Christchurch Feb 22nd Earthquake: A Personal Report by Charles Clifton - March 2011
Invited Guest's Comment byAssociate Professor of Civil Engineering Auckland University and Former Manager of HERA Structural Systems
When I joined the University of Auckland in 2008 after two decades as Senior Engineer in HERAs Structural Division, I became involved in a project called the Retrofit Project. It was focussed on the assessment and retrofit of earthquake prone buildings, with University of Auckland focussing on un-reinforced masonry and steel frames.
The project ran from 2004 to 2010 and three of us from the University have been holding a seminar series conveying the results around the country. This saw Professor Michael Pender, Associate Professor Jason Ingham and myself, along with an American forensic engineer, David Biggs, on the second floor of the Holiday Inn in Cashel Street, Christchurch, at 12.51 pm on Tuesday 22nd February. I was just hooking up the computer to the data show when the room started shaking. Immediately there was a collective groan of not another from the largely Christchurch audience, however the shaking rapidly grew very severe and people started moving for the doors. Quickly realising (probably without much justification) that I was safe, I concentrated on watching the movement of buildings out the window.A 14-storey reinforced concrete building opposite moved 1.5 metres sideways and back again in a classic first mode response and the top storey of a masonry wall moved a similar amount out of plane and back again.
Towards the end of the shaking we could hear the sound of collapsing buildings over the general earthquake noise. When the shaking stopped, all the wall to floor junctions in the seminar room had cracked, however the doors and stairs still worked OK. It was when we got into the street and looked across to the very badly damaged Grand Chancellor Hotel that we realised how severe the earthquake had been.
We made our way out of the city, past large numbers of damaged or collapsed older buildings, with Manchester Street looking like pictures of Tennyson Street, Napier, after the earthquake of February 1931 and before the fire, and finally ended up staying in a local motel which became our base for the rest of the week. While walking through the street pulling our luggage, I thought of my late and very loved father in law, dragging his damaged bike home from school in 1931 and dwelt on how sometimes history does repeat.
The days since then have been a blur of building assessments, some media interviews and looking with awe and sadness on the damage that 12 seconds of intense ground shaking can do to a wonderful city. The damage to the CBD was reminiscent to that from the 1995 Kobe earthquake.
The Christchurch earthquake generated a new fault 17 km long, had a focal depth of between 3 and 5 km and an epicentre 10 km from the CBD. The intensity of shaking was very high, corresponding to a greater than 2,500 year return period event for the city from the earthquake Loadings Standard, NZS 1170.5. [Christchurch Central Business District Spectra report from GNS dated 25 February]. This is the Maximum Considered Event (MCE) level for normal buildings, at which modern buildings are expected to suffer significant damage with a reasonable probability of remaining standing - they have. However, while the intensity was at MCE level, the duration of severe shaking was short, at 12 seconds, and this has prevented more widespread damage.
As far as the author is aware, this is a unique occurrence, being the first time that a first world city has been struck by two severe earthquakes in very close succession. The effect on the CBD has been much more severe in this second event as the following comparisons show (with the February earthquake figure first):
Peak Ground Acceleration (PGA) horizontal of 1g compared with 0.3g
So far nearly 800 buildings have been classed as unsafe compared with around 50 in September
Significant inelastic response in modern buildings compared with none in September
Two pancake failures of middle aged buildings compared with none
80% of the water and sewerage system severely damaged compared with 5%
Moderate to significant liquefaction through the whole city including the city centre in regions where none occurred last time
Where liquefaction has occurred, it is now 3 to 5 times worse It is likely every building in the CBD will have some structural damage, although in modern buildings it may well be minor.
From first hand observation, at least one of the high-rise reinforced concrete framed buildings has significant beam plastic hinging, corner column detachment from the floor system due to beam elongation and localised diaphragm cracking and it is likely this type of damage will be quite widespread. This will involve extensive and expensive repair.Given the intensity of the shaking, it is clear that modern buildings have performed as expected, showing that the concept of ductile design has worked in its first big test in New Zealand.
With regard to HERA and Steel Construction New Zealand, a key question is: How have structural steel and composite buildings performed? From observations immediately after the event it would appear very well; I inspected a 5-storey Eccentrically Braced Frame (EBF) car parking building and a 2-storey EBF rest home building that had no damage to the steel frame or suspended floors, although other components had suffered damage. The tallest steel frame buildings in the CBD appeared undamaged from the street although this does not rule out internal damage. It is planned to inspect some of these in the next few days but not in time for this article.
For Christchurch and the country, the question is where to from here? I hope and expect that Christchurch will be rebuilt more robust and more beautiful; however it will have a different look and feel to the old city. We are going to learn a lot about the repair of buildings designed for ductile response in earthquake and from this I expect a much greater emphasis to be put on low damage forms of earthquake resisting systems in future.
Most of the Un-reinforced Masonry (URM) and heritage stone buildings have been destroyed or severely damaged, however there are good examples where suitably strengthened URM and heritage buildings have performed very well and provide a model for such buildings in other cities as well of icons of hope for the new Christchurch. Some of the damaged old buildings have to be rebuilt Christchurch without its landmark Cathedral Spire would be a lesser place in spirit.
Finally, from an earthquake engineering viewpoint, we will have a unique data set of whole building performance against two different levels of severe earthquake response to compare our design procedures and numerical models against in order to further improve current seismic design best practice. We owe it to all, especially those who have died, to do this in order that the death and property destruction is lessened in future severe earthquakes.