Dynamic monitoring and model updating of a masonry bell tower in Pisa

Since the methods used to identify natural frequencies of existing structures have different resolution capabilities, depending on the algorithms adopted for the dynamic analysis as well as on the type of the dynamic forces used to excite the structure, an appropriate dynamic characterization procedure should combine several signal processing techniques. In the paper, a suitable natural frequencies identification methodology is illustrated in the framework of the dynamic identification of the bell-tower of St. Niccola’s church in Pisa, where classical methods, based on Fourier transform, are combined with more modern methods, based on wavelet transform, integrated by sophisticated finite element investigations as well. The proposed methodology allows to identify the field of application and the soundness of each signal processing technique, achieving an accurate estimation of the main parameters governing the dynamic structural response, especially when dynamic time histories are characterised by limited amplitude and/or are strongly contaminated by noise. 660 Structural Analysis of Historical Constructions This combined methodology, corroborated by experimental results, not only allows to verify the reliability and the field of application of each signal processing technique, but also to attain a good estimation for the main geometric, physics and mechanics parameters of the structure, governing its dynamic behaviour. Figure 1 : The bell tower of the St. Niccola’s church in Pisa. 2 THE BELL TOWER OF THE ST. NICCOLA’S CHURCH IN PISA The bell-tower of St. Niccola’s church in Pisa, considered a masterpiece of the Romanesque art, is characterized by a particularly complex architectural organization. In fact, the cross section, circular at the base, becomes octagonal to higher orders and finally hexagonal at level of the bell cell. The bell tower, standing to the left of the façade of the church, appears quite well preserved and doesn't show evident cracking. It is characterized by an inclination of about 1°13' with respect to the vertical line, caused by sinking of the ground, so that its maximum overhanging, referred to the road level, is about 100 cm (Fig. 2). The dating and the attribution of the bell tower are not certain and different hypotheses have been proposed. Vasari attributes the construction to Nicola Pisano, but recently several different hypotheses have been proposed, for example by Ragghianti, that attributes it to Diotisalvi (1170) and by Nannicini-Canale and Testi-Cristiani, that dates it between 1230 and 1250, while Frey assumes that it was built in two or three constructive phases, the first one starting around 1173 and the following ones between 1230 and 1250. Certainly, the bell tower, which was originally isolated, has been included in the adjacent convent of St.Augustin’s Fathers in 1295, during the extension works of the convent. Inside the bell tower there is a refined staircase in stone, supported by a helical gallery of elegant rampant arches, sustained by columns (see Fig. 3). Above the gallery, articulated in two fornices on each side of the octagon, the masonry is realized with stones perfectly trimmed, flattened and prepared in precise rows without joints. Each side is decorated with blind arches, sustained by angular pentagonal pilasters and containing deep lozenges, sometimes passing through. In the hexagon of the bell cell, built with rectangular cut sandstone, flattened and preM.L. Beconcini, P. Croce and M. Mengozzi 661 cisely lined up, a single lancet window for the bell is present in each side. Each lancet window is surmounted by an embedded circular arch and by flat angular pillars ending with three small circular arches. The inner structure is built with sandstone, the pyramidal hexagonal based roof is realized with masonry of full bricks, while the columns are realised with Apuan marble and Elba granite. Figure 2 : Bell tower cross section – (a) circular, (b) octagonal, (c) exagonal.