Code Reference
StructuralASCE 7-22

Section 12.12.1/Story Drift Limits

ASCE 7-22 Section 12.12.1 and Table 12.12-1 establish maximum allowable story drift limits that control lateral deformation during earthquakes.

What this section requires

The design story drift at each level must not exceed the allowable story drift from Table 12.12-1. For most structures in Risk Category II, the limit is 0.020 times the story height (2% drift). For Risk Category IV structures, the limit tightens to 0.010 (1% drift). The design story drift is computed by amplifying the elastic drift from the analysis by the deflection amplification factor Cd from Table 12.2-1.

Why this section exists

Story drift limits serve two purposes. First, they protect non-structural elements (curtain walls, partitions, mechanical piping) from damage during moderate earthquakes. These elements are not designed to accommodate large lateral deformations and will crack, leak, or fail if the building drifts too much. Second, drift limits prevent P-delta instability where the building's self-weight acting through lateral displacement creates additional overturning moments that can lead to collapse.

What plan reviewers look for

Plan reviewers check the drift analysis results against Table 12.12-1. They verify that the elastic drift from the structural analysis has been amplified by Cd (not just reported as elastic drift) and divided by the importance factor Ie. They also check that drift is evaluated at every story, not just the roof level, since an upper-story soft story can have excessive drift even when the overall drift is acceptable.

Common violations

Elastic drift reported without Cd amplification
The drift analysis shows elastic drift values from the structural model but does not amplify them by the Cd factor. The design story drift is Cd times the elastic drift divided by Ie, and this amplified value must be compared to the Table 12.12-1 limits.
Drift not checked at every story
The drift check is performed only at the roof level or as an overall building drift. Story drift must be checked at every level because a soft story condition can produce excessive drift at one level while the overall building drift appears acceptable.
Wrong drift limit for risk category
Using the 2% drift limit for a Risk Category III or IV structure. The drift limit for RC IV is 1%, and for RC III with certain structural systems it is 1.5%.
Compliance tip
Include a drift summary table in the structural calculations showing the elastic drift, Cd-amplified drift, and allowable drift at every story. Reference the risk category and the applicable limit from Table 12.12-1. Where drift controls the lateral design, note this on the structural general notes.

Related ASCE 7 requirements

Section 12.8.6 covers the calculation of the design story drift including the Cd amplification. Section 12.8.7 covers P-delta effects that become significant at high drift ratios. Table 12.2-1 provides the Cd values for each lateral system.

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Related sections

1604.4Analysis and Load PathIBC 202111.4.1Mapped Acceleration ParametersASCE 7-2212.8.1Seismic Base ShearASCE 7-22