Flexural Capacity of Composite Beams Subjected to Fire: Fiber-Based Models and Benchmarking

This paper presents the development and verification of a fiber-based modeling technique to predict the fundamental moment-curvature-temperature (M-I broken vertical bar-T) response of a composite steel beam cross-section in fire conditions. Experimental investigations and 3D finite element models can provide insight to the behavior of composite beams subjected to fire. However, these experimental and numerical approaches can be expensive and cumbersome for conducting extensive parametric studies. Therefore, a simpler 2D fiber-based modeling technique was developed for calculating the flexural behavior and moment capacity of composite steel beams at elevated temperatures. This simpler fiber-based approach can be implemented relatively easily and independently by designers. The 2D fiber-based approach is a simplified model that accounts for temperature dependent (steel and concrete) material properties and the level of slip at the concrete-to-steel interface at elevated temperature. The model does not account for membrane action or rupture of reinforcement, but it can be used to calculate the moment capacity of partially composite steel beams without an extensive finite element analysis. The fiber-based model was benchmarked using experimental results from composite steel beam tests subjected to fire, illustrating that it was capable of predicting moment capacity corresponding to the applicable limit state (i.e., full yield of the steel beam, concrete crushing or limiting stud slip) at elevated temperature. The fiber model predictions were typically conservative, and on average, the predicted moment capacity was 95% of the capacity determined from the experimental tests.