What happens to buildings when large earthquakes hit? What makes some structures more resilient than others? University of Auckland Associate Professor of Civil Engineering and Structures, Dr Charles Clifton, answers questions.

How are modern buildings designed to withstand severe earthquakes?

We design modern buildings for defined conditions of performance in three levels of earthquake.

These levels are called Limit States.

The Serviceabililty Limit State (SLS) earthquake is expected to occur with around 90 per cent probability in the 50-year design life of a typical building.

The Ultimate Limit State (ULS) earthquake is expected to have 10 per cent probability of occurrence 50-year design life of a typical building.

The Maximum Considered Event (MCE) earthquake is expected to have approximately a 2 per cent probability of occurrence in 50-year design life of a typical building.

The earthquake actions generated by these limit states, for buildings up to around 10 to 15 storeys high, are very large compared with those of other lateral loading conditions such as wind.

The SLS earthquake actions, for example, will be higher than the ULS wind, which has a 400-year return period; the ULS earthquake actions will be some five times higher than the SLS earthquake actions and the MCE earthquake actions some eight times higher than the SLS earthquake actions.

The ULS and MCE earthquakes are low probability events.

To design a typical building to remain undamaged in these events is a major cost and so buildings are typically designed to undergo controlled damage in these levels of earthquake, protecting the occupants at the expense of controlled structural damage.

Through this tradeoff of strength for damage, the ULS design actions for the building structural system can be lowered to the SLS level, but at the expense of structural damage being generated when the earthquake exceeds the SLS level.

A good analogy for this is design of a car.

The SLS earthquake is like a common minor accident in a supermarket carpark or the local high street.

There may be some panel damage but the car remains fully functional.

The ULS earthquake is the less likely accident on a major suburban road.

The car will be damaged and will need repair or at worst replacement but the occupants will be safe and expectedly uninjured.

Hopefully the occupants will survive okay but the building will be damaged beyond repair.

We have considerable experience now in New Zealand on how well our modern buildings perform in the different levels of earthquake intensity.

Generally, the performance is demonstrably better than these limits.

For example, the Christchurch 2010/2011 series of six damaging earthquakes - above SLS level, for instance - in the CBD comprised a MCE event in intensity and duration, but which took place over some 18 months instead of in one initial very large event with a series of typical aftershocks.

Only the CTV building collapsed; some modern medium- to high-rise buildings survived with no structural repair needed.

Other recent earthquakes have shown that for most buildings the damage threshold is about two to three times higher than the models would indicate.

How severe was the Kaikoura earthquake in Wellington?

The best measure of damage potential is the intensity, expressed by the Peak Ground Acceleration (PGA).

Duration also matters for severe earthquakes but is less critical.

The ULS design level PGA for a typical building in Wellington is 0.40g, where g is the acceleration due to gravity.

Aircraft turbulence on a very rough flight is around 0.2g to 0.25g.

The serviceability level is around 0.1g.

The damage threshold for a modern building designed for maximum controlled damage is around 0.15g.

The Kaikoura earthquake generated PGA values in Wellington of between 0.2g and 0.3g.
So the widespread level of structural damage in modern buildings designed for controlled damage in a severe earthquake is to be expected.

Why are some buildings more affected than others?

This depends on the size and shape of the building and the type of ground it is on.

Soft reclaimed ground moves more violently in an earthquake than solid ground, hence is more damaging to buildings.

If the ground shakes start to develop a resonant shaking frequency, such as you can get shaking a bowl of jelly, and this matches the vibration response of the building, the damage will be higher.

Also, earthquakes expose weaknesses in design or construction of a building, such as inadequate tying together of structural components.

All this leads to complex and considerable differences in building response, even in areas of close proximity.

This wide variation is especially the case in an earthquake of the intensity somewhere between the SLS and ULS limit states, as some buildings will remain elastic and others will be damaged.

When the earthquake gets very severe - as in the February 22, 2011, Christchurch earthquake, everything is damaged to some extent.