MS22.1 - Vibration-Based Assessment and SHM of Cultural Heritage Structure

Structural Health Monitoring (SHM) is nowadays common in many branches of engineering since it allows to have a continuous or periodic report of the structural conditions and therefore to intervene promptly if there are incipient damages. The first step to perform a SHM is the identification of the dynamic parameters, i.e. natural frequencies, damping ratios and modal shapes, and it is a crucial step since a modification of the structural parameters can be a direct consequence of structural damages. Among the structural identification methods, Operational Modal Analysis (OMA) methods have received increasing attention from the researchers since they do not require the knowledge of the structural excitation that is due to ambient vibrations and that is usually modeled as a white noise. This aspect makes this kind of methods cheaper and simpler than the classical Experimental Modal Analysis (EMA) methods. In this paper an innovative OMA method is proposed. It is a semi – automated method that allows to identify natural frequencies, damping ratios and modal shapes of a structural system and that can be used also from users that have not knowledge in stochastic dynamics and signal analysis. Specifically, first of all the modal shapes are estimated through the use of signal filtering techniques applied on the stochastic properties of the output process and then natural frequencies and damping ratios can be estimated from the mono – component analytical signals obtained by performing a decomposition of the analytical signals matrix. To assess the reliability of the proposed method several numerical simulations and an experimental test are reported also considering a comparison with the most popular OMA methods.

The conservation of reinforced concrete modern heritage structures requires careful consideration of multidisciplinary aspects. Among these, the structural safety evaluation plays a key role, and the assessment of the residual structural performances shall rely on a thorough knowledge of the current status of the structure [1]. A useful evaluation technique to achieve this makes use of the dynamic structural characterization, identification and monitoring of the structure that extract information on the actual structural behavior from the data acquired during vibration tests. Within this context, this study deals with the dynamic characterization of the Palazzetto dello Sport in Rome, one the most celebrated architecture by Pier Luigi Nervi [2]. Listed since 2021, the Palazzetto was conceived together with the architect Annibale Vitellozzi, and it was built in 1957 as part of the Nervi’s facilities built for the Olympic Games in 1960. The study is based on an experimental dynamic testing campaign recently carried out thanks to a research agreement with the Department for Sports and Youth Policies of Rome Municipality. The building is characterized by a reinforced concrete structure consisting of a shallow spherical dome with radius of 29.5 supported by 36 Y-shaped radially arranged trestles, inclined according to the tangent at the dome lower edge. Based on archive drawings and documents as well as a laser-scanner survey, an accurate initial 3D numerical model of the structure is developed. From the experimental data the modal identification is carried out through an Operational Modal Analysis (OMA) approach. The latter is tailored to cope with the peculiar dynamics of the dome, realised by Nervi as a ribbed spherical cap through a patented prefabrication process assuring the axial symmetry of the whole structure. In particular, high modal density is observed together with the dynamic features associated to the axial symmetry. Among them, the presence of non-axisymmetric modes with frequencies identical to the axisymmetric ones. In this case the degeneracy of the eigenfunctions corresponding to the spherical shell modes implies that the superposition of axisymmetric modes of identical frequency, but different orientations of axes, gives rise to new modes which are not necessarily symmetric with respect to any axes [3]. An ad-hoc FE model updating procedure including experimental modal frequencies and shapes is implemented. By resorting to pseudo-experimental response, the repeated modes analysis is performed by applying the space-MAC criterion validation. [1] Paolo Di Re, E. Lofrano, Jacopo Ciambella, and Francesco Romeo. Structural analysis and health monitoring of twentieth-century cultural heritage: the flaminio stadium in Rome. Smart Structures and Systems, 27:285–303, 02 2021. [2] Jae Hoon Kang. Vibrations of deep and shallow hemi-spheroidal domes with non-uniform thickness having a top cut-out. JVC/Journal Vib. Control, 22(11):2671–2686, 2016. [3] Erica Lenticchia, Rosario Ceravolo, and Cristiana Chiorino. Damage scenario-driven strategies for the seismic monitoring of XX century spatial structures with application to Pier Luigi Nervi’s Turin Exhibition Centre. Eng. Struct., 137:256–267, apr 2017.

The Brivio bridge, completed in 1917, consists of three reinforced concrete (RC) tied arches spanning 44.0 m each. The bridge crosses the Adda river on the route between Lecco and Bergamo and still represents a crucial node for the vehicular traffic of the region network. Hence, in the context of a research promoted by the Lombardy Region, the bridge has been equipped with a dynamic monitoring system, consisting of 8 seismometers per span. The response to operational excitations is collected at a sampling frequency of 100 Hz, with datasets of 3600s being created every hour for automated processing. The paper describes the dynamic monitoring system installed in the bridge and selected results obtained during the first two years of continuous monitoring. Several vibration modes are identified for each span, with the temperature significantly affecting the natural frequencies, whereas no remarkable changes of mode shapes are detected so far. The possible onset of structural changes is identified by through structural pattern recognition driven by both the changes in mode shapes and the cleansed natural frequencies. It is further noticed that two adjacent spans of the bridge exhibit a common mode, whose presence is determined by the interaction between the main girders of the two spans.