| The main objective of this research is to investigate the collapse performance of steel-framed buildings under fires and to contribute to the development of methods and tools for performance-based structural fire engineering. This research approach employs detailed finite element simulations to assess the strength of individual members and indeterminate structural subassemblies. One specific focus of the investigation is to assess the accuracy of beam and column strength design equations of the American Institute of Steel Construction (AISC). The simulation results show these design equations to be up to 60-100% unconservative. Alternative equations are proposed that more accurately capture the effects of strength and stiffness degradation at elevated temperatures.; The assessment technique is then extended to examine fire effects for indeterminate systems, including forces induced by restraint to thermal expansion and nonlinear force redistribution due to yielding and large deformations. Structural sub-assemblies are devised to examine indeterminate effects of gravity-framing in a 10-story building. Simulations on three types of subassemblies support the following observations and conclusions: (1) the rotational end restraint provided by the columns above and below the fire story have a significant stabilizing effect on gravity columns in the fire zone, (2) vertical restraint of the heated column, by floor framing above the fire story, does not significantly impact the strength limit state of the columns in the fire zone, (3) short of designing the building system with special redundant load paths, thermal insulation is essential to avoid progressive collapse of gravity columns, (4) thermal insulation requirements for beams can be reduced while preserving collapse resistance through enhanced connection details, employ slotted bolt holes to permit thermal elongation, and incorporate reinforcing bars in the slab.; Uncertainty in the collapse behavior under fires is evaluated considering variability in the gravity loading and structural response parameters. The collapse probability of the sub-assemblies is assessed by the mean-value first-order second-moment (FOSM) method, conditioned with respect to the scaled intensity of fire compartment gas temperature. The studies further show that the probability of column failure ranges from 4-38% for designs based on the AISC strength provisions (with &phis; = 0.9). These probabilities reduce to 0.5-3% (beta = 1.9-2.6) based on the proposed equations. |
