4 WEEKS ON : PRELIMINARY RECONNAISSANCE REPORT FROM THE CHRISTCHURCH 22 FEB 2011 6.3MW EARTHQUAKE

4 Weeks on : Preliminary Reconnaissance REPORT FROM THE CHRISTCHURCH 22 Feb 2011 6.3Mw Earthquake

Weng Y Kam[1] , Umut Akguzel[2] , Stefano Pampanin[3]


Download the full report (with figures) here.
**re-posted from the NZSEE Clearinghouse website.

1         Introduction

1.1      Note

This informal report provides an updated view of the various aspects in relation to the 22nd Feb 2011 6.3 Mw Christchurch earthquake event, 4 weeks after (written on the 25th March 2011). This is a preliminary report subjected to changes/amendments as new information is gathered.

1.2      General and services recovery (as of 15th Mar 2011)

The state of national emergency has been extended for the 5th time on the 31th Mar 2011, covering the territory of Christchurch City Council. Further information of the New Zealand’s National Civil Defence Emergency Management Plan [1, 2]is available at: http://www.civildefence.govt.nz/

110 of the 163 elementary and secondary (high) schools have reopened by the 14th of March 2011, with some schools reopened since the start of Week 2 (7th March).  

Water has been restored to 94 percent of households. 99.7 per cent of the city overall had power, with only 550 properties outside of the CBD still without power. Electricity was still off to around 50 percent of the CBD. Sewerage services are still compromised in many Eastern suburbs, with portaloos and chemical toilets being distributed as alternatives.

The bus services have been partially restored with two temporary bus terminus set up at the Hospital (Parkside) and the Bealey Avenue. 21 non-operational and 35 operational routes.

2    CBD Earthquake Records Acceleration Response Spectra

The elastic response spectra (5%-damped) from four recorded ground motions (of both the principal and secondary horizontal motions) from the Christchurch CBD are compared with the site seismic design coefficient in Figure 1. The NZS1170:5 [3]500-years and 2500-years design spectra for Christchurch site (Z/PGA=0.22g), distance R = 10km and soil class D (consistent with the four recording sites) is also plotted in Figure 1.

[IMAGE LINK] Figure 1: 5%-damped elastic acceleration response spectra in the Christchurch CBD and the NZS1170:5 design spectra (red solid) for Christchurch (soil class D, R=10km): a) Principal horizontal direction; b) Weaker secondary horizontal direction.

Some preliminary observations on the horizontal acceleration response spectra:

  1. In general, the shaking in the CBD was exceeding the 500-year motion design spectra, which is the typical ULS design motion in New Zealand.
  2. The principal direction of shaking is of the predominantly East-West component. The East-West components are approximately 15-30% higher in the periods ranging from 0-2.4s, except for the period range of 0.35s-0.6s in which the North-South components were stronger.
  3. The East-West components matched or exceeded the New Zealand loading standard NZS1170:5[3]2500-year motion 0.5s-1.75s (approximately 5-20 storey RC buildings).
  4. In the longer periods, the shaking demands have also exceeded the 500- year motion design spectra.

The equivalent vertical spectra from the NZS1170:5 [3]is plotted in Figure 2with the vertical response spectra from the four CBD recording stations. NZS1170:5 [3]used a multiple factor of 0.7 to determine the vertical spectra from the horizontal design spectra. At the short periods (T<0.3s) however, the vertical spectrum is assumed to be the same as the horizontal spectrum, to account for high-frequency content of vertical motions.

As it is difficult to determine the vertical stiffness of structures, it is hard to correlate the vertical acceleration demand to structural vertical response spectral demand. At very short period range (0.05s < T < 0.25s), the vertical response spectra greatly exceeded the expected 2500-year motion vertical spectra (according to NZS1170:5).

[IMAGE LINK]Figure 2: 5%-damped elastic vertical acceleration response spectra in the Christchurch CBD and the NZS1170:5 design spectra (red solid) for Christchurch (soil class D, R=10km).

3    CBD Earthquake Records DISPLACEMENT Response Spectra

The displacement response spectra give a better representation on the seismic displacement demand on the buildings [4]. The 5%-damped elastic displacement response spectra for the four CBD recording stations are plotted in Figure 3.

Assuming some levels of ductility and post-yield stiffness, effective period, Teff of a structure can be approximated from the initial period, Ti [4]. For instance, a ductile RC frame building (µ=4, r=0.02), effective period is approximately twice the elastic period.

Some preliminary observations:

  1. In the principal direction, there are significant displacement demands between 1.0s to 1.8s and 2.9s to 3.8s. This suggests, in-elastically responding RC buildings between 5 to 10 storey and 15 to 20 storey would have significant displacement demands and by extension possibly significant damage. The principal direction motion is exceeded the NZS1170:5[3]2500-year motion design spectra at these two period ranges.
  2. At all period ranges, the principal direction horizontal shaking exerted higher than the 500-year design displacement spectra.
  3. The two ‘amplification lumps’ observed in the principal direction motion are not apparent in the secondary horizontal direction motions. In addition, the spectral displacement demands in the secondary direction are significant lower when compared to the principal direction.

[IMAGE LINK] Figure 3: 5%-damped elastic displacement response spectra of four Christchurch CBD records and the NZS1170:5 design spectra (red solid) for Christchurch (soil class D, R=10km): a) Principal direction; and b) Secondary horizontal direction.

4     Civil Defence Emergency Response

4.1    Civil Defence / City Council Building Safety Evaluation Operations

Structural engineers are predominantly engaged with the Civil Defence / City Council Building Safety Evaluation (BSE) operations. Following are some of the ongoing operations that the authors are involved in or aware of:

  1. Critical Building Project– This project deals with the assessment and emergency stabilisation, shoring and deconstruction of buildings that are critical to the operation/re-opening of the city center – either critical to the major traffic way or critical in terms of their impacts to the surrounding buildings. At the time of writing, 30 buildings above 6-storeys of various construction age and lateral resisting systems are monitored under this project.

[IMAGE LINK] Figure 4: a) Critical building project team; b) Building evaluation and structural engineers briefing; c) Police vehicle recovery operation within the CBD; and d) 18th March 2011 Memorial Day.

  1. Operation CBD– The Operation CBD involves Level 1 and Level 2 building assessment according to the BSE procedure. The objective is to assess the building soundness for re-occupation or temporary re-entry. The level 1 tri-colour placard tagging was completed for the CBD area apart from three no-go areas around the Grand Chancellor, PGC and CTV buildings on the 5th Day (27th Feb 2011. At the time of writing, the Level 2 assessment was approximately 65% completed (verbal information). Figure 5summarises the CBD building tagging statistics as per 27th Feb, 8th Mar and 18th Mar 2011.

[IMAGE LINK] Figure 5: CBD buildings tagging statistics. Note: There are some duplicity of data points due to possible multiple assessments of a properties (with multiple addresses e.g. Apartments). Level 1 and 2 assessments are not discretised.

  1. Operation Suburbfocussed on: i) To ensure people are safe and well; and ii) To provide initial assessment of residential homes. Each team includes a building official, an EQC assessor and a welfare representative from either the Red Cross or the Salvation Army. If necessary, structural engineer will be called in to assess the residential buildings.
  2. Operation Shopsfocussed on essential commercial (malls, shopping centres, large hardware stores) and medical (clinics and pharmacies) buildings in the city-wide area, was completed on the 25th March 2011. 10-14 buildings were red-stickered.
  3. Operation USAR ‘deconstruction’– Deconstruction operation involves the partial or total deconstruction of the building for rescue or recovery purposes or because it presents an unacceptable safety risk from aftershocks. As per 18th March 2011, there were 114 buildings within the cordon which required deconstruction. The progress to date is as follow: a) 47 have been completed, b) 24 are in progress; and c) 43 are to be deconstructed.
  4. Project demolition– Under the national emergency legislation and the Canterbury Earthquake Emergency Act (2010), buildings that are hazards to public safety and city recovery can be demolished under an accelerated consent protocol.

