Abstract: An engineering design model of a large and complex installation, such as a chemical processing plant or a power generating station, including three-dimensional representations of ducting, beams, piping, valves, fittings, vessels and associated equipment. An assortment of ducting fabrication segments of various sizes and colors are used to form the ducting, each segment being a flat solid member of rectangular outline having longitudinal, step-shaped, nesting configurations extending along its two longest parallel edges. The segments are connected in groups of four with the nesting configurations interlocked to form hollow ducting representations of a large variety of sizes and proportions. Transparent segments can be color coded by marking their edges.