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9 Lessons from the Christchurch Earthquake - Implications for Building Code Changes
in Australia
The level of seismic hazard used in Australian loading codes to determine the design level
earthquake (the "rare" event) needs to be reassessed. The Building Code of Australia
(Australian Building Codes Board, 2011) stipulates the return period for design level
earthquakes to be 500 years for ordinary buildings (importance level 2), with a higher return
period for "more important" buildings, e.g. 1500 years for buildings of importance level 4.
This differs from recent U.S. practice reflected in the American Loading Code (American
Society of Civil Engineers, 2005) which attempted to create a uniform margin against
collapse at the design ground motion for all regions across the U.S. It has been recognised for
some time that there is a larger ratio, between the level of ground motions experienced in a
2500 year return period event to those in a 500 year return period event, in regions of low to
moderate seismicity, as compared with regions of high seismicity (Nordensen and Bell,
2000). To account for this, the ground motion hazards in the U.S. have been defined in terms
of maximum considered (MCE) ground motions. A lower bound estimate of the margin
against collapse in structures designed to the seismic provisions in the U.S. standard has been
taken as 1.5 in terms of ground accelerations. Hence, the design earthquake ground motion
was selected at a ground shaking level that is 1/1.5 (or 2/3) of the MCE ground motion. For
most regions in the U.S. the MCE ground motion has been defined with a uniform probability
of exceedance of 2 per cent in 50 years (return period of about 2500 years). There is a later
modification of the American Loading code (American Society of Civil Engineers, 2011) in
which the MCE terminology in the 2005 edition has been replaced by “Risk-targeted MCE”.
In this case the mapped ground motions were developed on the basis of risk of collapse,
however, the values themselves have only changed slightly.
More research is also needed by Australian seismologists to determine the ground motions in
the various capital cities that are likely to be experienced in a "very rare" event, especially in
zones with deep soft soils. Although Australia is located in a region of low to moderate
seismicity, it is one of the most active intraplate regions in the world due to strains created by
the Indo-Australian plate colliding with the Eurasian and Pacific plates. Most of the
Australian capital cities have known faults in their vicinity that are capable of generating
damaging shallow earthquakes; historical earthquakes of magnitude 6 or higher in Australia
have been caused by ruptures on shallow reverse faults and are similar to the event
experienced in Christchurch on February 22nd 2011. For example, the M w 5.7 1989
Newcastle earthquake occurred on a thrust fault and would cause losses of $3.2 billion if it
were to recur today. Australian earthquakes have sometimes occurred in clusters (three Mw
6.25 to 6.5 earthquakes occurred in one day in the 1988 Tennant Creek sequence (Bowman et
al., 1990)), and have been followed by aftershock sequences like that of the Canterbury
sequence (e.g. the sequence of events that occurred off the east coast of Tasmania near
Flinders Island in the late 1800s with magnitudes as large as M w 6.9 (Michael-Leiba, 1989).
Due to the vulnerable nature of the current building stock, if a magnitude Mw = 6.0 event or
higher did occur in an Australian capital city, it would be likely to cause extensive damage
and a large number of fatalities. This is recognised by the insurance industry which has
determined that Sydney presents one of the highest insured earthquake risks in the world due
to the universal coverage of earthquake losses provided by the Australian insurance industry
(http://www.captivereview.com/news/105276/australia-cat-bond-for-swiss-re-.thtml).
The seismic designs implemented in accordance with the Australian Earthquake Loading
Code AS1170.4 (Standards Australia, 2007) are typically "force-based" and the ULS
(Ultimate Limit State) 500 year return period PGA (Peak Ground Acceleration) in the capital
cities is about one third of that in Christchurch. The code-compatible ULS displacement
design spectrum for a soft soil site is said to have a transition period, T2, (at which the
variation in spectral displacement with period changes from linear to a constant value) of just
1.5 seconds, and hence a maximum value of 150 mm (much less than the 1.2 metres realised
at some sites in the Christchurch CBD). The current performance objective in Australia is to
achieve life safe performance or better in a rare event (currently defined as the 500 year
return period event). However, for most buildings, there is no provision made for a higher
level event. As happened in Christchurch, it is the very rare event that could cause major
damage, potentially rendering the CBD unusable for a long period. This is exacerbated in
Australia by the fact that material design standards such as the Steel Structures code
(Standards Australia, 1998) and the Concrete Structures code (Standards Australia, 2009) do
not require designers to use capacity design principles in their design; yet the implementation
of these design principles in New Zealand (since the 1980s) saved many lives in the
Christchurch earthquake. Most of the fatalities were due to the collapse of two older
reinforced concrete buildings with non-ductile detailing, the Pyne Gould and the CTV
buildings.
