Timber structures CLT
A short introduction to cross-laminated timber Cross-laminated timber – an innovative, sustainable option with numerous advantages for off-site construction – has a significant role to play within the 21st Century building industry. Here is a short guide to the material and its applications.
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he well-known environmental advantages of using timber as a construction material, coupled with the wider technical and performance benefits of products such as cross-laminated timber (CLT) and glued laminated timber (glulam), are enabling the construction industry to deliver innovative and costeffective timber buildings.
What is CLT? Cross-laminated timber is a factory-made construction product based on a manufacturing technique developed in the late 20th Century. Essentially, slices of softwood timber are assembled to form sheets, with the grain of each sheet set perpendicular to its neighbour. The sheets or layers (sometimes known as lamellas) are then glued together under pressure (laminated) to create a stable, structural product. The timber is kiln dried before lamination to reduce its moisture content to around 12%, simultaneously reducing the instability that could occur due to material shrinkage and minimising the risk of rot or fungal and pest attack.
The process of cross lamination limits the material’s crossgrain movement, resulting in a multi-layered, inherently stable, planar product. However, the panels do not have a single direction of structural span, performing instead in a multi-directional manner, and therefore providing a great degree of flexibility in how they can be used. It is important to note that the outermost direction of timber therefore does not necessarily correlate with the direction of structural span, although performance is enhanced if it does. The principal difference between CLT and the other wellknown innovation in engineered timber – glulam – is that in the latter the timber is glued in layers that are parallel with each other, rather than perpendicular. This gives glulam directional strength, which is why glulam elements are chiefly used for structural elements such as beams and columns. >>
The timber layers are assembled so that they are symmetrical around a middle layer. This results in panels that usually comprise three, five or seven lamellas, and range from 50mm to 300mm in thickness, although thicker panels are possible. Within the factory the resulting CLT panels are cut and processed to the designer’s specific requirements using a computer numerical control (CNC) machine that can produce precisely sized panels with cut-outs for windows, doors and building services as required, as well as any necessary rebates to the edge of panels. Panels are commonly available up to 16.5m x 2.95m in size and can, if required, be produced by some manufacturers to 20m x 4.8m, with the practicalities of transportation generally being the limiting factor. www.trada.co.uk
The perpendicular bonding of cross-laminated timber provides the material with inherent structural stability and the ability to span in two directions. Drawing: Ramboll UK Ltd
Timber 2016 Industry Yearbook
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