Future proofing our cities against earthquakes means being aware of the best mitigation technologies available, and encouraging their use on new design and retrofit, where economically viable.
“Damage avoidance” design represents a shift in focus in the seismic design of buildings, in which the goal is not merely to prevent sudden collapse and consequent human casualties, but also to allow the building to be reoccupied with minimal repair costs following a large earthquake.
Typically, damage avoidance involves designing carefully engineered joints or devices that can accommodate seismic movements, rather than relying on yielding of steel or crushing of concrete. Seismic base isolation is the most mature and widespread of these technologies, and has been employed in the seismic protection of thousands of buildings and bridges in Europe, the United States, Japan and New Zealand. A more recent application of damage avoidance design is to allow gaps to open up in the structure, thereby concentrating all the deformation imposed by the overall building movement in this gap rather than in large movements and damage in the structural components.
In parts of the world where seismic design codes are well developed and enforced, death tolls in earthquakes over recent decades have been relatively low, and casualties are mostly a result of the collapse of older building stock that predates modern codes. The 2010 and 2011 earthquakes in Christchurch, New Zealand, claimed 185 victims - not insignifi cant for a city of only 370,000, but relatively low considering the extreme levels of ground shaking experienced. However, the economic and cultural heritage losses associated with the subsequent large-scale demolition of the Christchurch city centre have been immense, not to mention the disruption experienced in whole residential suburbs that have had to be abandoned due to the risk of further liquefaction or landslides.
The example of Christchurch, and other earthquakes over the last decades in the US and Japan, has motivated a move towards damage avoidance design. A hospital building in Christchurch built before the earthquakes using such a system escaped relatively unscathed.
Until now, damage avoidance design has been mostly applied in buildings that serve an important role in post-earthquake recovery operations, such as hospitals or civil defence centres. However, in Christchurch, where the repair and rebuild costs associated with traditional design are now more widely appreciated by the general public, damage avoidance design is being employed even for conventional offi ce or residential occupancies.
It is important that engineers designing buildings in other areas of high seismic hazard learn from these examples, and clearly communicate seismic design issues to developers, architects and other stake-holders.
Dr Damian Grant is a senior earthquake engineer in Arup’s Advanced Technology and Research group. He was a member of the Earthquake Engineering Field Investigation Team group that visited Christchurch, New Zealand, following the earthquake last year. For more information, visit www.nce.co.uk/futureproofing