Influence of the effectiveness factors in assessing the shear capacity of RC beams strengthened with FRP

Shear failure of RC beams strengthened with composite textiles is often affected by the different failure modes characterizing the FRP reinforcement. The most relevant analytical models for evaluating the shear capacity of RC beams strengthened with FRP take into account these failure modes by introducing an effectiveness factor “R”, which reduces the ultimate FRP tensile stress. Moreover, the interaction between stirrups and FRP reinforcement leads to a reduced efficiency of the transverse steel reinforcement due to the brittle failure of composite textile which hinders the yielding of all the stirrups involved by critical crack. In this regard, some analytical models introduce an effectiveness factor “r”, aiming at reducing the yielding stress of stirrups. The procedures to calculate the above two parameters represent the main differences characterizing most of analytical models, significantly influencing their results. For this reason, the present paper focuses on the comparison of the different procedures to assess the effectiveness factors, proposing a new procedure for each effectiveness factor by modification of already existing formulations. Influence of the arrangement of composite reinforcement on the efficacy of stirrups, affected by brittle failure of FRP, is considered by means of the ratio between effective strain of composite to yielding strain of steel. The proposed procedures are employed in a design-oriented analytical model able to calculate the shear strength of RC beams retrofitted with FRP reinforcement arranged in any direction. The model is formulated aiming at representing an extension of EN1992 shear model to beams strengthened with FRP. The efficacy of the proposed procedures is assessed by comparing the experimental results against the predictions obtained via the design-oriented model and the above-mentioned analytical models.