Damage Evaluation of Composite Beams under Fire Conditions

Abstract

This paper presents a numerical method for damage evaluation of steel structures under fire conditions based on degraded material property values and global structural response to sustained loads. As the measure of structural damage to fire, two damage indices are proposed which can reflect the fire damage in a material level as well as in a structural level. The first damage index is formulated with the ratio of a volume-weighted average of yield strength at elevated temperatures to that of the yield strength at room temperature. The second damage index is defined with the deflection of the structure at elevated temperatures normalized by the deflection at room temperature. To investigate the feasibility of proposed damage indices, fire testing and the corresponding structural fire analysis of steel-concrete composite beams are performed using a design fire load. The fire is modeled using a standard fire curve proposed in American Society for Testing and Materials (ASTM) E119. Three composite beams, which consist of a regular beam and two different beams protected by fire-resistant materials, are considered for the fire damage evaluation. The damage state of the composite beams is evaluated using the damage indices at each time the fire progresses, and it is compared with actual fire damage of the structure obtained from the structural fire experiment. By the use of the proposed damage indices, the effect of fire-resistant materials on structural integrity is quantified, and it is investigated whether the damage state of each beam at the end of the fire load satisfies the design performance of composite beams used for nuclear power plant structures.