Evaluation of Connection Fracture and Hysteresis Type on the Seismic Response of Steel Buildings

Brittle fractures in steel moment connections during the 1994 Northridge, California, earthquake were observed in more than 200 buildings ranging from 1 to 21 stories. These widespread failures raised issues concerning inspection, evaluation and rehabilitation of existing structures. Research was conducted to develop a computer model that can accurately model the basic hysteretic characteristics of fractured connections in buildings. Redistribution of the unbalanced local moment caused by connection fracture makes the program unique. Results from analyses using this program provide some beneficial information about connection performance and overall system response, and aids in structural performance prediction and evaluation. As expected, buildings with fractured connections experience larger story displacements than buildings with connections that do not fail. However, this effect is smaller than expected indicating that most existing steel buildings are safe during large earthquakes. In addition, eight hysteresis models were developed for this programs. These were used to study the effect of hysteresis behavior on the response of multistory buildings. The models included various degrees of stiffness and strength degradation, pinching and nonlinear elastic behavior. Three-, 6- and 9-story buildings were investigated. The results indicated that the structures could be grouped into two broad categories, those with and without pinching in the hysteresis model. Less energy was dissipated by those with pinched hysteresis behavior, and this led to large displacements. Buildings with strength and stiffness degradation experienced maximum story drifts that were about 10% to 15% larger than for the same structures with bi-linear hysteresis behavior. Buildings with pinched hysteresis behavior experienced drifts that were 20% to 30% larger than those with hi-linear behavior.National Science Foundation Grant CMS 94-0222