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223© 2011 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin · Structural Concrete 12 (2011), No. 4

The 22 February 2011 Mw 6.2 Christchurch (Lyttelton) earthquakewas a particularly severe test for both modern seismically de-signed and existing non-ductile reinforced concrete (RC) build-ings. Some 16.2 % of 833 buildings with RC systems within theChristchurch central business district (CBD) were severely dam-aged. There were 182 fatalities, 135 of which were the unfortu-nate consequences of the complete collapse of two medium-riseRC buildings. As with the post-Northridge 1994 earthquake, thedesign performance of ”modern” structures is being scrutinized –with the inevitable question: is ”life safety” but irreparable dam-age still a valid performance target? This brief paper presents asummary of RC building damage from a broad performance-based earthquake engineering perspective. Several preliminarylessons, not all of them surprising, and the issues that havearisen will be discussed using case study buildings, with sugges-tions for urgently needed research areas.

Keywords: Christchurch 2011 earthquake, reinforced concrete, seismicperformance, earthquake reconnaissance, performance-based earthquakeengineering, field damage, New Zealand

1 Introduction

The 22 February 2011 Mw 6.2 Christchurch (Lyttelton)earthquake represents one of the most severe tests of“modern” and older non-ductile RC structures in a devel-oped nation with a strong seismic engineering back-ground. Some 16.2 % of 833 buildings with reinforcedconcrete (RC) systems within the Christchurch centralbusiness district (CBD) were severely damaged. Therewere 182 fatalities, 135 of these due to the complete col-lapse of two medium-rise RC buildings.

This brief overview paper discusses some preliminarylessons learnt from the observed seismic performance ofRC buildings in the 22 February 2011 earthquake. Thebackground to New Zealand RC seismic design and theseismic shaking of the Christchurch 2010–11 earthquakesare also briefly discussed. A full damage report on the seis-mic performance of RC buildings in the CBD will be avail-able in the near future in [1].

RC construction and design practice in New Zealandis similar to that in many other parts of the world. There-fore, some of the lessons learnt are directly relevant to theinternational community for further research, improve-ments to current seismic code design and performance cri-teria, and beg the implementation of most recent and mostadvanced technology.

2 Canterbury and Christchurch Earthquakes, 2010–112.1 The 4 September 2010 Mw 7.1 Darfield (Canterbury)

earthquake

The 22 February 2011 earthquake was an “aftershock” ofthe sequence of earthquakes triggered by the Mw 7.1Darfield (Canterbury) earthquake of 4 September 2010.The 4 September 2010 Mw 7.1 Darfield earthquake, withan epicentre approx. 35 km from the Christchurch CBD,generated peak ground accelerations (PGA) of up to0.2–0.3 g in the CBD area. This main shock resulted inwidespread liquefaction and damage to land, residentialbuildings and infrastructure in the Canterbury region [2].

Preliminary research has indicated that the associat-ed seismic demand was deemed to correspond to the ap-proximate 400–500-year return period motions of the NewZealand loading standards, NZS1170:5 [3] for structureswith periods of 0.3–1.0 s [4]. However, a reconnaissancereport on the seismic performance of the RC buildings inthe 4 September event [5] has indicated low to moderatelevels of damage for pre-1970s “brittle” RC buildings,somehow contradicting the expected level of damagegiven the magnitude of the earthquake and the recordedlevel of PGA.

Prior to the 22 February 2011 earthquake, it hadbeen demonstrated via a series of numerical non-lineartime history analyses and experimental shaking table teststhat the characteristics of the ground motions (recordedsignal itself) in the CBD after the 4 September event mighthave not be as demanding as compatibility with the designspectra (itself an approximate approach) would suggest[6].

2.2 Seismicity of the 22 February 2011 earthquake

The epicentre of the 22 February 2011 Mw 6.2 Christ-church earthquake was approx. 10 km south-east of theChristchurch CBD at a shallow depth of 5 km. The maxi-

Articles

The seismic performance of RC buildings in the 22 February 2011 Christchurch earthquake

Weng Yuen Kam*Stefano Pampanin

DOI: 10.1002/suco.201100044

* Corresponding author: [email protected],[email protected]

Submitted for review: 18 September 2011Revised: 18 September 2011Accepted for publication: 21 September 2011

mum felt intensity was MMI IX and the maximum PGArecorded was 0.4–0.7g in the Christchurch CBD. Fig. 1shows the fault rupture associated with the 4 Septemberand 22 February events and associated aftershocks asrecorded until 29 August 2011.

