PSI - Issue 78

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

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The essential prerequisite for such a strategy to be effective is that the period of the seismically isolated structure is equal to or greater than three times the period of the structure in its current condition. In other words, to achieve the intended decoupling, the structure (Ciborium) must be “sufficiently” stiff. The Ciborium in its pre-intervention condition was a “disarticulated” structure with strongly nonlinear dynamic behavior due to the “rocking” observed at the base of the cylindrical column. In other words, the stiffness of the system decreases (and the natural period increase) as the horizontal force grows. With increasing horizontal forces, the column rotates and lifts, its support becomes partialized and consequently the deformability of the system increases, up to the ultimate collapse limit. The natural period of the “disarticulated” system was likely particularly high and definitely higher than the limit value which would have rendered the seismic isolation strategy ineffective. On the other hand, making the system "monolithic"—where the columns act as single, base-fixed elements with a nearly "fixed-end" condition—would give the system sufficiently high stiffness. The “monolithic” system indeed had a frequency close to (or higher than) the frequency of 2.13 Hz recorded with microtremors, with a natural period less than 0.47 sec. In this way, consistent with how the monument was originally conceived, the seismic isolation strategy could be fully effective. By excluding or at least limiting invasive techniques such as drilling into decorated parts or disassembling/reassembling the system, the monolithic condition was achieved through a low-invasiveness approach, using epoxy adhesives placed between the contact surfaces of the various stone elements. In particular, the intervention was implemented in the weakest contact area, corresponding to the base of the cylindrical column. To carry out this operation, after securing and shoring up the Ciborium, a circumferential gap (no wider than a millimeter and at least 4–5 cm deep) was made between the cylindrical column and the quadrangular base, removing the thin layer of mortar present and pouring the liquid epoxy adhesive into the gap to achieve rigid bonding.

Fig. 4. Scheme of seismic isolation intervention at the Ciborium base