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The seismic performance of steel moment-framed buildings has been of particular interest sjnce bdttle fractures were discovered at the beam{olum[ colnections in a number of buildings following the M 6.7 Norrhridge earlhquake of January 17, 1994. A case study ofthe seismic behavior of an extensively instrumented 13-story steel moment tiame building located in the greater Los Algeles aroa of California is described herein. Response studies using frequency domain, joint time frequency, system identification, and simple damage detection an:rlyses are performed using an extensive strong motion dataset dating from 1971 to the present, suppofied by engineering drawings and results of postearthquake inspections. These studies show that the building's response is more complex than would be expected liom its highly symmetrical geometq,. The response is characterized by low damping in the fundamental mode, larger accelerations in the middle and lower stories than at the roof and base, extended periods of vibration after the cessation of stong input shaking, beating in the response, elliptical pafiicle motion, and significant torsion du ng strong shaking at the top of the concrete piers which extend fiom the basement to the second floor The analyses conducted indicate that the rcsponse of the structure was elastic in all recorded earthquakes to date, including Nofthridge. Also, several simple damage detection methods employed did not indicate any shctural damage or connection fracturcs. The combination of a large, real stucture and low inshl-lmentation density precluded the application of nany recently proposed advanced damage detection methods in this case study. Overall, however, the flndings of this study are consistent with the limited code compliant postearlhquake intrusive inspections conducted after the Nofihridge earthqunke, which found no connection fractures or other structural damage.

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Brittle fractures occurring at the beam-column connections of welded steel moment frames, such as those observed following the M w 6.7 1994 Northridge earthquake, result in sudden decreases in connection strength and stiffness. These changes lead to various types of transient dynamic behavior at the local and global levels. The effects on global acceleration include highly transient, high-frequency waveforms that occur immediately following fracture and decay quickly. The theoretical basis for the occurrence of these transient waveforms is examined and their presence in structural analysis results is demonstrated. Results from shaking table experiments on a simple steel moment frame with fracturing connections show that transient accelerations are consistently observed following fracture. These experiments and analyses show that, due to their short duration, the transient acceleration waveforms do not cause any sudden changes in the global lateral displacement response of typical building structures. Therefore, these global acceleration transients have relatively benign effects on overall system behavior despite their relatively large amplitudes.

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