Strength and stiffness of glulam beams reinforced with glass and basalt fibres

Timber has throughout history played an important role in all kinds of construction work as it is easy to work with and has many advantages compared to alternative materials. Through engineered timber products such as glulam, the natural variability in material properties of timber has been reduced, providing stronger, stiffer and more reliable structural elements. In recent years' new techniques have emerged to enhance these properties even further by using FRP materials to strengthen structural timber components. The FRP materials have more tensile strength and higher modulus of elasticity than timber. In recent years' experiments have been conducted in the Structural and Composite Laboratory at Reykjavik University (SEL) in order to investigate the effect of strengthening glulam beams using FRP materials. In these experiments, 24 glulam beams (3,2 m long, 65 mm wide and 167 mm high) were tested. Half of the beams were in strength class GL32h and the second half were in strength class GL30c. The beams were tested in a four-point bending test. The beams were randomly divided into groups with 3 or 4 beams in each group. For each group the bottom laminate of the beams was reinforced with slack basalt fibres, slack glass fibres, pre-stressed basalt fibres or with no reinforcement for comparison. The pre-stressing of the basalt fibre layers was done through cambering, where the beam was cambered to the point where the stresses in the timber reached 90% of authorized stresses. The test results were compared with an analytical model to predict the moment capacity. The numerical model considers the linear elastic behaviour and the plastic behaviour of the timber under compression. The experimental results compare relatively well with the mathematical model. Strengthening glulam beams on the tension side with FRP materials increases bending stiffness and bending strength. Increased reinforcement ratio, in the form of additional fibre cloth layers, had a significant effect on both the strength and the stiffness. Pre-stressing the FRP provides additional strength and higher stiffness compared to slack FRP strengthening. Strengthening the glulam beams on the tension side allows a possible reduction of the cross-section or lower timber grade while maintaining the same bending strength and stiffness as for the unreinforced beam.