AbstractThere is a growing trend toward utilizing high steel-reinforcement ratios (as high as 5%) in ultrahigh-performance concrete (UHPC) beams. This tendency is driven by the desire to take full advantage of the unique mechanical properties of UHPC, but results in construction that is expensive given the high cost of both steel reinforcement and UHPC. This paper focuses on UHPC beams with low reinforcement ratios and relatively low levels of fiber reinforcement in an attempt to reduce the cost of UHPC construction and hence broaden its appeal. Four-point bending tests were conducted on UHPC beams with steel-reinforcement ratios of ρ=0.85% and 1.54% and fiber volume fractions Vf=1.0%, 1.5%, and 2.0%. The experimental results showed that increasing the fiber volume fraction, which increases material tensile ductility, did not necessarily lead to enhanced structural ductility. This observation was confirmed through validated computational modeling using a hybrid fixed/rotating crack model. Synthesizing the test and computational results, this paper advocates for an underreinforced design philosophy and use of a fiber volume fraction that is just enough to ensure that crack localization does not occur under working conditions. This philosophy minimizes the cost of UHPC beams, making them more attractive for broader usage. The latter consideration ensures that steel rebars will have enhanced protection against corrosion under working conditions, which, coupled with the high durability of UHPC, ensures highly durable construction with low long-term maintenance costs.