Resistance factors for reliability based-design of fiber reinforced concrete suspended slabs in flexure

Abstract

Due to the inherent uncertainties in the properties of construction materials, fabrication processes, applied loads, and the approximate nature of analysis methods, modern design codes have adopted the Load and Resistance Factor Design format (LRFD) to design civil engineering structures. In this format, loads and resistance are factored to consider the random nature of design variables. For instance, the reinforced concrete suspended slabs design is covered by ACI 318 [(Committee 318, 2019) 11 code, which is currently based on the LRFD format. On the other hand, the design of fiber reinforced concrete suspended (FRC) slabs fall under ACI 544.4 [(Committee 544, 2018) 22, which is not based on LRFD; therefore, it warrants an appropriate resistance factor to be used in conjunction with the current ASCE 7 [(American Society of Civil Engineers, 2017) 33 load factors and load combinations. In this paper, a reliability-based design procedure is developed to calibrate the resistance factor for the flexural design of FRC slabs with and without conventional steel reinforcement. Statistical and probabilistic analyses of experimental data showed that the resistance model for the flexural design of hybrid FRC slabs obeys the lognormal distribution with a bias of 1.33 and a coefficient of variation of 0.24. But for FRC slabs without additional reinforcement, the bias factor and the coefficient of variation of the resistance model increase to 1.56 and 0.37, respectively. In the case of conventional reinforced concrete slabs, the current resistance factor is . In the case of FRC slabs, an additional resistance factor of shall be applied to account for the contribution of fibers. This factor consistently provides the minimum safety level (i.e., the reliability index is close to or larger than the target reliability). However, in the case of FRC slabs without conventional steel reinforcement, a resistance factor of 0.75 is recommended to consistently meet the target safety level. The effectiveness of fibers in hybrid reinforced concrete slabs is more pronounced (smaller reduction of compared to 25% for slabs with fibers alone), thus, supporting the synergetic effect between the two reinforcing materials.