A rich set of strong ground motion records were cap-tured in these earthquakes by the EQC-GNS GeoNet seis-mic hazard monitoring network, which has more than 50seismic instrumentation stations within 100 km ofChristchurch CBD and four permanent recording stations

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W. Y. Kam/St. Pampanin · The seismic performance of RC buildings in the 22 February 2011 Christchurch earthquake

Structural Concrete 12 (2011), No. 4

in the CBD. Fig. 2 shows a comparison of PGAs (horizon-tal and vertical) recorded by the GeoNet seismic monitor-ing network in the Christchurch CBD. A wide range of(medium to very high) horizontal accelerations wasrecorded, with peaks exceeding 1.6g at Heathcote Valleyand between 0.4 and 0.7 g at the CBD recording stations.A significant peculiarity of the 22 February earthquakewas the very high vertical acceleration, in the range of1.8–2.2 g on the hills, which were among the highest everrecorded worldwide [7]. In the CBD, the highest value of

Fig. 1. Fault rupture length and aftershock sequence for the 4 September 2010, 22 February 2011 and 13 June 2011 events (source: GNS Science)

Fig. 2. Peak ground accelerations recorded during the 22 February 2011 Christchurch aftershock (source: EQC-GNS Geonet)

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peak ground vertical acceleration recorded was between0.5 and 0.8 g.

Comparison of the recorded ground motions withthe probabilistic hazard model and various ground mo-tion attenuation models has highlighted the peculiarity ofthe extremely high shaking of the 22 February event [8].Preliminary seismological investigation indicates the com-plex seismic wave interaction in the deep alluvial soils un-derlying Christchurch (“basin effect”), the shallowness ofthe rupture and the directivity effects from the oblique-re-verse fault rupture mechanism resulting in severe groundshaking within the Christchurch CBD [9].

2.3 Response spectra of 22 February 2011 Mw 6.2Christchurch earthquake

The elastic acceleration response spectra (5 % damped) ofthe 22 February earthquake, from the four recordedground motions from the Christchurch CBD, are com-pared with the site seismic design coefficient in Fig. 3. TheNZS1170:5 [3] 500-year and 2500-year design spectra forthe Christchurch site (Z/PGA = 0.22g), distance R =20 km and soil class D (consistent with the four recordingsites) are also plotted in Fig. 3.

In general, it can be noted that the seismic shaking inthe Christchurch CBD significantly exceeded the 500-yearmotion design spectra, which is the design level in NewZealand for new buildings. The principal direction ofshaking was of the predominantly east-west component.The east-west components were approx. 15–30 % higher inthe periods ranging from 0 to 2.4 s, except for the periodrange 0.35–0.6 s, where the north-south components werestronger.

The east-west components matched or exceeded theNew Zealand loading standard NZS1170:2004 [3] 2500-year motion in the period range 0.5–1.75 s (approx. 5- to20-storey RC buildings).

It should be noted that the existing NZS1170.5:2004design spectra and the underlying probabilistic seismic

hazard model do not (yet) consider any near-fault effectsfor Christchurch CBD as there was no known active faultwithin 20 km of Christchurch CBD. The amplification ofspectra acceleration in the 0.5–1.5 s period range and theshift of the peak spectra acceleration “plateau” is typicallyobserved in ground motion records with forward directiv-ity effects [10], [11]. In addition, the soft-soil site amplifica-tion observed in the response spectra is also significantlyhigher than typical measurements for similar geologicalsites, which indicates that further urgent research is re-quired to quantify such site effects.

The 5 %-damped elastic displacement response spec-tra for the four CBD recording stations are plotted inFig. 4. In all period ranges, both the principal and sec-ondary directions of horizontal shakings were higher thanthe 500-year design displacement spectra [3].

