Inelastic Displacement of Structure

Inelastic displacement is the actual displacement, obtained by elastic analysis multiplied by a deflection amplification factor, Cd.

The deflection amplification factor, Cd, accounts for the increase in displacement due to the inelastic response of a structure that is not determined by elastic analysis and corrects for the reduction of forces introduced by response modification factor R.

If a structure remains elastic during an earthquake, the forces developed in the building are elastic (not reduced by R) which results in elastic displacements that do not account for system ductility and overstrength. If a structure is inelastic with initial stiffness equal to that of an elastic structure, then its maximum displacement will be similar to that of an elastic system.? On the other hand, structures having shorter fundamental periods are characterized by inelastic drifts larger than elastic drifts of structures of similar initial stiffness.

The deflection amplification factors values set in Table 12.2-1 may underestimate the inelastic drift, especially for seismic forces resisting systems that have Cd value less than the R-value. A research paper, Deflection Amplification Factor for Seismic Design Provisions, concluded that the deflection amplification factors (Cd) in the code are non-conservative and, to accurately capture the inelastic drift of system, the value of Cd shall be set equal to the value of R. Source from : https://www.structuremag.org/?p=14696