On the effect of span length on the fire resistance rating of post-tensioned concrete slabs

In this paper, we investigate the effect of increasing span lengths on the fire resistance rating (FRR) of post-tensioned concrete slabs under various fire scenarios using the finite element analysis. Three specimens of one-way post-tensioned slabs with unbonded parabolic tendons, spanning 4, 6, and 8 m, are designed for dead and live loads. These specimens are then subjected to the standard ISO 834 fire for two durations of 90 and 120 min, as well as four different natural fire scenarios, to determine their FRRs. Different failure criteria as such, mid-span deflection, the rate of deflection, and critical temperature for the prestressing tendons, are used. The results indicate that increasing the span length leads to a nonlinear reduction in the FRR, particularly influenced by the rate of deflection and the tendon's critical temperature. Specifically, a 40% decrease in FRR is observed for the 6 m specimen, and a 47% decrease for the 8 m specimen under natural fire scenarios. These findings underscore the importance of considering increased span lengths and natural fire scenarios in the design of post-tensioned slabs. Furthermore, the research indicates that the minimum concrete cover thickness does not guarantee the required FRR for post-tensioned slabs in either natural or standard fire scenarios. Additionally, the findings contributed to the development of a predictive model for the FRR of post-tensioned concrete slabs with span lengths exceeding 8 m. This model can inform modifications to regulations regarding FRR based on span length and fire conditions.