Flexural reinforcement of glue-laminated timber beams using prestressed FRP plates

Fibre-reinforced polymer (FRP) plates have been used successfully to enhance the structural performance of glue-laminated (glulam) timber beams. Unreinforced glulams usually fail in tension in a brittle fashion. The addition of reinforcement to the tension zone can result in plastic compression behaviour in the timber at failure. This paper examines the feasibility of further improving the flexural strength, ductility and stiffness of glulams by pre-tensioning the FRP prior to bonding. In practice, an additional timber board, referred to as a bumper laminate, is often bonded below the FRP to improve fire performance and aesthetics. An analytical model to predict the load-deflection behaviour and moment capacity of prestressed FRP reinforced glulams is outlined in this paper. The model incorporates the elastic-plastic compression behaviour of timber, the effect of the bumper laminate and the possible failure modes depending on the ratio of tension to compression strength. Plastic deflections are determined by applying the principle of virtual work to strain energy theory. The performance of FRP-prestressed glulams in comparison with unreinforced and nonprestressed FRP reinforced glulams is predicted. The FRP-prestressed glulams obtained a higher theoretical ultimate moment and deflection in comparison with the nonprestressed reinforced beams and an unreinforced control. The experimental programme to validate the results of the analytical model is ongoing. As a result, preliminary validation of the model is carried out using theoretical and experimental results for nonprestressed and prestressed FRP reinforced glulams reported in existing literature. Excellent agreement was found between the theoretical results. The model yields conservative values of moment capacity in comparison with the experimental results.