Applied Mechanics and Materials Vol. 430

Abstract: The method of testing the elastomeric isolators at shearing, in a system formed of two elements in parallel, is presented. The exterior actions can be represented by harmonic functions defined as:kinematic excitation through the instantaneous displacement of the form (t)=Asinωt;dynamic excitation through the instantaneous force of the form F(t)=F₀sintωt or P(t)=m₀rω²sintωt.The linear viscoelastic behavior of the elastomer enables the obtainment of hysteresis curves of elliptical shape for each harmonic loading.In the end, the dynamic stiffness as well as the equivalent damping can be evaluated, with the mention that the stand testing individualizes the dynamic system only through viscous, elastic and inertial forces. Due to this reason, the 1^st order differential equation eliminates the definition of the critical damping ratio as for a second order system, which leads to the introduction of the concept of equivalent damping, originating in the hysteretic damping or the loss factor.

Abstract: The paper presents the experimental results regarding the hysteretic behavior of elastomeric systems made in conformity with the European Standards SR EN 1337-3 and SR EN 15129. It is also mentioned that the hysteretic damping was determined by experimentally raising the hysteresis loop in a low-cycle harmonic regime, with a kinematic exterior excitation defined by a harmonic displacement law. In this context, it is mentioned that the area of the hysteretic loops was determined instrumentally (in analogical representation of the signals) as well as under digital form, through a sampling of the physical signal, which allows a high precision of the determination.

Abstract: The results of the experimental tests, in dynamic regime, on antiseismic elastomeric devices, evidenced the fact that the internal damping represents a significant parameter tightly related to the specific shearing deformation of the elastomeric elements. Consequently, based on the final obtained results, correlation curves were raised meant to represent the variation of the equivalent critical damping ration (dynamic system of order I).

Abstract: The aim of the paper is to present the results of applying the formulae of a simplified stochastic model for the calibration of some macroscopic parameters of the ground motion, on the basis of rather rough estimates. For this purpose the basic records combinations results obtained with the aid of computer programs (DaisyLab and LABView) are used, after digital recordings on the large span and rather isolated structure of the ROMEXPO Pavilion in Bucharest were performed. A stochastic model of stationary, low intensity, ground motion (referred to in literature as microtremors or ambient vibrations) was proposed by H. Sandi (1982, 2005). This lay at the basis of the specification of input for a consistent analysis of 3D earthquake induced motion of structures, adopted in the Romanian earthquake resistant design codes.

Abstract: The paper presents the problem of vibration reduction in designed discrete mechanical systems. The passive vibration reduction based on the synthesis method by using the Synteza application. The presented application has been developed by performing the algorithmization of formulated and formalized synthesis methods provided by the authors.

Abstract: The proper operation and reliability of numerous mechanical devices depends on reducing the vibrations of embedded components, modeled as multi-degree-of-freedom dynamic systems. This can be achieved by using various types of absorbers. The design of such devices is conditioned by the understanding of the variation of their technical characteristics with the constructive parameters. The present paper is dedicated to a special type of dampers, based on the use of the magnetic phenomenon. However, available empirical formulae for the damping characteristics of magnetic dampers do not reveal all significant dependencies. Based on theoretical, numerical and experimental methods, the paper brings a number of contributions concerning the relation between the damping characteristics of a magnetic damper and some of its constructive parameters. Variation curves, useful both for theoretical studies and practical applications, are plotted.

Abstract: For the study and design of the elastomeric seismic devices is essential to know the mathematical relation between the horizontal displacement and the force leading it. In this paper we present mathematical models for three types of devices: (i) natural rubber bearings, (ii) lead rubber bearings and (iii) hybrid device combining the two first mentioned bearings. For all devices the specific domains of operation are determined and for each domain the relations connecting horizontal displacement and stiffness is contrived, highlighting the hysteretic behaviour in respect to ground excitation. Finally we present numerical results and a comparison between the three devices, defining the opportunity to involve them in specific applications, in function of the type and nature of the isolated structure.

Abstract: The paper presents the results of a study performed on a large ground motion database, containing records obtained during the three strongest earthquakes that occurred during the past four decades in the Vrancea seismogenic zone. In order to express strength demands imposed by these earthquakes, constant-ductility nonlinear acceleration spectra were computed for two sets of seismic records, selected as representative for narrow frequency band and broad frequency band ground motions, respectively. The spectra, determined for various types of bilinear hysteretic models, were normalized with respect to peak ground acceleration and mean values, as well as coefficients of variation, were computed for each analysis case. The sensitivity of spectral values to the variation of strength hardening and stiffness degradation parameters was determined, with reference to the elastic-perfectly plastic model. Conclusions were drawn, separately for the two distinct types of ground motion frequency content, on the significance of the considered hysteretic model parameters for the assessment of seismic strength demands.

Abstract: Strength reduction factors are an important component of the force modification factors used in the calculation of seismic code forces, as they account for the effect of the nonlinear hysteretic behavior of the structure. They are expressed as the ratio between the elastic strength demand and the inelastic strength demand induced in a structure by the earthquake. The evaluation of strength reduction factors has applications in the substantiation of seismic force modification factors in building design codes and in the direct application of the capacity spectrum method. The paper presents a study performed on two sets of accelerograms having different spectral content types, i.e. narrow frequency band records and broad frequency band records. Based on constantductility spectra of strength reduction factors, computed with 50% and 10% probability of exceedance, a unified analytic formula is developed, to describe the variation with ductility and vibration period of these factors. By modifying the values of the coefficients of the variables, the formula can be adapted for each type of spectral content. The formula is further refined, by determining a new set of coefficients, to also account for the influence of the predominant period of the ground motion in the case of narrow frequency band records.