Abstract
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.
Highlights
Existing buildings in seismic-prone areas are usually exposed to sequential earthquakes, in which a mainshock is accompanied by several aftershocks that originate near its rupture zone and have smaller magnitudes than the mainshock (Raghunandan et al 2005)
The subscript AS was added to the intensity measure (IM) adopted in the following analysis to indicate that they represented the intensity of aftershocks
Conventional fragility assessment was performed on the steel moment frame using the standardised incremental dynamic analysis (IDA) procedure, where the undamaged steel frame was directly subjected to mainshocks and MS-AS sequences
Summary
Existing buildings in seismic-prone areas are usually exposed to sequential earthquakes, in which a mainshock is accompanied by several aftershocks that originate near its rupture zone and have smaller magnitudes than the mainshock (Raghunandan et al 2005). Many seismic events in recent decades (e.g., Niigata, Japan in 2004, L’Aquila, Italy in 2009, Christchurch, New Zealand in 2011 and Central Italy in 2016) have shown that aftershocks can have great impact on the seismic performance of existing buildings, resulting in severe damage accumulation and even collapse of structures that are already significantly damaged during the mainshocks. (Yeo and Cornell 2005) To this end, it is necessary to investigate the effects of earthquake sequences on the seismic behaviour of structures. Previous research has put great effort into the study of the impact of earthquake sequences on the behaviour of structures (e.g., Hatzigeorgiou 2010a, b; Di Sarno 2013; Di Sarno and Amiri 2019). Another study by Di Sarno (2013) compared the response spectra of single and multiple earthquakes and found that multiple ground motions tended to result in higher spectral acceleration for structures with short fundamental period and low ductility, as well as higher inelastic deformation demands than single earthquakes
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