Shape and construction of brick vaults. Criteria, methods and tools for a possible catalogue

The use of cloister vaults in the construction of noble buildings, as covering elements for square or rectangular rooms, is widespread and well-known. The geometric continuity at the intrados makes generally possible the execution all over the span of frescoes, stucco and decorations. The construction of brick vaults, from the late Middle Age, was sped up by limiting the centering to the wooden planks arches that were instrumental in the profile determination. Nowadays, the availability of several procedures, phases and tools for carrying out a survey allows to draw reliable assumptions about the construction methods and the execution time. It is mandatory to determine the properties of the binders, the shape and dimensions of the bricks, and to carry out a comparison between the geometry of the intrados surface and the evidences emerging at the extrados. The support of the laser scanner technique allows to accurately identify the surface profile and thickness. All these indications, in turn, are useful, in view of an interpretation of the structural behavior, to identify weaknesses, and to highlight contributing factors of instability (if any). In addition, Italian culture lacks systematic studies that indicate a reliable common pattern among the different types of vaults. Similar intrados shapes are often characterized by radically different patterns that are rarely taken into account from the structural point of view. The culture of stereotomy, in France and Spain, takes this interpretation of vaults for granted, while more systematic studies, in German-speaking countries, offer a broad and solid framework. The Italian technical literature was in Modern Age relatively scarce and showed limited attention to structural problems, and this fact contributes to widen this gap. Providing a variety of accurate surveys, aware of the aforementioned framework and the related shortcomings, will enable to overcome them. As a matter of fact, an element like the vault characterizes the Italian built heritage, but in the structural analysis there is often an oversimplification of some geometrical properties, limited to hurriedly measure thicknesses and intrados profiles, something that may lead to wrong conclusions. The paper focuses on a well-documented case, the Magio Grasselli palace in Cremona, where late medieval existing buildings were partially dismantled and modified in a modern aristocratic palace, designed by architect Francesco Pescaroli starting after 1658 until the end of the century. However, only the part of the building facing the main street and the part of the room overlooking the inner court were completed. The construction was resumed only after 1760: the wing facing the court and garden was completed, and all the rooms of the main floor, including the entrance hall, were covered with cloister vaults. Later on, the part of the building overlooking the main street was transformed. Here also, five rooms facing the street were vaulted, but only the central room, entirely painted by Giovanni Manfredini (1785), was completed, while three other unfinished rooms were subdivided in the Nineteenth century. The cloister vaults of two main rooms show different construction systems, although they were built almost at the same time. In the one overlooking the court, the bricks were laid on edge in concentric rows that seem parallel to the walls of the room, but are slightly arranged upwards. The irregularities are compensated by the longitudinal key joints. Wall-ribs systems at the extrados (the so-called Italian frenelli) connected to reinforcing arches, which could have served as base framework, contribute to the global stability of the structure. Thus, the vault seems to be constructed without continuous centering. Again, the large dimensions and the almost square shapes of the room overlooking the street suggest a construction through subsequent phases. Here the arches, visible from the extrados, were the centering outlining the profile of the vault, and enabled to build it without the support of planks. In order to understand the similarities and differences, the construction techniques and the state of damage of the two halls vaults, an integrated survey using laser scanners, photogrammetry and thermographic investigations was carried out. Such a survey allowed understanding the vault morphology and the masonry pattern, highlighting certain critical aspects of the structural elements, the kind of instability and decay, and the crack patterns mechanisms. More in detail, the three-dimensional model of the intrados of the hall, obtained from the laser scanner clouds, has been integrated with the three dimensional model of the masonry pattern, visible from the extrados, obtained from an elaboration of photogrammetric images. The current structure is a frame vault, characterized by two large and respectively transverse and longitudinal arches which divide the surface in nine squares and support thinner vaults, made of tiles (3-5 cm), widely used since the Sixteenth century. The investigation of easily detectable extrados parts makes a deeper knowledge of such constructions possible: it allows a better interpretation of recurring situations, even more problematic to be detected. This interpretation, however, cannot be carried out without an accurate survey integrated by thermography that can be extensively used at the intrados surfaces. In addition, the patient reconstruction and understanding of a constructive richness, made of recurrent elements and specific features, allow sketching a mixed pattern of workers and constructive knowledge. This process may result into current geographical updatable abaci, such as the French repertoires; from the point of view of innovation, it could lead to BIM libraries of vaulted elements that should be, however, not aimed at flattening and oversimplifying these valuable items to a unique parametric matrix, “copy-and-pasteable” at will, with mere changes of dimensions. Unfortunately, an oversimplification of complex elements (such as vaults) is the rule in the application of BIM to the cultural heritage. This oversimplification is the demonstration of incomplete knowledge and deviate interpretation of BIM that should be used, on the contrary, to better handle differences and peculiarities. Through the concept itself of instances consisting of unique and complex elements, “hic et nunc” to every object, the families are enriched; what is different is the point of view that should be aimed not at unifying, but at the comprehension and rendering of the acquired knowledge. As testified by some recent experiences, BIM models support this different approach to modeling, although still in a rather complex way (even if the rendering of a complex model is always complex). The availability of a detailed three-dimensional model allows a backward knowledge of the construction technique, maybe not in its entirety, but sufficient enough to explain and highlight that the provision and use of centering in the constructive phase, as previously mentioned, “changes” the geometric shape by creating multiple variations. The archival, metric and thermographic information concerning the Magio Grasselli palace are abundant and considerable, and are integrated with the plurality of data, essential to define an elevated standard of documentation, which allows to justify both detailed hypotheses on the construction techniques, and classification criteria. It is important, however, to subject the whole to a critical examination, in order to define principles effectively shared and aimed at building a reference system, whose absence was already pointed out.