Sara Sgobba

A Seismologically Consistent Husid Envelope Function for the Stochastic Simulation of Earthquake Ground-Motions

Earthquake-induced ground-motion may be realistically described as random processes that are intrinsically non-stationary in both amplitude and frequency content. In order to take into account the finite duration and the amplitude non-stationarity of earthquake-induced ground-motions, it is common practice to modulate a stationary stochastic process with a shaping window, or envelope function, in order to obtain a transient signal. Different shapes have been proposed in literature for such windows but the main problem until now has been how to correlate the parameters of the shaping window to the characteristics of some seismic design scenario (such as magnitude, distance, and site conditions). In this work, an envelope function is proposed that makes use of parameters that are commonly available in seismic design situations involving a scenario-type analysis. Such a scenario may be deterministically prescribed, or may be the result of a probabilistic seismic hazard analysis. The envelope function is also directly related to the Arias intensity of the ground motion and has a functional form closely related to that of a lognormal probability density function. The envelope function may be used, in conjunction with suitable peak factors, to predict the distribution of peak ground acceleration values corresponding to a given earthquake scenario.

Optimal Design Criteria for Isolation Devices in Vibration Control

Vibration control and mitigation is an open issue in many engineering applications. Passive strategies was widely studied and applied in many contests, such as automotive, aerospatial, seismic and similar. One open question is how to choose opportunely devices parameters to optimize performances in vibration control. In case of isolators, whose the main scope is decoupling structural elements from the vibrating support, optimal parameters must satisfy both vibration reduction and displacement limitation. This paper is focused on the a multi-objective optimization criterion for linear viscous-elastic isolation devices, utilised for decreasing high vibration levels induced in mechanical and structural systems, by random loads. In engineering applications base isolator devices are adopted for reducing the acceleration level in the protected system and, consequently, the related damage and the failure probability in acceleration sensitive contents and equipment. However, since these devices act by absorbing a fraction of input energy, they can be subjected to excessive displacements, which can be unacceptable for real applications. Consequently, the mechanical characteristics of these devices must be selected by means of an optimum design criterion in order to attain a better performance control. The proposed criterion for the optimum design of the mechanical characteristics of the vibration control device is the minimization of a bi-dimensional objective function, which collects two antithetic measures: the first is the index of device efficiency in reducing the vibration level, whereas the second is related to system failure, here associated, as in common applications, to the first exceeding of a suitable response over a given admissible level. The multi-objective optimization will be carried out by means of a stochastic approach: in detail, the excitation acting at the support of the protected system will be assumed to be a stationary stochastic coloured process. The design variables of optimization problem, collected in the design vector (DV), are the device frequency and the damping ratio. As cases of study, two different problems will be analysed: the base isolation of a rigid mass and the tuned mass damper positioned on a MDoF structural system, subject to a base acceleration. 20