Geomatic 3D modeling of a statue (also) for structural analysis and risk evaluation: the example of San Giovannino Martelli in Florence

This work has been developed among the researches of a PhD thesis in Civil and Environmental Engineering and Architecture of the University of Udine in cooperation with the GECO Laboratory of the University of Florence. It focuses on the interaction between Geomatics and Structural Analysis, both applied to cultural heritage, and expressly to artefacts and structures in stone materials, like the case study of this paper, the marble statue called “San Giovannino Martelli” (Saint John the Baptist) conserved in Florence. At the beginning, some interesting examples of surveying and structural analyses on statues are reported, in order to remind the complementary tasks and requirements of geomatics and structural analysis. Current laser scanning systems can accurately survey the geometry of a statue or any cultural heritage artefact, essential to understand their structural behaviour and resilience capability. Afterwards, following the few Italian regulations in this field, the possible risks of museum goods are described: topics of this part are more familiar for structural engineers as object classification, seismic reactions, damage mechanisms, possible movements (adherent, slipping and oscillation), dynamic domains, anyway necessary steps to evaluate the risk and so to define eventual interventions. The artistic description of the statue, its debated attribution to Donatello or/and to Desiderio da Settignano and its history is later recalled, remembering that the surveying has been done for the idea to 3D print a replica and to place it in the original place. Having used a close range laser scanner, the obtained 3D model has an impressive geometrical Level of Detail (LoD), whose geometric features are explained in the paper, underlying that such extremely detailed mesh is directly given as output from the laser scanner software. The model simplifications by four decimation are therefore explained and also changes to geometry, like shifts on centre of the mass or barycentre with respect to the original one, are evaluated: since these are pretty null, all the models can be used for structural analysis. Software Scan-and-Solve, a Rhinoceros plug-in, has been employed for Finite Elements Method (FEM) analysis, considering the sole weight and also a horizontal force, as a seismic event or an accidental push, that can synthesize the possible statue risks. The force intensity and geometry have been computed considering the resistance to the overturning for the adherence among statue and pedestal. The more numerically accurate results has been obtained with the more simplified model, having only 7% of initial triangles, since this situation better exploits the computational resources for solver precision and for congruent geometrical LoD and FEM resolution.