Computational 3D modeling supporting the preservation of historic timber roofs: the case of San Pietro's Cathedral in Bologna

This study analyzes the structural behaviour of historical roofs through an innovative non- destructive method to support their conscious maintenance and conservation. Starting from the case study of San Pietro's Cathedral in Bologna (Italy) - an impressive example of a 16th and 17th-century timber-framed roof preserved in all its authentic parts - this research aims to understand the causes and possible repercussions of structural deformations that timber trusses have undergone over their lifecycle and propose some criteria for their efficient preservation. Wooden structures have constantly been used worldwide in historical constructions with different configurations and technologies depending on the local availability of materials, their conception, and the expertise of workers. So they may present complex configurations as well as various ways of assembling linear elements, cutting joints, and connecting metal brackets and, above all, beams with irregular cross-sections. For this reason, the use of LIDAR (Laser Imaging Detection and Ranging) and TLS (Terrestrial Laser Scanning) techniques spread in the Cultural Heritage (CH) field in the last decades due to their precision and accuracy in surveying the geometry of elements that compose structures in short periods. While surveying methods are well-established in practice, the automatic transformation of laser-scanned point clouds into valuable information 3D models is still a recent issue. Visual Programming (VP) tools seem to be a leading technology to perform this issue since they require fewer programming skills than code programming; therefore, they are accessible also to experts in the Architecture, Engineering and Construction (AEC) domain without specific programming expertise. Gathering such information about these structural systems is critical for three main issues. First, studying their history and the transformations they have undergone throughout the centuries is essential for comprehending their material culture and holding their historical values. Secondly, knowing the current state of these objects is necessary for performing a conscious conservation design, planning maintenance and allowing the everyday use of buildings that they cover. Thirdly, investigating and monitoring their conservation state can help get information about the health of the underlying structures and the whole building. The method used for investigating the structural behavior of San Pietro's roof framing is based on a different approach from traditional Structural Mechanics, both from the instrumental and theoretical points of view. It mainly consists of archival studies, in-situ inspections, digital surveys, computational 3D modeling algorithms, and reverse engineering procedures. The premise behind the method consists of taking advantage of the large amount and accuracy of spatial information produced by the TLS survey. These data are used to enable a highly detailed analysis of timber trusses and, thus, to develop comparative information on their static behavior and preservation state. Moreover, the combination of processed geometrical data with historical research makes it possible to build up hypotheses on the structural behavior of the trusses. In addition, a monitoring phase could be performed to control structural displacements or deterioration of elements over time or after accidental events, such as fires, windstorms, or earthquakes. The method was applied, tested and validated by previous analyses that allowed acquiring and processing of consistent information about several wooden roofing systems in Bologna (Italy), belonging to a set of remarkable historic churches and theaters from the 16th and 18th centuries. This paper illustrates the most recent update of this research. Since the assessment method is continuously under development, it has been extended for San Pietro's Cathedral in Bologna, whose trusses are the most complex among the previously examined in terms of structural type and size (about 26 meters in span). Based on a few theoretical assumptions on the in-situ behavior of timber trusses reported in earlier studies, the trusses' original deformation state was determined by removing both inand out-of-plane displacements. The principal hypotheses accounted for the lateral bearings remaining in their original position, the projection of the centroidal axes of all beams onto the vertical plane of the truss, a slight bending deformation of the tie-beams, the inward translation and lowering of the joints between rafters and posts, the rotation of bottom rafters around the virtual center of the lateral bearings, and the absence of axial deformations of beams. The analyses conducted on the Cathedral's trusses have shown that most of them have similar structural behavior in terms of hypothetical deformations undergone since their original state. According to results, the maximum hypothetical vertical translation documented for the primary joints is equal to 13 centimeters. These deformations are modest considering that all the trusses cover a span of about 26 meters and comparing them with other case studies. These low displacements are probably related to the unique structural scheme of the San Pietro's trusses, as well as to their exemplary construction technique. Nevertheless, the asymmetrical behavior registered for most of the trusses must not be disregarded. Non-symmetrical loads on the roofing system could be provoked, for example, by the persistence of snow on the North-facing pitch after heavy snowfalls. They could trigger kinematics for which the trusses cannot respond, causing unexpected tension changes in the elements or excessive displacements of joints that could cause the loss of connection between the elements The whole analysis suggested that it would be appropriate to insert metallic straps connecting the primary truss beams, such as rafters, straining beams, tie beams, and posts, inhibiting the structure's vulnerability to non-symmetrical loads, and increasing the degree of the constraint of joints and notches. These minimal and reversible interventions are strictly necessary to preserve the cultural values of these fascinating construction systems and ensure the safety of the building. Programmatic maintenance and monitoring of the wooden roof are also required to keep the state of preservation of trusses under control, suggesting timely intervention and preventing deterioration phenomena from spreading to the collapse of the structures.