Rebuilding Christchurch – earthquake engineers call for rethink

Engineers are calling for a rethink in building design and standards in the wake of the Christchurch earthquakes. Designing to save buildings as well as lives would improve the resilience of crucial infrastructure following high magnitude quakes.

A major gathering of earthquake engineers from New Zealand and overseas kicks off in Auckland tomorrow (14-16 April). The Pacific Conference on Earthquake Engineering takes place once every four years, bringing together over 300 geotechnical experts, seismologists and engineers from NZ and abroad. Its theme this year is ‘Building an Earthquake Resilient Society’. The conference, which has been in the planning stages since before the September quake, has been adapted to address major issues emerging from Christchurch, including:

  • New perspectives in seismic hazard: from observation to quantification
  • Advances in seismic retrofit of reinforced concrete buildings
  • Engineering seismology: ground and site effects, seismic hazard and risk analyses
  • Social-economic issues: human behaviour, education, insurance and public policy
  • Geotechnical earthquake engineering: soils, liquefaction, foundation and soil-structure interaction
  • Structural engineering: design of buildings, bridges, lifeline structures
  • Lessons learned from recent earthquakes

In the lead up to the event, the Science Media Centre gathered comment from earthquake engineers about the challenges ahead.

To talk to these or other engineering experts, or for more information, contact the SMC (NZ).

Assoc Prof Greg MacRae, earthquake engineer at Canterbury University, comments:

“The most important factor to consider during rebuilding is seismic sustainability. That means that we build our structures so that we protect our infrastructure during a major earthquake.

“At at the moment we are building to protect lives, which means after a major earthquake like this, we expect this kind of damage.

“We can change our focus, and build to protect our infrastructure as well, using advanced technologies that are available for all materials.

“People have a choice. They can design and rebuild using 1980s technology, and they’ll get the kind of results we’ve seen. Or they can use modern technology and get very little damage.

“Some of these new technologies cost the same or less than existing construction techniques. Some of these, like base isolation, have been around for a number of years and are well-proven. Some are less robust, and require more extensive testing and research.

“However, when you’ve got that choice, I think it’s a no-brainer. The barrier to implementation is largely ignorance. A lot of it has also been lack of will. People have never really believed that an earthquake could occur here in Christchurch.”

Prof Michael Pender, geotechnical engineer at Auckland University, comments:

“[From a structural perspective] the MAJOR casualty after the September 04 earthquake was house foundations. The susceptibility of these to lateral spreading was emphasised again after February 22. We need revised standards for construction, and methods of site investigation that flag the risk of liquefaction.

“Another major casualty of both earthquakes was unreinforced masonry buildings. We have known about this problem since the 1931 Napier earthquake. We have worked at developing solutions but, with the benefit of hindsight, clearly with not enough urgency. There still remains this major problem in nearly every NZ city and town.

“During rebuilding, there will be an excellent opportunity for collaborative endeavours between the structural and geotechnical engineering teams. We have structural and foundation details of the buildings subjected to the earthquakes. We have soil profile details for the central business district. We have a reasonably good idea of the ground motions the structures were subjected to. We know how the structures and foundations behaved. This provides an important opportunity to analyse the behaviour of the structures and foundations, particularly those that performed well.

“These insights will be an ideal springboard for re-design. But to get best benefit it needs, as I said above, to be a joint geotechnical – structural endeavour. In other words, we need to considering the integrated design of structure-foundation systems, and not the design of a foundation to support a particular structure.

“From the engineering perspective, we can say that the two earthquakes have provided us with an enormous pool of data about the response of engineering systems. In other words, we are the beneficiaries of outputs from a huge natural laboratory. What we need to do now is winnow this information to obtain the best understanding of how to go forward.”

Dr Andy Buchanan, Prof of Civil and Natural Resource Engineering at Canterbury University comments:

“Lots of buildings in the Christchurch earthquake behaved exactly as expected, but they still ended up buggered.

“We designed buildings to withstand earthquakes with the expectation that they would be damaged. Unfortunately, many were damaged so badly that they will have to be pulled down.

“Looking to the future, what we have to do now within the research community is to start designing buildings so that no matter how big the earthquake, buildings won’t be damaged. This is the new paradigm. There are ways of doing this. We know how to do it, we just haven’t started doing it yet.”

Dr Charles Clifton, Assoc. Professor of Civil Engineering at Auckland University comments:

“There are three levels of earthquake design for modern structures (post 1975 approx). The first (serviceability limit state level) is the level of earthquake expected more than once during the lifetime of the structure, for which the structure is expected to remain fully functional. The second (ultimate limit state level) is the level at which a typical structure is expected to undergo controlled damage but must remain standing and allow evacuation. It is also likely to be repairable afterwards although this is not an explicit requirement of current design practice. The third (maximum considered event [MCE] level) is the strongest earthquake expected at that location and for which a modern structure should remain standing but will suffer considerable structural damage.

“For most of the Christchurch CBD the intensity of this earthquake was close to the MCE but the duration was shorter than would have been expected for an MCE event.

“For reinforced concrete multi-story structures, modern buildings remained standing, which meets the criteria for MCE response. However, I would have expected them to be repairable and at least two of the highrise buildings have not met this criterion. There are some specific areas of failure, e.g. shear walls failing in compression, which were not expected.

“Unreinforced masonry buildings were subjected to between 5 and 6 times their design limit so it is surprising there were not more collapses. Retrofitted buildings have performed well, which is very credible given the level of acceleration was some 2.5 to 3 times design level.

“Failure of shear walls in compression and patterns of plastic hinge cracking in reinforced concrete beams (small number of wide cracks instead of larger number of small cracks) and beam elongation causing floor failures and column detachment from floors are problem areas needing addressing in modern concrete structures.

“The intensity of shaking was amongst the highest recorded in a city. It has already led to a rapid review of the current measure of seismicity for Christchurch (the zone (Z) factor), and this is expected to increase.

“However, this shows that the seismology modelling has considerable uncertainties and it is the response of the built environment to the event that is critical. This response must be best understood and improved given that we cannot predict the time or severity of a severe earthquake at a given location in other than general terms.”

Dr Rolando Orense, Senior Lecturer in engineering at Auckland University and specialist in liquefaction, comments:

“There are many methods of remediating the areas affected by liquefaction and prevent re-liquefaction in the future. Densification techniques (increasing the density of soil), making the skeleton of soil particles more stable (through injection of cement grouts and chemicals), dewatering techniques (to lower saturation) and improvement of resistance to deformation (through sheet pile walls, diaphragm walls, etc.) among others. Lateral spreading (near the rivers) can be prevented through compacted walls, stone columns, piles, etc.

“The technology is available and has been implemented in many parts of the world; it is just a question of cost.

“The buildings and residential houses affected by liquefaction can be retrofitted to make them stronger and to resist damage from future liquefaction.”