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Assessment of the Performance of Steel Moment Frame Buildings During the 1994 Northridge Earthquake: Task 3 of Sac Steel Program

by James O. Malley

Malley, James O. (1995). Assessment of the Performance of Steel Moment Frame Buildings During the 1994 Northridge Earthquake: Task 3 of Sac Steel Program. SMIP95 Seminar on Seismological and Engineering Implications of Recent Strong-Motion Data, p. 83 - 92.

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Following the January 17, 1994 Northridge earthquake, more than 100 steel buildings with welded moment-resisting frames were found to have experienced beam-to-column connection fractures. The damaged structures cover a wide range of heights ranging from one story to 26 stories; and a wide range of ages spanning from buildings as old as 30 years of age to structures just being erected at the time of the earthquake. The damaged structures are spread over a large geographical area with the highest concentration of reported damage near the epicentral region. Discovery of these extensive connection fractures, often with little associated architectural damage to the buildings, has been very alarming. The discovery has also caused some concern that similar, but undiscovered damage may have occurred in other buildings affected by past earthquakes. Indeed, there have been isolated reports of similar damage having been found in buildings following both the 1971 San Fernando and 1989 Loma Prieta earthquakes.

Welded steel moment frame construction is used commonly throughout the United States and the world, particularly for mid- and high-rise construction. Prior to the Northridge earthquake, this type of construction was considered one of the most seismic-resistant structural systems, due to the fact severe damage to such structures had rarely been reported in past earthquakes, and that only notable collapse of such a structure, the Pino Suarez failure in the 1985 Mexico City earthquake, had ever occurred. That collapse was attributed by investigators to large axial column demands, induced by overly strong bracing in this dual system structure. Subsequent editions of U.S. building codes adopted provisions specifically intended to prevent such failures, and it was presumed by many that buildings designed to these later provisions would be largely collapse resistant. However, the widespread severe structural damage which occurred to such structures calls for re-examination of this premise.