Historic Timber Trusses in Europe. The case of St. Peter in Bologna

The paper aims at giving an original contribution in the field of historic timber construction in Europe. The work is based on a recent survey and subsequent analysis of the roofing system of the Metropolitan Cathedral of St. Peter in Bologna, Italy. The width of the central nave is one of the largest of the Roman Catholic Church: the eighteen trusses of the roof have probably the longest span in European churches until that period, overcoming the size of the biggest church in Rome, that is St. Peter in the Vatican City. Considering the concept of simple truss, made with rafters, tie-beam, king post and raking braces, the 24,68 meters span was covered with a combination of invention and practice. The re-sult was the construction of extremely elegant and almost unique structures. Given the long span, some supporting beams were placed under the tie-beam to contrast the bending moment. A “false collar beam” was included in the scheme of the trusses; these are composed of compressed brac-ing elements which create an arched system, without interrupting the king post. Another peculi-arity of the structure was the use of soft german timber (fir), which is probably the only suitable species of wood for such a slender solution. The Cathedral of Bologna was extended over a period of fifty years, starting from 1570. There is no evidence of the origin of the design and there is no clear reference in any publication of the period; a similar scheme was proposed by Giuliano da Sangallo in a drawing fifty years before; similar – even bigger – trusses are also present in the Farnese theatre in Parma, built by the architect Giovanni Battosta Aleotti in the same years. The presence in Italy of this type of trusses should be further addressed. Nevertheless, the first goal of the study was already achieved, in having accurately surveyed such a complex roof-ing structure. The paper offers a technical interpretation of these structures and suggests a classi-fication. Besides, the survey served to check the conditions of the structures and to understand the operating principles of the elements during construction. Too little attention is usually placed on the role of joints; instead, a close observation of the details has shown the influence of the connections in distributing the loads, according to the expertise of timber master builders and carpenters.