The structural design paradigm has evolved from an engineering-oriented approach to a holistic one that accommodates various concerns including form, function, behavior, economy, efficiency, and elegance. Because as many aspects as possible must be considered in the early design stages to reduce downstream problems afterward, the design tool for this stage has to be iterative and interactive with designers to manipulate various alternatives. In addition, the advancement of computer graphics software together with generative algorithms provide rapid dynamic simulation that enables efficient interactive design. This research has focused on rapid prototyping for the development of a computer-supported design tool that helps to perform the conceptual design of suspension bridges from the designer's concept to computer simulation during the conceptual design stage. The design information and tasks in conceptual design are formalized in terms of entities using the concept of an entity-based model, and these entities are further implemented into the generative algorithms of graphic software. Thanks to the dynamic simulation of the implemented design tool, conceptual design can be done interactively and iteratively while generating and selecting design alternatives following a graphical approach.