Dimensioning adhesively bonded timber joints

Timber architecture is embracing free-form designs. Traditional joints are not ideal for this. They often spoil the overall impression of the timber frame and are insufficient in terms of structural efficiency. From an architectural point of view, adhesively bonded joints offer a valid alternative. They can generally bear heavier loads and are more rigid than joints with mechanical elements. Before adhesively bonded joints can be used more widely, we need methods to accurately predict their load-bearing capacity and rigidity.

In principle, wood has excellent mechanical and thermal properties. But it is a naturally grown material with certain weaknesses (branches, irregular fibres, etc). Wood therefore has a wide range of mechanical properties, which is problematic when it is used for load-bearing, structural elements. To make wood a reliable and economical construction material, we need to describe its properties as accurately as possible.

The project developed a novel design concept for adhesively bonded joints in timber construction covering all classic load scenarios. These include static short-term, long-term, vibration and climate pressures. The researchers re-evaluated the entire information chain in connection with the design of timber joints. In so doing, they considered key data and calculation methods with regard to the materials used (wood-based materials and adhesives) as well as with regard to the joints and the final designs.

The development of novel adhesively bonded joints will enable a new design vocabulary in timber construction, either as pure wood or in mixed hybrid systems (wood with steel and/or concrete). The successful development and implementation of the design tool will make a major contribution towards increasing the share of wood in construction and improving the competitiveness and overall ecological balance of the Swiss construction sector.

On the basis of an experimental investigation, we were able to determine the correlation between E-modulus, rigidity and strain at fracture. In addition, we could assess the volume changes (as a result of the variable length) for these properties.

In addition to the experimental testing, we analysed the load-bearing properties of adhesively bonded timber-timber joints using various numerical methods. In so doing, we focused on rigidity, load-bearing capacity and ductility. The main parameters were the geometry of the joint (overlap), type of wood (spruce or beech) and the adhesive. We ran a numeric exploration of three different types of adhesive (epoxy resin, polyurethane glue and acrylic resin). Acrylic resin showed the greatest potential for ductile bonding. Further analysis therefore focused on acrylic resin. The results of the tensile testing of adhesively bonded timber joints using epoxy resin and acrylic resin, which were well mapped by numeric simulations, show that acrylic resin joints exhibit greater ductility.

Design tool for adhesively bonded timber joints