Earthquake Induced Eccentrically Braced Frame Link Fracture in the Christchurch Hospital Parking Garage

A rigorous forensic examination of the first documented field fractures of an eccentrically braced frame (EBF) observed worldwide is presented in this thesis. The fractures occurred in the Christchurch Hospital parking garage during the 6.3 magnitude Christchurch Earthquake of February 22, 2011. These fractures are especially important because they are unexpected; EBFs are considered highly ductile structural systems. Moreover, EBFs are commonly implemented in the seismically active regions of the United States, and other countries, and have historically performed well during seismic events.;It was determined that fracture may be attributed to a combination of three factors: inadequate steel material properties, misalignment of EBF features during fabrication and installation (i.e. detailing error), and exceedance of design demands produced by an extreme seismic event. A comprehensive testing and simulation program was performed to ascertain the relative impact of these factors. These simulations ranged from nonlinear time-history simulations of the full structure (to establish deformation demands), to finite element simulations of the individual fractured frames to quantify capacities and determine when fracture occurs with respect to loading and geometry. All these simulations were based on link samples procured from the fractured components. Thus, the material properties (including fracture toughness) and as-built details were implemented into this study to replicate the conditions that precipitated fracture.;Preliminary assessments of the fractures attributed them to poor detailing, wherein stiffeners in the link were accidentally misaligned with respect to the flange of the brace framing into the link, causing a stress concentration. However, the frame simulations conducted during this study indicate that the link rotation demands were significantly (by over 100%) in exceedance of expected (design) values, suggesting that even well detailed connections would be subject to fracture under the recorded level of shaking. On the other hand, the finite element simulations indicate that even with the stiffener misalignment, the link may have survived design level demands.;The study presents a demonstrative simulation methodology that encompasses several scales, as well as several types of simulations, which are validated by test data. Recommendations for mitigation of these types of fracture are provided based on the results of the research.