In Australia, due to the lack of thought given to strength hierarchies within a building and the
failure to incorporate weak ductile zones that allow the building to safely deform into the
plastic range, the performance of some buildings is likely to be poor in a very rare earthquake
event. The robustness clause in the loading code AS1170.0 is intended to ensure that the
damage caused by an event is not disproportionate to the magnitude of that event. The
question here is what type of damage would the structural engineering community view as
appropriate for a very rare earthquake event. It is the author's opinion that "prevention of
collapse" is the minimum performance objective that should be considered.
10 Conclusions
A general summary of the damage caused by the February 22nd Christchurch earthquake has
been given with considerable emphasis on the behaviour of reinforced concrete buildings,
and some emphasis on the effects of liquefaction and vertical accelerations. Some types of
damage have not been discussed here: damage due to rock falls, damage to masonry
buildings (both unreinforced and reinforced), pounding of one building against a
neighbouring building or of a bridge deck against its abutment, failure of glass windows and
others. Many of these have been covered in the detailed report produced by (Chouw et al,
2011) and in a special edition bulletin of the NZSEE (NZSEE, 2011).
Any major earthquake event leads to a reassessment of design philosophies, and the
Christchurch earthquake is no exception to that. As has been discussed here, the New Zealand
engineers and authorities are in the process of thoroughly reassessing their design philosophy
and detailing requirements. Australia has a large stock of unreinforced masonry buildings
including many highly regarded heritage structures. This is a type of construction that is
known to be especially vulnerable to earthquakes, and the need to address this has been
covered in (Ingham et al, 2011). There are several other important lessons for Australia from
the Christchurch earthquake:
 The level of seismic hazard used in Australian loading codes to determine the design
level earthquake (the "rare" event) needs to be reassessed.
 The level of seismic hazard associated with a "very rare" event should be defined and
codified.
 The minimum performance objective for a "very rare" event should be stipulated in
the Australian Building Code; the author believes the objective of "prevention of
collapse" under this level of event would be appropriate for most buildings in
Australia, although consideration should be given to achieving even higher levels of
performance as has been advocated by some engineers in New Zealand (Buchanan et
al, 2011).
 Following on from this, some revision of the material design standards may be
required to ensure compliance with the newly defined performance objective for a
"very rare" event, with consideration given to strength hierarchies and capacity design
principles.
Structural engineers in New Zealand are currently trained to incorporate reliable ductility in
buildings so as to enhance the building's ability to withstand a very rare earthquake event
without collapse, and there is no reason why Australian engineers cannot be trained to do the
same where it is necessary. Displacement-based methods to assess building performance
when subjected to a very rare earthquake event should become a routine part of the structural
design. This approach will lead to more resilient types of building construction (with better
details) being favoured by designers, and the adoption of newly developed forms of
construction including "low damage" solutions for some buildings of high importance.
Consideration of these issues will result in a building stock which will be more robust than it
is now when and if a major earthquake does strike one of the Australian capital cities in a
hundred years time or more.
It is anticipated that if engineers were forced to make these considerations, cost increases
would be likely to be marginal. In Sesoc (2011) it is said that the cost of multi-storey office
buildings are marginally more expensive in Auckland than in Wellington or Christchurch, but
are all within 2% over a range of building types. They think that this suggests that regional
material and labour cost factors more than compensate for any cost difference as a
consequence of seismic loading.
11
References
American Society of Civil Engineers (2005) ASCE/SEI Standard 7-05: Minimum Design
Loads for Buildings and other Structures, Reston, Va.
American Society of Civil Engineers (2011) ASCE/SEI Standard 7-10: Minimum Design
Loads for Buildings and other Structures, Second printing incorporating errata identified
through April 6, 2011, Reston, Va.
