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Moon, Lisa, Dizhur, Dmytro, Senaldi, Ilaria, Derakhshan, Hossein, Griffith,Michael, Magenes, Guido, & Ingham, Jason(2014)The demise of the URM building stock in Christchurch during the 2010-2011 Canterbury earthquake sequence.Earthquake Spectra, 30(1), pp. 253-276.
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https://doi.org/10.1193/022113EQS044M
1 The Demise of the URM Building Stock in2 Christchurch during the 2010–20113 Canterbury Earthquake Sequence
4 Lisa Moon,a) Dmytro Dizhur,b) Ilaria Senaldi,c) Hossein Derakhshan,d)
5 Michael Griffith,e) M.EERI, Guido Magenes,f ) and Jason Ingham,g) M.EERI
6 The progressive damage and subsequent demolition of unreinforced masonry
7 (URM) buildings arising from the Canterbury earthquake sequence is reported.
8 A dataset was compiled of all URM buildings located within the Christchurch
9 CBD, including information on location, building characteristics, and damage
10 levels after each major earthquake in this sequence. A general description of
11 the overall damage and the hazard to both building occupants and to nearby
12 pedestrians due to debris falling from URM buildings is presented with several
13 case study buildings used to describe the accumulation of damage over the
14 earthquake sequence. The benefit of seismic improvement techniques that had
15 been installed to URM buildings is shown by the reduced damage ratios
16 reported for increased levels of retrofit. Demolition statistics for URM buildings
17 in the Christchurch CBD are also reported and discussed. [DOI: 10.1193/
022113EQS044M]
18 INTRODUCTION
19 There have been over 11,000 earthquakes and aftershocks associated with what is
20 referred to here as the 2010–2011 Canterbury earthquake sequence (Bradley et al. 2013).
21 The Christchurch Central Business District (CBD) was defined by the Canterbury Earth-
22 quake Royal Commission (2011) as the area bounded by the four avenues (Bealey,
23 Fitzgerald, Moorhouse and Deans) and Harper Avenue (Figure 1a), with Bradley et al.
24 (2013) reporting that the intensity of shaking recorded at the CCCC site (refer Figure 1a)
25 was representative of that experienced across most of the CBD. Ground motion in five
Earthquake Spectra, Volume 30, No. 1, pages 1–24, February 2014; © 2014, Earthquake Engineering Research Institute
a) Doctoral researcher, School of Civil, Environmental and Mining Engineering, University of Adelaide, South
Australia 5005, Australiab) Research Fellow, Department of Civil and Environmental Engineering, The University of Auckland, Private Bag
92019, Auckland 1142, New Zealandc) Doctoral researcher, ROSE Program, UME School, IUSS, Via Ferrata 1, Pavia, Italyd) Research Fellow, School of Civil, Environmental and Mining Engineering, University of Adelaide, South
Australia 5005, Australiae) Professor, School of Civil, Environmental and Mining Engineering, University of Adelaide, South Australia
5005, Australiaf) Associate Professor, Department of Civil Engineering and Architecture, University of Pavia and European
Centre for Training and Research in Earthquake Engineering (EUCENTRE), Via Ferrata 3, Pavia, Italyg) Professor, Department of Civil and Environmental Engineering, The University of Auckland, Private Bag 92019,
Auckland 1142, New Zealand
1
26 of these events, with a combined duration of approximately 325 seconds, caused horizontal
27 ground accelerations in excess of 0.15g (Figure 1b–d) at the CCCC site.
28 For completeness it is noted that while the Darfield earthquake (4/9/2010) had a greater
29 magnitude (Mw 7.1), its epicenter was located much further away (approximately 40 km)
30 from the Christchurch CBD than was the Mw 6.3 Christchurch earthquake (22/2/2011)
31 whose epicenter was only 10 km from the Christchurch CBD. Thus, from a ground accel-
32 eration perspective, the Darfield earthquake elastic spectra for sites in the CBD were roughly
33 consistent with the elastic design spectra for a 500-year return period event for Christchurch.
34 In contrast, the spectra for the same sites in the February 2011 event corresponded roughly to
35 the 1/2500 year design earthquake (Bradley and Cubrinovski 2011).
