Most existing reinforced concrete (RC) buildings in regions of low and moderate seismicity have been designed only considering gravity loads. Gravity-designed RC flat plate frames may be more vulnerable than RC frames with beams and columns owing to the potential for abrupt and brittle punching shear failure at slab-column connections. This study evaluates the seismic performance of gravity-designed three- and seven-storey flat plate frames, which were assumed to be located at sites having low, moderate and high seismic risks. An analytic model for simulating the hysteretic behaviour of slab-column connections obtained from experiments was proposed. It was found that the level of gravity load as measured by the gravity shear ratio significantly affects the inelastic behaviour of flat plate frames. The frames satisfied the basic safety objective (BSO) specified in AISC 41 subjected to ground motions at the Boston site, but not at the Seattle and LA sites.