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Story Drift Calculations

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I’m a bit confused over the allowable story drift calculations.

Section 1630.10.2 indicates that Story Drift Limitations should be 0.025 times the story height for structures having a fundamental period of less than 0.7 seconds and 0.020 times the story height if the fundamental period is higher.


Section 1633.2.4 states “Expected deformations shall be determined as the greater of the Maximum Inelastic Response Displacement, Delta M, considering P-delta effects determined in accordance with Section 1630.9.2 or the deformation induced by a story drift of 0.0025 times the story height.”


I am assuming that the first section 1630.10.2 is what I would use in the flexible diaphragm analysis for comparison of story drift to determine whether the diaphragm is flexible or rigid. The next paragraph seems to be applicable for the design of components of a building.


Can someone help me with this one? Normally, I make the assumption in residential construction that the diaphragms are flexible and I increase the base shear calculation by the Simplified Static design and ignore the diaphragm deflection calculations.


Your comments would be appreciated.



Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant



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