In recent years, the use of timber as a structural material in the construction industry has grown considerably. Thanks to new possibilities provided by current production techniques, increasingly complex geometries and structural systems have been achieved. Concerning the connections, timber is rarely used as a structural material alone, often being reinforced with other materials such as steel. Current digital manufacturing techniques, however, promote the exploration of new typologies of timber-to-timber connections, while giving increased structural value to timber and limiting at the same time the use of additional materials.
Over the last years, the structural analysis of timber elements in relation to the service state has evolved significantly. Indeed, several static calculation procedures based on elastic models and empirical relationships have been developed. On the contrary, up to now, no standard procedure has been developed to support the static calculation concerning the ultimate limit state of timber elements, which focuses on the definition of the collapse load.
The research aims to contribute to the development of the static calculation procedure of the ultimate limit state of timber structures under combined loads. The ultimate limit state is determined with the support of the theory of plasticity, and particularly through the lower bound theorem. At first, the yield conditions of natural and engineered wood are developed. The yield conditions, governed by a few material-related parameters, are composed of different regimes, each representing a different failure mode. By introducing anisotropic yield conditions, the plastic theory allows considering the anisotropy of the material effectively. Afterwards, the defined yield conditions are implemented in the analysis and design of interlocking timber-to-timber connections. Using the lower bound theorem, the forces thus calculated are lower, or at most equal, to the collapse load, while assuming the necessary deformation capacity. Strategies to ensure the necessary deformation capacity are developed with the support of mechanical tests on various types of connections. The use of the theory of plasticity allows determining in an illustrative way the respective stress state in the element, thus facilitating the proper development of the connections and the integration of structural performance with the requirements provided by the multidisciplinary approach of the NCCR-dfab project.

The research is part of the National Centre of Competence in Research in Digital Fabrication (NCCR-dfab) "Spatial Timber Structures - Bespoke Digital Fabrication" project funded by the Swiss National Science Foundation (SNSF). PIs: Prof. Gramazio (leader), Prof. Kohler (leader), Prof. Dr. Schwartz, Prof. Dr. Coros, Prof. Dr. Frangi