Seismic design and damage evaluation of buildings with steel moment resisting frames

Unprecedented cases of brittle and unpredictable failure of welded moment resisting connections in steel moment resisting frames were revealed during recent earthquakes, such as 1994 Northridge Earthquake in California and 1995 Hyogo-Ken Nanbu Earthquake in Japan. It raised doubts about the seismic performance of steel moment resisting frames. As a result, an investigation was performed in an attempt to answer two critical questions regarding the seismic resistant design of steel moment resisting frames: (1) How to improve the ductility of moment resisting connections to eliminate the brittle behavior during an earthquake ground motion? (2) How to assess the level of damage to buildings with steel moment resisting frames after a seismic event?; In the first phase of this research, a modified moment resisting connection with enhanced ductility was proposed. It was based on the idea that a section, weak in bending near the end of the beam, but not at the beam-to-column interface, could be created to reduce the stress developed at the beam-column interface during a cyclic loading process. Also, the influence of these modified connections on the overall seismic behavior of moment resisting frame was studied.; To evaluate the damage level of moment resisting frames, a systematic damage assessment technique was developed in the second phase of this research. The damage evaluation technique started with the local damage evaluation of each individual moment resisting connection. A low-cycle fatigue damage model for moment resisting connections was established based on the hysteretic energy dissipated in the connections. The values of the low-cycle fatigue parameters were obtained statistically through a study of available full-scale test data on connections of ordinary member sizes. Then, weight-averaging techniques were employed to combine the local damage indices to form the damage index for a story and/or the entire frame.; The analytical results indicated that the proposed modification to the beam ends could be an effective means to mitigate brittle connection damage that could occur in moment connections during seismic events without inducing any unfavorable effects on the overall seismic behavior of a moment resisting frame with these connections. This research provides a basis for in-depth experimental investigation of full-scale modified connections. The overall damage assessment approach made it possible to assess quantitatively damage to steel moment resisting frames. This damage assessment technique will provide the structural engineers with another valuable tool for the design, retrofit and post-earthquake damage evaluation of steel frame structures.