Development of structural laminated veneer lumber from stress graded short-rotation hem-fir veneer

The key objective of this work was to investigate the feasibility of using short-rotation western hemlock (Tsuga heterophylla (Raf.) Sarg) and amabilis fir (Abies amabilis (Dougl.) Forbes) from coastal Britich Columbia, Canada for manufacturing structural laminated veneer lumber (LVL). Fourteen hem-fir logs were sampled, bucked and conditioned. Hem-fir veneer was then peeled, clipped, dried and visually graded. Combined hem-fir veneer was further segregated into three E grades based on dynamic modulus of elasticity (MOE). LVL billets were manufactured from each E grade and from two grade mixes, and evaluated for flatwise and edgewise bending MOE and modulus of rupture (MOR), and longitudinal shear strength. The results demonstrated that hem-fir veneer visual grade yield was about 15% B with the remainder C. No correlation existed between hem-fir veneer visual grades and E grades. Hem-fir LVL made from high E1 grade, medium E2 grade and low E3 grade could meet 2.2E, 1.8E and 1.5E product market requirements, respectively. Higher E grade veneer yielded higher LVL bending MOE, but not necessarily higher LVL shear strength. A good correlation was found between the LVL bending MOE and veneer mean MOE, and between the LVL bending MOR and MOE. Thus, the bending performance of the hem-fir LVL can be predicted based on veneer E grade and product lay-up. Pressing time changed clearly with the LVL lay-up with higher E grade requiring longer pressing time. Pressing time of mixed grade LVL fell within the times required for the two single grade lay-ups. Mixed grade LVL had a greater enhancement in flatwise than in edgewise, and was therefore more suitable for fabricating flanges of I-joists. Using a grade mix in the product lay-up allows low E grade veneer to be fully utilized for increased value recovery. The results of this study implied that without species segregation, the combined short-rotation hem-fir veneer can be successfully stress graded to manufacture structural LVL for building applications.