This paper focuses on the performance-based structural fire engineering of steel buildings subjected to realistic design-basis fire scenarios. Medium-rise office buildings were designed in accordance with U.S. building codes and standards. These buildings were designed with steel gravity frames and different lateral force resisting systems: (1) interior rigid core walls, and (2) perimeter moment resisting frames. Code-based prescriptive approaches were used to design the passive fire protection for the structural members of the steel buildings. The structural performance of these buildings, when subjected to realistic design-basis compartment fire scenarios (one-hour fire with cooling), was evaluated by conducting detailed nonlinear inelastic finite-element analysis of the complete three-dimensional (3D) building system. The analysis results indicated that steel gravity columns were the most vulnerable component, susceptible to inelastic buckling during the fire events. This could further precipitate the partial or overall collapse of the building during the fire event. This fundamental understanding of structural performance and vulnerabilities of the steel buildings (designed according to prescriptive approaches in codes) was leveraged to optimize the layout and distribution of passive fire protection in the steel buildings, leading to performance-based structural fire engineering. Detailed analysis results indicated that moving the fire protection from intermediate filler beams in the floor systems to the gravity columns improved structural performance, fire resistance, and collapse resistance of the steel buildings. This exemplifies the potential of performance-based fire protection engineering linked with structural engineering analyses to optimize design and economy. © 2019 American Society of Civil Engineers.