This paper focuses on the stability behavior of steel building structures under fire loading. A 10- story office building is designed according to current U.S. building codes and design specifications. The structural layout consists of interior gravity frames and perimeter moment resisting frames (MRFs). A 3D finite element model of the building is developed and analyzed for two fire scenarios: (1) a corner compartment of fifth story is under fire, and (2) the whole 5th story is under fire. Structural analysis is conducted using nonlinear implicit dynamic analysis in ABAQUS. The results indicate that thermal expansion of the composite floor system produces large out-of-plane displacements and inelastic stresses in the perimeter columns. These columns, however, do not undergo inelastic buckling failure or collapse. Failure occurs due to inelastic buckling failure of the interior gravity columns because they are much lighter sections than the perimeter columns. The normalized story capacity of the building structure can be predicted with reasonable accuracy using the normalized axial load capacity of a three-column subsystem. © 2009 ASCE.