This paper presents an object-oriented architecture for structural design software. The architecture's novel features are the representation of an artifact with distinct levels of idealization, a hierarchy of classification within each of these levels, and the appropriate separation of software components. These enable seamless integration of geometric modeling and structural analysis in an interactive environment, extensibility of modeling and analysis capabilities, and integration of interactive multi-objective optimization. The paper presents a design environment implemented on the basis of the architecture, and demonstrates the benefits of refocusing engineering software from analysis to design. (C) 2012 Elsevier Ltd. All rights reserved.