The use of timber as a construction material throughout the ages, and what technological advancements have brought it back to the forefront of the construction industry?
Edwin Westwell - N0521458
Cross laminated timber framing
Cross laminated timber (CLT), also known as X-LAM, is a structural two-way spanning, prefabricated panel used to form environmentally sustainable structural walls, roofs and floors across a wide range of structural applications for all manner of buildings. Thanks to the gluing of both longitudinal and transverse layers, which reduces the working behaviour of the wood to a negligible degree, CLT meets the standards required by modern building materials (Unknown, 2016).
CLT (fig9) is produced by layering timber boards, each layer running perpendicular to the last and polyurethane glue being applied between each layer. CLT is generally made using an odd number of layers; most commonly three, five or seven layers of timber are used to produce a finished solid piece. More layers can be used dependent upon the use intended for the finished piece. Once layered, the large timber sheets are pressed together in what is known as an X-press or cross laminated timber press. This not only applies pressure from the top but the sides and ends also, aligning and compressing the timber from all directions. This makes the CLT sheets as compact and structurally able as possible. Once pressed, the CLT sheet is fed through a machine that adds high frequency energy to the sheet, curing the glue in the quickest and safest manner possible (Unknown, 2016).
Fig 9, Cross laminated timber. (Puumarket.ee, 2016)
A typical CLT sheet made up of five layers has the capacity to span a length of 20 metres, more layers meaning more capabilities. Sheets of CLT are custom made for a specific purpose. Leaving out openings for windows and doors for a building not only saves on timber, but with CLT being a solid structural material, there is no need for lintels to support openings as the whole CLT panel is self-supported (Unknown, 2016).
CLT construction, in comparison to medieval and modern timber framing, is extremely simple. Once the custom CLT wall panels have been made off site they are put together on site in a jigsaw-like fashion (fig 10). Damp proof course is laid between the foundations and the CLT first floor wall panels, which are then fixed to the foundations using steel angle brackets. Adjoining wall panels are screwed together using large torque screws or angle brackets. Once this is all in place, the first floor envelope is complete. (Stora Enso, 2012).
Fig 10, CLT construction. (Smartlam, 2016).
Large CLT floor panels then span the ground floor walls creating the second floor; again these are fixed to the first floor wall panels using large torque screws. As per the first floor, the second floor wall panels are fixed to the floor panels using steel angle brackets and to one another using torque screws. Traditional roof trusses can be used to span the walls of the second floor, similarly to that of any modern construction, supported laterally by purlins and by a ridge beam at the peak. Alternatively, there have been cases where the roof of a CLT construction has been made using large CLT panels that are cut at angles along each end. These panels are then fixed to the wall panels and at the peak where they butt together using torque screws, distributing the weight down the external load bearing walls as roof trusses do. The robust nature of the CLT panels means that no bracing is needed, as it is with common roof trusses (Stora Enso, 2012).