In the seismic design of reinforced concrete frames, plastic hinges are allocated to beams such that a ductile beam-sway mechanism will form in preference to other less ductile mechanisms in the event of a major earthquake. This is achieved by ensuring that the flexural strength of columns is greater than that corresponding to the maximum likely flexural strength of beam plastic hinges. Recent experimental studies in New Zealand have shown that elongation of ductile beam plastic hinges, and its interaction with nearby floor slab containing precast-prestressed floor units, increases the strength of beams much more than that specified in New Zealand and American Concrete standards. This level of strength enhancement has raised concern on the adequacy of the current design provisions. To further investigate this problem, a research project was initiated to examine the strength of beam plastic hinges in reinforced concrete frames containing precast-prestressed floor units. In this research, the strength of beam plastic hinges was assessed through experimental and analytical studies. A three-dimensional, one-storey, two-bay reinforced concrete moment resisting frame with prestressed floor units and cast-in-situ concrete topping was tested under quasi-static displacement-controlled cyclic loading. The experimental results provided insight into the mechanics associated with frame-floor interaction. Subsequently, improved design specifications were proposed based on the observed behaviour. To analytically predict the beam-floor interaction, a ductile reinforced concrete plastic hinge multi-spring element was developed and validated with experimental results from cantilever beam and frame sub-assembly tests reported in the literature. The comparisons have demonstrated the ability of the proposed plastic hinge element to predict the flexural, shear, axial, and most importantly, elongation response of ductile plastic hinges. The proposed plastic hinge element was implemented into an analytical model to simulate the behaviour of the frame-floor sub-assembly tested in this research. Specially arranged truss-like elements were used to model the linking slab (the region connecting the main beam to the first prestressed floor unit), where significant inelastic behaviour was expected to occur. The analytical model was found to be capable of predicting the non-linear hysteretic response and the main deformation mechanisms in the frame-floor sub-assembly test. The analytical frame-floor model developed in this study was used to examine the effect of different structural arrangements on the cyclic behaviour of frames containing prestressed floor units. These analyses indicated that slab reinforcement content, the number of bays in a frame and the position of frame in a building (i.e., perimeter or internal frame) can have a significant influence on the strength and elongation response of plastic hinges.