Story drift is the horizontal displacement of a floor relative to the one below it during the design earthquake. The NSR-10 limits it to 1% of the story height in reinforced concrete, steel and timber structures, and to 0.5% in masonry structures (table A.6.4-1). On a 3 m story, that is 3 cm or 1.5 cm.
Why it is limited
A building can withstand the forces and still move too much. Large drifts break walls, windows and facades, pound against neighboring buildings and increase the second-order effects on the columns. Controlling them protects the structure and also what surrounds it.
How it is controlled
With stiffness: shear walls, larger columns and beams, or bracing. Often it is the drift, not the strength, that sets the size of a building's members. When the analysis uses cracked sections, the code allows the calculated drifts to be multiplied by 0.7 before comparing them with the limit (A.6.4.1.1).
What the independent reviewer checks
That the drifts were calculated in both directions, on every floor, with the correct stiffnesses and masses, and that they meet the limit. It is one of the first points of an independent design review.
Source: NSR-10, Title A, chapter A.6.
Frequently asked questions
What is the maximum story drift under the NSR-10?
1% of the story height for reinforced concrete, steel and timber structures, and 0.5% for masonry structures (table A.6.4-1).
What happens if a design does not meet the drift limit?
The structure has to be stiffened, with walls, larger sections or bracing, until it complies. It is one of the points the independent review checks.
