Predicting the demand and plastic capacity of axially loaded steel beam-columns with thermal gradients

This study evaluates the adequacy of different methodologies to predict the plastic capacity and response caused by non-uniform thermal gradients through the depth of beam-columns that are loaded only axially at the centroid. Three models with different levels of complexity were used to evaluate the fire response of beam-columns under non-uniform temperature gradients: (1) code-based equations; (2) a fiber-beam element model; and (3) a shell element model that discretizes the full cross section and length and is capable of capturing local (i.e. plate) instability. The code-based equations do not predict the response satisfactorily since these equations do not properly consider temperature gradients. The fiber-beam element and shell model results correlate well to the thermal and structural response of the beam-columns tested experimentally with varying parameters. If local buckling is not expected at ambient temperature, complex shell elements are not necessary when the failure mode is fully plastic and fiber-beam elements, which are simpler and less "computationally expensive" than shells, suffice. The experiments and models also validated equations that consider thermal gradients and predict the plastic capacity and structural response of these members, which includes a moment reversal due to a shift in the section center of stiffness with increasing temperatures.