On-site Stress Measurements of a Flying Buttress in the Palma de Mallorca (Spain) Cathedral

Abstract

Construction of the Palma de Mallorca Cathedral dates to the beginning of the XIVth century. This cathedral represents an excellent example of the gothic style in Spain despite several modifications due to structural problems. Because of the observed inner structural deformations, special attention was focused on determining the role of the flying buttress by measuring the acting stresses in one of them. The in situ stresses have been experimentally determined using the Hole Drilling technique. This technique allows for the determination of the principal stresses and their directions in the stone masonry. This study presents the applicability of the Hole Drilling technique on flying buttresses as structural elements. In this sense, the paper is a very specific and original case study of rock assemblies. The Palma de Mallorca Cathedral was built over a long period spanning 300 years (1306–1600) and was later subjected to significant repairs and reconstructions. This monument (rock construction) shares major structural features with other buildings of this era, such as spaciousness, high lateral naves, and extremely slender octagonal piers. Because of the large dimensions intended for the nave, the builders provided this structure with a double battery of flying buttresses to stabilize the high vaults (Fig. 1). The mechanical stresses of these elements are still unknown. The central nave spans 19.9 m and reaches up to 43.9 m at the vaults’ keystone. The octagonal piers are connected with the lateral vaults. In a few piers of the Palma de Mallorca Cathedral, vertical or oblique cracks have developed across the stone, extending in some cases to several rows. The vaults are known (thanks to historical research) to have experienced severe problems leading to some reconstruction during the 18th century and at other times. The piers show significant lateral deformation, reaching, in some cases, up to 30 cm, i.e., 1/100 of the height at the springing of the lateral vaults. In previous works (Roca et al 2008), both the chronology of the structural elements and the mechanical properties of interest for this work were obtained. Pulse radar and seismic tomography provided valuable information about the internal morphology, while the material properties were mainly determined by laboratory mechanical tests on stone samples extracted from the original quarries, which were identified by a historical investigation. Microscopy and diffractometry were performed on small amounts of both mortar and stone sampled throughout the building. In the cathedral, a clear correlation was found between construction stages and different stone varieties. This procedure confirmed the hypothesis that the construction process occurred as the historical research suggested. The fact that construction was interrupted for almost a century, after the erection of the 4th bay, is signaled by a change in the material used in the two phases. The substitution of all flying arches during the 18th century is also recognizable because they are built with the same variety of local sandstone, which, in turn, is not present in the rest of the building. Information has also been gained about the extent of historical repairs (in particular, joint repointing). & Santiago Sanchez-Beitia santiago.sanchez@ehu.es; santiago.sanchez@ehu.eus