Development and Experimental Evaluation of a Scarf Joint for Solid Timber Ribbed Slab Beams

Sammanfattning

Ribbed floor systems are already considered material-efficient structural elements. However, the maximum available length of solid structural timber, typically 5.4 m, limits their application in long-span structures. This study investigates an optimised scarf joint as a production-feasible joining method for solid timber beams in ribbed slab floor systems. The joint geometry was designed based on the stress distribution in the cross-section, using a peel stress criterion to determine the optimal inclination angle as a function of cross-sectional height. Three configurations with overlap lengths of 600 mm, 900 mm and 1200 mm were manufactured using CNC milling and adhesively bonded with a one-component polyurethane adhesive. The assembly was carried out in a single production step where the same pressing forcewas used to simultaneously press the scarf joint and the web-to-flange bond, in a manner representative of the production process used for ribbed floor elements. Experimental testing was conducted using four-point bending according to EN 408 on 15 test specimens with T-shaped cross-sections, representing a single rib of a ribbed floor system, with five specimens per joint configuration. The results show that bending stiffness was not significantly influenced by the overlap length, with all three variants reaching values close to the 95 % acceptance level for strength class C24 according to EN 14081-3. Failure mode analysis showed that the 600 mm joint was governed by bond-line failure in three of five specimens, while the 900 mm and 1200 mm joints consistently failed at knot defects in the timber. Based on the observed failure modes and bending strengths, a 900 mm overlap is considered the minimum configuration required to ensure that the beam is not governed by the capacity of the joint. The 1200 mm overlap provided no structural advantage over the 900 mm configuration. The environmental impact of the three scarf joint variants was compared with that of a reference plywood gusset plate joint. The comparison was conducted in terms of GWP-fossil over life cycle stages A1–A3.The 900 mm scarf joint reached approximately 46 % of the GWP-fossil of the plywood joint, with the difference primarily attributable to the higher embodied carbon intensity of birch plywood relative to solid sawn timber. The study showed that the adhesively bonded scarf joint, designed based on the cross-sectional stress distribution, can achieve bending stiffness and strength consistent with solid C24 structural timber. The 900 mm joint is identified as the most suitable overall configuration, combining sufficient structural performance with low GWP-fossil.