Properties of 10 Ghanaian high density lesser-used-species of importance to bridge construction – Part 2: mechanical strength properties

Logs from sixty trees of ten high density Lesser Used Species (LUS) of potential importance to bridge construction were extracted from four forest reserves - Bobiri, Pra-Anum, Nueng, and Subri River in four different ecological zones. The logs from the trees were converted on a horizontal bandmill to 27 and 53 mm thick boards. One-half of the boards were used in the green state and the other half air dried. Mechanical strength test specimens were prepared and tested in accordance with the British Standard BS 373:1957. The range of mean strength values in N/mm2 in the ‘green’ [and dry] conditions for the ten wood species were as follows: - Modulus of Rupture: 69-132 [104-188], Modulus of Elasticity: 7,970-13,740 [9,740-17,620], Compression parallel to grain: 31-63 [54-92], Shear parallel to grain: 8.8-22.1 [15.1-15.1], and Hardness in kN: 3.5-11.3 [5.1-17.0]. The 10 species in decreasing order of strength was as follows: Lophira alata (Kaku) > Cynometra ananta (Ananta) > Strombosia glaucescens (Afina) > Celtis mildbraedii > Nauclea diderrichii (Kusia) ≥ Celtis zenkeri ≈ Piptadeniastrum africanum (Dahoma) > Nesogordonia papaverifera (Danta) > Combretodendron africanum (Essia) > Sterculia rhinopetala (Wawabima). Mean ratios of dry to ‘green’ MOR and MOE were 1.44 and 1.28. There was a good correlation (84.4~98.2%) between the Density (X) and the various mechanical strength values (Y). Regression equations in the form: Y = aXb were derived with R2 values of 0.72-0.93.