Story drift is the lateral displacement of one floor of a building relative to the floor below it, caused by seismic or wind forces. It is a critical design parameter because excessive drift damages non-structural elements, causes instability, and is often the governing limit state in seismic design codes.
Story drift (Ξ) is the horizontal displacement of one floor level relative to the floor immediately below it, produced by lateral forces such as earthquake or wind. It is distinguished from story drift ratio (or inter-story drift ratio), which is the story drift divided by the story height (Ξ/h) and is the more commonly used design parameter because it is dimensionless and independent of building size. Modern codes typically limit inter-story drift ratios to 0.5%β2.5% under design-level seismic forces, depending on the structure's occupancy category, structural system, and the code being applied. These limits are intended to prevent damage to non-structural components (cladding, partitions, glazing, piping) and to ensure the structure remains stable under large deformations.
Story drift is governed by the balance between stiffness and strength. A structure that is strong enough to resist seismic forces but not stiff enough to limit drift will suffer damage to non-structural elements and may experience P-delta instability β a geometric effect where gravity loads acting on a laterally displaced structure create additional moments that amplify the drift. The P-delta stability coefficient (ΞΈ) is calculated as the ratio of the secondary moment from gravity loads to the primary moment from lateral forces; codes typically require that ΞΈ be less than 0.10, with stricter limits for higher values. For tall, flexible structures, drift β not strength β often governs the design, leading engineers to increase stiffness (through shear walls, braced frames, or larger columns) rather than just adding material strength.
Story drift is distributed non-uniformly through a building and is sensitive to irregularities. Soft stories β floors with significantly less stiffness than those above them β concentrate drift and are a well-known cause of collapse, as seen in the 1995 Kobe and 1999 Turkey earthquakes. Torsional irregularities create uneven drift distribution across the plan, with one side of the building drifting much more than the other. Modern codes address these irregularities through amplification factors, dynamic analysis requirements, and in extreme cases, prohibition of the irregular configuration. Story drift is computed from linear elastic analysis (with appropriate R and C_d factors to account for inelastic response), modal response spectrum analysis, or nonlinear response history analysis. In performance-based design, drift limits are directly linked to performance levels: immediate occupancy typically corresponds to drift ratios below 0.5%, life safety to around 1.5%, and collapse prevention to limits that depend on the structure's ductility and failure mode.