42 C O N T E X T 1 3 4 : M A Y 2 0 1 4 Richard Harris,WenShao Chang and Peter Walker are at the BRE Centre for Innovative Construction Materials at the University of Bath. Jon Shanks is CSAW Research Fellow at the school of architecture and design at the University of Tasmania. yielding of a metal dowel, the plateau of load before final failure gives prevents a sudden failure and enables a similar approach.The method used the expressions that form the basis for the methods adopted by the Eurocode and in very wide use for metal dowels (Johansen K 1949). A model for calculating connection stiffness was also developed (Shanks 2005). The tests showed that an average ultimate strength of the pegged connections loaded in double shear was approximately 12kN (Shanks 2005). Significant deflection and energy absorption is observed with the pegged connections at the ultimate strength load, achieving the desired ductile failure mode. The research project included tests on full-size frames. The methods of evaluation of joint load and stiffness were used to predict the frame performance and the results were published (Shanks and Walker 2006).The method uses ‘embedment values’, which are a measure of the bearing strength of the dowel on its hole.This was evaluated using bearing tests developed for the purpose.The method leads to good predictions of load, although more validation work is needed if they are to be used on real structures. The conclusions of the work, as set out in Shanks andWalker 2005, were summarised as: • The three commonly used peg types in mortice-andtenon connections (cleft tapered, cleft-die driven and turned) have similar stiffness and strength characteristics in pull-out loading. • Pull-out failure of a traditional pegged mortice-andtenon connection is ductile. • The effect of long-term loading and creep has not been investigated as part of this research. Joints were tested under very short term loading, but would be less stiff under much-longer-term loading. • The EC5 method for calculating steel dowelled connections can be used to reasonably predict the failure load of timber-dowelled connections. • The peak load resisted by the connections in the pull-out tests is related to the dry density of the timber used to fabricate the dowels. • Moment resistance of a pegged mortice-and-tenon connection can be related directly to the pull-out characteristics of the joint. •The fit of the tenon within the mortice has an important influence on the load carrying capacity and action of the mortice and tenon in pull-out, bending and shear. The research into traditional joints has continued internationally at the University of Kyoto (Shanks et al 2008), and University of Tasmania (Turbett 2013). Furthermore, it has led to a fertile theme of follow-up research at the University of Bath.The mode of failure applies to all fibrous dowels. The BRE Centre for Innovative Construction Materials (BRE CICM) has continued the work in developing modern joints with GFRP dowels) (from Thomson et al 2010). These behave in a similar manner to oak dowels, enabling a joint with strength and stiffness approaching steel but for lower cost, and better fire and corrosion resistance. The work shows that traditional methods, developed over years of trial and error, can inform and inspire the development of modern methods. However, the main output of the work is in enabling structural engineers to make better predictions of the behaviour of traditional frames. The most common ‘failure’ for contemporary reinterpretations of traditional green-oak framing is for the frames to fail to meet the serviceability limits for the structure.That is, the frames move under wind load, or creep over time beyond levels acceptable to the client.These deflection limits are often far more stringent than in historical structures because of modern building performance requirements such as sealing doors and windows. Proper understanding should allow engineers to make better assessment of the strength and stiffness of these frames, ensuring that traditional methods can be used for repair and ensuring that intervention is made only when absolutely necessary. ELEVATION PLAN Draw bole – solid line is hole in mortice, dashed line is hole in tenon 89mm 19mm 19mm 100mm 150mm 3mm 200 x 200mm 38 mm 38 mm Peg hole 100 x 150mm Tenon member Mortice member Four-hinge failure of oak dowel in a traditional joint (Shanks 2005) A load-deflection plot, showing a joint with correct detailing producing a stiff initial response, followed by a long plateau of load as the wedged tenon pulls out A typical joint geometry tested
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