Reinforced concrete (RC) columns, under fire conditions, can experience biaxial bending which can arise due to eccentricity in loading, uneven spalling, or due to significant thermal gradients generated through 1-, 2-, or 3-sides of fire exposure. High strength concrete (HSC) columns are especially prone to such biaxial bending due to fire induced spalling. However, there is limited data and understanding in the literature on the effect of fire-induced biaxial bending on the response of RC columns under realistic loading and fire conditions. To overcome some of these knowledge gaps, a comprehensive study was undertaken to develop an understanding on the fire performance of RC columns under biaxial bending. A numerical model, in the form of a computer program, is developed for tracing the fire response of RC column. The model, based on macroscopic finite element approach, utilizes time dependent moment-curvature (M-κ) relationships for establishing the response of RC columns under fire conditions. The model is applicable for both rectangular and circular RC columns, and accounts for fire-induced spalling, biaxial bending, various strain components, and high temperature material properties. Fire-induced biaxial bending is incorporated in the analysis through M-κ curves generated along both axes of bending while spalling is accounted through hydro-thermal analysis. The model generates critical response parameters such as cross-sectional temperatures, axial and lateral deformations, failure time and failure modes, extent of fire-induced spalling, and strains across the cross-section, which can be used for evaluating failure of an RC column based on different limit states.For validating the numerical model, fire resistance tests were carried out on six RC columns under realistic loading, and fire scenarios. The columns comprised of one NSC, three HSC and two HSC columns with polypropylene fiber, and were tested under a standard and two design fires. Results from the tests show that HSC columns exhibit lower fire resistance due to occurrence of spalling and faster degradation of strength. But these HSC columns can survive burnout conditions (no failure) under typical design fire scenarios encountered in buildings. Also, the addition of polypropylene fibers mitigates spalling and thus significantly enhances fire resistance of HSC columns. Data from these fire tests, as well as that reported in literature, was utilized to validate the above numerical model by comparing concrete and rebar temperatures, extent of spalling, axial and lateral deformations, and failure times.The validated model was applied to carry out parametric studies to quantify the effect of various factors on the fire response of RC columns. Data from parametric studies show that the significant factors that influence fire resistance of RC columns are fire scenario, concrete strength (permeability), biaxial bending arising from 1-, 2-, or 3-face exposure, load eccentricity, or uneven spalling, load ratio, and slenderness ratio. Results from the parametric studies are utilized to develop a rational design approach for evaluating fire resistance of RC columns. The proposed approach comprises of evaluating fire resistance under standard fire conditions and then establishing equivalency between standard and design fire scenarios. The fire resistance under standard fire is evaluated through an equation that accounts for critical factors, such as fire scenario, spalling, load ratio, load eccentricity (uniaxial and biaxial), different face exposure (1-, 2-, 3-, or 4-side), concrete strength, and slenderness ratio. Then, an equivalency between standard and design fire scenarios is established based on the area under the fire curve concept, where in the survivability of column under design fire evaluated. The validity of the proposed approach is established by comparing resulting fire resistance predictions with those obtained from detailed finite element analysis and fire tests. The comparison shows that the proposed approach gives good estimates of fire resistance and also these fire resistance predictions are better than those obtained from current code provisions. Further the proposed approach expresses fire resistance in terms of commonly used structural parameters and thus can easily be integrated into structural design.
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