Review and classification of international URM macroblock failure mechanisms and application to the Canterbury earthquakes building inventory

Earthquake-induced damage in unreinforced masonry (URM) buildings is often characterised by cracking that delineates relatively undamaged masonry macroblocks, whose interaction forms localised failure mechanisms. The reported study presents an international review of URM macroblock failure mechanisms based on post-earthquake observations, experimental testing, and numerical modelling reported in the literature. The compiled macroblock framework is subsequently applied to a dataset of 318 URM buildings surveyed following the 2010/2011 Canterbury earthquakes in Christchurch, New Zealand, enabling systematic classification of observed damage and large-scale vulnerability screening. Out-of-plane mechanisms represented 65% of all observations, confirming their predominant contribution to URM seismic vulnerability, with rocking vertical cantilevers (chimneys and parapets) and top-storey façade overturning identified as the most frequent failure mechanisms. A notable finding was the high occurrence of two-way bending wall failures despite the limited availability of practical seismic assessment approaches for this behaviour, highlighting the need for further research and development of engineering assessment tools. Pounding was also frequently encountered (102 observations) yet remains rarely addressed in common seismic assessment practice. A directionality analysis of observed macroblock mechanisms showed no clear correlation with earthquake directionality from the epicentres to the Christchurch CBD, indicating that URM macroblock failure mechanisms should not be used as indicators of seismic loading direction.