36 OBSERVED PERFORMANCE OF URM BUILDINGS
37 The results from two companion studies are presented below. The first study considered
38 all URM buildings in the Christchurch CBD and was undertaken at the request of the
39 Canterbury Earthquakes Royal Commission (Ingham and Griffith 2011c). The second
40 study specifically focused on stone URM buildings, due to both the disproportionately
(a) Location of CCCC recording station (b) Horizontal component N64E
(c) Horizontal component N26W (d) Vertical component
-0.5
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|<-- 127 seconds -->| 25 s |<-- 47 s -->|<-- 63s -->|<-- 64 -->|
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Figure 1. Details of combined earthquake recordings in the Christchurch CBD.
2 MOON ET AL.
41 large number of stone URM buildings in the Christchurch region when compared to all of
42 New Zealand and because of the particular heritage significance of many of these buildings
43 (Senaldi et al. 2013). A third companion study considering the seismic performance of 112
44 Christchurch churches (including 20 clay brick and 32 stone URM churches) has been
45 reported elsewhere (Leite et al. 2013).
46 MATERIAL PROPERTIES
47 The general observation from the debris of collapsed URM walls was that the kiln fired
48 clay bricks were of sound condition, but that the mortar was in poor condition. In most cases
49 the fallen debris had collapsed into piles of individual bricks, rather than as larger chunks of
50 masonry debris, and when rubbed the mortar readily crumbled when subjected to finger pres-
51 sure (refer Figure 2), suggesting that the mortar compression and shear strengths were very
52 low. Subsequent testing (Lumantarna 2012) of 293 mortar samples and 67 clay bricks col-
53 lected from 51 and 23 damaged URM building sites respectively located across the greater
54 Christchurch area resulted in average compressive strengths of 1.75 MPa and 23.6 MPa for
55 the mortar and bricks, confirming the initial observation that mortar was frequently in
56 poor condition (i.e., soft) but that the bricks were reasonably strong. These findings are con-
57 sistent with results obtained from testing of other URM buildings throughout New Zealand
58 (Lumantarna et al. 2013a, b).
59 When undertaking a post-earthquake review of archived files reporting the design of
60 earthquake strengthening interventions for URM buildings in the Christchurch CBD
61 there was little evidence found on comprehensive site investigations conducted by practicing
62 engineers in order to determine appropriate masonry material proprieties (i.e., mortar bed
63 joint shear tests and anchor pull-out tests) to be used in their designs. Instead, it appeared
64 that most conclusions regarding building condition and material properties were based on
65 visual observations. In contrast, extensive material investigation on existing buildings is
Figure 2. Condition of masonry rubble.
THE DEMISE OF THE URM BUILDING STOCK IN CHRISTCHURCH 3
66 routinely conducted in the U.S. in order to establish both the existing condition of the build-
67 ing and accurate and reliable material properties for use in structural designs.
68 TYPICAL FAILURE MODES AND DAMAGE PATTERNS
69 Comprehensive descriptions of the damage caused to URM buildings by the Canterbury
70 earthquake sequence have been previously reported (Dizhur et al. 2010, Dizhur et al. 2011,
Table 1. List of the most commonly observed URM building failure modes
Component Failure mode: Damage
Chimney Frequent flexural failures, falling brickwork, damage to roof and/or adjacent
structures (most commercial buildings had no chimneys, or chimneys that
were not visible from the street, so numbers were not recorded)
Gable end wall Out-of-plane failure due to missing or excessively spaced ties (63% of
buildings with gables (120∕189) had partial or full collapses of gables)
Parapets Frequent collapse of both unrestrained and restrained parapets. Unrestrained
parapet failure usually occurred along the roofline if wall–roof connections
were intact, otherwise in conjunction with out-of-plane wall failure. Restrained
parapet failure was often due to missing or ineffective restraint either in
adhesive anchor connection to masonry or punching shear failure in the
masonry around the connection (66% of buildings with parapets (324∕491)
had partial or full collapses of parapets)
Awnings Falling parapets landed on awnings, causing failure of the awning canopy or
awning support when tied back into the URM (42% of buildings with an
awning (125∕299) had partial/full awning collapse)
Out-of-plane walls Many examples of out-of-plane wall failures. Almost universally the masonry
fell in the outward direction (70% of buildings were observed to have out-of-
plane wall damage (438∕627))
Anchorages Anchorage failures at wall-to-floor and wall-to-roof connections were common,
as well as anchorage failures for parapets and awnings (this information is not
currently in the database)
Corners Vertical connections between walls at building corners were observed to fail in
some cases (66% of buildings were observed to have some corner failure
(414∕627))
In-plane walls Relatively infrequent in-plane wall failures, typically at spandrel and pier
locations. Failures were observed mostly in walls containing a large number
of openings (56% of buildings showed signs of in-plane wall damage
352∕627–of these only 32∕352 (9%) showed extreme damage)
Pounding Several cases were observed of damage caused by pounding of one URM
building against its neighbor (see Cole et al. 2012) (this information is not
currently in the database)
