What Is Plan Irregularity in Seismic Design?
Plan irregularities (the ASCE 7 Table 12.3-1 — the Types 1-5) are the torsion (the 1a/1b), the reentrant (the 2 — the 15%), the diaphragm (the 3 — the discontinuity), the offset (the 4 — the out-of-plane), and the nonparallel (the 5 — the systems'). The triggers: the Ax (the 1a's), the 3D (the 1b's), and the SDC's (the prohibitions). The plan's — the code's flags.
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The five types
The torsion (the 1a/1b — the drift's; the 1.2/1.4x), the reentrant (the 15% — the projection's; the 2), the diaphragm (the 50% — the discontinuity's; the 3), the offset (the out-of-plane — the 4; the vertical's), and the nonparallel (the systems' — the 5; the concurrent's).
Design consequences
The Ax (the 1a's — the 1.0-3.0; the ELF's), the 3D (the 1b's — the required; the modal's), the diaphragm (the 3's — the designed; the 25%'s), the collectors (the 4's — the Omega's), and the SDC E/F (the 1b/5's — the prohibited).
Avoiding
The rectangular (the plan's — the regular; the preferred), the separation (the joints' — the split; the independent), the diaphragm's (the continuous — the detailed), and the symmetry (the stiffness' — the balanced). The plan is the seismic's — the regularity designed.
The bottom line
Check the five plan irregularities per ASCE 7 Table 12.3-1 — torsional (1a/1b) triggers Ax amplification and 3D analysis; extreme versions prohibited in SDC E/F.
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