In the principal direction, there are significant dis-placement demands at 1.0–1.8 s and 2.9–3.8 s. This sug-gests RC buildings of 5–10 storeys and 15–20 storeys re-spectively would experience significant displacementdemands and thus possibly significant damage. The prin-cipal direction motion exceeded the NZS1170:2004 spec-tra corresponding to a 2500-year return period event (con-sidered the Maximum Credible Event, MCE) at these two“amplification lumps” period ranges.

A more thorough discussion of the response spectraof the 22 February 2011 earthquake in comparison withthe 4 September 2010 and 26 December 2010 earthquakesis given in [12].

3 RC buildings in Christchurch3.1 Development of New Zealand seismic design

of RC buildings

The first national building by-law, the 1935 Model Build-ing By-Law NZS95:1935 [13], was published by the NewZealand Standards (NZS) in the wake of the NapierHawke’s Bay 1931 Mw 7.9 earthquake, which resulted in256 fatalities. Unreinforced masonry buildings were dis-

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Fig. 3. 22 February 2011 Mw 6.2 earthquake: elastic acceleration response spectra (5 % damped) in the Christchurch CBD after the 22 February event andthe NZS1170:2004 design spectra (solid red line) for Christchurch (soil class D, R = 20 km): a) principal horizontal direction (generally east-west component),b) secondary horizontal direction (generally north-south component) [12]

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couraged and masonry walls were required to be tied tothe floor diaphragms.

The 1955 revision of the NZS Standard Model Build-ing By-Law (NZS95:1955) introduced an inverted triangu-lar distribution of horizontal load with seismic coefficientsof 0.12 g and 0 at the top and bottom of the structure re-spectively. Explicit definitions of deformed and plainround bars were given, but limited bond advantage of de-formed reinforcement was accounted for. The provisionfor shear resistance of concrete elements was tightenedand the requirement of 135° anchorage for links was in-cluded.

The NZS1900:1964 code [14], [15] was a significantevolution from its predecessors. Three seismic zones withthe maximum seismic coefficient ranging from 8 % (zoneC) to 12 % (zone A) were introduced to achieve a betterrepresentation of the regional seismicity of New Zealand.The magnitude of seismic force was formed as a functionof a building’s natural period. The concept of structuralductility was introduced with the stated assumption of5–10 % of damping for structural ductility of 4 for RCstructures. However, no provision for ductile RC detailingor modern capacity design considerations was included.

In the late 1960s, following the 1966 SEAOC recom-mendations [16] and the 1971ACI-318 code [17], practisingengineers in New Zealand began to develop “ductile” rein-forced concrete design and detailing. J. P. Hollings intro-duced a step-by-step design procedure to achieve beam-hinging inelastic mechanism in RC frames under seismicloading, which pre-dated the concept of capacity design,in 1969 [18]. The 1970 Ministry of Work’s Code of Prac-tice for Design of Public Buildings [19], [20] adopted manyductile detailing recommendations, including a recom-mendation for beam-column joint reinforcement.

The provisional NZS3101 code, published in 1972[21], also adopted many parts of the 1971 ACI-318 code[17] and some of the recommendations were from thedraft version of ref [22]. In their seminal publication [22],

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Park and Paulay outlined many concepts of modern seis-mic RC design and detailing, including a rigorous designprocedure for RC frames using the capacity design philos-ophy and quantification of the ductility capacity of RCbeam, column, wall and joint elements.

The introduction of the NZS4203:1976 loading stan-dards [23] represented a quantum change in the seismicload requirements. NZS4203:1976 quantified the soil am-plification factors, with higher seismic coefficients speci-fied for softer soils. The ultimate strength design approachwas codified as the preferred design method. Concurrent-ly, the 1982 edition of the NZS3101 concrete design stan-dard [24] was developed and written. NZS3101:1982 in-cluded now well-accepted concepts of ductile RC detailingand capacity design philosophy.

NZS3101was reviewed and updated in 1995 [25] and2006 [26] to reflect the further knowledge gained fromresearch and the lessons learnt from the Loma Prieta,Northridge and Kobe earthquakes. NZS3101:1995 in-creased the requirements for minimum seating lengths forprecast floor elements and ties between precast walls andfloors. The column transverse reinforcement requirementwas increased, whereas the shear reinforcement requiredin the joint cores of ductile frames was lowered inNZS3101:1995.