For non-heritage-listed buildings with six storeys and less, standalone (not connected to adjacent buildings) and built after 1945, may be demolished without a demolition building consent.

For other buildings (six storeys or more, joined to other buildings or built before 1945), a process is in place which would includes structural engineer’s assessment report and demolition plan, peer-review by another independent structural engineer, Christchurch City Council’s review and recommendation, and the final decision by the Civil Defence National Controller on whether or not the building can be demolished. This process can override existing heritage law.

  1. Project CSR (Customer Service Request)– to inspects suburban residential buildings which are considered a serious threat to people or property, as per reported by neighbouring residents.

4.2    Welfare centres and Recovery Assistance Centres

On the first night (morning of 23rd Feb), up to 950 people were housed at the Hagley Park and Burnside High emergency welfare centres. Temporary shelters, blankets, food, sanitation are supplied at the centres. By the 4th day (26th Feb 2011), three welfare centres housed 329 people. Welfare centers are now all closed by the 2nd week (11th March 2011), with the last welfare centre (Pioneer Stadium) being converted into a Recovery Assistance Centre.

Recovery Assistance Centres are one-stop-shops where people can access welfare information and services. They bring together central, local government and non government organisations. In addition, Recovery Information Kiosks have been opened throughout Christchurch. The kiosks are places within communities where people can get information on the help available following the Christchurch Earthquake.

4.3   CBD and business recovery and continuity

Business recovery and continuity is a major issue.

Map of recovery centers, info centers, emergency management centers.  

[IMAGE LINK] Figure 6: a) Cordon reduction zone as per 14th March 2011; b) Removal of essential business items from damaged buildings within the cordoned Red Zone.

5    CBD Distribution of Damage and State of Recovery

5.1   CBD building damage (BSE tagging) distribution

Table 1and Figure 8presents the statistics of the Building Safety Evaluation of the CBD buildings, as per 18th March 2011, divided into different construction types. While the building tagging status is not a direct representative of damage, it is the best-available indicator index

[TABLE LINK] Table 1: Building safety evaluation tagging status for CBD buildings as per 18th March 2011 (Data courtesy of the Civil Defence).

The predominant construction types within the CBD are timber and reinforced concrete constructions. Timber construction is typically used for residential buildings at the fringe of the CBD area. In general, these low-rise timber structures performed very well.

Unreinforced masonry generally performed poorly, with significant number of buildings red-tagged. However, only 289 addresses were assessed to be unreinforced masonry, which is somewhat lower than expected. From field observation, it was understood that many unreinforced masonry buildings did not require a BSE tagging due to the considerable damage and collapse.

The distribution of damage and the high number of ‘red – unsafe’ tagged buildings confirmed the expected high number of damaged structures given the level of shaking intensity.

5.2State of recovery

A major effort to deconstruct dangerous and unsafe structures, as well as to clean up the CBD streets has been on-going. Following is a short pictorial comparison of various heavily damaged section/streets of the CBD – taken at the 2nd day (23rd Feb) and three weeks after (14th Mar).

[IMAGE LINK] Figure 7: Before and after photos of the heavily damaged Manchester-High-Lichfield Streets corner.

[IMAGE LINK] Figure 8: Building safety evaluation tagging status for CBD buildings as per 18th March 2011.

[IMAGE LINK] Figure 9: Before and after photos of the heavily damaged Manchester-Gloucester streets corner.

[IMAGE LINK] Figure 10: Before and after photos of the heavily damaged Colombo-Kilmore Street corner.

5.3     Urgent stabilisation / deconstruction work

Urgent stabilisation work was carried out either by the USAR operation or by contractors, acting either on behalf of the Civil Defence’s Critical Building Team or the building owners. Following are some notable examples.

[IMAGE LINK] Figure 11: Temporary stabilisation work to restore the axial and shear integrity of heavily damaged basement columns at a multi-storey RC building. The steel encased reinforced concrete improvements confinement and shear capacities of the damaged columns.

 [IMAGE LINK] Figure 12: Concrete encasement for temporary stabilisation of a shear-damaged RC wall of a multi-storey. The photo on the left shows the first phase of the stabilisation. In the 2nd phase, the RC wall will be completely jacketed to full-height, with sufficient anchorage to the 2nd floor slab. The left-side photo is courtesy of Ruaumoko Solutions. 

