Following the 1994 Northridge earthquake, many welded, moment-resisting steel frame buildings were discovered to have brittle fractures of their beam-column connections. This behavior had not been widely anticipated, was contrary to the intended ductile behavior, and called to question the ability of such structures to reliably resist future earthquakes. On behalf of the Federal Emergency Management Agency, the SAC Joint Venture embarked on a program of analytical and laboratory investigations to determine the causes of this behavior, better understand earthquake response of frame structures, and develop design and construction recommendations that would provide structures capable of more reliable earthquake resistance. Extensive laboratory investigations provided statistical information on the hysteretic behavior of beam-column connections. Analytical investigations provided statistical information on the response of frames incorporating such connections and the ability of analytical techniques to predict this response. These investigations permitted development of a reliability-based approach for evaluation of the probable performance of moment-resisting steel frame structures. This approach forms the basis for recommended design, evaluation, repair and upgrade criteria. These design and evaluation criteria employ a Demand and Resistance Factor Design (DRFD) format that accounts for the randomness and uncertainty inherent in earthquake demand and structural capacity prediction. Two performance levels are addressed - a state of incipient collapse, termed Collapse Prevention, and a state of incipient damage. Interstory drift is adopted as the primary performance-prediction parameter. Acceptance criteria (resistance) are developed both for local (connection or element) and global (frame) behavior. Resistance factors, dependent on structural configuration, connection type and construction quality are applied to account for uncertainties and randomness in capacity prediction. Computed drift demands are factored, based on the analysis used, modelling assumptions and hazard characteristics to account for the uncertainties and variability inherent in drift demand response prediction. The quotient of factored capacity and demand indicates the level of confidence that desired performance will be attained within specified probability of non-exceedance. Application of these procedures to buildings designed in accordance with current United States design standards for moment-resisting steel frame construction indicates that acceptable margins against global collapse are generally provided by these procedures. However, beam-column connections must be significantly more robust than previously anticipated to provide suitably high confidence that endangerment of life safety will not occur due to local failure of the gravity load carrying system. Also, these structures are far more susceptible to damage in more moderate, and frequent earthquake events than previously imagined.
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