Structural performance and analytical modelling of hybrid pine-poplar glulam beams through efficient use of resources

This paper presents an experimental and analytical study on the structural performance and resource efficiency of hybrid glulam beams manufactured from pine ( Pinus nigra ) and poplar ( Populus × euramericana, clone MC ). In hybrid glulam beams, the modulus of elasticity is inherently non-uniform, varying both longitudinally and transversely because individual boards exhibit spatial stiffness variability along their length and across the cross-section. The main objective is to quantify the mechanical benefits of pine-poplar hybridization and to develop a predictive formulation for the beam modulus of elasticity accounting for these longitudinal and transverse stiffness distributions. Single-species and hybrid glulam beams were manufactured from pine and poplar boards and characterized by non-destructive testing and four-point bending tests according to UNE-EN 408. A new analytical formulation was developed to predict the beam modulus of elasticity from the spatial distribution of board elastic moduli, explicitly considering the strategic placement of the highest-stiffness boards in the outer lamellas, where bending stresses are maximum. The results show that this selection and placement increases the modulus of elasticity of hybrid glulam beams by 21 %, reaching values comparable to single-species pine beams, while also increasing flexural strength by 18 % and reducing beam density by 22 %. These findings are relevant for both researchers and the structural timber industry, enabling efficient, lightweight, and competitive hybrid glulam solutions for structural applications.