Strength and Elastic Properties of Two-Species Laminated Wood Beams

A series of tests were performed to determine the validity of the transformed section (transformation theory) principle of calculating the strength and elastic properties of two-species beams. The test was ASTM bending on 2- by 2- by 30-inch specimens. All of the seven species used came from the American tropics. The core material in all specimens was Cordia alliodora (0.48 sp. gr.). The faces were higher density species (from Magnolia sororum, 0.56 to Manilkara bidentata, 1.03). There were six variations of face thicknesses– 1/32, 1/16, 1/8, 1/4, 3/8, and 1/2-inch. The moduli of elasticity were not equal to those calculated. A statistical analysis indicated that significant differences existed among face stiffness (species) groups, but there was no trend in the MOE ratio data as the thickness of the face material varied. The ratio of actual to calculated MOE was expressed as 0.954. The modulus of rapture value, were not equal to those calculated. Significant differences existed among both face species and face thickness groups. To each group of data, a straight line was fitted by the least squares method and the slope of each regression line examined. When these data were examined by a multiple regression analysis the results were: Y = 1.061 – 0.2469 X1 – 0.00000589 X2 Where Y = ratio of M of R to calculated, X1 = face thickness (inches), and X2 = modulus of rupture (psi). A table of MOE correction factors was presented for any of 17 combinations of face thickness (0.50 to 0.98 ratio of core depth to total depth) and any of 12 combinations of face strength (0.400 to 0.950 ratio of strength of core species to face species). Fiber stress at proportional limit values were not equal to those calculated. Significant differences existed among face species groups but none existed among face thickness groups. It was recognized that fiber stress at proportional limit values are not very reliable. The conclusions state that calculations of strength and elastic properties by the equations originally developed for plywood and adopted to two-species laminates do not accurately predict modulus of elasticity, modulus, of rupture, or stress at proportional limit.

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