Period elongation of deteriorating structures under mainshock-aftershock sequences

The elongation of the fundamental period of structure is a typical indicator of structural inelasticity during seismic events and of consequent damage accumulation. This study investigates the lengthening of the fundamental periods of reinforced concrete (RC) structures subjected to mainshock-aftershock sequences. RC structures are schematized herein as nonlinear SDOF (Single Degree Of Freedom) systems with deteriorating properties. Inelastic response spectra are developed considering both ground motion characteristics (e.g. site condition, epicentral distance, PGAs ratio of aftershock to mainshock, and duration) and structural properties, such as ductility, softening, pinching, accumulated damage and unloading stiffness. Furthermore, analytical equations estimating the elongated fundamental period are proposed as a function of the elastic (undamaged) vibration period and the significant structural parameters. The proposed equations can be reliably used for the design of structures against mainshock-aftershock sequences in future generations of seismic design codes.