How can a renewable material such as timber best meet the requirements of resource-saving and environmentally friendly construction techniques? One solution lies in the use of cross-laminated timber. Internationally known as Cross Laminated Timber (CLT), this revolutionary construction material was developed around 25 years ago by leading researchers from the Institute of Timber Engineering and Wood Technology at TU Graz together with holz.bau forschungs gmbh Competence Centre. It consists of several layers of solid timber, which are glued together cross-wise, mostly using conifers such as spruce, larch or pine, but also deciduous species such as birch, ash and beech. These layers of wood are processed to form solid timber elements of up to 20 metres in length and 4 metres in width, while the number of layers used determines their thickness. In recent years, CLT has meanwhile developed from a niche product into an internationally much sought after and industrially manufactured product. As a result, it is now possible to construct multi-storey buildings in addition to single-family /multi-family dwellings and buildings for commercial and industrial use. According to Gerhard Schickhofer, Head of the Institute of Timber Engineering and Wood Technology, this solid timber building technique is in great demand all over the world:
These cross-wise bonded solid timber elements can be used for buildings of up to ten storeys, like the one in Melbourne, Australia. In Vienna, even a 24-floor building is currently in the planning stage. We are talking about timber buildings in really large cities, cities that are already prioritising this construction technique.
Solid timber elements are manufactured from layers of wood that are glued together orthogonally, allowing spaces for windows and doors to be cut out easily.
An ever-increasing number of buildings in urban areas is being built with cross-laminated timber: hotels, schools, offices and multi-storey residential housing. Building with this fabulous product has great potential!
The production of large span wall, roof and ceiling components allows for a very high level of prefabrication. Loadbearing structures can be erected in a very short time.
Head of the Institute of Timber Engineering and Wood Technology
8010 Graz, Austria
Univ.-Prof. Dipl.-Ing. Dr.techn.
Phone: +43 316 873 4600
holz.bau forschungs gmbh
8010 Graz, Austria
Buildings made from cross laminated timber are climate-friendly, sustainable and economical. CLT offers a whole host of other advantages, too:
inferiorquality, the boards were derived from the outer parts of the trunk, which are especially strong and stiff and particularly well suited for processing. Meanwhile, due to increased demand, so-called centre boards are chiefly used.
Lots of convincing reasons, therefore, why Japan, for instance, has decided to realise solid timber construction using cross-laminated timber on a comprehensive scale and to co-operate closely with researchers from TU Graz!
In order to exploit maximum technical potential of the solid timber construction technique, researchers from both Graz research institutions are refining the CLT method even further. With a focus on application-oriented research, but also including basic research projects, they:
construction kitsand/or components that are delivered in a prefabricated state and connected either at the factory or on-site using innovative joining techniques such as self-tapping wood screws or system connectors.
CLT panel made from birch in a bending test
Testing of loadbearing, deformability and vibration behaviour and the structural parameters of cross-laminated timber elements and alternative wood species. The above photo, for example, shows a point-loaded CLT panel measuring 2.5 x 4 metres in a test.
Cyclic testing of a CLT wall connection
Ascertaining vibration parameters of CLT panels in the lab
One of the building industry’s key future requirements will be to develop cost-efficient and affordable buildings. This could be implemented, for instance, by providing the highest possible degree of prefabrication at the factory in combination with defining clearly structured construction systems, for which CLT is particularly well suited.
The current research programme in the frame of the FFG-COMET K project
focus_solid_timber-solutions (focus_sts), which is led by a consortium (hbf), is subdivided into three areas based on the following leading concept:
Construction system (or components) =
element + joining technique
Development of components or systems for roofs, floors (ribbed floors and hybrid structures) and walls as well as stiffening and stabilising systems consisting of cross-laminated timber:
In the process, the vibration behaviour of CLT flooring systems is tested in the lab. Target-oriented leading details are derived from these findings to help standardise the construction technique, for example, in order to meet requirements of proper construction such as piping and HVACR. Implementation of a user-friendly and dynamically serviceable Wikipedia System for CLT and programming of a free CLT measuring programme (CLTdesigner) are already in the pipeline.
Questions relating to (further) product development, testing procedures and calculation of CLT:
resource-saving CLT production and characteristic rolling shear properties of CLT are in focus. Production of CLT from various deciduous species, e.g. birch, was investigated and successfully validated. Additional research was done on CLT shear parameters. The
Formulation of Loadbearing Models work programme involves investigation into combined stresses as well as stability behaviour (buckling, bending and warping) of CLT components. Topics concerned with local load transmission in walls and floors, and mechanical description of the load-carrying behaviour of ribbed base plates (combination of CLT and laminated timber) have already been completed. One special highlight was to test point-supported CLT floors in standard dimensions as well as perform the respective structural modelling.
Research and development of joining techniques (VT):
This involves investigating joining techniques in general and specifically for CLT. CLT is excellently suited for use under dynamic stress forces (e.g. earthquakes, see photo 4). Since modern and efficient timber construction would be inconceivable without fully threaded screws, a number of tests were carried out to that extent: single and group testing of lateral and narrow surfaces, block shears, fatigue behaviour, hydrogen embrittlement, combined pull-out and shear loads and development of a micrometer screw gauge, amongst many other things. Besides developing system connectors and designing a monitoring system, screw press adhesion is of great relevance for the future of the product CLT as a construction system. Researchers with special interest in cost efficient construction techniques are investigating possibilities of gluing in steel sheets and timber materials at the factory and then joining the components, for instance, just like steel connections.
In order to achieve specific research targets, engineers and scientists from the Institute of Timber Engineering and Wood Technology at TU Graz collaborate with holz.bau forschungs gmbh Competence Centre in the Sustainable Systems Field of Expertise at TU Graz, and internationally with numerous other research institutions.