Hybrid wood species design for structural elements: experimental investigations at two scales

The growing demand for wood in construction increases pressure on forest resources, among others, due to reliance on a limited range of species. Expanding the use of underutilised, low-density, fast-growing species such as grand fir could reduce this pressure and create new opportunities for construction. In this context, this paper investigates a hybrid wood species design approach that combines fast-growing, low-density softwood with reclaimed hardwood for structural applications. Experimental investigations were carried out on pure grand fir and hybrid specimens composed of grand fir and reclaimed oak, arranged in various cross-sectional configurations and tested at both small clear wood and structural timber scales. Analytical estimations were performed at both scales, employing a transformed-section method to predict bending resistance. In the small clear wood tests, the compressive and bending properties of both pure grand fir wood and hybrid specimens were examined. The hybrid specimens demonstrated significant potential for improving mechanical properties, with compressive strength, modulus of rupture, and modulus of elasticity increasing by up to 63.8%, 78.6%, and 41.2%, respectively. For the structural-sized timber specimens, three-point bending tests were carried out. One pure grand fir and two hybrid cross-sectional configurations were selected based on the results of the small clear wood tests. The hybrid specimens exhibited up to a 30.9% increase in bending resistance compared to the pure grand fir specimens. Furthermore, good agreement between the experimental results and the analytical estimations was observed at both scales. The study demonstrates the potential to enhance the mechanical performance of structural elements through hybrid wood species design, as well as the feasibility of predicting the bending resistance of structural-sized timber from small clear wood properties.