1b's — the 12.3.3's; the 3.0's), the max/avg drift (the ratio's — the 1.2's; the 1.4's), the 12.3.3 (the 1a's — the 1b's; the Table's), and the 1a (the 1.2's — the 1b's; the 1.4's).
The Ax trigger
The Ax (the 1.0-3.0's — the 12.8.4's; the 1a's — the 1b's), the 1.0 (the 3.0's — the 12.8.4's; the minimum's), the 3.0 (the 1.0's — the 1b's; the 12.8.4's), the 12.8.4 (the Ax's — the accidental's; the 5%'s), and the 1a (the Ax's — the 1.2's; the 1b's — the 1.4's).
ELF restriction
The 1b (the 1.4's — the 12.6's; the ELF's), the 12.6 (the 1b's — the D's; the F's), the D-F (the 12.6's — the 1b's; the SDC's), the 1.4 (the 12.6's — the 1b's; the D's), and the 12.3.3 (the 1a's — the 1b's; the Ax's). The 1a's — the 1.2's Ax; the 1b's — the 1.4's 3.0.
The bottom line
Check torsional irregularity per ASCE 7 Table 12.3-1 (1a >1.2, 1b extreme >1.4 max/avg drift ratio) — triggers Ax 1.0-3.0 amplification of accidental torsion per 12.8.4; 1b also restricts ELF in SDC D-F.
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