Investigating the use of bow for prestressing lamellas of glulam beams made with young Eucalyptus grandis timber

Abstract

Young plantation forests can be used as a resource for structural sawn timber products to help reduce the embodied carbon emissions of buildings. Eucalyptus spp. are amongst the most promising species due to their adaptability and high growth rates. However, young Eucalyptus forests often produce timber containing defects such as excessive warp (due to growth stresses) and knots. We hypothesize that using lamellas that are bowed can result in prestressed glulam beams and, in this way, improve their load capacity and lower the impact of knots. Eighteen (18) glulam beams were manufactured with lamellas of varying bow distortion levels. Nine of the beams had the pre-tensioned side coincident with the tension side of the beam, and the other nine with the compression side. Results indicate that lamellas with increased bow did have better inherent flexural properties than straight lamellas, and hence more bowed lamellas resulted in beams with improved bending strength and stiffness irrespective of pre-tensioning. On the other hand, the prestress added to the beams through the straightening of lamellas did not influence the beams’ stiffness or strength. In total, 80% of the lamellas had bow above the limits established by a usual standard for hardwood timber which would normally result in a rejected product. Utilizing bowed timber for laminated beams result in a high value product and reinforces the potential of young fast-growing plantation forests to provide raw material to the timber and construction industries.