Evaluation of reinforced concrete columns resistance with eccentric loading under non-uniform heating regimes

Current design guidelines for structural fire safety generally focus on the resistance of structural members for a specified period under uniform heating in a standardized, harmonised fire scenario. A potential shortcoming of this method is that it neglects the impacts of non-uniform thermal loading, as well as the behaviour of structural members in the cooling phase of the fire. In this paper, a nonlinear finite element model has been developed to evaluate the behaviour of reinforced concrete columns with square cross sections under non-uniform thermal loading applied locally on one face. The numerical simulations have been performed in two steps: a heat transfer analysis followed by an evaluation of the structural behaviour of the axially loaded columns exposed to the heat flux. Due to the importance of second-order effects on the behaviour of the columns, axial loads with different eccentricities were considered, together with a user-defined subroutine developed to implement the irreversible concrete damage during cooling. Comparing the numerical results with the experimental data suggests that the proposed model is capable of reasonably predicting the thermal history at different locations within the column cross-section, and the history of mid-height lateral deflection of the columns during heating and cooling. It was observed that the column response is strongly influenced by the initial eccentricity of the load, and that its response when heated on the face subjected to higher compression stresses is drastically different from the response when heating takes place on the face with lower compression stresses. The columns loaded to 60% of their ambient temperature capacity and heated on the face subjected to higher compressive stresses exhibited lateral deflections during cooling up to 2.5 times the deflections at the end of the heating phase or even failed in cooling after a few hours. The analyses presented demonstrate the complexity of the structural response of concrete columns under non-uniform heating. Such complexity, which is not captured by conventional methods of assessing fire resistance, validates advanced numerical modelling techniques to predict structural response under more complex (and potentially realistic) conditions.