Interstory drift estimates for low-rise flexible diaphragm structures

Current seismic codes allow regular structures to be designed using an equivalent lateral force procedure if the interstory drifts calculated on the basis of the design lateral forces are less than the specified allowable story drifts. While this approach assures that structures have some minimum lateral stiffness, calculated interstory drifts may be significantly less than actual peak interstory drifts particularly for structures with flexible diaphragms. Consequently, the gravity framing systems in such structures may be exposed to interstory drift demands several times greater than would be expected on the basis of design calculations, calling into question their perceived safety. In this paper, a simple method to more accurately estimate peak interstory drifts that accounts for higher mode effects is described for low-rise perimeter shear wall structures having flexible diaphragms or even for stiff diaphragms. The proposed method is based on the principal modes obtained from a principal components analysis (PCA) of computed dynamic response data. The method, applicable to both elastic and inelastic response, considers the shape of the design response spectrum and gives interstory drift estimates for use for preliminary design of the structure as well as for use with the approaches proposed in the companion paper for determining the required diaphragm shear and flexural strengths.