This paper is concerned with the development of a knowledge-based system for the economical design of tubular trusses using a fabrication-led approach. For this purpose the system employs a cost model for estimating the likely cost of fabrication for these trusses. The cost model, which was developed using the object-oriented methodology, is used for estimating the cost of fabrication of one or a number of trusses, joint detailing, members and individual fabrication operations. Therefore, the relative cost of one design option compared with its alternatives can be obtained. Using the model, the consequences of design decisions on the cost of fabrication can be appraised. This will assist in obtaining economical designs that take into account not only the cost of material but also that of fabrication. Copyright © 1996 Civil-Comp Limited and Elsevier Science Limited.