A simplified design formula for intermediate debonding failure in RC beams externally strengthened by FRP

Strengthening interventions on RC beams through externally bonded FRP plates or fabrics are among the most effective ways for improving the flexural strength of RC members in bending. Plenty of studies, both experimental and theoretical in nature, have been carried out in the last decade and, more recently, several Codes of Standards have been released for ruling the use of composite materials for structural strengthening. In particular, huge research activities have been carried out on intermediate debonding phenomenon that is one of the most common and peculiar failure modes observed in FRP strengthened beams. Code provisions against this topic are sometimes simplistic. In some other cases they are based on complicated procedures that are neither handy for designers nor completely consistent under the mechanical standpoint. The present paper, after a comprehensive discussion based on both numerical simulations and experimental data available within the scientific literature, is basically aimed at deriving a design formula for intermediate debonding failure. It considers explicitly some of the parameters controlling the mentioned phenomenon, managing both the uncertainties deriving by the neglected ones and the randomness of the mechanical properties of materials. Some general behavioural observations are firstly derived by previous studies based on utilizing a numerical procedure implemented and validated by the authors. Such observations, basically related to the relationships between starting of yielding within the steel rebars and the occurrence of intermediate debonding failure, are confirmed by a large number of experimental results (about two hundred) collected by the authors within the scientific literature. The above experimental results are finally utilized for calibrating two possible design formulae: the first one is based on the above observation between the strict connection existing between rebar yielding and intermediate debonding failure, the second one adopts and recalibrate a more usual formulation for estimating the available contribution of external composite reinforcement to the total bending resistance