An interior three-storey frame structure with a column and 4 beams in each floor is investigated. The vertical and horizontal (seismic) forces, normal forces and bending moments as well as elastic interstorey drifts are calculated. The welded box columns and rolled I-section beams are designed for minimum weight and cost. The beam-to-column connections are selected from a number of structural versions improved for seismic resistance. The fabrication costs are calculated in detail. Design constraints relate to interstorey drifts and to the stability of column parts and beams loaded by compression and bending. Calculations show that, after a connection type is selected, the fabrication cost has little effect on the optimum design, since it varies proportionally with the mass according to the present calculating method. Thus, the minimum weight design gives suitable results.