Issue 46

S. Ivorra et alii, Frattura ed Integrità Strutturale, 46 (2018) 203-215; DOI: 10.3221/IGF-ESIS.46.19 204 I NTRODUCTION he preservation of the cultural heritage is a topic of great importance for our societies. Thus, several researchers have focused their attention on the techniques, which allow the definition of a refined description of the behavior of such structures with the aim of evaluating their vulnerability and/or designing retrofitting interventions [1–5]. Among the typologies which are wide spread in the Italian building patrimony, the towers are the ones characterized by a high slenderness and, consequently, may be more vulnerable to horizontal forces like the ones induced by wind, earthquake or bells’ swinging. Due to this characteristic, these structures may be investigated by means of in-situ dynamic tests, which have the advantage of allowing a description of the global behavior. In situ tests are compatible with almost all geometrical characteristics of the examined tower, and also with almost all the service conditions [6–8]. Indeed, the use of “classic” testing techniques cannot be adopted because they provide only “local” information, which cannot be sufficient for the definition of a reliable numerical model, the crucial goal for the evaluation of the seismic vulnerability and safe level of the examined structure. As historical material, reinforced concrete (RC) has more than one century of life and many singular structures have been designated with this structural material. Many studies have been carried out adopting the aforementioned dynamic experimental techniques but almost all the examined cases deal with the evaluation of the modal parameters of masonry towers [9–11]. Many of these researches use the results of the modal parameters’ identification to evaluate the unknown mechanical characteristics. This aim is obtained by updating a numerical model of the examined tower in order to match the experimental results. Almost all the cases described in the technical literature regard masonry structures, while few studies refer to structures realized with different structural materials. In [9] a tower realized in cyclopic concrete is examined, moreover [8] deals with the dynamic identification of a tower whose walls are made of a concrete core covered with stone blocks. However, by the Authors’ knowledge, in the literature no studies regard towers with concrete moment resisting frames under dynamic loads generated by bells with a high level of accelerations. On such basis the here examined bell tower has been chosen because it is characterized by a concrete frame. Moreover, during the bells’ swinging the bell tower shows high amplitude vibrations. Some authors studied the effect of these vibrations on the interaction with the bell tower such as in [12-14]. T HE SS. M EDICI BELL TOWER he bell tower was realized later with respect to the construction of the Pontifical Basilica of the SS. Medici, the latter started in 1960. Bishop Marena entrusted the task of designing the great panoramic bell tower to the bitontine architect Prof. Antonio Scivittaro. On 23 February 1974, Msgr. Aurelio Marena blessed the first stone of the panoramic bell tower in the presence of civilian and military authorities, and a multitude of devotees [15]. The bell tower is a framed reinforced concrete structure, with masonry walls made with gasbeton blocks and covered with rectangular bricks (23x12x5 cm) of red color. It is about 60 meters tall and consists of eight floors with a quadrangular base, morphologically it is a pyramidal trunk. Inside, a large staircase of 250 stone steps, which leads to the bell tower, is situated in the East side and an elevator shaft is located in the West side. The tower has a quadrangular cross section oriented to the N-S direction. In each side, near to the corner, two variable section columns support the tower. These RC elements have a cross section of 0.6 m x 1.7 m at the base, while at the seventh floor the cross section reduces to 0.3 m x 0.5 m (Fig. 1). The main access is situated in the North side. In 1990, the top of the tower was refined by a clock tower from Trebino Company in Treviso, placed here by gift from the town of Bitonto. There is a Belvedere floor at 36.5 m, realized by a reinforced concrete terrace, from which it is possible to enjoy an unlimited panorama, including the Adriatic Sea. Eight bells are situated in two different floors, the four bigger bells are at 44 m (Fig. 1 – right). They were cast by the Fondelli Marinelli of Agnone in Molise Region. These bells are enriched with original and magnificent bas-reliefs that decorate their surfaces. Tab. 1 shows the main characteristics of these four bigger bells, and Fig. 2 shows in detail the position and direction of swinging of each bell, all in the N-S direction. From November 1974 the bells started to swing, spreading their song along all the town. T T