Steel structures have been always considered as a suitable solution for constructions in high seismicity areas, due to the very good strength and ductility exhibited by the structural material, the high quality assurance guaranteed by the industrial production of steel shapes and plates and the reliability of connections built up both in workshop and in field.
In spite of these natural advantages, researchers are concerned about the necessity that, in order to ensure ductile structural behaviour, special care must be paid mainly in conceiving dissipative zones, which have to be properly detailed, assuring stable hysteresis loops, able to dissipate the earthquake input energy with high efficiency.
As a confirmation, during the recent seismic events of Northridge and Hyogoken-Nanbu, even if the cases of collapse of steel buildings have been extremely rare, steel moment frame buildings, considered as highly ductile systems, exhibited an unexpected fragile behaviour. They presented many failures located at the beam-to-column connections, challenging the assumption of high ductility and demonstrating that the knowledge on steel moment frames is not yet complete. Hence, in order to improve constructional details and to propose new design solutions for achieving a correspondence between the design requirements and the actual structural response, the scientific community began to deepen the reasons of this poor behaviour: does it depends on the material quality, on the design concept, on the structural scheme, on the constructional details, on the code provision, or on the seismic input occurred?