Stability of Composite Axial Members Under Fire Loading

Composite axial members (Concrete-filled Steel Tube (CFT) columns and Composite-Plate Shear Walls-CPSWs) may be subjected to a combination of loading conditions, such as gravity, wind, seismic or fire. Under a combination of fire and gravity loading, the member would experience degradation of material properties and non-uniform stresses through the cross-section due to a temperature gradient. A time-temperature study is thus necessary to determine the member capacity and stability at elevated temperatures. This paper presents the results of benchmarked numerical studies conducted on CFTs and CPSWs. The studies involved simulating fire conditions by exposing steel faceplates to increasing temperatures (ASTM E-119 Time-Temperature Curve) while maintaining a constant axial load. A simplistic fiber-based numerical analysis tool was developed to model the specimens. The tool was verified using experimental data for CPSWs and benchmarked finite-element models. Additional experimental tests for CFTs have also been conducted. The effect of various parameters such as column slenderness (5-50), column aspect ratio (1-4), section slenderness (30-120), and material properties on the behavior of the members was studied. Additionally, a layer of fire protection was modeled to understand its effect on column failure (failure time and surface temperature at failure). It was observed that a layer of fire protection significantly delayed the time to failure but caused the member to fail at marginally lower steel-surface temperatures. The fiber-model can be used as a tool for performance-based design of CFTs. An equation to estimate the capacity of CFTs at elevated temperatures is also proposed based on the analysis results.

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