Low-cycle fatigue life evaluation of buckling-restrained braces based on cumulative plastic deformation curves

To quantitatively evaluate the low-cycle fatigue life of buckling-restrained braces (BRBs), an evaluation method for BRBs based on the combination of the CPD curves and CPD measurement meter (CMM) is established herein. The difference between two CPD curves (i.e., the CPD curves under constant strain amplitude loading history (C-CPD) and under random strain amplitude loading history (R-CPD)), and their selection criteria to evaluate the low-cycle fatigue life of BRBs are discussed. An example under the variable strain amplitude (VSA) loading history and an example under the multiple earthquakes are carried out. Finally, the CPD limit values of BRBs in the ANSI/AISC 341-10 and FEMA-450 are discussed by statistical analysis of the test results of BRBs. The analysis results show that the C-CPD curve can be used to evaluate the low-cycle fatigue life of BRBs under the VSA loading, and the R-CPD curve can be used to evaluate the low-cycle fatigue life of BRBs under the multiple earthquakes. The degree of reliability of the current CPD limit value is not enough. A CPD limit curve is recommended in this study to quantify the low-cycle fatigue performance of BRBs.

Fragility assessment of existing low-rise steel moment-resisting frames with masonry infills under mainshock-aftershock earthquake sequences

This paper presents the fragility assessment of non-seismically designed steel moment frames with masonry infills. The assessment considered the effects of multiple earthquakes on the damage accumulation of steel frames, which is an essential part of modern performance-based earthquake engineering. Effects of aftershocks are particularly important when examining damaged buildings and making post-quake decisions, such as tagging and retrofit strategy. The procedure proposed in the present work includes two phase assessment, which is based on incremental dynamic analyses of two refined numerical models of the case-study steel frame, i.e. with and without masonry infills, and utilises mainshock-aftershock sequences of natural earthquake records. The first phase focuses on the undamaged structure subjected to single and multiple earthquakes; the effects of masonry infills on the seismic vulnerability of the steel frame were also considered. In the second phase, aftershock fragility curves were derived to investigate the seismic vulnerability of infilled steel frames with post-mainshock damage caused by mainshocks. Comparative analyses were conducted among the mainshock-damaged structures considering three post-mainshock damage levels, including no damage. The impact of aftershocks was then discussed for each mainshock-damage level in terms of the breakpoint that marks the onset of exceeding post-mainshock damage level, as well as the probability of exceeding of superior damage level due to more significant aftershocks. The evaluation of the efficiency of commonly used intensity measures of aftershocks was also carried out as part of the second phase of assessment.