What Is Story Drift in Seismic Design?
Drift (the 12.8.6's — the Cd's; the 2%'s) is the displacement (the Cd*δxe/Ie's — the 12.8.6's; the 0.02's) limited to (the 2%'s — the 12.12's; the 1.5%'s) via (the 12.12's — the 0.02's; the 0.015's). The drift: the 12.8.6's (the Cd*δxe/Ie's — the 2%'s; the 0.02's), the 2%'s (the 12.12's — the 0.02's; the I-II's), the 1.5%'s (the 12.12's — the 0.015's; the III's), and the 1%'s (the 12.12's — the 0.010's; the IV's). The story's — the Cd drifted.
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How drift is computed
The Cd*δxe/Ie (the 12.8.6's — the 2%'s; the 0.02's), the 12.8.6 (the Cd's — the Ie's; the δxe's), the δxe (the 12.8.6's — the Cd's; the elastic's), the Ie (the 12.8.6's — the Cd's; the 1.0's — the 1.5's), and the 12.8.6 (the 2%'s — the 0.02's; the Cd*δxe/Ie's).
The 12.12's limits
The 0.02 (the 2%'s — the 12.12's; the I-II's), the 0.015 (the 1.5%'s — the 12.12's; the III's), the 0.010 (the 1%'s — the 12.12's; the IV's), the 12.12 (the 0.02's — the 0.015's; the 0.010's), and the I-IV (the 2%'s — the 1%'s; the 12.12's).
P-delta
The θ (the 12.8.7's — the 0.10's; the 0.25's), the 0.10 (the 12.8.7's — the θ's; the 0.25's), the 0.25 (the 12.8.7's — the 0.10's; the θmax's), the 12.8.7 (the θ's — the 0.10's; the 0.25's), and the 2% (the 12.12's — the 12.8.7's; the θ's). The drift's — the 12.8.6's Cd; the θ's — the 12.8.7's 0.10.
The bottom line
Compute drift per ASCE 7 12.8.6 (δ = Cd·δxe/Ie) and check 12.12 limits (2% Risk I-II, 1.5% III, 1% IV) — run P-delta per 12.8.7 (θ ≤0.10, θmax 0.25); drift usually governs tall-moment-frame sizing.
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