ABSTRACT
The work presented EN338 (2009) strength classification for some common Nigerian timber species namely: Antiaris africana, Guarea cedrata, Piptadeniastrum africanum, and Isoberlinia tomentosa. The work also assessed the suitability of the Nigerian timber species for use as simply supported bridge beams according to EN1995-1-1(2004), EN1995-2(2004), EN1991-2(2003) and BS EN1991-1(2002). In developing the EN338 strength classes, samples of the species were obtained for laboratory experimental measurements in order to determine the physical and mechanical properties of the selected timber species in accordance with EN13183-1(2002), ISO3131(1975), EN408(2003) and ASTM D143(2000) using four point and three point bending tests. The generated properties were used to obtain the characteristic values of the physical and mechanical properties according to BS EN 384(2004) with the aid of EasyFit (2010) statistical package. Other material properties such as tensile and compressive strengths parallel and perpendicular to grains, shear strength as well as shear modulus were obtained in accordance with the EN338 recommendations. Antiaris africana, Guarea cedrata, Piptadeniastrum africanum and Isoberlinia tomentosa were assigned to D18, D35, D40 and D40 strength classes respectively. Significant variability within and between the physical and mechanical properties obtained on sub-samples collected for each species was identified using One-Way ANOVA and measures of dispersion. Appreciable sensitivity of the physical and mechanical properties on change in moisture content of the selected timber species was observed. Distribution models namely: Gamma, Gumbel, Normal, Log-normal and Weibull were fitted to each specie reference material properties and the goodness of fit investigated using Kolmogorov-Smirnov fitness test at 5% significance level. The fitness test results revealed that density, bending strength and modulus of elasticity of all the species investigated can best be modeled using Normal, Log-normal and Weibull distributions respectively. The outcome of structural reliability and sensitivity analysis on the selected timber species as simply supported bridge beams using First-Order Reliability Method considering bending, shear, deflection and end bearing failure modes revealed the order of criticality of the failure modes investigated and optimal utilization of each specie for this purpose under the specified operating conditions with for example, Piptadeniastrum africanum found safe and reliable over a simply supported span of 4500mm, depth of 600mm, breadth of 200mm and end bearing length of 300mm. The probabilities of failure for the species under the above failure modes and operating conditions are: 0.16 X 10�’2, 4.15 X 10�’3, 2.07 X 10�’3and 2.64 X 10�’2 respectively.