1. A method for building a computer model of a longitudinal structure using a graphical user interface display, comprising: providing a catalog of subassemblies, wherein each subassembly is represented as a data component that stores modifiable parameters, and a logic component that generates a parameter-specific geometry of the subassembly for use in a cross-section of the model; executing the logic component, for a subassembly selected from the catalog, to generate the parameter-specific geometry for the subassembly; and integrating the subassembly into the cross-section of the model, based on the parameter-specific geometry.