Strain driven mode-switching analytical framework for estimating flexural strength of RC box girders strengthened by prestressed CFRP plates with experimental validation

•Early research attempt on the analytical model of prestressed CFRP plates strengthen RC box girders.•Prestress losss, shear lag effect and different failure modes are considered in the model proposed.•Analytical predictions validated by experimental results. This paper presents a pioneering study that developed the first analytical model to analyse and design reinforced concrete (RC) box girders strengthened by prestressed CFRP plates. The proposed analytical model considered and addressed prestress loss, shear lag effect and failure modes under different design configurations at elastic, elastoplastic and plastic stages. An experimental study was also conducted to validate the proposed analytical model. The study was motivated by the increasing demand for the structural strengthening of aged and over-used hollow RC box girders in transport and other infrastructure systems, as well as the lack of previous attempts to incorporate prestressed CFRP plate strengthening for hollow RC beams. It is very common in many developing countries, the traffic flow increased dramatically due to the economic growth. The original design underestimated the traffic loads. When demolition and re-building may not always be the best option, thus, strengthening and enhanced maintenance have become promising alternatives. But the lack of existing analytical models that can guide the engineers to analyse and design this type of structures effective, has become an urgent need from the industry. In the experimental study, eight box girders with different types, cross-section sizes, and prestress levels were prepared and tested. Two samples were preloaded to create damaged beams before strengthening to simulate the aged or over-used members. The experimental results are in good agreement with the analytical prediction. The proposed analytical framework provides a comprehensive yet practical method for designing the prestressed CFRP strengthened RC box girders in bending and laid the foundation for further studies on shear and torsion behaviours.