Performance of Beam-Column Joints in the 2010-2012 Christchurch Earthquakes

The design of beam-column joints in reinforced concrete moment frames is an area where USA and New Zealand standards have diverged for many years. USA design guidelines, and ACI 352 in particular, implicitly accept damage in the form of shear cracking, bar slip and possible column hinging for joints subjected to large lateral load reversals. Since the 1980's, the New Zealand approach has been to minimize that type of damage and to concentrate the deformations in plastic hinges in the beams by careful detailing of the joint and adjacent beam regions, thus keeping columns essentially elastic. The recent February 22, 2011 Christchurch earthquake and its associated swarm present an excellent opportunity to contrast these approaches in terms of visual performance for a variety of New Zealand structures detailed and built before and after the newer, more stringent joint design guidelines came into effect. The main lesson from the Christchurch experience is the importance of providing both some degree of lateral resistance, e.g. via beam-column joint moment-resisting capacity, and an increased level of displacement capacity in secondary or gravity-frames in order to improve the overall building's robustness and seismic resilience in response to earthquake demands beyond the code design level.