Rotational capacity of steel I-beams under fire conditions Part II: Numerical simulations

This paper describes the use of finite element models to study the inelastic behaviour of a steel I-beam in terms of its rotational capacity at elevated temperature. Two main objectives of this study are to investigate the feasibility of applying the finite element method to study the moment-rotation relationship of steel I-beams at elevated temperature and to investigate the main parameters affecting rotational capacity at elevated temperatures through parametric study. The finite element (FE) model was validated against published test results [Lukey AF, Adams PF. Rotation capacity of beams under moment gradient. Journal of the Structural Division, ASCE 1969;95(ST6):1173-88] at ambient temperature and test results reported in Part I of the paper. It is demonstrated that the finite element analysis gives reasonable accuracy compared to test results, providing an efficient, economical, and yet accurate tool to study the rotational capacity of beams in fire. An extensive parametric study was then undertaken using this validated model. Finally, a simple moment-rotation relationship at elevated temperature was developed for use in design. As a result of this study, the ductility of beams in fire can be based on the concept of member behavioural classes instead of the current EC3:1.2 [European Committee for Standardization (CEN). Eurocode 3: Design of steel structures, part 1.2: General rules - structural fire design. EN 1993-1-2. Brussels (Belgium); 2005] concept of cross-sectional classes, which tend to greatly oversimplify the problem. In addition, this paper shows that plastic theory exercised with due care can also be applied to fire engineering design when it considers the more limited rotational capacity of members under fire conditions.