SUMMARYNear-fault ground motions are characterized by long-period horizontal pulses and high values of the ratio between the peak value of the vertical acceleration, PGA V , and the analogous value of the horizontal acceleration, PGA H , which can become critical for base-isolated (BI) structures. The objective of the present work is to check the effectiveness of the base isolation of framed buildings when using HighDamping-Rubber Bearings (HDRBs), taking into consideration the combined effects of the horizontal and vertical components of near-fault ground motions. To this end, a numerical investigation is carried out with reference to BI reinforced concrete buildings designed according to the European seismic code (Eurocode 8). The design of the test structures is carried out in a high-risk region considering (besides the gravity loads) the horizontal seismic loads acting alone or in combination with the vertical ones and assuming different values of the ratio between the vertical and horizontal stiffnesses of the HDRBs. The nonlinear seismic analysis is performed using a step-by-step procedure based on a two-parameter implicit integration scheme and an initial-stress-like iterative procedure. At each step of the analysis, plastic conditions are checked at the potential critical sections of the girders (i.e. end sections of the sub-elements in which a girder is discretized) and columns (i.e. end sections), where a bilinear moment-curvature law is adopted; the effect of the axial load on the ultimate bending moment (M-N interaction) of the columns is also taken into account. The response of an HDRB is simulated by a model with variable stiffness properties in the horizontal and vertical directions, depending on the axial force and lateral deformation, and linear viscous damping.

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The insertion of damped braces proves to be very effective for enhancing the performance of a framed building under seismic loads. For a widespread application of this technique suitable design procedures are needed. In this paper a design procedure which aims to proportion damped braces to attain a designated performance level of the structure, for a specific level of seismic intensity, is proposed. In particular, a proportional stiffness criterion, which assumes the elastic lateral storey-stiffness due to the braces proportional to that of the unbraced frame, is combined with the displacement-based design, in which the design starts from a target deformation. To check the effectiveness and reliability of the design procedure, a six-storey reinforced concrete plane frame, representative of a medium-rise symmetric framed building, is considered as primary structure. This, designed in a mediumrisk seismic region, has to be retrofitted as in a high-risk seismic region by the insertion of braces equipped with either metallic-yielding dampers or viscoelastic ones. Nonlinear dynamic analyses of unbraced and damped braced frames are carried out, under real (set A) and artificially generated (set B) ground motions, by a step-by-step procedure. Frame members and hysteretic dampers are idealized by bilinear models, while the viscoelastic dampers are idealized by a six-element generalized model describing the variation of the mechanical properties depending on the frequency, at a given temperature.

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The insertion of steel braces equipped with viscoelastic dampers (VEDs) ('dissipative braces') is a very effective technique to improve the seismic or wind behaviour of framed buildings. The main purpose of this work is to compare the earthquake and wind dynamic response of steel-framed buildings with VEDs and achieve optimal properties of dampers and supporting braces. To this end, a numerical investigation is carried out with reference to the steel K-braced framed structure of a 15-storey office building, which is designed according to the provisions of Eurocodes 1 and 3, and to four structures derived from the first one by the insertion of additional diagonal braces and/or VEDs. With regard to the VEDs, the following cases are examined: absence of dampers; insertion of dampers supported by the existing K-braces in each of the structures with or without additional diagonal braces; insertion of dampers supported by additional diagonal braces. Dynamic analyses are carried out in the time domain using a step-by-step initial stress-like iterative procedure. For this purpose, the frame members and the VEDs are idealized, respectively, by a bilinear model, which allows the simulation of the nonlinear behaviour under seismic loads, and a six-element generalized model, which can be considered as an in-parallel-combination of two Maxwell models and one Kelvin model. Artificially generated accelerograms, whose response spectra match those adopted by Eurocode 8 for a medium subsoil class and for different levels of peak ground acceleration, are considered to simulate seismic loads. Along-wind loads are considered assuming, at each storey, time histories of the wind velocity for a return period T r = 5 years, according to an equivalent spectrum technique. 157 multiple frequency components and it is also warmed depending on the energy dissipation during loading. The steady-state response of a VED, for a sinusoidal motion of amplitude 0 and circular frequency (Figure 1(a)), can be idealized by using the force-displacement (N D − D ) law: