In early stages of the design of a free-plan building different structural layouts should be evaluated to achieve a satisfactory level of seismic performance. However, the urgency of initial stage decisions needed by architects and developers makes the use of complex 3D structural models impractical. For this reason, a tool that can evaluate with acceptable accuracy the expected seismic performance of these configurations is necessary. This paper introduces a simplified building model that includes the flexural contribution of the slabs, and models the shear wall cores using a wide column analogy that includes warping effects. The model introduces kinematic constraints to account for the interaction between walls and slabs. Comparison of modal and response-histories with those given by a 3D finite element model shows that the proposed model has good accuracy, leading to errors usually less than 15%. Finally, the model has been devised to include other shear-wall structural configurations, energy dissipation systems, and the inelastic behavior of walls.