The achievable material strain on assumed ductiledetailing was quantified in the 2006 revision of theNZS3101. The overstrength from slab and diaphragm con-tributions to the beam flexural capacity was refined. Spe-cific clauses relating to beam elongation and precast floordetailing were also incorporated in NZS3101:2006, includ-ing disallowing the use of cold-worked brittle wire mesh asdiaphragm ties into seismic elements [27]. The use of joint-ed ductile connections (post-tensioned/rocking dissipa-tive) systems has been introduced (NZS3101:2006 Appen-dix B) along with the possibility (for these specialstructures) of designing according to a displacement-basedapproach.

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Fig. 4. 22 February 2011 Mw 6.2 earthquake: 5 %-damped elastic displacement response spectra of four Christchurch CBD records and the NZS1170:2004design spectra (solid red line) for Christchurch (soil class D, R = 20 km): a) principal horizontal direction (generally east-west component), b) secondary hor-izontal direction (generally north-south component) [12]

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3.2 Damage statistics for RC buildings in Christchurch

Christchurch CBD consists of predominantly commercialand light industrial buildings (58 %) but also a significantnumber of residential buildings (42 %). The majority (ap-prox. 81 %) of the buildings are of one and two storeys.There are 127 buildings with at least six storeys, with thetallest building being 22 storeys (86 m). Based on the12 June 2011 CCC Building Safety Evaluation (BSE) statis-tics, there are at least 3000 buildings within theChristchurch CBD. Fig. 5 presents the summary of build-ing damage and building type/age statistics for RC build-ings within the Christchurch CBD.

Pre-1970s RC buildings generally performed poorly,with about 60 % of these buildings being given yellow orred placards to indicate restricted or unsafe buildings. Aspart of this class of buildings, those relying upon RC wallsas the primary lateral-resisting system generally showedsome higher level of structural redundancy such that se-vere damage (e.g. section 4.2) did not lead to collapse ofthe buildings. Pre-1970s buildings with a significant RCframes gravity and lateral system are observed to be moresusceptible to damage leading to structural instability (andlikely collapse).

A construction boom in the 1980s led to a large num-ber of medium- to high-rise RC buildings in ChristchurchCBD, with precast concrete ductile perimeter frame sys-tems being widely used. Fig. 6 illustrates some of the no-table RC medium- to high-rise structures in ChristchurchCBD. Some of these high-rise buildings were previously re-ported to be damaged during the 4 September 2010Darfield earthquake [5], [29].

After the 22 February 2011 earthquake, a large num-ber of these 1980s RC buildings performed as expected ofthem in a severe earthquake, with formation of plastichinges in the beams (section 4.1). However, unexpected is-sues such as brittle failure of cold-worked mesh requiredfor diaphragm action (section 4.5), excessive residual dis-

placements, beam elongation effects (section 4.5) and col-lapse of precast staircases (section 4.6) resulted in exces-sively expensive repairs and consequently the need for thedemolition of a number of these buildings.

Whereas for the more recent RC buildings (builtsince 1990, particularly RC-wall and high-rise buildings,some unexpected performance such as significant lique-faction-induced differential settlement (section 4.7) andpoor performance of “ductile” shear walls (section 4.3) wasobserved.

4 Preliminary lessons and damage observed

Some of the unexpected performance and preliminarylessons from the observed damage to RC buildings in thissevere earthquake event are discussed below. Many com-plex issues and surprising (and unsurprising) lessons werelearned and these issues will be briefly highlighted for fur-ther research.

Detailed discussion of the observed damage patternand damage statistics for RC buildings during the 22 Feb-ruary Christchurch earthquake is presented elsewhere[1], [28], [30]. Several preliminary observations and likelychanges to design practice have also been summarized by the New Zealand Structural Engineering Society [31].

4.1 Capacity design and ductile-response RC buildings

Many RC buildings were moderately and severely dam-aged in the 22 February 2011 earthquake. A large majorityof the RC buildings, particularly those with capacity-de-sign consideration, performed as expected of them in a se-vere earthquake, with formation of plastic hinges in thebeams, coupling beams and at bases of walls and columns(see Fig. 7a and 7b). Nevertheless, in many cases, thebuildings are considered too expensive to be repaired. Inaddition, there are also no New Zealand guidelines for the

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Fig. 5. RC (wall and frame systems) building stock “damage tagging” data and distribution in terms of construction year in Christchurch CBD based on 12June 2011 CCC Building Safety Evaluation (BSE) statistics

assessment and estimation of the residual ductility of plas-tic hinges. This makes it difficult to design the repairs aswell as estimate the building capacity and safety under se-vere aftershocks.

