A series of compartment fire experiments was conducted on long-span steel-concrete composite floor beams designed and constructed following U.S. building codes and standards. The test program consisted of five 12.8 m long composite floor beam specimens with various end support conditions. Each specimen was constructed as a partially-composite beam consisting of a W18 35 steel beam and an 83 mm thick lightweight concrete slab cast on top of 76 mm deep ribbed steel decking units. Test variables included two types of simple shear connections (shear-tab and welded/bolted double-angle connections) and the slab continuity over girders. One specimen with the double-angle connections at the ends was tested at ambient temperature and the remaining four specimens were tested under simultaneous mechanical and fire loading. This report, Part 1, presents details of the test setup, specimens, design basis of fire loading, instrumentation, and the behavior of the composite beam with double-angle connections at ambient temperature. The ambient temperature test indicated that the composite beam specimen failed by a shear stud near the west end, followed by concrete breakout failure and yielding of the steel beam. The measured moment capacity was approximately 80% of the calculated flexural strength. The double-angle connection at the west end failed by weld fracture, which caused collapse of the composite beam. The ambient behavior of the composite beam specimen presented herein will serve as a baseline to compare with the composite beam assemblies tested under combined mechanical loads and fire exposure, which are presented in a subsequent report; Part 2 (Choe et al. 2019). The datasets obtained from these tests provide technical information to advance performance-based design of composite floor assemblies in steel-framed buildings subject to fire.