Historic timber roofs modelling: prosthesis and resin repairs

1.1. Structural analysis of old timber frameworks Nowadays, a considerable number of timber structures require structural intervention due to material decay, improper maintenance of the structure, faulty design or construction, lack of reasonable care in handling of the wood, accidental actions or change of use. While the assessment of old timber structures is complex, it is an essential precursor to the design of the reinforcement of the joints. Owing to a lack of knowledge or time, the species and/or grade assumed are often an overly conservative estimate which can lead to unnecessary replacement, repair and retrofi t decisions along with associated superfl uous project costs. Timber frameworks are one of the most important and widespread types of timber structures. Their confi gurations and joints are usually complex and testify to a high-level of craftsmanship and a good understanding of the structural behaviour that has resulted from a long evolutionary process of trial and error. A simplifi ed analysis of old timber frameworks, considering hinged joints and only plane parts of the system, is often hard to realize. Old timber structures are usually highly statically indeterminate structures. This means that loads applied to the structure have different pathways to reach the supports. Resolving the indeterminate system involves looking for additional equations that actually express the relative stiffness of all those pathways. To illustrate how the differential stiffness of elements, joints or supports may infl uence the behaviour of the structure, a simple collar-braced roof is presented in Fig. 1. In the absence of buttressed walls, under vertical loads, the collar (or the tie-beam) is under tension because it prevents the roof from spreading. If buttressed walls restrain the feet of the rafters, the collar is in compression. The only difference between these situations is the horizontal stiffness of the supports (zero or infi nite). The mass of the walls to resist the outward thrust is not the only infl uencing factor. Most of the time, principal rafters are connected to wall plates that have to be stiff enough to act as a beam in the horizontal plane spanning between two fi xed ends in the walls. If the rafters are notched, for example, with birdsmouth joints, over the plate at the top, the roof can be hung from the ridge purlin, depending on the stiffness of the wall plate. The stiffness determines the ability of the wall plate to act as an additional support. This is valid for most types of carpentry joints as they usually are statically indeterminate. This simple example illustrates how the stiffness of joints may infl uence the force distribution inside the structure. This also points out that when restoring Thierry Descamps*, Coralie Avez**, Olivier Carpentier***, Emmanuel Antczak***, Gi Young Jeong****