To study the initial collapse mechanism of multi-story steel structures under localized fire

the experimental study on four full-scale steel frame structures was performed.The experiments were conducted in a three-dimensional fire test furnace under elevated temperature at constant load mode.The first floor column and the steel beams connected with it were subjected to fire during the test

whereas the middle floor column and beam-column joint were fire protected.The bottom of the column under fire was connected with the developed force-measuring bearing using the bolt joint.The temperature distribution and the displacement response of the columns and beams were obtained

and the variation of axial force of the interior column was obtained by using the force-measuring bearing

and the initial collapse mechanisms of structures were obtained through the fire experiment.