In line with current seismic design practice, steel structures may be designed to EC8 according to either non-dissipative or dissipative behaviour. The former, through which the structure is dimensioned to respond largely in the elastic range, is normally limited to areas of low seismicity or to structures of special use and importance; it may also be feasible if vibration reduction devices are incorporated. Otherwise, codes aim to achieve economical design by employing dissipative behaviour in which considerable inelastic deformations can be accommodated under significant seismic events. In the case of irregular or complex structures, detailed non-linear dynamic analysis may be necessary. However, dissipative design of regular structures is usually performed by assigning a structural behaviour factor (i.e. force reduction or modification factor), which is used to reduce the code-specified forces, resulting from idealised elastic response spectra. This is carried out in conjunction with the capacity design concept, which requires an appropriate determination of the capacity of the structure based on a pre-defined plastic mechanism, often referred to as failure mode, coupled with the provision of sufficient ductility in plastic zones and adequate over-strength factors for other regions.