Fire resistance design of un-protected FRP-strengthened RC beams

In the strengthening of reinforced concrete (RC) structures with fiber-reinforced polymer (FRP) composites, fire resistance design is an important issue. If fire insulation is not provided, the fire resistance of an FRP-strengthened RC beam can be approximated as that of an equivalent bare RC beam with a much higher load ratio as the mechanical resistance of the unprotected FRP strengthening system disappears within minutes of fire initiation. Due to this difference in load ratio, existing fire resistance design methods for conventional RC beams cannot be used in the fire resistance design of these equivalent bare RC beams. This paper aims to address this deficiency in existing research by presenting a reliable design method for the fire resistance of bare RC beams that provides close predictions for a wide range of load ratios, particularly those of high load ratios. Numerical results obtained using an advanced finite element approach developed by the authors are presented to examine the effects of various parameters on the fire resistance of bare RC beams, including the load ratio, the concrete cover depth, the reinforcement ratio of tension steel rebars, the distribution ratio of tension steel rebars, the cross-sectional dimensions, and the aggregate type of concrete. A design equation for predicting the fire resistance period of bare RC beams under standard fire exposure, formulated on the basis of the numerical results, is proposed. It is demonstrated that the proposed equation provides reasonably close fire resistance predictions for bare RC beams with a wide range of load ratios.