The rise and rise of CLT buildings

Dr Keerthi Ranasinghe discusses how cross-laminated timber is helping architects go to new lengths

Architects’ desire to create sustainable structures has pushed technological innovations beyond the imaginable. Cross-laminated timber, or CLT for short, has been drawing a lot of attention in this area in recent months.

A highly sustainable and versatile construction material, CLT has been incorporated in a range of buildings, from small detached homes to tall multi-storey towers. Just 10 years ago, glued laminated timber (glulam) would have been considered the best solution for large, wide-span structures, but today CLT provides an equally attractive option.

Typically manufactured from European softwoods, the creation of the material itself is a key highlight. Similar to glulam, the product is formed by the layering of timber planks (known as lamellas), yet unlike glulam, where all the lamellas are set parallel to the grain, those in CLT are set mostly perpendicular to each other.

They are then glued together under pressure (laminated) to create stable structural panels, whose size is seemingly limited only by transportation restrictions. The panels are shaped using Computer Numerically Controlled (CNC) machines to specific dimensions. This guarantees exceptional levels of accuracy, allowing for panels to be produced with window and door cut-outs in the exact positions, permitting much more freedom around the positioning and design of these features, and ultimately leading to a more creative result.

Taking cues from the development of glulam, CLT has greatly enhanced structural properties thanks to its lattice style lay-up. While glulam offers substantial directional strength for use in beams and columns, the product has limitations with issues including shrinkage, fractures and fissures.

CLT on the other hand has enhanced strength and stability characteristics with less issues of shrinkage and enabling the creation of larger structures.

Green credentials

From its origins as raw timber, which locks away carbon dioxide as it grows, right up to the installed product used in energy-efficient buildings, CLT contributes to sustainability. CLT can also be quickly and easily transported and installed, providing significant cost savings and a reduction in environmental impact. A faster build programme and reduced preliminaries mean reduced costs across the board.

Exposing the timber elements is a design touch applied in many modern buildings. The visual appeal of CLT fits the bill with an attractive pale appearance that gradually yellows through UV exposure.

CLT around the world

While we still consider CLT as a ‘new’ method of building, it’s been applied in projects across the globe such as swimming pools, museums and high-rise developments. Ultimately, CLT has formed a crucial part of some designs and has made for a very visually satisfying end product. While architects can specify much larger timber structures than before, transportation to site remains an obstacle. But even this can be overcome – for example, by delivering large CLT panels by helicopter.

This quality of the material has benefitted an ambitious 20-storey development in Canada, which has been made possible by the sheer size CLT panels can be made to and the combined strength properties it has when used with a concrete basement and core structures.

Canada has also recently published its CLT design guidance as part of its building code, while in Australia the National Construction Code was changed to allow the use of timber construction systems under the Deemed-to-Satisfy (DTS) Provisions for buildings up to 25 metres in height. The UK is currently one of the biggest users of CLT within Europe and there are a number of eight or nine-storey CLT buildings either being built or already built in London.

There is significant potential for the use of CLT in the mid-rise residential and commercial sectors where steel would usually be the product of choice. However, unlike other products architects have known for years, work is still being done on research and standards for CLT. An important milestone would be the introduction in 2020 of a unified European circulation method as part of Eurocode 5.

Nevertheless, designers are able to find solutions for their individual projects and this allows additional freedom for truly remarkable bespoke buildings. It’s clear that CLT will become a more popular choice in the future, with standards becoming harmonised and the benefits becoming more appreciated.

A crucial benefit of CLT for designers is that buildings are not confined to the standard ‘box type’ constructions of the past – instead, they allow architects to be more creative in building design.

Dr Keerthi Ranasinghe is the principal structural engineer at Exova BM TRADA