Author: J. Miller (Ramboll)
1 December 2015
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J. Miller (Ramboll)
The greatest threats to the structural integrity of timber are from attack by rot and insect; therefore, in the damp British Isles, those working in conservation will often need to reach for the sketchpad to record and re-detail areas damaged by the effects of moisture.
Interventions to historic timberwork are also necessary when a building is converted. This happens, for example, when floor joists are reframed or loading is assessed for a new use. While philosophically this is different to a simple repair, it nevertheless requires similar skillsets to achieve the best, most sensitive results.
This article looks briefly at these matters, first from the aspect of conservation philosophy and material choice to establish some ground rules, and then by showing some of the details typically in use in the UK today. In order to focus on these, it does not consider survey and diagnosis.
The Timber Engineering Notebook series concludes by examining the use of glued-in rods for timber connections. The increased use of manufactured timber, such as glued laminated timber (glulam) and laminated veneer lumber (LVL), with improved mechanical properties and the ability to produce cross-sections of almost unlimited size,has driven the timber engineering industry to come up with improved connection systems. Glued-in rods and plates have been used in the UK as a method of connecting timber since the 1970s. They offer the possibility of creating concealed connections that are capable of transferring large forces and moments with minimal slip due to their high stiffness.
In conservation work and like-for-like repair on older masonry, lime mortar is the only recommended material. The thick, plain or lightly punctured walls that make up most historic buildings have few concentrations of load. Calculations of stress in such cases are often needless and, subject perhaps to the check of any critical element, we can generally lay aside our concerns about mortar strength. In contrast, the need to maintain a balance of moisture and flexibility in the body of an old wall is essential. Ignoring this will lead to the classic error of repointing old structures in brittle, impermeable Portland cement (OPC) mortar. The mortar provides the route for evaporation from the core and should be more permeable than the brick or stone. To reverse this by sealing the joints with a hard finish can only lead to trouble.
This article is the first of two which will discuss the problem of corrosion of steel frames behind masonry elevations. It aims to provide an introduction to this form of construction and to consider the ways in which lack of maintenance can lead to corrosion of the steel frame, before setting out how remedial work should be approached.