PSI - Issue 44

Fabio Mazza et al. / Procedia Structural Integrity 44 (2023) 1172–1179 Fabio Mazza / Structural Integrity Procedia 00 (2022) 000–000

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system including coupling of horizontal and vertical motions, change of critical buckling load due to significant horizontal displacement, tension buckling, cavitation, and post-cavitation phenomena (Mazza and Mazza (2021); Mazza and Labernarda (2021)). Moreover, uncoupled damping axial forces are considered with the equivalent viscous damping in the horizontal direction depending on the shear deformation and a constant value in the vertical one. A lumped plasticity model is adopted for RC frame members of the superstructure, based on a piecewise linearization of the axial load-biaxial bending moment elastic domain of the cross-sections. To account for the plastic deformations along the beams, each is discretized into two sub-elements of equal length with uniformly distributed mass. Mean of the maximum values, evaluated for each earthquake at the mid-span section of the beams, are considered for curvature ductility demand and vertical acceleration. Firstly, ductility demand and vertical acceleration at all the floor levels of the BI.H structures are plotted in Figures 2a,b, respectively, assuming nominal, upper, lower and mixed design properties of the HDRBs corresponding to α Ke =2400 (see Table 1). It is worth noting that for many earthquakes the analysis terminates prematurely, because the cavitation load of some HDRB is exceeded. Increasing values of both structural and non-structural parameters are observed along the building height, with a maximum value at the top floor, but only slight variations are found when different approaches are applied for the base-isolation system. Similar graphs are reported in Figures 2c,d for the mixed design approach of the base-isolation system, considering BI.H and BI.HV structures, the latter corresponding to eight values of a Ke (see Table 2). It is interesting to note that decreasing values of plastic deformation and vertical acceleration are obtained at the mid-span section of beams for decreasing values of a Ke , trending towards an almost constant vertical distribution. Moreover, cavitation effects on the HDRBs decrease steadily until they disappear when a Ke =50 and a Ke =100 are considered.

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(c) (d) Fig. 2. Structural and nonstructural damage parameters for base-isolated structures with different design approaches.

Continuous wavelet transforms of the vertical floor acceleration time histories at the mid-span section of beams at the top floor provide clear information about energy content of each record in time and dominant vibration periods, also defining width of resonance region and accounting for moving resonance due to nonlinearity effects. As an