A building in seismic design category C requires a horizontal irregularity analysis. Which of the following plan configurations would trigger this requirement?
Correct Answer
A) T-shaped building with significant wing projection
T-shaped buildings with significant wing projections create plan irregularities that require special analysis due to potential torsional effects during seismic events.
Why This Is the Correct Answer
ASCE 7 defines horizontal structural irregularities including Type 2 (re-entrant corners) and Type 3 (diaphragm discontinuity). A T-shaped building with significant wing projections creates re-entrant corner conditions where the wing projections exceed 15% of the plan dimension in that direction — a defined trigger for horizontal irregularity analysis. The asymmetric mass and stiffness distribution also creates potential torsional response under seismic loading.
Why the Other Options Are Wrong
Option B: L-shaped building with equal wing lengths
An L-shaped building with equal wing lengths creates a re-entrant corner, but equal wings minimize torsional effects and may not exceed the 15% projection threshold depending on proportions. The key is 'significant projection' — symmetry in an L-shape reduces irregularity concerns compared to a T-shape with large wings.
Option C: Square building with central courtyard
A square building with a central courtyard creates a diaphragm opening (Type 3 irregularity) but is not the classic re-entrant corner irregularity. The courtyard introduces diaphragm discontinuity concerns, but the plan shape itself is symmetric, reducing torsional risk.
Option D: Rectangular building with 2:1 length-to-width ratio
A rectangular building with a 2:1 length-to-width ratio is a standard, regular plan geometry. ASCE 7 does not flag simple rectangular plans as horizontally irregular regardless of aspect ratio, as long as the structure is symmetric and there are no re-entrant corners or diaphragm discontinuities.
Memory Technique
ASCE 7 Horizontal Irregularities mnemonic — 'TRDO': Torsional, Re-entrant corners, Diaphragm discontinuity, Out-of-plane offsets. The T-shaped building with big wings = Re-entrant corners = irregularity trigger. Asymmetric projections create torsion during earthquakes.
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