Rocco Alaggio

Bio: Rocco Alaggio is an academic researcher from University of L'Aquila. The author has contributed to research in topics: Nonlinear system & Structural engineering. The author has an hindex of 17, co-authored 59 publications receiving 835 citations.

Non-linear oscillations of a four-degree-of-freedom model of a suspended cable under multiple internal resonance conditions

Abstract: A four-degree-of-freedom model able to capture the main phenomena which are likely to occur in the non-planar finite dynamics of an elastic suspended cable subjected to external forcings and support motions is developed from the continuum equations. It contains two in-plane and two out-of-plane components, one symmetric and one antisymmetric. An order two multiple time scales solution is obtained for the case of primary external resonance and three simultaneous internal resonances (cable at crossover frequency). Possible classes of steady state (planar/non-planar, unimodal/multimodal) regular motions of the system are identified, and their linearized stability analysis is performed. Some results are presented to highlight the role played by key perturbations on the onset of various classes, the rich behaviour of the system and the influence of some control parameters on the response.

Sensitivity-based operational mode shape normalisation: Application to a bridge

Abstract: Recently, an innovative sensitivity-based technique was introduced for the normalisation of operational mode shapes purely on a basis of output-only data The technique is based on the use of a controlled mass modification experiment and does not involve any analytical models Moreover, it allows to extend the applicability of many modal analysis based applications towards the domain of in-operation modal testing Previously, this method was successfully tested by means of experiments on various mechanical engineering structures The focus of this contribution is the validation of the sensitivity-based normalisation technique on a civil structure For this purpose, measurements were performed on a bridge

Experimental Investigation of the Nonlinear Response of a Hanging Cable. Part II: Global Analysis

Abstract: An experimental model of an elastic cable carrying eight concentrated masses and hanging at in-phase or out-of-phase vertically moving supports is considered. The system parameters are adjusted to approximately realize multiple 1:1 and 2:1 internal resonance conditions involving planar and nonplanar, symmetric and antisymmetric modes. Response measurements are made in various frequency ranges including meaningful external resonance conditions. A ‘local’ analysis of the system response is made on the basis of numerous amplitude-frequency and amplitude-forcing plots obtained in different ranges of the control parameter space. Attention is mainly devoted to the detection of the main features of the regular motions exhibited by the system, and to the analysis of the relevant phenomena of nonlinear modal interaction, competition, and local bifurcation between planar and nonplanar regular responses. The resulting picture appears very rich and varied.

Extension of Generalized Bouc-Wen Hysteresis Modeling of Wood Joints and Structural Systems

Abstract: A critical analysis of the hysteresis models of wooden connections has led to an alternative analytical formulation of the pinching phenomenon. Starting from the generalized Bouc-Wen model,...

Sensitivity analysis of subspace-based damage indicators under changes in ambient excitation covariance, severity and location of damage

Abstract: It is common practice in Structural Health Monitoring (SHM) to perform damage detection by detecting changes in subspace-based damage indicators. They are computed by comparing a reference state with the current (possibly damaged) state through a matrix based on the computation of residuals originated from the orthogonality defect of projected subspaces. In the last decades, several scalar indicators have been proposed based on the residual matrix. Still, a multivariate sensitivity analysis was not carried out to estimate the sensitivity of each indicator to the excitation changes. In the current paper, a covariance-based sensitivity analysis applied to a spatial truss structure is carried out to evaluate the sensitivity of eight damage indicators to ambient excitation covariance, damage severity, and damage location. The analysis aims at identifying the ranges of applicability of damage indicators within a simple statistical framework, possibly extendable to more general applications. It is argued whether any equivalent class of indicators can be obtained from the manipulation of the residual matrix. The role of damage location in the probabilistic assessment of damage is discussed: a methodological Bayesian approach, based on the results of the sensitivity analysis, is proposed for the development of a structural reliability analysis driven by damage indicators.

The Method of Proper Orthogonal Decomposition for Dynamical Characterization and Order Reduction of Mechanical Systems: An Overview

Abstract: Modal analysis is used extensively for understanding the dynamic behavior of structures. However, a major concern for structural dynamicists is that its validity is limited to linear structures. New developments have been proposed in order to examine nonlinear systems, among which the theory based on nonlinear normal modes is indubitably the most appealing. In this paper, a different approach is adopted, and proper orthogonal decomposition is considered. The modes extracted from the decomposition may serve two purposes, namely order reduction by projecting high-dimensional data into a lower-dimensional space and feature extraction by revealing relevant but unexpected structure hidden in the data. The utility of the method for dynamic characterization and order reduction of linear and nonlinear mechanical systems is demonstrated in this study.

System Identification Methods for (Operational) Modal Analysis: Review and Comparison

Abstract: Operational modal analysis deals with the estimation of modal parameters from vibration data obtained in operational rather than laboratory conditions. This paper extensively reviews operational modal analysis approaches and related system identification methods. First, the mathematical models employed in identification are related to the equations of motion, and their modal structure is revealed. Then, strategies that are common to the vast majority of identification algorithms are discussed before detailing some powerful algorithms. The extraction and validation of modal parameter estimates and their uncertainties from the identified system models is discussed as well. Finally, different modal analysis approaches and algorithms are compared in an extensive Monte Carlo simulation study.

Reference-based combined deterministic-stochastic subspace identification for experimental and operational modal analysis

Abstract: In this paper, the use of the combined deterministic-stochastic subspace identification algorithm for the experimental modal analysis of mechanical structures is discussed. The algorithm requires artificial forces to be applied to the structure, so it can be used for experimental modal analysis (EMA). The algorithm can also be used for operational modal analysis (OMA), since the excitation level of the artificial force(s) can be low compared to the excitation level of the ambient forces. Both the modes that are artificially excited and those that are excited by the ambient forces are identified. This type of operational modal analysis is called an OMAX analysis (Operational Modal Analysis with eXogenous inputs) [1]. The main advantages of OMAX over OMA are that the modes that are excited by the artificial forces can be scaled to unity modal mass and that a higher number of modes can be identified.

Reference-based combined deterministic–stochastic subspace identification for experimental and operational modal analysis

Abstract: In this paper, the use of the combined deterministic-stochastic subspace identification algorithm for the experimental modal analysis of mechanical structures is discussed The algorithm requires artificial forces to be applied to the structure, so it can be used for experimental modal analysis (EMA) The algorithm can also be used for operational modal analysis (OMA), since the excitation level of the artificial force(s) can be low compared to the excitation level of the ambient forces Both the modes that are artificially excited and those that are excited by the ambient forces are identified This type of operational modal analysis is called an OMAX analysis (Operational Modal Analysis with eXogenous inputs) [1] The main advantages of OMAX over OMA are that the modes that are excited by the artificial forces can be scaled to unity modal mass and that a higher number of modes can be identified

Closure of "Rocking of Rigid Blocks Due to Harmonic Shaking"

Abstract: The dynamic behavior of rigid-block structures resting on a rigid foundation subjected to horizontal harmonic excitation is examined. For slender structures, the nonlinear equation of motion is approximated by a piecewise linear equation. Using this approximation for an initially quiescent structure, safe or no-toppling and unsafe regions are identified in an excitation amplitude versus excitation frequency plane. Furthermore, several possible modes of steady-state response are detected, and analytical procedures are developed for determining the amplitudes of the predominant modes and for performing stability analyses. It is shown that the produced stability diagrams can be beneficial to assessing the toppling potential of a rigid-block structure under a given amplitude-frequency combination of harmonic excitation; in this manner the integration of the equation of motion is circumvented.