We describe a system, Prompt, used to design physical systems. Prompt employs a multi-level approach to design. When simple constraint propagation over prototypes [Adda85] fails, Prompt can significantly modify prototypes by reasoning about their structure and physics. Prompt derives the behavior of a prototype from its structure using knowledge of physics stored in a Graph of Models. It then uses heuristics called Modification operators to control the process of modifying the prototypes. In this paper we describe how our system works in the domain of structural design. We describe the kinds of analysis Prompt performs on beams and how it makes innovative changes to prototypes.