Danilo Gargaro

Integrated non-destructive assessment of relevant structural elements of an Italian heritage site: the Carthusian monastery of Trisulti

The analysis of historical structures in need of preservation and restoration interventions is a very complex task due to the large uncertainties in the characterization of structural properties and detailing in view of the structural response. Moreover, the predictive performance of numerical analyses and simulations depend on the availability of information about the constructional properties of the architectural complex, crack patterns and active degradation phenomena. In particular, local changes in material properties or damage due to past events (such as earthquakes) can affect individual structural elements. They can be hardly detected as a result of the maintenance interventions carried out over the centuries and the possibility to carry out limited or even no destructive investigations due to the historical relevance of the structure. Thus, non-destructive investigations play a fundamental role in the assessment of historical structures minimizing, at the same time, the invasiveness of interventions. The present paper deals with an explanatory case study concerning the structural investigations carried out in view of the seismic assessment of an Italian historical monument, the Carthusian monastery of Trisulti in Collepardo, erected in 1204 under Pope Innocenzo HI. The relevance of the case study is due to the application, in combination, of different NDT methods, such as sonic tests, and active and passive infrared thermography, in order to characterize relevant masonry elements. Moreover, an advanced system for the in-situ nondestructive vibration-based estimation of the tensile loads in ancient tie-rods is described and the main results obtained from its application for the characterization of the tie-rods of the cloister are presented.

Predicting the variability of natural frequencies and its causes by Second-Order Blind Identification

Structural aging, degradation phenomena, and damage due to hazardous events are common causes of failure in civil structures and infrastructures. The increasing need of extending the structure lifespan for sustainability and economic reasons motivated the rapid development of remote, fully automated structural health monitoring systems. Different approaches have been developed for damage detection based on the incoming data. Modal-based damage detection is probably one of the most popular procedures for structural health monitoring of civil structures, also thanks to the development of robust automated operational modal analysis algorithms in the last decade. However, the sensitivity of modal parameter estimates and the associated damage features to environmental and operational factors represents a significant drawback to the extensive application of this technology. Thus, effective damage detection cannot skip the preliminary compensation of the effect of those variables on modal properties. Different approaches to compensate the environmental influence on modal property estimates are reported in the literature. In this article, the use of Second-Order Blind Identification is proposed. It is applied to a number of case studies in order to validate its effectiveness in the presence of one or more environmental or operational variables. Results demonstrate that it can model the variability of natural frequency estimates in operational conditions and, above all, it can give a fundamental insight in determining the causes of such variability.

Vibration-Based Continuous Monitoring of Tensile Loads in Cables and Rods: System Development and Application

The estimation of axial load in prismatic members using flexural vibrations has been investigated for long. Several methods are available in the literature. They take advantage of experimental estimates of the parameters of flexural modes to solve an inverse problem and identify the axial loads in the presence of a number of assumptions and eventual additional unknowns. These methods represent a valuable solution for the estimation of the tensile force in operation. A common drawback is the need to periodically carry out a modal identification test, usually exciting the member by an impact hammer and manually processing the collected dataset. This makes these techniques suitable for periodic checks rather than continuous monitoring. Taking advantage of the most recent developments in the field of Operational Modal Analysis (OMA), an automated system for continuous monitoring of axial loads based on dynamic measurements has been developed.

Structural health monitoring of existing bridges in earthquake prone areas: Laboratory validation

Road infrastructures and particularly bridges can suffer structural damage due to earthquakes threatening the efficiency of the transportation network and the possibility to ensure prompt rescue operation. This particularly applies to existing bridges, most of which have been designed and built according to outdated codes. Structural Health Monitoring (SHM) systems can support the prompt assessment of bridges after seismic events. However, reliability of modal based damage detection currently depends on the accuracy of modal parameter estimates automatically obtained from the analysis of the operational response of the monitored structure, and on the capability of the measurement system to resolve low amplitude as well as strong motions, eventually associated to saturation of sensors. In the present paper, the performance of a modal-based SHM system for existing bridges in seismic areas is assessed by shaking table tests on a 1:3 scale single span bridge representative of existing highway bridges built in the 60s in Italy. Results show that hidden damage can be identified on a remote basis, thus demonstrating the interesting applicative perspectives of modal based SHM for fast assessment of existing bridges in the early earthquake aftershock. The resilience to earthquake shaking of the SHM system has been also assessed. Finally, specific data processing procedures for earthquake response data are tested and compared with the results of laboratory measurements.

Damage Detection of a Bridge Model After Simulated Ground Motion

Vibration-based damage detection techniques offer interesting opportunities for Structural Health Monitoring of strategic structures and infrastructures, such as bridges, and they foster the shift from time-based maintenance to condition-based maintenance. However, effective vibration-based damage detection requires accurate estimates of the modal parameters of the monitored structure automatically obtained from the analysis of the operational response of the structure.

Statistical tools for the characterization of environmental and operational factors in vibration-based SHM

In recent years the interest in permanent monitoring of civil structures has raised because of the needs of controlling the ageing of a huge number of existing infrastructures. The recent advances in sensing technologies and data processing have made the installation and operation of permanent monitoring systems more and more attractive. Vibration-based monitoring is based on the analysis of the evolution in time of damage features. A lot of these features are obtained from experimental estimates of the modal parameters. However, these estimates are usually influenced by environmental and operational factors. The variations they induce in the estimates may hide the small changes due to damage, so their influence has to be appropriately considered in practical applications.

Stand-alone NDT system for tensile force estimation in cables and tie rods

Non-destructive techniques represent a very relevant tool in many engineering fields and play a relevant role in the case of preservation of cultural heritage and historical structures. This is particularly true in those countries exposed to seismic risk and other relevant natural hazards. Health conditions of masonry constructions often depend on the interaction between curved structures, such as arches and vaults, and tie-rods installed to neutralize the related horizontal forces. In the present paper, key aspects related to the development of a stand-alone vibration-based monitoring system for tensile load estimation and monitoring in cables and tie-rods are presented in the light of the available technical literature. Sample experimental results obtained after the installation of the monitoring system on one of the cables of a sample arch steel roof are finally illustrated.