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In recent years, the emerging damage-resisting solu-tions for precast concrete structures, typically referred toas PRESSS technology, have been successfully introduced,further refined, developed, codified [26], [32] and imple-

Fig. 6. Notable medium- and high-rise buildings in Christchurch CBD in 1978 and 1990 [28] (photo sketch courtesy of CCC)

Fig. 7. Ductile-response RC systems: a) coupling beams of 1960s coupled-walls building; b) beam plastic hinges on 1980s perimeter RC frames building;c) ductile behaviour of 1980s RC walls building and d–f) “low-damage” PRESSS post-tensioned frames building built in 2010

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mented in practice in New Zealand. This approach relieson jointed ductile connections as an alternative to themore traditional monolithic emulative of cast-in-place so-lutions [33].

New Zealand South Island’s first PRESSS building,completed in 2010, is a three/four storey (plus basementparking) concrete building with post-tensioned coupledwalls in one direction and post-tensioned frames in theother. The hospital building sustained the very severe(and beyond design level) sequence of earthquake from 4September, through 22 February and 13 June, with no evi-dent structural damage [34]. Given the suddenly appreci-ated importance of damage-control design and also the ac-tually proven cost-efficiency of such systems, it does notcome as a surprise that such technology in precast con-crete is likely to be one of the milestones in the post-earth-quake reconstruction of the city.

4.2 Pre-1970s buildings brittle response

For the older RC frame buildings, the observed perfor-mance was generally poor and the observed inelastic dam-age pattern was typically characterized by brittle mecha-

nisms such as beam lap-splice/shear, column shear, beam-column joint shear and/or wall shear failures (e.g. Fig. 8).Unfortunately, pre-1970s “non-ductile” RC buildings canrespond with a binary “on – safe, off – collapse” mecha-nism, as observed in several partially or totally collapsedbuildings in Christchurch, e.g. the Pyne Gould Corp.building (Fig. 9). For pre-1970s buildings with significantredundancy and regular plan form and height, generallysatisfactory behaviour was observed. The detailed analysisof brittle and inelastic-response RC buildings remains achallenging task, pre- and post-earthquake.

4.3 Ductile shear walls detailing and design

Prior to the NZS3101:1982 standard, walls were not de-tailed for ductility, thus resulting in inadequate horizontaland vertical reinforcement, particularly at critical regionsin the walls. In particular, the older walls generally haveno reinforcement to prevent brittle confinement or buck-ling failure. Fig. 8c and 8d illustrate the typical wall dam-age observed for pre-1970s designs.

Perhaps due to the apparent increase in sophistica-tion in design and structural analysis in recent years, a

Fig. 8. Pre-1970s RC buildings brittle failure mechanism: a) near collapse of a frame/wall building with beam-column joint failure, b) short column shear andbeam-column shear damage, c) lightly reinforced 1950 RC wall compression failure, d) shear/axial failure of composite RC wall/steel columns system

a) b) c) d)

Fig. 9. The collapsed Pyne Gould Corp. 1950s core wall/frames building

large percentage of the recently constructed RC walls con-sists of thin slender walls with a minimum level of rein-forcement and a higher level of axial load ratio. RC wallsdesigned for limited or nominal ductility in accordancewith NZS3101:2006 [26], with limited boundary zone con-finement reinforcement, e.g. Fig.10b, have been observedto fail in brittle shear-compression or premature reinforc-ing tensile/compressive fracture, leading to irreparablebuildings.

The high number of severely damaged reasonablynew RC wall buildings (see Fig. 10a to 10c) has indicatedthat the current design for slender RC walls with inade-quate confinement steel at the non-boundary zone (possi-bly in the core region, too), and with inter-panel groutedlap-splice, is inadequate. In addition, many reinforced con-crete walls suffered premature compression failure (e.g.Fig. 7c and Fig. 10c), particularly for L-, T- and V-shapedwalls. The effects of the high vertical acceleration of the

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22 February 2011 earthquake could have also amplifiedthe compression force demand on RC walls with alreadynon-negligible axial load. The lack of a distributed crack-ing pattern in the plastic hinge zone of the RC walls is alsoan unexpected observation that requires further research.

