The paper reviews the whole design process of large−scale trusses of overhead lines for power transmission. It describes the modules of the system of interactive computer programs made by the authors. According to the iterative nature of the design process special graphic support helps the interaction of the designer. The effect of assumptions used during the design process, imperfections due to the fabrications and erection of the structures can only be detected by the loading tests carried out on newly designed set of towers. Some experiences learned by these tests are also detailed in the paper.