In human reasoning about the behavior of the physical world, the importance of the role that diagrams play cannot be over-emphasized. We are undertaking a project aimed at studying the power of visual reasoning and developing techniques for making use of visual reasoning in a specific problem solving context in physical domains. The problem we will solve is determination of the deflection shape of the frame structure of a building under a load. We are constructing a computer program that solves this problem using a diagram in a way similar to human engineers. Given a specification of a frame structure and a load, the program will produce a diagrammatic representation of the deflected shape of the structure. It modifies and inspects the deflected shape using structural engineering knowledge until it finally produces a stable deflection shape under the given load.