Insights into the Response of Multi-Storey Eccentrically Braced Frame Steel Structures to Earthquake Ground Motion

Abstract

Insights were gained from the 2010 Christchurch earthquake series into the performance of eccentrically braced frame (“EBF”) buildings up to around 20 storeys high. An elastic-plastic finite element analysis method using ABAQUS software was developed to assess the amount of cyclic plastic strain accumulated (“PEQ”) in the active links of EBF. Ground motion records were filtered to remove low and high frequency noise. Peak displacements calculated from elastic time history analyses were moderated to peak displacement observations in the building at movement joints post-earthquake. Visual assessments of PEQ in the active links based upon Luders cracking of mill scale were found to be broadly consistent with the analytical results. The amount of PEQ expected to develop from the application of the design rules in the NZS 3404:1997 was assessed by subjecting the full building model to a cyclic imposed displacement series used for laboratory testing of the ductility of structural steel assemblies. The upper limit of PEQ was set using the local strain limits in ASME BPVC VIII.2:2021. Leeb hardness testing results were used to identify comparative pre-earthquake and post-earthquake yield stress in the active links. The elastic-plastic analyses showed that the locations of maximum strain hardening vary with active link configuration. Difficulties were found in using the Leeb hardness testing results to infer PEQ due to this and also to the effect of strain-aging. Analytical PEQ was also used to estimate the changes in Charpy V-Notch impact energy (“CVN”) and the steel’s changed susceptibility to brittle fracture.