New simulations challenge how engineers calculate safety of wooden structures
Researchers used computer modeling to test whether current design standards for timber connections—the bolts and dowels that hold wooden beams together—accurately predict when they'll fail. The findings could reshape safety calculations for everything from timber buildings to hybrid steel-wood construction, affecting engineering codes and project costs.
Originaltitel: Numerical simulation of the effective number of fasteners in dowelled timber connections
Dagens dimensioneringsstandarder för träkopplingar med tappar underskattar risken för sprödbrott i stål-trä-förband och dragbelastade förbindelser. Forskarna vid Minho-universitetet, tillsammans med Linnaeus University och Politecnico di Milano, validerade en numerisk modell baserad på linjär elastisk brottmekanik för att testa befintliga empiriska uttryck för effektivt antal tappar. Simuleringarna visar att förbindelstyp och belastningsriktning påverkar brottbeteendet väsentligt — stål-trä-kopplingar och dragbelastade förband misslyckas tidigare än trä-trä-förbindelser under tryck, vilket ger lägre effektivt antal tappar. Dagens designekvationer fångar inte dessa kritiska faktorer tillräckligt väl. För konstruktörer och materialvetare innebär detta att standarderna behöver uppdateras för att minska överbelastningsrisken i moderna hybridkonstruktioner. Resultaten efterfrågar både experimentell validering och probabilistisk analys innan nya dimensioneringsregler kan implementeras.
Abstract The capacity of dowel-type timber connections is generally governed by brittle failure modes, such as splitting and row shear failures. In design standards, this is indirectly addressed through the effective number of fasteners concept. This parameter accounts for both the non-uniform load distribution and the interactions among fasteners, which can induce brittle failure before the fasteners’ capacity is fully mobilised. Although the current expressions for the effective number of fasteners in European design codes are based on tests on timber-to-timber connections, they are also applied to other configurations, such as steel-to-timber connections. The present study investigates, through numerical simulation, the validity of existing empirical expressions under various loading conditions and connection arrangements. The numerical model adopted was previously proposed and validated by the authors and is grounded in linear elastic fracture mechanics. The model validation performed herein on the original experimental dataset used to derive the empirical expressions for the effective number of fasteners further strengthens its validity. The parameters influencing the connection response across different configurations are further evaluated, including the fastener slenderness ratio and the spacing between fasteners. The findings reveal that both connection arrangement and loading direction (compression vs tension) substantially affect failure assessment and, by extension, the effective number of fasteners. Specifically, steel-to-timber and tension-loaded connections tend to fail earlier than timber-to-timber connections under compression, thereby leading to a lower effective number of fasteners. These critical factors are not adequately captured in current design equations, underscoring the need for revised approaches, supported by further experimental validation and probabilistic assessment, to improve the reliability of design expressions for the effective number of fasteners.