STIFFNESS AND FRACTURE CHARACTERISTICS OF THE NORTHRIDGE STEEL MOMENT RESISTING CONNECTIONS

The Northridge fractures demonstrated the inability of the welded beam-to-column connection, typically used in California prior to the 1994 earthquake, to fully develop the beam's plastic moment and, in some cases, to even reach yield. The unanticipated character of these failures implies that loads at which fracture occurs under Northridge-type conditions as well as the effects the latter has on a building's stiffness need to be quantified. This paper addresses these issues in a quantitive manner. To simplify matters, the behaviour of a simple single degree-of-freedom (SDOF) frame, with connection characteristics modelled in a way that reflects the Northridge conditions, is examined by taking into account the random nature of the connection's resistance to fracture. The fracture event is found to result in a modest reduction in the frame's lateral stiffness, with the post-fracture behaviour being governed by the behaviour of the remaining connecting elements.