Development of stress field models for determining the load-bearing capacity of screwed connections in timber engineering based on plastic theory

• Applicant: Professor Dr.-Ing. Timo Claus
• Subject Area: Structural Engineering, Building Informatics and Construction Operation
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 564881048

The variety of materials and connection techniques used in timber construction has increased significantly in recent years. The load-bearing capacity of connections is often determined empirically or by using individual calculation methods, parameters and correction factors. Standardised design methods are often not available, even for very similar failure mechanisms. The lower limit theorem of plastic theory is already used in timber construction for the design of dowel-type fasteners according to Eurocode 5 and for the design of bracing structures. (Tanadini, 2023) derived the first general formulations for determining the ultimate loads of timber connections by determining the yield conditions from various failure mechanism. The stress field models were then used to determine the ultimate loads for interlocking timber connections of different geometries. On the other hand, a high- definition fiber-optical sensor (HD-FOS) measurement screw with a continuous sensor has been developed by (Claus et al., 2022). These measurement screw can be used to determine the bond stress along screws very accurately in experiments. It has been shown that higher bond stresses could be transmitted under additional compressive stresses from a biaxial stress state. This shows that the load carrying capacity of fully-threaded screws can be highly depending on the external conditions, such as the arrangement of direct or indirect supports. These effects are currently not considered in the design of screw fasteners, yet. The aim of this project is to better understand the behaviour of fully-threaded screws and timber connections and to develop a general approach to the determination of the load bearing capacity of connections in timber construction based on the theory of plasticity. The calculation method can be used to describe specific phenomena in different materials. It is therefore in contrast to the development of individual theories for individual phenomena, as currently found in standards and building approvals. Experimental results from the bonding behaviour of fully-threaded screws and comparative numerical calculations using linear finite element modelling (FEM) with evaluation of stress trajectories will be used to develop a systematic approach for designing stress fields for anisotropic wood-based materials. In particular, the formation of stress nodes, load distribution and the introduction of stress fields in the area of screws (bonding) or direct supports will be considered. Finally, the ultimate load from the stress fields will be determined using the yield conditions from the failure mechanisms for the anisotropic material behaviour of wood. In addition, the specimens will be implemented with the FEM non-linear contact elements to determine the calculated failure load of the screwed joints. Finally, the results will be compared with the load capacities obtained and with those reported in the literature.

• DFG Programme: Research Grants