Australian Building Codes Board (2011) National Construction Code (NCC) 2011: Complete
Series.
Baird, A., Palermo, A. and Pampanin, S. (2012) Façade damage assessment of concrete
buildings in the 2011 Christchurch earthquake, Structural Concrete, Journal of the fip, Vol.
13, Issue 1, pp. 3-13
Beca Report (2011) Investigation into the Collapse of the Pyne Gould Corporation Building
on 22nd February 2011, prepared for Department of Building and Housing (DBH) by Beca
Carter Hollings and Ferner Ltd (Beca), 26th September 2011.
(http://canterbury.royalcommission.govt.nz/Technical-Reports)
Bowman, J.R., G. Gibson, and T. Jones (1990) Aftershocks of the 1988 January 22 Tennant
Creek, Australia intraplate earthquakes: evidence for a complex thrust-fault geometry,
Geophys. J. Int., 100, 87-97.
Bradley, B.A. (2012) Ground Motion and Seismicity Aspects of the 4 September and 22
February 2011 Christchurch Earthquakes, Technical Report Prepared for the Canterbury
Earthquakes Royal Commission, 13 Jan. 2012, Christchurch, New Zealand.
(http://canterbury.royalcommission.govt.nz/Technical-Reports)
Bruneau, M., Clifton, C., MacRae, G., Leon, R., Furnell, B. (2011) Steel Building Damage
from the Christchurch Earthquake of Feb 22, 2011, NZST.
(http://db.nzsee.org.nz:8080/web/chch_2011/structural)
Buchanan, A.H., Bull, D., Dhakal, R.P., MacRae, G., Pampanin, S. (2011), Base-Isolation
and Damage-Resistant Technologies for Improved Seismic Performance of Buildings, Aug.
2011, Royal Commission website.
(http://canterbury.royalcommission.govt.nz/Technical-Reports)
Bull, D. (2005) Earthquake Engineering in Regions of Low-Moderate Seismicity, Proc. of the
2005 Conference of the Australian Earthquake Engineering Society, Albury, New South
Wales, Australia.
Chouw, N., Hao, H. and Goldsworthy, H. (2011) Some Observations of Damage in the 22nd
February Christchurch Earthquake.
(http://www.aees.org.au/News/110222_CHCH/Christchurch_report_May_2011.pdf)
Department of Building and Housing (2012) Structural Performance of Christchurch CBD
Buildings in the 22 February 2011 Aftershock, Expert Panel Report to the New Zealand
Royal Commission of Inquiry into Building Failure Caused by the Canterbury Earthquakes.
(http://canterbury.royalcommission.govt.nz)
Galloway, B.D., Hare, H.J. and Bull, D.K. (2011) Performance of multi-storey reinforced
concrete buildings in the Darfield Earthquake, Proceedings of the Ninth Pacific Conference
in Earthquake Engineering, 14-16 April, 2011, Auckland, New Zealand.
GNS (2011) The Canterbury Earthquake Sequence and Implications for Seismic Design
Levels, GNS Seismic Consulting Report, 14th October 2011, Royal Commission website.
(http://canterbury.royalcommission.govt.nz/Technical-Reports)
Goldsworthy, H. and Hao, H. (2011) AEES Reconnaissance Mission to New Zealand: Part 1
(Preliminary Report).
(http://www.aees.org.au/News/110222_CHCH/Mission-Helen.html)
Henry, R. and Ingham, J. (2011) Behaviour of tilt-up precast concrete buildings during the
2010/2011 Christchurch earthquakes, Structural Concrete, Journal of the fip, Vol. 12, Issue 4,
pp. 234-240
Ingham, J., Moon, L. and Griffith, M.C. (2011) Performance of Masonry Buildings in the
2010/2011 Canterbury Earthquake Swarm and Implications for Australian Cities, Proc. of the
Australian Earthquake Engineering Society Conf., 18-20 Nov, Novotel Barossa Valley
Resort, South Australia.
Ingham, J.M. and Griffith, M.C. (2011) The Performance of Unreinforced Masonry Buildings
in the 2010/2011 Canterbury Earthquake Swarm, Technical Report Prepared for the
Canterbury Earthquakes Royal Commission, August 2011.
