Seismic ratcheting of structures - rethinking structural design

Structures may predominantly inelastically deform in one direction during major seismic events, causing greater structural damage and a higher probability of collapse. This phenomenon is known as “Seismic Ratcheting”. Structures with overhangs on only one side are likely to exhibit such behavior, such as the Hotel Grand Chancellor building in New Zealand. The hotel’s eastern bay was cantilevered, causing seismic ratcheting to occur in the cantilever direction during the 22 February 2011 earthquake, resulting in a large area around the hotel being cordoned off for months due to the possibility of collapse during aftershocks. Post-earthquake investigations into the hotel’s design highlighted issues with modal response spectrum design methods, as the fundamental assumption of balanced stiffness in the forward and backward directions were violated due to the eccentric gravity loads. Despite most building codes having similar issues and not explicitly considering seismic ratcheting effects, structures with overhangs in only one direction are still being built globally. This paper summarizes findings from research performed by the author since 2010 to highlight: (i) the mechanics of seismic ratcheting, (ii) limitations of building codes to consider seismic ratcheting effects, (iii) potential mitigation methods, and (iv) ongoing challenges to address seismic ratcheting effects.