Dr. Chia-Ming Uang, Professor at Department of Structural Engineering, University of California, San Diego
Seismic design of steel structures in the United States has made significant advances in the past half century. This development can be broadly divided into three eras. Up to the 1985 Uniform Building Code (UBC), limited design requirements were provided in the Uniform Building Code; these requirements, mainly for ductile moment-resisting space frames, were “borrowed” from those of the AISC plastic design. Based on research conducted in the 1970s and 1980s, the 1988 UBC marked the start of the second era, when both the ductility and capacity design concepts were introduced. New systems, like the Eccentrically Braced Frame, were added to UBC, but the number of steel seismic force-resisting systems was still limited. The ductile Special Moment-Resisting Space Frame (SMRSF) was regarded as the premium system for seismic resistance for its high ductility and energy dissipation capacity so more specific design requirements were added to UBC and the prescriptive welded flange-bolted web moment connection was codified. Unfortunately, the excellent reputation of steel buildings for earthquake resistance was in serious doubt, by design professionals, after widespread damage, in the form of brittle fractures of the moment connections in many SMRSF buildings, was discovered after the 1994 Northridge earthquake. This event marked the start of the third era as it profoundly impacted the research, design code development, and construction of not only steel moment frames, but also of other systems for high seismic applications. This presentation will provide a historical overview and assessment of the development which leads to the modern ASCE 7 and AISC seismic design provisions.