Punching-shear design equation for two-way concrete slabs reinforced with FRP bars and stirrups

Recently, the Canadian Standard Association (CSA) provided its first punching-shear equation in the CSA S806-12, which represented a step forward in designing two-way concrete slabs reinforced with fiber-reinforced-polymer (FRP) bars. Yet neither the new CSA S806-12 equation nor any other design guide addresses the contribution of FRP stirrups as shear reinforcement for such elements due to the very limited research work. This study proposes a preliminary design equation to predict the punching-shear resistance of two-way concrete slabs reinforced with FRP bars and stirrups. The proposed equation is based on adopting the concrete contribution equation from the CSA S806-12 and modifying the stirrup contribution equation of the CSA A23.3-04 to reflect using FRP materials instead of steel. The proposed modification was verified against the test results of an extensive experimental work conducted at the University of Sherbrooke on full-scale two-way specimens measuring 2500 mm × 2500 mm × 200 mm or 350 mm. The accuracy of the predictions was compared to the test results. The results of this study supported the strain limit of 4000 microstrains in the FRP stirrups for calculating the contribution of the FRP stirrups to the punching-shear capacity, as provided by ACI 440 Committee and CSA S6S1-10. In addition, the proposed punching-shear design equation shows good yet conservative predictions with a tested-to-predicted punching-shear ratio (Vtest/Vpred) of 1.17 ± 0.21.