Rigid-Plastic Bound to the Seismic Inelastic Response of Flexible Elastoplastic Oscillators

Rigid-plastic models may well approximate the peak plastic response of igid enough elastoplastic oscillators. The more flexible the oscillator, however, the less reLiable the approximation is expected to be. Contrary to this expectation, the present paper shows thctt, under seismic conditions, a rigidplastic model may also give a good estimate of the peak plastic response of quite Jlexible elastoplastic oscillcttors. In particular, it is shown that the rigid-plastic approximation is even conservative when the natural period T of the elastoplastic oscillator falls within a particular range, the ends of which are marked by two characteristic values. A simple graphical procedure is pntvided to obtain these characteistic periods directly from the elastic respotlse spectrum of the earthquake, for a given value of the oscillator yield acceleration. It spots the range in which the simpler rigid-plastic model can be adopted to bound the plastic response of elastopLastic oscillators and the range where, on the contrary, such a model underestimates the same response. The procedure also gives the value of the peiod above which the earthquake is no longer able to produce plastic yielding in elastoplastic oscillators of a given yield accelerotion. These results can be useful to estimate the earthquake inelastic demand in a simple way.