GNGTS 2013 - Atti del 32° Convegno Nazionale

N2 method gives only one inelastic spectrum, and this does not depend on the hysteretic law adopted; in Fig. 2a the calculated inelastic spectrum is close to that curve, while the overdamped spectrum method gives too conservatives acceleration curves. These last are, on the contrary, dependent on the dissipated energy amount. For the ST case, the numerical results have shown an intermediate response between EP and FS systems. In general, the differences of the studied systems can be further investigated with their energy spectra. Figs. 2b and 3a,b depict the energy spectra. It can be observed that, while for the EP behaviour there is a great percentage of hysteretic energy (Fig. 3a), for the FS behaviour the SDOF moves with higher velocity and the dissipation is mainly due to the 5% damping. The hysteretic energy percentage is quite different: the FS SDOF system dissipates far less hysteretic energy than the EP (Fig. 3a) and ST (Fig. 3b) ones. Conversely, the FS SDOF system shows a great percentage of kinetic and viscous energy to balance the poor dissipative capacity with higher velocity of motion. Obviously, in this case the viscous energy cannot be correctly evaluated considering an equivalent damping ratio determined on the basis of the dissipated energy. In fact, for SDOF systems with the same ductility level, the hysteretic dissipation was found not to markedly affect the structural response, as can be seen on the inelastic spectra reported in Fig. 2a. Instead, the N2 method, which considers the effects of the ductility in all cases, provides a sufficiently accurate evaluation of the structural response. Conclusions. In the paper the accuracy of the N2 method and of the overdamped spectrum method is investigated through a series of non-linear analyses conducted on SDOF systems. Three different hysteretic laws were considered, and different levels of dissipative capacity were analysed. It was found that the overdamped spectrummethod overestimates in general the acceleration, particularly when the dissipation is limited. Conversely, the design spectrum obtained in accordance with the N2 method, reducing the elastic spectrum by the behaviour factor q on Fig. 2 – Average ADRS spectra for EP and FS SDOF systems with µ = 2 (a); average energy spectra for FS behaviour with C F = 0.01 (b). Fig. 3 - Average energy spectra for EP behaviour (a) and for ST behaviour (b) with µ = 2. 135 GNGTS 2013 S essione 2.1