A three dimensional, approximately half scale experimental test has been carried out to investigate the effect of precast-prestressed floor units on the seismic performance of reinforced concrete moment resisting frames. This paper gives an overview of the experiment and summarizes the results obtained from the test. The paper focuses on the level of strength enhancement and elongation within the plastic hinges. The results show that the presence of prestressed floor units partially restrains elongation of plastic hinges such that elongation in the exterior and interior plastic hinges differs significantly. This increases the strength of the beams more than that specified in the major structural design codes. This is a cause of great concern as this underestimation of beam strength may result in brittle column failure modes in a major earthquake.