There is a range of building products for load-bearing applications made of wood: solid structural timber (KVH), duo/trio beams, laminated veneer lumber (LVL), glulam, cross laminated timber (CLT) and others. Here the production process of mass timber products - glulam and cross laminated timber - is explained.
Glue laminated timber (Glulam) is an industrially manufactured building product for load-bearing structures. It is made of board lamellas, which are finger-jointed lengthwise and then glued together with parallel fibres. This produces an elongated beam, similar to a solid wooden beam. Compared to this, glulam as a building product has decisive advantages: On the one hand, the load-bearing capacity is greater than with solid wood, as defects (knots etc.) are cut out in advance and a homogeneous cross-section is created by the gluing. On the other hand, finger-jointing allows longer beam lengths to be realised. Thus, column-free spans of roof trusses of up to 60m are possible. The glued joints from glulam must be produced with special care. For example, a suitable climate (temperature and humidity) must be ensured during bonding.
Glulam is produced as standard product or according to a list in any desired length and dimension. In addition, there are also custom building elements which exceed the standard dimensions, but also shapes such as raised beams, pent roof beams, arched trusses and free shapes. Ceiling elements can also be produced from glulam. In addition, there are also country-specific structural elements for load-bearing purposes such as the Post & Beam construction method in Japanese timber house construction.
Glued laminated timber has developed into a high-quality, universally usable product in timber construction. Around 7-8 million m³ of beams are produced globally. The main producing countries are Japan, Austria and Germany. The USA, Russia and Finland follow at a distance. Well-known manufacturers include Mayr-Melnhoff, Hasslacher, Binderholz, Stora Enso and Chugoku Mokuzai. For large spans, companies such as WIEHAG and Derix are well known.
Cross laminated timber (CLT) is a panel-shaped mass timber product, which is built up from at least three crosswise glued layers. The layers each consist of panels made of wooden lamella. According to the same principle as for veneer plywood, a high degree of dimensional stability is achieved by the crosswise structure: each layer of wood prevents the dimensional change of the adjacent layer at right angles to it, which occurs with single boards due to changes in wood moisture.
CLT is mostly used as ceiling and wall element in timber construction. Another field of application is the construction of temporary paths, for example in the extraction of oil and gas, off paved roads (rig mats). They are used as static load-bearing and non-load-bearing elements in residential, commercial and industrial construction. Rig mats can be used not only for the construction of exterior and interior walls and roof and ceiling elements, but also for staircases and balcony slabs. Due to their light weight, they are often used for extensions to existing buildings. Insulation, facing sheets and facade elements can be easily fixed to the cross laminated timber. There are also special applications such as wooden towers for wind turbines.
CLT can be combined well with other materials - for example concrete. It is therefore an ideal material for multi-storey timber construction. In Brumunddal, Norway, the current (March 2019) tallest house made of CLT is located. It has 18 storeys and a height of over 80 m. Commercially available are panels with three to seven layers and a total thickness of up to about 50 cm. Boards up to 20 m length and 6 m width can be produced. Another common name on the market is X-Lam.
Properly designed, built and protected against weather and moisture, a CLT building can last for centuries. The cross laminated timber market is growing very dynamically around the globe. In 2019, the production capacity was around 2.5 million m³. By 2022, this is to be increased by around another 2 million m³.
The production processes of glulam and CLT are largely similar. There are no differences in finger-jointing. The core element of both processes is the press. The design of glulam presses and cross laminated timber presses is naturally different.
Glued laminated timber (glulam) and cross laminated timber (CLT) consist of board lamellas, which have been technically dried to a certain wood moisture content. The typical types of construction timber are used, i.e. softwoods such as spruce, fir, pine, larch or Douglas fir. More and more often, hardwoods such as eucalyptus are also used.