A New Zealand earthquake expert has told a London lecture audience that no large high building can be absolutely safe in a major quake.
Dr Damian Grant says engineers must communicate better with the public on the degree and nature of risks involved in building — and they must also be clear about the limits on what can be done to avert the risks. Our UK correspondent Tom Aitken was at the lecture and here’s his report:
Can we anticipate and minimize quake effects?
At a NZ Universities Graduates’ Association lecture in New Zealand House, Dr Damian Grant, , discussed the reasons for the differing levels of destruction in the Christchurch ’quake last February. His talk was succinct and wittily expressed.
Dr Grant, a New Zealander earthquake engineering lecturer at University College, London, said that over the past 40 years New Zealand universities in general, and Canterbury in particular, have done valuable research into uses of structural engineering in countering quake effects.
Particularly significant factors in the impact of the earthquake on Christchurch
were ‘heritage’ masonry buildings, liquefaction and collapsing reinforced concrete.
It makes little sense, he said, to talk about building ‘earthquake-proof’ structures. Such an achievement is theoretically possible but only at prohibitive expense.
Not all earthquakes are created equal, and it is impossible to anticipate the strength and variousness of their effect.
Buildings can be built to cope with a shake of a particular strength, but if the jolts that come are, say, twice that strength, the building will probably collapse.
There are other, even more unpredictable factors. Will ground rise or fall vertically? Will it lurch sideways? How fast and with what level of acceleration will these things occur?
Will there be liquefaction? We know what causes this phenomenon––loose, waterlogged sand and silt, as produced by slow, meandering rivers like the Avon––and that it may cause soil under the surface to slide downhill on surfaces not themselves liquefied, again at an unpredictable rate.
No structural engineer would claim that buildings will not suffer any damage at all during a ‘quake. What engineers do try to do is design buildings that will not collapse.
Apart from aesthetic qualities and particular requirements re arrangement of internal spaces and the like, two main considerations apply in today’s design codes: ductility and brittleness.
‘Ductility’ relates to the extent to which a material or construction method is able to bend without breaking.
‘Brittleness’ applies to materials that are strong provided they are not forced to bend.
Parts of a building that must be strong, but are also brittle, such as floors and walls, need to be contained in a framework that is ductile.
Damian illustrated this ingeniously, using a straightened out paper clip and a stick of uncooked sphagetti. The paper clip could bend a long way without breaking, whereas, of course, the sphagetti snapped immediately under pressure.
Of three major phases in New Zealand’s requirements for large, especially multi-story buildings, the first, building almost entirely with ‘brittle’ materials such as brick, predates
the Napier Earthquake in 1931.
Next came a period when such buildings had to be strengthened by the use of additional frameworks in other, less brittle materials, leading to much use of reinforced concrete.
Various shortcomings in these methods became apparent, and since 1982 there has been a legal requirement for workable and affordable combinations of ductility and strength.
No large, high building, Damien repeatedly stressed, can be absolutely ‘safe’.
Damian applied these considerations to the buildings affected by the Christchurch ’quake.
The great majority of those which collapsed were of brick, and many dated from before 1931.
Buildings made later of reinforced concrete did, all things considered, fairly well but there were tragic exceptions. The Canterbury TV and Pine Gould Buildings in which upper floors collapsed into the building had not been built to cope with ’quakes of this magnitude.
Most structures built from 1982 onwards did quite well, although in one case a building survived but all its staircases collapsed. This could have had very much more serious consequences than it did had the occupants already begun to use them as an escape route.
In future he thought, engineers must communicate much more effectively with the public on the degree and nature of risks involved in building. They must also be clear about the limitations on what can be done to avert those risks.
Developers must be prevented from building in very risky areas.
He concluded by mentioning new engineering methods that can do a little to make older buildings safer.
Various sorts of ‘retrofit’––metal ties in walls and floors––can be applied to brick and other old buildings to strengthen them. But they cannot be made more ductile.
An exciting but very expensive new method is called ‘base isolation’. In this the building sits on a platform of lead/rubber bearings, allowing it to move as a unit without having its parts move in relation to each other.
Another method is to use ‘unbounded tension pendants’––‘basically, large rubber bands’, says Damian as he tips a model skyscraper back and forth on his lectern.
Both these, he says with pride, are Kiwi developments. People in Italy and other earthquake-prone areas are looking to a rebuilt Christchurch to show the world the earthquake resistant buildings of the 2040’s.
PS. The last speaker from the floor told us that the 19-storey structure at the top of which we sat, because of its diplomatic status, complied with the Wellington building code. It was probably by some distance the most earthquake proof building in London.