Lecture on May 13, at the IOMAC reception, Admiral Hotel
Carmelo Gentile, Professor, Politecnico di Milano, Italy:
OMA-based structural health monitoring of historic structures
Structural Health Monitoring (SHM) is generally defined as a multi-disciplinary process involving: (a) the repeated or continuous measurement of the response of a structural system through arrays of appropriate sensors; (b) the extraction from measured data of features, which are representative of the health condition and (c) the statistical analysis of these features to detect any novelty or abnormal change in the investigated system. Among the different SHM strategies, the fully non-destructive nature and the minimum impact of the vibration monitoring makes the OMA-based approach especially suitable to address the preservation of Cultural Heritage structures and also to avoid, in some cases, inappropriate strengthening interventions. The main ideas of OMA-based SHM of historic structures are presented and exemplified through the application to ancient towers and complex historic buildings.
For ancient towers, exhibiting cantilever-like dynamic behavior, few accelerometers installed in the upper part of the building allow a successful monitoring of the dynamic characteristics and, most important, the assessment of the effects of changing temperature on natural frequencies and the detection of small damages from frequency shifts. Subsequently, the focus is moved to the vibration monitoring of complex Cultural Heritage buildings, involving the installation of a large number of sensors and more complex SHM strategies. The main expectations are exemplified by using data collected in the first months of continuous monitoring of the Milan Cathedral.
Lecture on May 14, at the conference dinner, Langelinie Pavillonen
Svend Gade, M. Sc., former Application Specialist at Brüel & Kjær, Denmark:
35 Years of Modal Analysis
The modern era of structural measurements began in the early 1980s. The first international modal analysis conference IMAC I in Orlando, Florida, USA was held in 1982. At that time, modal analysis changed from being a pure scientific concept to be a practical engineering tool. Many tests were then limited to single-input/single-output (SISO) measurements. In 1983 the first dual channel signal analyser was released by Brüel & Kjær. Today dual-channel FFT analysis is the established method of performing frequency response function measurements used for what we might call classical mobility based modal testing and analysis. Over the years, progress and development in all fields of the measurement chain has occurred. Developments include the input vibration transducers via signal conditioning to the analogue to digital conversion, real-time and post-processing of functions followed by modal parameter estimation and applications of the modal model. This paper deals with some of the most important steps achieved in this progress. Today it is not unusual that modal analysis is performed using several hundred high dynamic range measurement channels connected to dedicated smart modal accelerometers. These have built-in transducer electronic data sheets (TEDS) doing multiple-input/multiple output (MIMO) measurements or output only measurements with automated modal parameter estimation and model correlation with large finite-element models.