Effectiveness of timber beam strengthening with the near-surface-mounted technique

Strengthening and retrofitting of existing structures represent an important challenge for the conservation of the built heritage. Among structural members traditionally used in heritage buildings, timber beams often require strengthening and/or retrofitting due to their deformability, durability and damage issues, and increase of applied loads. A possible solution to improve the bending load-carrying capacity and stiffness of existing timber beams is the application of near surface mounted (NSM) reinforcement. NSM reinforcement, comprised by steel or composite profiles bonded with organic resins, was shown to be effective in improving the structural performance of timber beams. However, limited research was performed to accurately determine the contribution of the NSM reinforcement to the beam capacity. In this paper, the bending load-carrying capacity of timber beams strengthened with the NSM technique is investigated using an analytical model based on the cross-section equilibrium. The variables studied are the wood type (fir, chestnut, larch, and oak), NSM reinforcement material (steel or carbon FRP) and strengthening configuration. In addition, the possible crushing failure of the beam ends is studied considering masonry supports with two different stiffness. The results obtained provide an insight on the potential of the NSM technique in improving the stiffness and bending load-carrying capacity of timber beams.