Lashed, Pegged, or Notched: Computed Strengths for Timber Joints Without Metal, and a Selection Rule

Timber can be joined without any metal at all, by lashing members together with rope, by driving wooden pegs through them, or by cutting one member to fit into a notch in the other. Builders who work where nails, bolts and plates are scarce or absent choose between these three families largely by habit. This paper computes the working capacity of each family, under stated assumptions, for tension, for shear and for bending moment, and turns the three sets of numbers into a selection rule. The study is simulated, not measured, and no joint described here has been load-tested. A lashing of 10 turns on a 100 mm member resists only 2.1 kN in slip and 4.2 kN in tension in this model, and its moment capacity is 0.105 kNm — an order of magnitude below the other families in every load case. A single hardwood peg of 25 mm diameter in a 100 mm member carries 4.42 kN in single shear and 8.84 kN in double shear, governed by shear across the peg rather than by bearing in the member, for which the corresponding figure is 15.0 kN. A notch cut one quarter of the depth into a member retains only 56.2% of the member's unnotched bending capacity, but its remaining capacities are still the largest of the three families in tension and in direct shear, provided the shear-out length beyond the notch is generous: a 150 mm member notched to that depth still carries 135.0 kN in tension and, with 200 mm of wood beyond the notch root, 33.0 kN in shear-out. The selection rule originated here is therefore not a ranking of good and bad joints. It is a matching of load case to mechanism: use a notch where the geometry allows a long shear-out length and the load is tension or direct shear; use pegs, preferably in double shear and in groups, where members must be joined face to face or taken apart; use lashings only for light, temporary, or easily inspected shear and positioning loads. For one load case none of the three is adequate: transferring a substantial bending moment through a joint in a 100 mm member. Against a demand of 2 kNm the best of the three, a peg pair at 200 mm spacing, offers 1.502 kNm, a notch offers 0.938 kNm and a lashing offers 0.105 kNm. In that case the builder should not search for a stronger version of any of these joints. The geometry itself must change, so that the moment is carried as a pair of opposite forces in triangulated members, or the joint is moved to a point where the moment is small.