C O N T E X T 1 3 4 : M A Y 2 0 1 4 41 A joint before and after shrinkage Failure modes for dowelled joints in timber: mild steel (left) and oak (right) A typical brace-post joint in a traditional frame compression load requires considerable movement. On the other hand, carpenters will point out that there are no historic precedents of the carefully crafted pegged joints failing in tension. Thus, the simplifying assumption of full-load transfer in compression and zero in tension is at odds with carpentry knowledge. Shanks’ work has revealed a different mode of failure from that generally assumed. Modern joints in timber are, by and large, made with metal connectors (nails, screws and bolts). The engineering method of design for metal dowels assumes that the joint fails through a combination of bearing failure of the wood and plastic yield of the metal connector. Howevr, failure of the traditional timber-pegged joints occurs almost solely in the oak peg. On being loaded, oak-pegged joints develop large shear deflection, which leads to the oak peg being trapped in the space between the faces of the wood. In a traditional joint this is the space between the mortice wall and the tenon.The question the research posed was whether or not the same method of evaluating connector capacity could be modified for use in traditional joints.The model used for steel dowels is a good one. The research has revealed that, although an oak peg behaves in a very different way, a similar method can be applied to evaluate the strength of oak-pegged joints. Design of structures should avoid sudden brittle failure. A key revelation of the research is that, because of the wedging effect of the peg, following initial failure there is a quasi-ductile pull-out as the tenon is withdrawn from the mortice. The Green Oak Carpentry Company, Carpenter Oak and Woodland Company, and Oakwrights fabricated mortice-and-tenon test connections, comprising a 150 x 100mm stud tenoned into a 200 x 200mm beam.The mortice was 150mm long, 40mm wide, 100mm deep and inset 38mm from the face of the beam. The tenon was 150mmwide, 38mm thick and 89mm long, and also inset 38mm from one face of the stud.The connections were pegged with 19mm die-driven dowels, tapered pegs or turned pegs inserted through 19mm-diameter holes. The tenon hole was offset from the line of the mortice holes by 3mm, towards the face of the beam, to induce a ‘draw’ which tightens the joint.The joint dimensions and draw follow common carpentry practices in the UK. Using well-established carpentry rules for the joints is important. The tenon is restrained in the mortice, and adequate wall thickness for the mortice prevents the tenon splitting and the peg pulling out prematurely. If the peg is to pull out through the tenon, a section of the tenon, or ‘relish’, must be sheared parallel to grain and correct end distance for the peg in the tenon prevents this. The research led to a proposed design method for connection strength.This is based on a modified version of the method used for metal dowels. While the shear deflection of an oak peg is very different from the plastic Shoulder opening from di erential shrinkage Brace after shrinkage Brace before shrinkage POST POST Shoulder opening from di erential shrinkage Brace after shrinkage Brace before shrinkage POST POST References Shanks JD andWalker P, 2005, ‘Experimental performance of mortice and tenon connections in green oak’, Structural Engineer, 6 September Shanks JD andWalker P, 2006, ‘Lateral strength of green oak frames: physical testing and modelling’, Structural Engineer, 6 September Shanks, J, Chang,W-S, and Komatsu, K, 2008, ‘Experimental study on mechanical performance of all-softwood pegged mortice and tenon connections’, Biosystems Engineering, doi:10.1016/j. biosystemseng.2008.03.012 Thomson A, Harris R, Ansell M andWalker P, 2010. ‘Experimental performance of non-metallic mechanically fastened timber connections’, Structural Engineer, 7 September Turbett, S, ‘Native Tasmanian all-timber connections’, bachelor engineering dissertation, University of Tasmania, 2013
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