This chapter uses generic models for the displacement response of multistory buildings to seismic actions to interpret limits on interstory drift and strength reduction factors for the proportioning of lateral force-resisting systems. In the conventional approach, an increase in reinforcement content is assumed to increase strength without affecting stiffness, and hence the slope of the load–displacement curve and associated period are constant. Structural design according to the Eurocodes is based on the use of limit states, which limit values of engineering demand parameters such as stresses, strains, deformations, and drifts experienced by the structure under prescribed loading conditions. Peak interstory drift is determined as the maximum of the absolute values of interstory drift during the analysis. Interstory drifts during dynamic response are influenced to varying degrees by higher modes. The drift limits are applied to drifts determined in both static and dynamic methods of analysis for the design-level ground motion.
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