Time-dependent degradation of structures during fire - Analysis and design issues

This paper describes the behavior of structures subject to high temperatures during a fire. Floor systems made up of reinforced concrete slabs with or without intermediate beams were investigated under application of a loading that consisted of dead load and fire exposure. The effect of high temperature on material strength and behavior; creep and cracking in concrete; formation of localized failures; and structural degradation resulting from loss of stiffness were investigated using a nonlinear finite element analysis. The analysis simulated the loading process and followed the step-by-step structural degradation of floor systems until a predetermined collapse criterion was reached. The results revealed a dramatic loss of structural stiffness after a temperature increase of about 400-500 °C. The paper also describes the significance of fire loads in structural analysis and design. Issues pertinent to design include: (1) prevention; and (2) design for safe performance in a fire. These issues as related to a performance-based design code are reviewed and discussed.