Mechanical stress grading of tropical timbers without regard to species

Abstract

Some reports have shown that for single species the c,)rrelation between modulus of elasticit\· (MOE) and modulus of rupture (MOR) in bending is quite high. Tropical timbers consist of hundreds of species that are difficult to identify. This report deals with the mechanical stress grading of tropical timber regardless of species. Nine timber 'species or groups elf spc:cies with a total Dumber.of 109.+ pieces measuring 60 x 120 x 3000 mm, were tested in static bending. The MOE \\'a~ measured flat wise, while MOR was tested edge \\'ise. Statlsllcal analysis of linear regression with a dummy model and anillysis of covariance were used to analyze the role of l\lOE and the effect of species on prediction of MOR. The analysis sho'1ed that using MOE as a single predictor caused under/overesti'mation for one or more species and/or ;groups of species. The accuracy of: prediction would be increased with species identification. An allowable stress and refei'ence resistance for species and/or groups of species \\ere provided to compare with the prediction of strength through timber grading. The timber strength class for species and/or groups of species was also established to support the application of mechanical timber grading.