The effects of floor slabs on the flexural strength of beams in reinforced concrete buildings under seismic action were verified through tests of frame assembly specimens. A series of experimental and analytical investigations were conducted from 2010 to 2014 in order to further validate the current design practices in Japan. Loading methods in the past beam component tests were reviewed with probable effects of floor slabs. A special loading set-up was used for the frame assembly specimens consisting of four columns and four beams with lengths of one span and two half spans in two directions. The four columns were loaded laterally and independently at mid-height of the upper storey and supported at mid-height of the lower storey with pin-fixed and pin-roller so that axial elongation of the beams and slab would not be constrained by the lateral forces. It has been found from these new loading tests that the tensile stresses in the floor slab reinforcing bars are generally uniform at the beam critical sections and up to the full slab width for the flexural strength when the slab is subjected to tension bending around one percent storey drift, which is much wider than assumed in the current design evaluation.