Joints in traditional carpentry often rely on compression to keep the faces of the timber in close contact without anything other than notches in connected members.
A widespread classic connection at the heel joint in timber trusses is the front notched joint, with a single tooth in the strut, a notch in the top face of the bottom chord and sometimes metal fasteners. This joint directs the horizontal component of the internal compression force in the strut to the bottom chord, through compression of the front notch and a shear surface in the toe of the chord.
These joints rely on the compression internal forces to keep facing surfaces in close contact. Metal fasteners are more common in Western cultures than in Oriental joint craft.
Without these this joint relies on the friction developed between the timber elements in the front notch, which is highly dependent on the compression level in the strut, and cannot support alternating connection forces.
Eurocode 8  requires compression members of carpentry joints to “be designed in such a way that they are prevented from separating and remain in their original position”. This may mean that metal elements are necessary to comply with the safety requirements, assuring the contact between friction surfaces and stabilizing the performance under cyclic actions. On the other hand, often the metal parts are used to strengthen the joints, without adequate knowledge on the consequences of such actions on the joint and on the overall structural behaviour.
The heel joint is usually considered as hinged and the internal forces acting in the connection are the axial compression in the strut, the tension in the chord and the vertical support reaction. The widespread method, commonly presented in the related literature to assess the strength of this type of joint relies on this assumption. This implies checking the compression stresses at an angle to the grain in the frontal notch surface (if the strut and the chord timber belong to different strength classes this must be done in both the strut and the chord) and shear stresses in the timber beyond the notch.
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