Seismic performance of concrete bridges during Canterbury earthquakes

In less than six months, the city of Christchurch, New Zealand, experienced two major earthquakes: on 4 September 2010 and 22 February 2011. The former was generated by the rupture of the previously unknown Greendale fault, releasing a magnitude Mw 7.1 earthquake 30–40 km away from the Central Business District (CBD); the latter event, of magnitude Mw 6.2, was less than 10 km from the CBD on an unknown buried fault at the edge of the city. There was widespread damage to the lifelines covering not only Christchurch City but also the closest districts of Selwyn and Waimakariri. The different nature of the fault ruptures and locations of the two events resulted in a variation in damage levels between the earthquakes throughout the region. Teams from various organizations performed inspections on over 800 bridges throughout the affected Canterbury region. No major collapses were registered among concrete bridges and only 20 bridges required closure due to damage caused by the two earthquakes. Owing to the nature of the Canterbury soils, extensive liquefaction and lateral spreading occurred throughout the region. It was this lateral spreading that caused most of the traffic disruption and closure of bridges, due to damage to the abutments and approaches, foundation settlement and rotation. The authors aim to give a detailed overview of the damage assessment and seismic performance of the Canterbury bridges during these two earthquakes, emphasizing unexpected issues that are still not properly detailed in New Zealand and overseas standards.