Giuseppe Abbiati

Vibration-Based Monitoring and Diagnosis of Cultural Heritage: A Methodological Discussion in Three Examples

ABSTRACT Modern monitoring techniques contribute to accurately describing the structural health conditions of historical buildings and to optimizing the plan of maintenance as well as the restoring intervention. Particularly, dynamic testing gives knowledge about global structural behavior and can be used to calibrate numerical models and to predict the response to dynamic and earthquake loading. In some circumstances, vibration-based monitoring can also help in evaluating safety conditions. The present article proposes a discussion about the methodological multidisciplinary approach to modal testing when applied to architectural heritage buildings and structures, along with the description of selected case studies. These examples were chosen to cover the various issues connected to test design and interpretation.

Identification, Model Updating, and Validation of a Steel Twin Deck Curved Cable-Stayed Footbridge

To perform a realistic reliability analysis of a complex cable-stayed steel footbridge subject to natural hazard and corrosion, this article addresses a rational process of modeling and simulation based on identification, model updating, and validation. In particular, the object of this study is the Ponte del Mare footbridge located in Pescara, Italy; this bridge was selected as being a complex twin deck curved footbridge because it is prone to corrosion by the aggressive marine environment. With the modeling and simulation objectives in mind, a preliminary finite element (FE) model was realized using the ANSYS software. However, uncertainties in FE modeling and changes during its construction suggested the use of experimental system identification. As a result, the sensor location was supported by a preliminary FE model of the footbridge, although to discriminate close modes of the footbridge and locate identification sensor layouts, Auto Modal Assurance Criterion (AutoMAC) values and stabilization diagram techniques were adopted. Modal characteristics of the footbridge were extracted from signals produced by ambient vibration via the stochastic subspace identification (SSI) algorithm, although similar quantities were identified with free-decay signals produced by impulse excitation using the ERA algorithm. All these procedures were implemented in the Structural Dynamic Identification Toolbox (SDIT) code developed in a MATLAB environment. The discrepancies between analytical and experimental frequencies led to a first update of the FE model based on Powells dog-leg method that relied on a trust-region approach. As a result, the identified FE model was capable of reproducing the response of the footbridge subject to realistic gravity and wind load conditions. Finally, the FE was further updated in the modal domain, by changing both the stationary aerodynamic coefficients and the flutter derivatives of deck sections to take into account the effects of the curved deck layout

Time Domain Identification of Structures: Comparative Analysis of Output-Only Methods

AbstractThe focus of this work is on methods for modal identification of civil structures using output data only. An important family of time domain methods uses autoregressive time series models and exploits formulations developed in the field of system control. Another strategy consists of using methods well tested in the identification of structures on the basis of impulse response or free decay and extending them to the analysis of response signals generated by excitations of a more general nature. A third option refers to the Ho-Kalman minimal realization algorithm, which was extended by Akaike and Aoki to stochastic systems. These approaches, or their combinations, include a sizable proportion of the methods actually used in output-only identification of civil structures subjected to natural excitation, and most of them are based on stationarity assumptions. The question that prompted this study was as follows: what degrees of reliability and accuracy can such methods ensure when they are used, as i...

Dynamic Characterization of Complex Masonry Structures: The Sanctuary of Vicoforte

Forecasting the response to dynamic and earthquake loading, for safety assessment as well as for control purposes, may benefit from vibration measurements, which are indispensable to update a mathematical model. The viability of ambient vibration testing for complex masonry structures is demonstrated here in a relevant case study of the Sanctuary of Vicoforte, near Mondovì, Italy. This monument is of great architectural significance, characterized by the largest elliptical dome in the world (axes 37.15 m × 24.80 m). Conceived in 1596 by Duke Carlo Emanuele I to serve as the mausoleum of the Savoy dynasty, the monument was affected since the earliest stages of its construction by the settlements of foundations, resulting in the opening of extensive cracks. In 1983, its dome-drum system was strengthened for fear of an imminent collapse. Since 2001 a research project has been conducted to evaluate the state of conservation and structural safety of the building. After a short description of the activities carried out to determine the structural conditions of the building, this study illustrates the salient stages of the dynamic characterization.

Pseudo-Dynamic Testing Based on Non-linear Dynamic Substructuring of a Reinforced Concrete Bridge

The assessment of the seismic performance of an old concrete bridge was conceived within the RETRO Transnational Activity funded by the SERIES research project. The installation of isolation devices -one per column- for each pier portal frame interposed between the cap beam and the deck was proposed as seismic retrofit, fully complying with Eurocode 8 requirements. A comprehensive set of hybrid simulations was conceived to simulate the dynamic response of the existing 400 m span Rio Torto Viaduct, both in the isolated and the non-isolated cases. In order to support the pseudo-dynamic test design, a refined fiber based FE model of the bridge was implemented in the well-known OpenSEES software; time history analyses highlighted appreciable nonlinearities in the dynamic response of the system under the Serviceability Limit State (SLS). As a consequence, a Numerical Substructure (NS) capable of reproducing such nonlinear behaviour was deemed necessary. Nonetheless, implementation issues relevant to the typical solving time dictated by the controller time step, made the use of complex fiber based FE models not suitable for testing purposes. In this perspective, the paper presents the design of a suitable NS, which is based on a rational and nonlinear reduction of the aforementioned OpenSEES Reference Model (RM). Moreover, in order to impose a consistent degradation of physical and numerical piers, a testing procedure based on recursive identification and model updating sessions is proposed. The numerical tools and simulations developed in the project are presented and discussed.

