Comparative assessment and strain limit optimization for predicting shear contribution of FRP stirrups in concrete beams

Due to the brittle nature of FRP reinforcement and concrete, the catastrophic shear failure of FRP-reinforced concrete beams makes the shear design a critical topic. Various prediction models and approaches have been developed to predict the shear capacity of the FRP-reinforced concrete beams, yet there has been limited attention paid to the prediction effectiveness of the shear contribution of FRP stirrups to the overall shear capacity. Various prediction models and approaches have been developed to predict the shear capacity of FRP-reinforced concrete beams, yet there has been limited attention paid to the prediction effectiveness of the shear contribution of FRP stirrups to the overall shear capacity. To overcome the limitations of conventional coupled evaluations, this study introduces a decoupled approach to independently assess the stirrup contribution and highlights the theoretical necessity of shifting from constant to variable strain limit. This paper presents comparative analyses focusing on the shear contribution of FRP stirrups in six codified shear design provisions and design approaches suggested by four groups of researchers for FRP-reinforced concrete beams and the corresponding approaches to determine the design strength of FRP stirrups. It shows that by excluding the influence of shear contributions from concrete, most of the codified design approaches underestimate the shear contribution from FRP stirrup by up to eight times and the design approaches proposed by four groups of researchers provide more satisfying predictions for the shear contribution from FRP stirrup. By adjusting the design strain limits for FRP stirrups, the predictions for the shear contribution from FRP stirrup could be improved but simply using a constant strain limit could result in inconsistent predictions. The optimal method to determine the strain limits has been found as the two used in JSCE and ISIS guideline, which are calculated based on the design configurations and are able to show the non-linear relations between shear contributions of FRP reinforcement with shear reinforcement ratio. With a more comprehensive understanding of the predictions for the contribution of FRP stirrups in the existing shear design approaches, more reliable predictions could be made for practices and related research.