DAMAGE TO DUCTILE MOMENT RESISTING STEEL FRAMES IN THE CALIFORNIA NORTHRIDGE EARTHQUAKE

Ductile steel frames have long been considered the most earthquake resistant type of buildings. Modern steel structures with adequately designed structural Systems, good seismic detailing (particularly .at beam-column and bracing connections), and adequate quality control and engineering inspection have performed well in recent major earthquakes. However, after recent Northridge earthquake, damage was reported to a significant number of steel structures. Various types of local failures were observed in those damaged steel frame buildings, which can be grouped under the following three classifications: a) failures in beam-to-column welded connections at steel moment resisting frames; b) failures due to member and connection buckling of braces at steel braced frames; and c) failures of column base plates and anchors in braced and moment resisting frames. These types of failures have not been widely reported following other earthquakes and their discovery in Southern California occurred only during the Northridge earthquake.This paper focuses on connections failures of ductile moment resisting steel frames which occurred in bolted-web and welded - flange moment connections. It discusses the types of joint failures observed, and suggests possib connection failures. It also proposes repair and new design concepts to improve the performance of moment resisting steel frames. Finally, the paper describes the results of full scale testing of steel moment connections performed at the University of Texas in Austin.