Flat plates are widely used in reinforced concrete buildings. Their design is usually based on the shear forces and bending moments produced by the gravity loads. During seismic activities, the lateral building deformations induce additional shear forces and bending moments that they must withstand. To evaluate the seismic moment capacity of a flat plate system, an effective slab width needs to be defined. In this paper, grillage analysis is utilized to predict the nonlinear lateral behaviour of flat plate buildings. A comprehensive parametric study is used to evaluate the effective slab width contributing to the lateral strength of residential interior flat plate connections. The studied parameters include span length, bay width, column dimensions, and level of column axial load. Both gravity load designed frames and moment resisting frames are analysed. The effect of the material safety factors is assessed by conducting two sets of analyses using nominal material properties and factored material properties. Equations to estimate the effective slab width are proposed.