A Fiber-Based Approach for Modeling Beam-Columns under Fire Loading

The work described herein emphasizes a new fiber-based approach to modeling the response of structural frames subjected to realistic fire conditions. The proposed approach involves the development and validation of two finite elements that can be used collectively to simulate the thermal and mechanical response of structural frames at elevated temperatures. To model the thermal response, a special-purpose fiber heat transfer element is introduced. The first of its kind, the fiber heat transfer element uses a combination of finite element and finite difference methods to provide an accurate and highly efficient solution to the three-dimensional thermal problem. To simulate the mechanical response, a flexibility-based fiber beam-column element is used. The element presented here extends the formulation of Taucer et al. (1991) to include thermal effects, geometric nonlinearities, and residual stresses.

Both fiber elements are implemented in ABAQUS (2007) using the user-defined element (UEL) subroutine. The element formulations are verified by analyses of benchmark experimental tests and comparisons with traditional finite elements. Results indicate that both elements offer superior accuracy and computational efficiency when compared to traditional methods of analysis. Analyses of structures subjected to non-uniform heating emphasize the advantages of the fiber-based approach.

To demonstrate a realistic application of the proposed approach, the work concludes with an investigation of the response of unprotected steel beams subjected to localized fires. Because realistic fires are considered, the treatment of strain reversal upon cooling is also addressed. The analyses are used to demonstrate that the standard fire test is generally unconservative at predicting the time at failure of a structure subjected to realistic fire conditions, since failure depends more on the evolution of temperatures within the steel beams than the duration of fire exposure. The analyses also show that critical temperatures from the standard fire test are conservative and thus offer a better means for predicting failure in steel structures within the scope of the standard fire test.