Relevance of absolute and relative energy content in seismic evaluation of structures

A reassessment of input energy measures taking into consideration the characteristics of near fault ground motions is presented. The difference between absolute and relative energy input to structural systems is shown to be more significant for near-fault than far-fault records. In particular, the coherent velocity pulse contained in near-fault records resulting from a distinctive acceleration pulse rather than a succession of high frequency acceleration spikes produces sudden energy demand in the early phase of the response and is typically larger than the total energy accumulated at the end. Studies using idealized pulses indicate that input energy is a function of the shape and period of the velocity pulse. For spectral periods shorter than pulse period, greater absolute energy is input into the system rather than relative energy, while the reverse is true for spectral periods larger than the pulse period. The discrepancy between two energy definitions is initiated by the phase difference in ground velocity and system relative velocity, and it tends to be minimal as the pulse period approaches to system vibration period. The significance of these findings, based on linear SDOF simulations, is further investigated by examining the nonlinear seismic response of a group of realistic buildings subjected to near-fault recordings with and without apparent acceleration pulses. This study concludes that selection of appropriate energy measure for near-fault accelerograms should be based on the shape and period of dominant pulse in the record, and the vibration properties of the structural system.