Assessment of the Seismic Behaviour of a Retrofitted Old R.C. Highway Bridge Through PsD Testing

The RETRO project aims at studying the seismic behaviour of existing reinforced concrete (RC) bridges and the effectiveness of innovative retrofitting systems. A typical as-built RC highway viaduct, designed primarily for gravity loads, has been analysed experimentally. The objective of the laboratory test program is twofold: (1) improve the knowledge of the non-linear behaviour of RC framed piers without seismic detailing and (2) assess the effectiveness of seismic isolation systems as a structural mitigation measure. Two large scale framed piers with two and three levels were re-designed and tested using the PsD method with hybrid (analytical and experimental) simulation. Two test configurations were considered: (1) the as-built bridge layout and (2) the viaduct retrofitted by means of Friction Pendulm (FP) isolators. The seismic performance evaluation was carried out at two limit states, i.e. at serviceability and ultimate limit states. The experimental tests stressed the high structural vulnerability of the viaduct, confirmed by the extensive shear damage in the transverse beams and the fix-end rotation effects generated by the bond slip of plain steel bars in the columns embedded in the foundation. The effectiveness of the isolation system with FP devices was also demonstrated. Nevertheless, the friction of the FP devices is a critical response parameter which may vary significantly because of the pressure due to vertical loads, strong motion velocity and temperature. The reliable evaluation of the friction parameter is of paramount importance to prevent the onset of damage within the framed piers.

Refined and Simplified Numerical Models of an Isolated Old Highway Bridge for PsD Testing

RETRO’ project aims at studying the seismic behaviour of existing reinforced concrete (RC) bridges and the effectiveness of retrofitting systems based on seismic isolation of the deck of the viaduct. The research program focuses on a typical non-compliant bridge system for earthquake loading, designed for gravity loads only. The prototype structure is the Rio Torto bridge system, which is located in a region of medium seismic hazard in Italy. The seismic vulnerability of the as-built framed pier bridge is first assessed. A typical seismic isolation system, employing slide spherical bearings, is then designed as a passive control retrofitting measure. The present chapter discusses the non-linear response of the Rio-Torto viaduct in the “as-built” and “isolated” configurations. The seismic performance assessment is carried out by utilizing refined non-linear structural models implemented in an advanced and reliable computer platform. The earthquake behaviour of refined models used for the sample structure accounts for non-linear phenomena of the viaduct, e.g. strain penetration of plain bars, shear deformation of transverse beams, flexural deformations in columns and beams. The finite element models are calibrated on the basis of experimental tests results. The assessment of the seismic response system is investigated in terms of local and global response parameters. In addition, the effectiveness of the isolation systems used as a retrofitting system is also investigated numerically. The outcomes of the comprehensive nonlinear analyses are used to simulate the seismic response of the viaduct in the as-built and isolated configurations during Pseudo-dynamic testing, which is illustrated in a companion chapter.

Pseudo-Dynamic Heterogeneous Testing With Dynamic Substructuring of a Piping System Under Earthquake Loading

In recent years, both pseudo-dynamic and real time heterogeneous testing with dynamic substructuring — hybrid testing — have gained significant popularity for their applicability to testing several types of nonlinear structures/systems. In a hybrid test, a heterogeneous model of the emulated system is created by combining a Physical Substructure (PS) with a Numerical Substructure (NS) that describes the remainder of the system. Nevertheless, an efficient implementation of this technique requires overcoming certain problems, e. g., proper dynamic substructuring, reduction of external forces and actuator delay compensation. This paper presents a pseudo-dynamic test campaign undertaken by the University of Trento, Italy, on a typical full-scale industrial piping system subjected to earthquake loading in order to investigate its seismic performance. Some challenges faced during the implementation are shown and strategies adopted to overcome these problems are illustrated. Experimental activities will be described and performances of different components of the piping system, i.e., elbows, tee-joints, bolted flange joints and straight pipes under earthquake loading with the presence of an internal pressure of 3.2 MPa will be presented and commented.Copyright

A Procedure for the Approximated Response History Analysis of Linear Thermoelastic Structures

ABSTRACT This article presents a procedure for the approximated response history analysis of linear thermoelastic structures subjected to pure thermal loading based on eigenvalue analysis. The underlying assumption is that the mechanical system response does not affect heat transfer. Typically, the mechanical response of a structure subjected to pure thermal loading is such that inertia forces can be neglected but there are instances, however, where this is not the case. An approach to make this determination is also presented for a generic pair of retained mechanical and thermal modal coordinates and then extended to the multiple degree-of-freedom case.

Hybrid Simulations of a Piping System Based on Model Reduction Techniques

The deficiency of seismic design standards for piping systems, their components and support structures necessitates experimental investigations of such structures under earthquake loading to extract valuable information for the amendments/development of relevant design guidelines. This paper describes an experimental test campaign carried out at the University of Trento, Italy, on a full-scale piping system in order to evaluate its seismic performance. In particular, a typical industrial piping system containing several critical components, such as elbows, a bolted flange joint and a Tee joint, was tested under different levels of earthquake loading corresponding to serviceability and ultimate limit states suggested by performance-based earthquake engineering standards. Hybrid Simulations with Dynamic Substructuring (HSDS) in both pseudo- and real-time were adopted to conduct seismic tests. Experimental results displayed a favorable performance of the piping system and its components; they remained below their yield limits without any leakage even for the Collapse Limit State. As a result, the proposed model reduction techniques were fully justified.