Experimental and numerical analysis of a stop-splayed traditional timber scarf joint with key

Traditionally constructed heavy timber trusses employ scarf joints to transfer tensile forces between lower chord members. Scarf joints significantly impact the global stiffness of heavy timber trusses, yet little is known about their structural behavior. This paper describes a study recently completed on the commonly used stop-splayed scarf joint with a key. Experimental tests were performed on scarf joints replicated from a covered wooden bridge in Pennsylvania and compared to the numerical results of full, three-dimensional finite element models created using ABAQUS. The authors determined that the key has the most influence on scarf joint behavior as its orientation causes it to be loaded in compression perpendicular to grain. Experimental tests also revealed the importance of clamping bolts, without which a limit state of “key rolling” was observed. With two clamping bolts in place, the limit state was shear failure parallel to grain, which provides a more stable failure and a higher ultimate strength. The authors recommend to engineers that the scarf joint not be analyzed as a pure tension member; rather, it should be considered a member subject to combined tension and bending forces.