Building structures are occasionally subjected to extreme events such as fire loading. Significant experimental and analytical research has been conducted on the behavior of building structures under fire loading. However, most of this research has focused on the performance of individual structural components under standard ASTM E119 fire loading. These standard fire tests have been conducted on a wide range of structural components and assemblies with different material, geometric and loading parameters. The standard fire test results are used to compute the design fire resistance rating (FRR) values for the structural components etc. The current building codes emphasize the prescriptive fire resistant design approach, while allowing for performance based design. The prescriptive design approach is more prevalent and commonly used. It specifies the required FRR values for the structural components of the building depending on its size, occupancy, construction materials, importance and other parameters. Fire resistant design is achieved by selecting structural components and assemblies with design FRR values greater than or equal to the required FRR values. It is evident that the prescriptive design approach does not account for the effects of realistic fire loading and thermal and structural interactions between the building components during the fire events. Recent events, for example, the 9/11 WTC Tower collapses, and the following analytical investigations directed by NIST, have highlighted the need for fundamental knowledge of the behavior and stability of building structures under realistic fire loading. The realistic fire behavior of building structures depends on several parameters, the most important of which are: (a) the structural configuration and layout, (b) fire intensity, duration, and spread, (c) structural loading and boundary conditions, and (d) fire protection distribution. This paper presents an analytical approach for simulating the realistic fire behavior of steel building structures. It focuses on the behavior of a hypothetical 10-story office building located in Chicago, and designed according to current U.S. building codes. The structural layout consists of interior gravity frames and perimeter moment resisting frames (MRFs). It is expected that the behavior of other structural configurations will be different from those presented in the paper. However, the paper provides a fundamental primer for simulating and evaluating the realistic fire behavior and stability of steel building structures. This primer will be used to further investigate the behavior of other structural configurations and layouts in this research. © 2008 ASCE.