4.4 Plan and vertical irregularity

In general, buildings with significant plan and/or verticalirregularity were found to perform very poorly. For exam-ple, RC walls that discontinued above the basement levelwere observed to induce severe damage in the transferslabs and basement columns and walls. Plan irregularityas a consequence of inelastic behaviour of perimeter later-al-resisting systems (walls or frames) leading to inelastictorsion amplification was another problem commonly ob-served. Further discussion of the observed damage due toplan and vertical irregularity is presented in [1], [30].

Fig. 10. Post-1970s RC structural wall buildings collapse/damage patterns: a) nine-storey residential apartment block built in 2000–10 with severe shear fail-ure of the long slender wall (photo courtesy of Ken Elwood), b) 14-storey residential apartment block built in 2007 with buckling of the boundary reinforce-ment, c) seven-storey 1980s office block with significant compression failure of V-shaped RC shear wall

a) b) c)

Fig. 11. Extensive damage to floor diaphragm and loss of floor support due to the beam elongation effects of concrete frame inelastic response

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4.5 Displacement incompatibility, brittle diaphragmreinforcement and beam elongation

Displacement incompatibility of lateral load-resisting sys-tems and the “gravity” elements such as floors and transferbeams have been recognized as a critical structural weak-ness in recent research [27]. In particular, the adverseelongation effect of expected ductile plastic behaviour oflateral systems (e.g. RC frames) on the structural integrityof the diaphragm of the precast flooring elements is welldocumented [35], [36].

Fig. 11 illustrates an extreme example in which ex-tensive floor diaphragm damage and loss of precast floor-ing unit support occurred due to the beam elongationeffect. The use of cold-formed brittle wire mesh for thediaphragm shear transfer has resulted in uncertainty overthe remaining structural life of the diaphragm.

Whereas Fig. 11 represents an extreme example ofbrittle diaphragm behaviour, there are many buildings withminor to moderate cracking in the slabs-to-lateral system(beams or walls). It was found that even for residual crackwidths < 1 mm, the cold-worked mesh across the floor slabtopping (which is meant to act as the seismic diaphragmshear transfer) could be fractured. The uncertainty of theresidual diaphragm capacity and the difficulty of assessingthe remaining strain life of the “brittle” mesh indicate thatbuildings relying on cold-formed mesh for the diaphragmaction may require urgent remedial retrofits.

4.6 Precast staircases in multi-storey buildings

Severe damage and collapse of precast staircases havebeen observed in many instances in the 22 February 2011earthquake. In many buildings, staircases exhibited signifi-cant damage in buildings where the inter-storey move-ments of the staircases were restrained. Complete or par-tial internal precast concrete staircases collapses havebeen reported for four multi-storey high-rise buildings (e.g.Fig. 12a). Minor to moderate levels of staircase move-ment/damage (e.g. Fig. 12b and 12c) were observed inmany other medium- to high-rise buildings.

The staircase damage observed in multi-storey build-ings indicates that the deformation allowance they had

been designed for (even when compatible with the coderequirements at the time) was typically inadequate to sus-tain the very high seismic demand. Considering that stair-cases represent a critical means of escape in buildings, it isclear that a major reconsideration of the design philoso-phy of staircases in multi-storey buildings (RC or other-wise) will be needed. An interim approach to the assess-ment and retrofitting of existing stairs has been developedby an Engineering Advisory Group to the Department ofBuilding and Housing [37]. Further description of the ap-proach to staircase design in New Zealand is given in [37],[12].

4.7 Liquefaction, lateral spreading and building foundationdamage

Severe widespread liquefaction and lateral spreading wereobserved in Christchurch and surrounding suburbs in the4 September 2010 Mw 7.1 Darfield earthquake as well asthe 22 February 2011 Mw 6.2 Christchurch earthquake.However, limited or partial liquefaction manifestation wasobserved within the Christchurch CBD in the 4 Septem-ber event, whereas severe liquefaction was observed inparts of the Christchurch CBD in the 22 February earth-quake.

