What Is the Seismic Drift Limit — and Why It Matters?
The seismic drift limit caps how far each story displaces relative to the one below during the design earthquake — typically 1-2% of the story height (ASCE 7 Table 12.12-1: 0.010-0.025 depending on the occupancy and the system). The drift limit protects what the strength design doesn't: the cladding (the facade's survival), the partitions (the interior's integrity), and the building's neighbors (the pounding). The serviceability of the survival.
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What drift damages
Cladding (the curtain wall's interstory movement — the 2-inch drift shattering the rigidly connected panels; the seismic joints accommodating), the partitions and the stairs (the gypsum cracking — the cosmetic vs. the egress-threatening; the stair's differential movement — the stair detaching), the elevators (the rails' misalignment — the post-earthquake entrapment), and the pounding (the adjacent building's impact — the separation joints sized for the combined drifts). The drift damage is the earthquake's economic loss — the structure survives, the building doesn't function.
The code's drift limits
ASCE 7 Table 12.12-1 (the allowable story drift — 0.025 for the ordinary low-occupancy, 0.015 for the high-occupancy, 0.010 for the essential; the Risk Category scaling the protection), the calculation (the elastic analysis's displacements amplified by Cd — the inelastic drift estimated; the P-delta — the gravity's amplification included), and the irregularities (the torsional — the drift at the flexible edge; the soft story — the concentrated drift).
Designing for drift
The stiffness (the drift often governing the member sizes — the moment frame's beams sized for the drift, not the strength; the shear wall's length; the braced frame's configuration), the detailing (the cladding's seismic joints — the accommodated movement; the partition's slip tracks — the interior surviving), and the separation (the building separations — the anti-pounding joints). The performance-based refinement (the PBSD's explicit drift targets — the owner's chosen performance). The drift design is the damage design — the engineer's control of the earthquake's economic consequences.
The bottom line
Seismic drift limits (1-2% per ASCE 7 Table 12.12-1) protect cladding, partitions, and neighbors — design the stiffness for drift (often governing over strength) and detail joints accommodating the movement.
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