[IMG LINK] Figure 13: A post-tensioned RC foundation slab was erected to provide foundation support for a 2nd phase propping of the historical Victoria Clock Tower.

[IMAGE LINK] Figure 14: Urgent controlled demolition of the heavily damaged RC infill frames St Elmo Courts, as it was uneconomical and unviable to save the building.

[IMAGE LINK] Figure 15: The historical Old Trust and Loan building and Olympia building was demolished recently in order to clear the collapse hazard along the Hereford Street. The facade walls of both buildings were badly damaged.

6      Reinforced concrete Car park BUILDINGS and vehicle recovery

Since 15th of March, the police with the Civil Defence USAR operation have been organising a phased vehicle recovery process from car parks within the CBD. Nevertheless, three car park buildings were assessed to be unsafe for the police vehicles recovery process, including the collapsed Smiths City car park, and the damaged 33 Lichfield Street car park and Farmers Oxford Terrace car park. Further information is available here: http://www.police.govt.nz/christchurch-vehicle-recovery.

[IMAGE LINK] Figure 16: Localised collapse of 1st floor at the Bedford-Lichfield St car park (with many of the vehicles recovered on the 20th March 2011).

The Smiths City car park had pan-cake collapse at several levels. Punching shear failure of the 220mm thick flat-slab on wide columns (~1200x450mm) can be observed at the South section of the collapsed building. A section of the building over the Dundas Street, consisted of in-situ prestressed RC beams had also pan-cake collapsed. 

[IMAGE LINK] Figure 17: The pancake collapse of the Smiths City Dundas St car park due to punching shear failure of the columns.

7     CONCLUSION

It has been a month since the end of the search and rescue operation (3th March 2011) and nearly three weeks since the end of the USAR recovery operation (9th March 2011). The Civil Defence and the Christchurch City Council authorities are focussing the resources towards the recovery of essential services, support of local business (by reopening CBD zones and facilitating entry to condoned area), social and essential welfare support and the transition into the recovery phase (end of the national emergency period).

This very preliminary field report is prepared to provide immediate information for interested parties. However, this is a draft document and will be subjected to future amendments/corrections. Please kindly acknowledged for the use of any photographs. The report above is not peer-reviewed. Please kindly forward your thoughts/comments to me at weng.kam@canterbury.ac.nz.

Authors would like to acknowledge the excellent support provided by the Civil Defence and the Christchurch City Council on the field reconnaissance. In addition, the Civil Defence vision to collate and share of the information and lessons from this nationally and internationally significant earthquake in order to prevent and mitigate future hazards is commendable.

REFERENCES

[1].MCDEM. National Civil Defence Emergency Management Plan. Wellington, NZ: Ministry of Civil Defence & Emergency Management (MCDEM); 2005.

[2].MCDEM. The Guide to the National Civil Defence Emergency Management Plan 2006. 1.2 ed. Wellington, New Zealand: Ministry of Civil Defence & Emergency Management (MCDEM); 2006.

[3].NZS1170. NZS 1170.5:2004 Structural design actions. Wellington, NZ: Standards New Zealand; 2004.

[4] Priestley MJN, Calvi, GM, Kowalsky, MJ. Displacement-based Seismic Design of Structures IUSS Press, Pavia, 2007.



[1]
Research Associate (of Assoc. Prof. Stefano Pampanin), Dept of Civil and Natural Resources Eng, Uni.of Canterbury, Christchurch, New Zealand. weng.kam@canterbury.ac.nz

[2]Research Associate (of Assoc. Prof. Stefano Pampanin), Dept of Civil and Natural Resources Eng, Uni.of Canterbury, Christchurch, New Zealand. umut.akguzel@canterbury.ac.nz

[3]Assoc. Prof. Stefano Pampanin, Dept of Civil and Natural Resources Eng, Uni.of Canterbury, Christchurch, New Zealand. stefano.pampanin@canterbury.ac.nz  

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