Project 1.1

After 15 years since the building product cross laminated timber (CLT) was introduced, and after partly turbulent phases during product launch, this product is now on best way to become a building system. In general, a system is always composed of several components. Regarding the bearing function of building products and systems these components can be identified as (i) CLT-elements (see Area 2), as well as (ii) the corresponding and target- oriented joining technique (see Area 3). Despite the fact that CLT-elements stressed in plane, e.g. if used as wall-elements, or stressed out of plane, e.g. if used for roofs and ceilings, are nowadays common in use, up to now a systematic application in respect to economic efficiency and in particular in regard a building system is not identifiable. This delayed development can be argued by the fact that only recently standardisation of layer-thicknesses and lay-ups as well as standardisation of the product with consideration of the necessity of consistent and general accepted test configurations and design models took place.

Beside clarification of further research questions concerning production and design of CLT- elements main focus is on analysis and development of joining techniques optimised for CLT- elements. Beside that it is intended to address a systematic examination of single components with the aim to define a building system as much as possible standardised for CLT. Therefore a comprehensive analysis, involving aspects of production, design and joining of CLT- elements as well as of building physic and economical boarder conditions, is required. Due to nowadays world-wide application of CLT regional aspects have also to be taken into account. For example in some regions it is also required to give answers to the resistance of components and systems stressed by fatigue (e.g. wind towers) or earthquake actions.

For roof elements and beside standard applications it is intended to develop systems for wide- span constructions, for example for multi-storey buildings and halls erected as solid timber construction. For both examples powerful constructions in CLT can be expected. Beside constructions of by steel suspended tied CLT-elements, in particular folded panels can be mentioned. Thereby folded panels constitute powerful constructions which combine stresses in and out of plane. A general application of this construction principle fails due to lack of sufficient basis for design and the availability of an adequate joining system. Beside this, a further future oriented field for CLT-roof elements is seen in conjunction with integrated photovoltaic and solar panels, enabling a high degree in pre-fabrication.

In the field of ceiling elements it is required to differentiate building components according their field of application, their maximum span and action, e.g. in (i) solid constructions, (ii) ribbed plates, and (iii) box girders. Thereby also the possibilities of product combinations (e.g. a combination of engineered timber products) material combinations (e.g. timber with concrete) have to be allowed of and analysed as well. The development of combinations of ultra high performance concrete (UHPC) and CLT and / or glulam can lead by all means to powerful constructions opening new possibilities for constructing in CLT. Nevertheless, in all cases technical possibilities have to be balanced with economical efficiency.

Concerning wall elements need for research is given in respect the enhancement of the degree of pre-fabrication. Furthermore and in regard to the intended goal to come up with a building system in CLT, constructive solutions for connections between walls, including more than one function (e.g. load bearing together with air tightness), and joining of wall elements by means of e.g. timber / engineered timber products and / or metal plates, together with a high degree of pre-fabrication have to be developed. Requests from engineers further indicate the necessity for the development of design principles for openings, in particular if CLT- elements have to resist earthquake actions. It is also intended to consider the inclusion of “plug-and-play” facade elements, which are however analysed and developed externally. Overall only a comprehensive view of all relevant aspects in a system can lead to successful results.

Due to their assembly CLT-elements are able to bear stresses in different directions. Thereby they constitute modules for an integrative building system. If compared to common timber engineering constructions which are characterised by a 3D-structure assembled by plane structures (primary structure), structural components like girders (secondary structure) and bracings, CLT-elements are capable to include more than one of these functions in only one single element. The combination of load bearing in and out of plane with the functional aspects of bracing and partitioning of space lies thereby on hand. The need for research is thus in particular given for clarifying the design and construction of adequate connection details.