PSI - Issue 78

Riccardo Vetturini et al. / Procedia Structural Integrity 78 (2026) 734–744

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The structure has a square footprint, with a side length of approximately 4.00 m. The columns rise to a height of 3.00 m from the base to the top of the capitals, from which the circular arches spring with an intrados radius of about 1.75 m. The tiburium above the four arches reaches a height of 5.65 m, on which the hemispherical dome rests, giving the architectural complex a maximum height of almost 8.00 m. Internally, the hemispherical vault is connected to the arches by pendentive vaults, also made of stone. With regard to construction principles, it should be noted that the structural system of the Ciborium, especially the columns, is an assembly of stone elements presumably joined together with metal pins and lead infill, according to the construction methods of the time, as shown in Figure 1. 3. Dynamic characterization In-depth experimental investigations were conducted by the Department of Civil and Environmental Engineering of the University of Perugia — Scientific Coordinator Prof. Filippo Ubertini - working group: Eng. Nicola Cavalagli and Eng. Enriche Garcia-Macias. These investigations focused on the dynamic characterization of the Ciborium of Santa Maria Maggiore in Spello, with related numerical analyses aimed at identifying the frequencies corresponding to its first vibration modes and their modal shapes. An initial dynamic analysis was conducted by recording the accelerations of the monument induced by ambient microtremors, which allowed the following natural frequencies and corresponding vibration periods to be identified:

Tab. 1. Excerpt from UNIPG Report: Identified vibration modes

Mode

Frequency

Period

Description

[Hz] 2.13 2.44 3.52

[s]

1 2 3

0.47 0.41 0.28

traslazionale “x” traslazionale “y”

rotazionale

Fig. 2. extract from UNIPG Report – First three vibration modes and corresponding shapes and frequencies

In order to evaluate the dynamic response of the structure for larger amplitude oscillations, a series of tests were carried out by subjecting the structure to free oscillations through several release tests, that confirmed the disarticulation of the system under even modest horizontal actions. The test consisted of applying an increasing horizontal force at the height of the springing of the arches (at the top of the columns) by means of a rope manually pulled. The purpose of the test was to verify possible nonlinear effects and to analyze the free oscillation motion, both in terms of vibration frequencies and displacement amplitudes. The maximum horizontal forces applied during these pulling tests were in the range of 50–100 kg, applied horizontally at the top of the columns. Based on the measurements performed, it was observed that there is a clear increase in the natural period of the structure under larger amplitude free oscillations, linked to a rocking motion between the elements constituting the monumental Ciborium, particularly at the columns.