Author: J. Ruddy (CARE & Capstone Consulting Engineers Ltd)
2 November 2015
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J. Ruddy (CARE & Capstone Consulting Engineers Ltd)
Timber Engineering Notebook (TEN) No. 11 and No. 12 provided a detailed introduction to the applications and use of cross-laminated timber (CLT) as a structural timber product, together with information on the manufacture, detailing and erection of CLT constructions. This article presents detailed advice on the material properties and structural design of CLT based on current UK practice.
Timber and stone are the oldest known building materials. Our most ancient buildings are characterised by their use. So it is no surprise that an engineer looking after historic fabric will regularly encounter the need to repair timberwork. 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.
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.