Local fracture‐induced phenomena in steel moment frames

Brittle fractures were observed at the welded beam‐to‐column connections of a number of steel moment frame buildings following the M6.7 1994 Northridge earthquake. Such fractures cause a rapid loss of connection strength and stiffness, as well as a sudden release of the strain energy stored by the connection at the time of fracture. Immediately following the fracture, a number of highly transient phenomena occur locally in the members adjacent to the connection, as well as globally in the structure as a whole. Four significant local phenomena were observed locally during shaking table tests of a one‐third scale, two‐story, one‐bay steel moment frame in which quasi‐brittle beam‐to‐column fractures occurred: (a) change in beam deflected shape; (b) change in moment distribution in adjacent members; (c) generation and propagation of elastic waves; and (d) initiation of dynamic modal response at the member level. Owing to the highly transient nature of these phenomena, they were observed to have second‐order effects on overall behavior of the system. In comparison, the reductions in local strength and stiffness caused by the fractures had much more significant effects on system behavior. Copyright © 2008 John Wiley & Sons, Ltd.