Just one year before the Hyogoken-nanbu earthquake, the Northridge earthquake took place. Under these two earthquakes, moment resistant steel frames showed a typical mode of fracture at the beam-to-column connections. Thus, the fractural mode of failure became a fundamental mode of failure in steel structures under earthquakes. In this paper, referring to current Japanese design practice, main causes for the fractural mode in the Hyogoken-nanbu earthquake are estimated to be an excessive seismic input and an overlook on stress concentration at connections.
