What was the performance of the building stock in Christchurch?

Col Gurley, FIEAust, 29 March 2011

I am retired in Sydney now and only know what I have managed to find from the local press/TV and on a 6-year old computer over the internet.

Consider (1) URM buildings and (2) multi-storey concrete buildings with/without a steel frame.

New URM buildings have not been permitted in NZ for more than, say, 50 years. There is still a huge investment in them and they did constitute most of the serious damage in the ChCh quake. There were many 2 and 3 storey URM commercial buildings (shops and offices) on the main roads leading away from Cathedral Square.

In ChCh, many (perhaps most) of those were damaged and there is probably now the political will to demolish many of them. This is perhaps, not true for, say Auckland where there are many more URMs.

Perhaps, as engineers we need some sort of yard-stick for discriminating risk while recognising that that varies from "lower" to "dreadful". Not so much a structural analysis tool as a paradigm for considering issues such as: (1) Number of storeys 1,2,3 and total floor area and (2) Occupied for how many hours per day and (3) Occupied at what density m2/person and (4) Concrete versus timber floors and coming out of all that: (5) How many lives are at serious risk?

Clearly family homes will rank low on this list because they are usually occupied at a low density say 50 m2/person as against office buildings at up to 10 m2/person. Three-storey walk-up medium density housing will have higher risk than houses because of higher densities, say 25 m2/person and because collapsing concrete floors supported on URM are much more likely to kill people than timber but, of course, timber floors do not have a fire-rating. This sort of new building is still being constructed in very large numbers in all of the Australian capitals and, as I remember, the earthquake risk in Auckland is not much greater than that of Adelaide. Commercial shops/offices in large URM buildings will have the highest risk. There were a lot of these in ChCh.

Turning now to reinforced-concrete buildings which are usually 8 storeys or more. Clearly the potential death-toll is larger because of the larger plot-ratio (occupied floor-area/site area). On the other hand we should now be able to design new buildings to withstand, perhaps, all/all but the very largest earthquakes. Some useful data will emerge from ChCh relating to buildings built there before the Los Angeles (San Fernando) earthquake of 1971 which initiated a major reassessment of structural codes thru the 70s. More will emerge from the Japanese event of 11 March 2011 because of the size of that event variously reported at 9.0 and 9.1. My memory is the Japan has tended to put relatively more emphasis on strength (loads) as compared to ductility and as compared to the US and NZ codes. This view may be out of date.

There will be some quite hazardous old concrete buildings around and they do to be identified but this will be tempered by the lower risk in Auckland and Adelaide.

There is a risk now of over-reaction in the codes for the design of new buildings. The ASCE Report on the collapse of the Oklahoma Federal Building (1996) made the point that detailing to the Californian rules in place at the time of original construction would have increased the total cost of the building by 2% - 3% and reduced the extent of collapse and presumably the (167?) death-toll by 80%. Three percent sounds entirely reasonable. Note that this is 3% of the total cost of the building which will be 10% - 12% of structural content cost and 30% - 40% of rebar cost. I do suggest that, for a well-conceived building, the sizes for gravity and wind loads should generally be sufficient for earthquake and that most/all of the extra cost should be in more rebar and better detailing. Engineers can sell this to architects/clients on the basis that it is a choice between seismic safety and, say, the Carrara marble facings in the lift-lobby. PS Does anyone know what has happened to the 15-storey hotel (originally Noah's) on the north-eastern corner of Worcester and Oxford Terrace overlooking the Avon.