Comparison of ASTM D 198 and five-point bending for elastic constant ratio determination

Current design of structural composite lumber (SCL) assumes that the ratios of Young's modulus (E) to in-plane shear modulus of rigidity (G) are equivalent to values associated with solid-sawn lumber. Yet there is no published research presenting a direct comparison of the elastic constants for SCL and solid-sawn lumber. Accordingly, a study was conducted to better define elastic constants and appropriate E:G ratios evaluated for three kinds of SCL, including laminated veneer lumber (LVL), parallel strand lumber (PSL), and laminated strand lumber (LSL), and one type of solid-sawn lumber. This paper reports on data collected applying the ASTM D 198 test standard and the non-standard five-point bending test (FPBT) for elastic and shear moduli determination. Data collection included edgewise and flatwise loading in the longitudinal direction to determine the modulus of elasticity (E-1) and planar shear moduli (G(12) and G(13)). The edgewise E-1:G(12) from LVL was the only SCL elastic constant ratio comparable to solid-sawn lumber values. Large differences in the SCL shear moduli values were observed compared to solid-sawn lumber. This research concludes that there are differences between I he E:G ratios of solid-sawn lumber and SCL materials. Comparison of solid-sawn and LVL ASTM E:G ratios with previously published literature was favorable. A comparison of the ASTM D 198 and FPBT results shows that all solid-sawn lumber elastic constants as well as the SCL elastic moduli were less than 10 percent different. The FPBT yielded less test measure variability than the ASTM testing. The FPBT was felt to provide an accurate and simple evaluation of the shear modulus properties for wood and wood composites.