Influence of the manufacturing process on the stress state of curved glulam beams

This study examines the spring-back effect and residual stresses in curved glued-laminated timber (glulam) beams during the manufacturing process. In addition to curving along the longitudinal axis ($X$), a cup-deformation develops in the transverse direction ($Z$) due to the Poisson’s effect. This deformation, combined with the glue-hardening process and the release of pressure, leads to the development of residual stresses in all three axial directions, as well as shear stresses within individual boards and at their interfaces.

Besides the well-known factors such as longitudinal elasticity, board thickness, and inner radius, the study reveals that the number of layers ($n$) and Poisson’s ratio (${\nu }_{LT}$) significantly influence the magnitude of residual stresses. However, aside from longitudinal stresses, the impact of $n$ and ${\nu }_{LT}$, as well as other residual stresses, have been scarcely studied and are not adequately addressed in current design standards.

A Monte–Carlo analysis of the growth-ring effect is conducted, taking the pith location of different board layers as the input random variable. Strong influences can be identified on the residual stresses in both radial and tangential directions, with intensified maximum values and more scattered distribution inside the cross-section. The time- and moisture-dependent relaxation analysis using the rheological wood model shows a significant influence of the temperature and relative humidity.