A 12.8 m-long composite floor beam consisting of a steel reinforced concrete slab with profiled metal decking supported by a W18×35 steel beam was tested under multi-point bending load at ambient temperature. Material strain and temperature were measured using fiber optic sensors. Measurements were taken along more than 150 m of fiber optic cables using Pre-Pump Pulse Brillouin Optical Time Domain Analysis (PPP-BOTDA) and Optical Frequency Domain Reflectometry (OFDR). Along the beam centerline, three strain and two temperature fiber loops were placed to characterize the strain and temperature distribution along and through the depth of the concrete slab. The neutral axis depth was evaluated using the measured data and compared with theoretic predictions for a fully-composite beam and a non-composite beam. The theoretic prediction was based on an elastic steel and elastic cracked concrete section and was found in general agreement with the test data.