This paper presents the results of experimental investigations conducted to determine the fundamental behavior of steel members under fire loading. A total of eleven full-scale steel members were tested under combined thermal and structural loading. First, five A992 steel beam-columns (W10 × 68) were tested to determine their fundamental moment-curvature responses at elevated temperatures and different axial load levels. The experimental approach involved the use of radiant heating and control equipment to apply the thermal loading, and close-range photogrammetry combined with digital image processing techniques to measure the deformations (curvature) in the heated zone. Next, six A992 steel wide-flange (W8×35 and W14×53) columns were tested to determine their inelastic buckling behavior and axial load-displacement responses at elevated temperatures. A self-reacting test frame was designed to subject the column specimens to axial loading and heating. The thermal loading was applied by using the same type of radiant heating and control equipment as the beam-column specimens. The measured behaviors (and strengths) of the tested beam-column and columns specimens are presented and then compared with those obtained from detailed 3D finite-element analyses. The experimental investigations showed that the fundamental behavior and strength of steel members is governed mostly by the steel surface temperature, and the strength and stiffness of steel columns decreases significantly with increasing temperatures, particularly in the range from 500-600°C. The elevated temperature behavior of steel members can be predicted reasonably by using detailed 3D finite-element models. These verified models are recommended for conducting analytical parametric studies. The experimental approaches are recommended for evaluating the fire behavior of other structural members and loading conditions. © 2011 American Society of Civil Engineers.