Forensic Analysis of Link Fractures in Eccentrically Braced Frames during the February 2011 Christchurch Earthquake: Testing and Simulation

Abstract

The earthquake on February 22, 2011, in Christchurch, New Zealand, resulted in the first documented field fractures of links in eccentrically braced frames (EBFs). A comprehensive forensic analysis of these fractures, which occurred in the parking garage of the Christchurch Hospital, is presented. The analysis is based on mechanical and spectrochemical testing and three-dimensional (3D) scanning of procured physical samples of the fractured links. The analysis features nonlinear time history simulations to characterize deformation demands and continuum finite-element simulations to determine the capacities based on a sophisticated fracture mechanics model. The exercise represents a multiscale end-to-end simulation of the system and provides insight regarding the observed fractures. The analysis reveals the inherent challenges in determining the proximate cause of the fractures because the fractures occurred because of a confluence of several interacting factors, primarily the intensity of shaking (several times the intensity that was expected during a design-level event) and the frame geometry, which severely amplified the imposed demands. In addition, the fractured links also suffered from an erection (fit-up) error, in which the link stiffener was not located (as specified) directly above the brace flange, producing a severe strain concentration. This flaw significantly reduced the deformation capacity; however, the simulation of hypothetical scenarios indicates that, even with this flaw, the links would (with high likelihood) have survived a design-level event. Strategies for mitigation include stricter tolerances for stiffener location, consideration of frame geometry to reduce amplification of rotation, and enhancement of the seismic hazard used for design. The limitations of the study are outlined.