To integrate existing CAD capabilities more closely into structural-design practice, the paper presents a top-down approach to the geometric modelling of building structures. A network database with newly developed data abstraction is utilized to organize the geometric and topological information obtained, so that it can provide a central communication area between applications in structural engineering. To support the top-down structural-design process, facilities for intersecting higher-level primitives and splitting them into lower-level entities, and for evolving simple wireframe models of entire buildings to complex solid models of their components, were developed. The system is a geometric modeller for the integrated structural design of buildings, providing the designers with easy construction of the geometric model, and effective management of the model data of buildings.