AbstractA special testing apparatus was designed and employed to explore the fire behavior of a steel–concrete composite floor system with boundary restraints and to measure the variation in the restraint forces induced by fire. Two full-scale fire tests of slab specimens with different secondary support conditions were conducted. Detailed experimental results were obtained, including the furnace temperature, temperature distribution, vertical and horizontal deflection, crack patterns, and restraint forces. The furnace was shut off after 270 and 300 min when the insulation criterion for failure was met. Although the two composite floor systems were exposed to lengthy fire durations, they exhibited better fire resistance than the isolated composite slabs evaluated in previous tests in terms of aspects such as relatively limited deflection, higher deflection recovery, and excellent concrete slab integrity. The restraint moment at the slab corners was determined by the measured internal forces of steel beams in the steel frame using mechanical analysis of the relevant spatial structures. Significant warping forces were observed at the slab corners during the tests. This implied that the warping force posed a potential hazard to the adjacent columns above the composite floor system subjected to fire.