Liquefaction land damage induced differential settle-ment of buildings, resulting in foundation damage andbuilding permanent tilting [38]. There is clear evidence ofbuilding damage/tilting as a consequence of liquefaction-induced settlements and ground movement as shown inFig. 13. Variable soil profiles underneath these buildingswith varying foundation designs are some of the complex-ities, resulting in mixed (good and bad) performance ofvarious CBD buildings within the same segments of lique-faction-damaged streets.

Preliminary observations indicate buildings with pilefoundations generally exhibit less differential settlementand liquefaction-induced tilt [38]. High-rise multi-storeybuildings on shallow foundations with significant liquefi-able soil depth generally exhibit substantial settlementand liquefaction-induced tilt. The soil-foundation-struc-ture interaction is a very complicated subject that remainsin the forefront of earthquake engineering research.

Fig. 12. Precast concrete staircases in multi-storey buildings: a) collapse, b) top landing damage, and c) bottom landing damage

a) b) c)

5 Conclusions

This paper has presented a brief summary and overview ofpreliminary lessons learnt from our observations of the seis-mic performance of RC buildings in the 22 February 2011Christchurch earthquake. Due to the concise nature of thepaper, it is impossible to discuss all relevant aspects in de-tail. Readers are encouraged to read more detailed recon-naissance reports presented elsewhere [1], [9], [30], [31].

At the time of writing, the Canterbury EarthquakesRoyal Commission and various investigations into theseismic performance of severely damaged and collapsedRC buildings are ongoing. This paper has therefore re-frained from any discussion of the buildings that form partof this current investigation. Readers are encouraged toread the outcomes of the inquiry at the Royal Commissionwebsite [39].

The unique and unprecedented series of severeearthquake events in Christchurch and the substantialdamage observed to “modern” and “well”-designed rein-forced concrete buildings (in addition to the expecteddamage to and collapse of unreinforced masonry build-ings and older ”non-ductile” RC buildings) has promptedthe consideration that current performance criteria adopt-ed for both new building and older building stock mightnot be adequate, certainly not in fulfilling the expectationsof modern society.

Although it has to be recognized that the 22 Febru-ary Christchurch earthquake caused particularly high seis-mic shaking well beyond the current code design levelsand that the event was caused by an unknown blind fault,the devastating reality of the facts is that the bar shouldundoubtedly be raised (at international level).

Such a step-up can be achieved not only by obvious-ly increasing the seismic coefficients or hazard levels, but– and probably just as importantly – by:– increasing the performance requirements (e.g. moving

towards a low-damage or damage-control approach evenunder design-level or MCE-level earthquakes) for bothnew and, to a lesser extent, existing structures, and

– favouring the implementation, and continuing the re-search and development, of higher seismic-performance

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concrete technology capable of giving society what itnaturally believes was already an ”earthquake-proof”building.

Acknowledgements

Special thanks go to the numerous professional structuralengineers, Urban Search & Rescue Teams and NewZealand Civil Defence who assisted in various forms dur-ing the critical emergency period of these earthquakes.Any conclusions and inappropriate mistakes in reportingmade in this contribution are nevertheless to be consid-ered entirely those of the authors.

The assistance and data provided by ChristchurchCity Council, Dr. Ken Elwood and Dr. Umut Akguzel aregratefully acknowledged. Thanks are also given to theFRST Retrofit project postgraduate students (SahinTasligedik and Patricio Quintana-Gallo) for their contri-butions in the field data collection.

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Fig. 13. Liquefaction-induced differential settlement resulting in significant tilting of medium- to high-rise buildings with various foundation and soil details:a) four-storey with shallow foundation, b) six-storey with shallow foundation, c) two high-rise buildings with substantial differential settlement and tilting

a) b) c)

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Stefano PampaninChair of the Structures/Geotechnical Cluster, Associate Professor (Reader)Department of Civil & Natural Resources Engineering, University of Canterbury, Private Bag 4800, Christchurch 81240, New ZealandEmail: [email protected]

Weng Yuen KamBeca Carter Hollings & Ferner Ltd, Auckland, New ZealandFormerly Research Associate Department of Civil & Natural Resources Engineering, University of Canterbury, Christchurch, New ZealandEmail: [email protected]

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