Bio-mechanical properties of Klainedoxa Gabonensis and the strength performance of its Dovetail and Mortise-Tenon joints in leg-and-rail construction

Wood product demand far outstrips timber supply. Secondary timbers, likely-substitutes for endangered traditional hardwoods, lack information about their engineering applications. Furniture strength depends on their joints, design and timber suitability. Thus, the extent to which furniture producers utilize non-traditional timber species was ascertained by interviewing 300 Timber Firms from Kumasi and Accra using the stratified random sampling technique. Key bio-mechanical properties of Klainedoxa gabonensis (Lesser-Utilized-Species [LUS]) and Entandrophragma cylindricum (a notable building and furniture material), which was the control as well as the strength of two dovetail and mortise-tenon joint-designs from the two timbers were further investigated based on BS 373, ASTM D 1037-06a (24), EN 252 and the International Association of Wood Anatomists (IAWA). Most manufacturers (85%) hardly use any LUS; 44% of these attributed this to lack of information on their properties and prospective uses and 32% to their non-availability on the markets. K. gabonensis moisture content and density (at 12% mc) were greater than those of E. cylindricum. The former’s Tangential-Radial ratio for swelling (1.31–1.38) and shrinkage (1.58–1.63) are within acceptable thresholds (<1.6 and <2.5 respectively) for structural timbers. Anatomically, K. gabonensis is diffuse-porous, has simple perforation plates, sclerotic tyloses, prismatic crystals and thick-walled fibres forming the greatest proportion (42.4±4.5–45.6±4.5%) of all its tissues. Its fibre diameter (20.1±0.2–22.7±0.2 μm) and double wall thickness (8.9±0.3–9.7±0.3 μm) were greater than those of E. cylindricum (19.6±0.4–19.7±0.3 μm and 8.6±0.2–9.0±0.2 μm respectively). Vessel lumen diameters for K. gabonensis (144.8±2.2–176.9±4 μm) and E. cylindricum (115.6±1.4–184.4±2 μm) compare with those reported for tropical diffuse-porous timbers (<200 μm). K. gabonensis heartwood and sapwood were found very durable (Visual Durability Rating: 1; mass loss: 4.8±0.3%) and iv durable (1; 8±0.6%) respectively against termite attacks, E. cylindricum heartwood was durable (2; 10±0.7%) and sapwood was moderately durable (3; 13.1±0.6%). Dovetail joints were strongerh (e.g., K. gabonensis heartwood: 913.8±49.2) than mortise-tenon (e.g., K. gabonensis heartwood: 745.9±59.7). For both joints, the design with longer, wider and thicker tails and tenons (Type LS) was stronger (e.g., K. gabonensis heartwood dovetail: 913.8±49.2 Nm) than those with shorter, narrower and thinner tails and tenons (Type SS) (e.g., K. gabonensis heartwood dovetail: 745.9±59.7 Nm). K. gabonensis joints were also stronger (e.g., heartwood Type LS dovetail: 913.8±49.2 Nm) than those from E. cylindricum (heartwood Type LS dovetail: 759.6±16.8 Nm). To improve on the level of LUS utilization including K. gabonensis, wood workers must be supplied with comprehensive information about their properties and economic values. The superior physico-mechanical and biological properties of K. gabonensis, its abundance (> 396 m3km-2) and great amount of biomass (diameter: 120-150 cm; height: 45-50 m) make it a suitable material for the building/construction sector, the wood and other related industries. Its use will contribute to reducing pressure on the primary timbers, ensuring consistent supply of wood and keeping the timber sector always operational. K. gabonensis chairs designed with dovetail joints and longer, wider and thicker tails would resist bending forces better and ensure greater strength of furniture than mortise-tenon.

Source: KNUST