This paper presents the findings of a probabilistic evaluation of a doubly symmetric I-steel beam's bending, shear, and deflection limit states. The design adhered to BS 5950, Part 1, 2000. Failure equations for flexure, shear, and deflection were derived, while random variable probabilistic models were sourced from the literature. Optimization using the First-Order Reliability Method (FORM) yielded design points, reliability indices, and sensitivity analyses. The results revealed that the reliability index decreased as beam span increased, with negative indices observed at a load ratio of 1.0 and beam span of 8.5m. Moreover, increasing the beam span to an overall depth ratio above 42 compromised reliability. The design achieved material savings in the plastic section modulus for a target reliability index of 3.0 but increased the modulus for a target index of 3.80 over a 50-year period. The design proved critical in bending, safe in deflection, and satisfactory in shear.