BIM for built cultural heritage: the case of the Baptistery of San Giovanni in Florence.

The 3D virtual modeling of any artifact involves the organized composition of digital elements. This process is common to any 3D modeling workflow but, when applied to BIM systems, shows a good correspondence in the construction processes between the virtual and the real world. In fact, any building can be considered as a coordinated set of simple technological elements, linked together by design logics and construction techniques that are quite predictable and controllable. Even existing buildings can be "deconstructed" into recurring elements, especially if the architectural survey operations reveal the regularity of certain geometric patterns; however, parametric and informative modeling of built artifacts is much more difficult, both in terms of geometric transposition of the continuity of the real world and of its qualitative and semantic description. One of the main problems refers to the concept of "ideal model". While in new buildings the ideal model coincides with the coordinated set of technological elements established "from scratch" by the designer, for existing ones this approach is obviously not applicable. The first objective of the H-BIM project is to try to answer the following "simple" question: how can we build this model? Clearly, trying to recover the original conformation of the building, not only in terms of geometry but also of materials, and from the constructive point of view. From this point of view, historical research is an essential means of selecting the necessary information. According to an "additive" approach, in fact, this modeling phase should be able to incorporate new information into the BIM model as additional layers that overlap, intersect or replace the original structures; the building phases that made up the monument. This process, which for historical heritage normally spans a period of centuries, ends with today's "as built", ie the current condition and consistency of the artifact, which in the field of restoration is called synchronic analysis. In this framework, the presented study becomes an application opportunity for the theoretical issues highlighted above, using the Baptistery of San Giovanni in Florence as a case study. Specifically, we intend to analyze not only how to discern the constructive elements through their semantic segmentation, but above all how to connect them and make them interact with each other. As a result, we aim at providing a representation of the different evolutionary phases and above all propose a more effective meaning for the Levels of Development parameter when applied to built heritage.