1. A computer-implemented method for building a three-dimensional (3D) computer model of a longitudinal structure using a graphical user interface display, comprising: providing a catalog of two-dimensional (2D) subassemblies, wherein each 2D subassembly is represented as a data component that stores modifiable parameters, and a logic component that generates a parameter-specific geometry of the 2D subassembly for use in a 2D cross-sectional representation of the 3D computer model; displaying an image associated with a first 2D subassembly in a first portion of the graphical user interface display; displaying the 2D cross-sectional representation of the 3D computer model in a second portion of the graphical user interface display; detecting that the image associated with the first 2D subassembly is dragged and dropped from the first portion of the graphical user interface display to the second portion of the graphical user interface display; in response to the detecting, executing the logic component associated with the first 2D subassembly to generate the parameter-specific geometry for the first 2D subassembly based on default parameter values associated with the first 2D subassembly; and the first 2D subassembly based on default parameter values associated with the first 2D subassembly; and integrating the first 2D subassembly into the 2D cross-section of the 3D computer model based on the parameter-specific geometry.