This paper investigates the behavior of an innovative steel column section both at room temperature and under the transient heating regime of building fires. The column consists of Grade 1200 ultra-high strength steel (UHSS) tubes welded to the corners of Grade 304 stainless steel plates, forming a square-shaped hybrid cross-section. First, through performing compressive tests on 780 mm long stub columns of the hybrid section at room temperature, its vulnerability to local buckling is assessed; accordingly, decision is made on the appropriate width of the plates. Then, six transient fire tests are conducted on the hybrid stub columns under three different compressive load levels. Due to the very fast heating rate of standard fire curves such as the ISO 834, the thermal creep of steel columns may be overlooked using these heating regimes, which may lead to an overestimation of the column's resistance against building fires. Thus, a distinctive heating regime is adopted for the fire tests in this study; at any load level, two fast and slow heating rates are applied to the specimens. In this way, by comparing the results obtained, the effect of thermal creep on the fire resistance of the columns can be assessed. More importantly, the fire tests are used to verify finite element (FE) models developed for these hybrid columns in ABAQUS software; specifically, the creep model already developed for the UHSS tube and stainless steel plate materials is verified for component-level simulations. The verified FE modeling framework is then employed to perform an extensive parametric study on the effects of length, utilization factor, axial restraint stiffness ratio, and heating rate on the hybrid columns' fire resistance. Finally, based on the results obtained from the parametric study, simplified design equations in the temperature domain are obtained for the axially-restrained hybrid steel columns under transient fire.
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