The effects of nonlinear viscous damping on the dynamic response of single-degree-of-freedom (SDOF) structural systems are analyzed. This kind of damping characterizes a special class of fluid viscous dampers recently utilized in the field of vibration control as base-isolation devices or viscoelastic elements included in steel braces of framed structures. The analytical relationship adopted to reproduce the mechanical behavior of the fluid viscous dampers is a fractional power-law of the velocity, the exponent of which ranges between 0.1 and 0.2. This function had been previously calibrated on the results of a special experimental survey carried out at the University of Florence. The dynamics of the classical linear-viscous SDOF oscillator is herein reformulated on the basis of the above-mentioned fractional viscous damping (FrVD) relationship. In particular, the transient and steady-state responses are examined in both free and forced vibration conditions. The magnification and transmissibility factors are analytically determined for different damping levels. Moreover, the relation between the viscous damping coefficient and the frequency ratio (i.e., the ratio of the dynamic load to the oscillator frequencies) is defined. The diagrams describing these functions provide direct correlations between the damping as well as the elastic properties of the system and the frequency content of the dynamic action. © ASCE.

The study of two motion control-based seismic retrofit solutions for a low-rise reinforced concrete school building is presented in this paper. The building was assumed as a benchmark structure for a Research Project financed by the Italian Department of Civil Protection, and is representative of several similar public edifices designed with earlier Technical Standards editions, in Italy as well as in other earthquake-prone European countries. The two solutions refer to the alternative earthquake protection strategies based on the concepts of supplemental damping and seismic isolation, respectively. Namely, they consist in the installation of: (1) a dissipative bracing system incorporating pressurized fluid viscous spring-dampers; and (2) a base isolation system including double friction pendulum sliding bearings. The structural characteristics of the building, and a synthesis of the investigation campaigns developed on it, are initially presented. The mechanical parameters, dimensions, locations and installation details of the constituting elements of the two protective systems are then illustrated, along with the performance assessment analyses carried out in original and rehabilitated conditions according to a full non-linear dynamic approach. The results of the analyses show a remarkable enhancement of the seismic response capacities of the structure for both retrofit hypotheses. This allows reaching the mutual high performance levels postulated in the two rehabilitation designs with remarkably lower costs and architectural intrusion as compared to traditional rehabilitation interventions designed for the same objectives. © 2014, Springer Science+Business Media Dordrecht

A study concerning the evaluation of seismic response of statues exhibited in art museums, and a base-isolated floor strategy for their enhanced protection, are presented in this paper. Attention is particularly focused on statues made of small tensile strength materials, whose behaviour is simulated by a smeared-crack finite element approach. Seismic performance is assessed by referring to four levels specially postulated herein, and namely: (1) Rest conditions; (2) No rocking; (3) Damage control; and (4) Collapse prevention. The response is investigated via incremental dynamic analysis, by progressively increasing the amplitude of the ground motion histories adopted as inputs, and by relating output data to the limit conditions fixed for the above-mentioned performance levels. The assessment procedure is applied to a demonstrative case study, represented by a marble statue to be exhibited in the museum wing situated at the ground level of a medieval castle in Italy, according to an architectural hypothesis of partial rebuilding and reuse of the stronghold. The design solution for the base-isolated floor consists in a system of double-friction pendulum isolators. The finite element model constitutive laws and parameters, the dynamic analyses carried out in fixed-base and base-isolated floor conditions, and the practical implementation of the assumed performance assessment criteria are reported for the statue examined, along with a selection of technical details of the floor design. © 2014, Springer Science+Business Media Dordrecht

SUMMARYAn experimental investigation on a base isolation system incorporating stainless steel-Teflon bearings as sliders, and pressurized fluid viscous spring dampers, is presented in this paper. In the system examined, dampers are connected to the base floor of an isolated building to provide the desired passive control of response in the superstructure, as well as to guarantee that it re-centres completely after the termination of a seismic action. Two types of experiments were conducted: sinusoidal and random cyclic tests, and a pseudodynamic test in 'substructured' configuration. The cyclic tests were aimed at characterizing what follows: the hysteretic and strain-rate-dependent response of the considered highly non-linear spring dampers; the normal pressure-and strain-rate-dependent frictional behaviour of steel-Teflon bearings, manufactured in compliance with the latest standards for this class of sliders; and the combined response of their assembly. The pseudodynamic test simulated the installation of the protection system at the base of a 2:3-scale three-storey steel frame structure, already tested in unprotected conditions by an earlier experimental campaign. Among other findings, the results of the performed tests, as well as of relevant mechanical interpretation and numerical simulation analyses, confirmed the linear additive combination of the dissipative actions of spring dampers and sliders in this mixed installation, and the high protective performance of the considered base isolation/supplemental damping system in a realistic earthquake simulation.

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