Structural Analysis of Gothic Roof Trusses in View of their Conservation. Case Study of St. Paul's Cathedral, Liège (Belgium)

Abstract

This study focuses on the structural behaviour of gothic roof structures. Starting from the case study of St. Paul’s Cathedral, Liège (Belgium), the aim of the research was to identify and understand the reasons and possible consequences of past and future structural distortions for the conservation of this historical timber roof structure.The wooden structure of St. Paul’s Cathedral is an interesting example of a 13th and 14th century timber-framed roof. The entire structure has mainly preserved its authentic parts even if transformations and reinforcements were made until mid-20th century. Today, these interventions must be evaluated from a critical point of view in order to preserve the authenticity of the structure. Two interventions have been studied more particularly. Firstly, the effects of an important neo-gothic restoration made in the 19th century and secondly, the consolidations of the frames, made after the bombings of WWII. Preliminary studies have proved that these interventions were poorly documented. As often in the past, hidden under the roofs, historic wooden structures were forgotten. However, we found some very interesting texts which have been confronted to an architectural analysis of the building and a study of the pathologies. These surveys have been used to undertake a structural analysis of the gothic roof structure in order to better understand and explain distortions and reinforcements of the frames.Due to biological deterioration, hypotheses on the resistance of wood have been implemented in the 3D models. We paid special attention to the nodes connections in order to evaluate their resistances to shear forces. Our models highlight critical areas which correspond to visible deformations and breakings produced after a bombing in 1945. The influence of the neo-gothic masonry, which encloses the wooden structure, has been also discussed. Due to a high humidity level, important structural degradations are visible on the base of the frames. Therefore, we also focused our attention on the evaluation of resistance of the principal-rafter, directly in contact with the masonry.