Comparison of Simple and Advanced Methods of Analysis in the AISC Specification for Fire-Resistant Structural Design

AISC Specification Appendix 4 provides criteria to aid structural engineers in structural design for fire conditions. It includes an advanced method of analysis and a simple method of analysis. The simple method provides straightforward equations to determine member capacities at elevated temperatures. However, determination of demands on the structure (forces and deformations) due to elevated temperatures is less clear. This paper aims to minimize these gaps in knowledge. A comparison of these methods will be articulated through the analysis of a 10-story office building. The building is a steel structure with perimeter moment frames and a composite floor system that was designed for hazards in Chicago, Illinois. In order to conduct the advanced analyses, a three-dimensional (3D) finite element method building model was developed using ABAQUS. This model can simulate inelastic deformations, instability failures, connection damage at elevated temperatures, and the effect of temperature on strength and stiffness of materials. The simple analyses were conducted using SAP2000, a commercially available structural analysis and design software. A comparison of results from each method of analysis shows that for gravity framing members, the advanced method produced the longest fire-resistance rating. The fire-resistance rating determined from the simple method was more conservative, resulting in a shorter resistance rating. The simple method was also found to be the most conservative approach for the moment-resisting frame members, making it a less desirable method for designing the lateral system for fire than the prescriptive approach. Use of the simple method may be most advantageous for gravity framing applications only and may be overly conservative for considerations of the lateral framing system.