(http://canterbury.royalcommission.govt.nz/Technical-Reports)
IPENZ (2011) Fact Sheets
(http://www.aees.org.au/News/110222_CHCH/110311_ChChFactSheets.pdf)
Kalken, E. (preface by Kalken, E. and then papers by many different authors) (2011) Special
issue on the Christchurch earthquake, November/December 2011, Vol. 82, pp. 765-964.
Kam, W.Y. (2011a) Day 02 Field Report from the Christchurch 22 Feb 2011 6.3Mw
Earthquake: Structural Perspectives.
(http://db.nzsee.org.nz:8080/web/chch_2011/structural)
Kam, W.Y. (2011b) Day 03 Field Report from the Christchurch 22 Feb 2011 6.3Mw
Earthquake: Critically Damaged Multi-Storey RC Buildings.
(http://db.nzsee.org.nz:8080/web/chch_2011/structural)
Kam, W.Y., Akzugel, U. and Pampanin, S. (2011a), 4 weeks on: Preliminary Reconnaissance
Report for the Christchurch 22 Feb 2011 6.3Mw Earthquake.
(http://db.nzsee.org.nz:8080/web/chch_2011/structural)
Kam, W.Y. and Pampanin, S. (2011b) The Seismic Performance of RC Buildings in the 22
February 2011 Christchurch Earthquake, Structural Concrete, Journal of the fip, Vol. 12,
Issue 4, pp. 223-233
Michael-Leiba M.O. (1989) Macroseismic effects, locations and magnitudes of some early
Tasmanian earthquakes, BMR Journal, 11, 89-99.
Nordensen, G.J.P. and Bell, G. R. (2000) Seismic Design Requirements for Regions of
Moderate Seismicity, Proceedings of the 12th World Conference in Earthquake Engineering,
Auckland, New Zealand, Paper No. 825.
NZS1170.5: (2004) Structural Design Actions Part 5: Earthquake Actions - New Zealand.
Wellington: Standards Association of New Zealand.
NZS4203: (1976) Code of Practice for General Structural Design and Design Loadings for
Buildings. Wellington: Standards Association of New Zealand.
NZSEE website
(http://db.nzsee.org.nz:8080/web/chch_2011/home)
NZSEE (2011) Special Edition Bulletin on the Observations of the February 2011
Christchurch Earthquake Sequences, Vol. 44, No. 4, December 2011.
Pampanin, S., Kam, W.Y., Tasigedik, A.S., Quintera Gallo, P., Akguzal, U. (2011)
Considerations on the Seismic Performance of pre 1970s RC buildings in the ChCh CBD
during the 4th Sept 2010 Canterbury Earthquake: Was that really a big one?, Proceedings of
the 9th Pacific Conference on Earthquake Engineering, 14-16 April, Auckland, New Zealand.
Priestley, M.J.N., Calvi, M. and Kowalsky, P. (2007) Displacement-Based Seismic Design of
Structures, IUSS Press, Pavia, Italy.
SeSoc (2011), Preliminary Observations from CHCH Earthquakes, Structural Engineering
Society, New Zealand, Royal Commission website.
(http://canterbury.royalcommission.govt.nz/Technical-Reports)
Smyrou, E., Tasiopoulou, P., Bal, I.E., Gazetas, G., Vintzileou, E. (2011) Structural and
Geotechnical Aspects of the Christchurch(2011) and Darfield (2010) Earthquakes in New
Zealand, 7th National Conference on Earthquake Engineering, 30 May-3 June 2011, Istanbul,
Turkey.
Standards Australia (2007), AS1170.4-2007: Structural design actions, Part 4: Earthquake
actions in Australia.
Standards Australia (2009), AS3600-2009: Concrete Structures.
Standards Australia (1998), AS4100-1998: Steel Structures.
Tonkin and Taylor (2011) Darfield Earthquake 4 Sept. 2010 Geotechnical Land Damage
Assessment and Reinstatement Report - Stage 1 Report, Earthquake Commission.
(http://www.tonkin.co.nz/canterbury-land-information/docs/T&T-Stage%201%20Report.pdf)