Seismic stability of steel welded moment frames damaged during the 1994 Northridge earthquake

In the aftermath of the 1994 Northridge earthquake, the authors conducted an emergency research to investigate the life safety hazard posed by the steel moment frames with fractured welds. Three structures were selected for the study. The structures were a 4-story, a 14-story and a 27-story welded special moment frames. Al three structures were damaged during the Northridge earthquake. Relatively realistic computer models of the moment frames representing the welded moment frames in undamaged and damaged conditions were constructed. The computer models were then subjected to a number of ground motion records including the Northridge-Newhall, Miyagi-ken-Oki and Taft earthquakes. Various intensities of these ground motions were used in the analyses. The undamaged and damaged connections were modeled as best and as realistically possible, with the available modeling technology, to represent the available strength and stiffness of the connections before and after damage. Various response parameters such as story drift, base shear, member forces and others were obtained and analyzed. The following conclusions could be drawn from the analyses: The taller the structure, the less damage occurred. No tendency to collapse was observed in these three structures with ground motions used. The damaged structures behaved in a somewhat "semi-rigid" manner and did not attract as much energy of the earthquake as similar but undamaged structures. As a result, in most cases, the drift and base shear of damaged structures were less than the corresponding values for undamaged structures.