Impact of steel-to-timber joints with screws on the failure modes and reliability of a structurally indeterminate timber beam

Artikel i vetenskaplig tidskrift, 2026

The nonlinear behaviour of timber joints is considered in Eurocode 5 by prescribing a stiffness reduction ratio; specifically, the joint stiffness at the ultimate limit state is calculated as two-thirds of its serviceability limit state value. The system effects, i.e. how the joints interact within the global structure, are not taken into account. These simplifications may be unreliable for modern statically indeterminate structures, potentially leading to non-conservative or overly conservative designs. Steel-to-timber joints with self-tapping screws are widely used and offer high stiffness and load-carrying capacity. This study aims to assess the effects of the current design rules on the reliability of a statically indeterminate timber beam with steel-to-timber joints with self-tapping screws and subjected to a uniformly distributed load. The uncertainties in material properties, the load, and the nonlinear behaviour of joints are considered in the probabilistic model of the structure. For semirigid joints with laterally loaded screws, applying the code stiffness reduction ratio of 2/3 roughly doubles the probability of failure compared to the design target. Moreover, the required stiffness reduction ratio is found to be a function of relative joint-beam stiffness and the load-to-screw axis angle. The reliability-based calibrated stiffness ratio ensures that the target probability of failure is met. The framework can also be extended to other joint typologies for safer design.