4 MOON ET AL.
71 Ingham et al. 2011, Ingham and Griffith 2011a, 2011b, 2011c, 2011d). It should be noted that
72 the most commonly observed failure modes (listed in Table 1) sustained by URM buildings
73 were typical of those seen in other earthquake-affected cities around the world. Even before
74 the February 2011 earthquake, a significant number of wall–diaphragm connections had been
75 seriously damaged, with obvious out-of-plane bed joint sliding, although in some cases com-
76 plete wall failure had not yet developed (see Figure 3).
77 On the few occasions that building owners or occupants were in attendance it was
78 possible to gain access to the interior of URMbuildings and often observe that some separation
79 had occurred between the floor and/or roof diaphragms and the masonry walls (in the out-of-
80 plane direction). This damage was not easy to detect from the outside of a building, so that the
81 damage reported from external building surveys in the first 72 hours is assumed to be a lower
82 bound estimate of structural damage toURMbuildings (Ingham et al. 2011,Moon et al. 2012).
83 There were also many cases of buildings that were structurally sound but were yellow
84 or red-tagged owing to falling hazards from neighboring buildings. In some cases it was
85 clear that a parapet or chimney from a neighboring building had fallen onto or through
86 the roof, being the only damage to the structure. In other situations, a building abutting
87 a taller building with a damaged parapet or gable side walls or chimney was given a yellow
88 card (no public access) due only to the falling hazard posed by the adjacent structure. These
89 examples of collateral damage and risk and the associated business interruption costs ele-
90 vated the financial impact of this earthquake sequence beyond just the cost of rebuilding
91 (Cousins et al. 2012).
92 DAMAGE STATISTICS FOR CLAY BRICK URM BUILDINGS IN
93 CHRISTCHURCH CBD
94 The data reported in this section are for clay brick load bearing masonry buildings
95 located in the Christchurch CBD. These buildings were mostly (77%) of solid two or
Figure 3. Wall-roof anchorage partial failure.
THE DEMISE OF THE URM BUILDING STOCK IN CHRISTCHURCH 5
96 three wythe thick wall construction with timber floors and roofs, although some (23%)
97 brick cavity construction was also observed. It is clearly shown in Figure 4a that the
98 sizeable percentage (43%) of green-tagged URM buildings in September 2010 signifi-
99 cantly reduced (to just 1%) after the February 2011 earthquake. In the data that follows,
100 it was not possible to distinguish between two and three wythe walls because most mul-
101 tistory URM buildings were comprised of three wythe walls in the lower levels and of
102 two wythe walls in the upper levels. Nevertheless, in every case the out-of-plane damage
103 started in the upper levels where the accelerations were highest and the walls were their
104 thinnest (i.e., two wythe).
105 In addition to collating the placard data as reported in Figure 4, data were collected on
106 many attributes of the URM building stock in the Christchurch CBD, such as number of
107 stories, building footprint area, and building occupancy type (see Ingham and Griffith
108 2011d for full details). The observed extent of damage was correlated against both the
109 Wailes and Horner (1933) Damage Scale, which was specifically developed to describe
110 damage to URM buildings, and against the damage levels prescribed in ATC 13 (ATC
111 1985), because of the widespread use of this survey form in post-earthquake damage assess-
112 ments. Interestingly, no discernible trends regarding the extent of building damage were
113 apparent for any parameter, such as indicated in Figure 5 where it can be seen that an
114 approximately uniform extent of damage was observed independent of the number of
115 stories.
116 Based on the position of fallen debris, the risk to building occupants and to pedestrians
117 located outside but in close proximity to the 340 URM buildings in the CBD was assessed as
118 shown in Figure 6. The data are consistent with the documented location of fatalities (39)
119 associated with URM buildings (both clay brick and stone) within the CBD, where the
(a) 4 September 2010 earthquake
(361 entries)
(b) 22 February 2011 earthquake (361
entries including 21 (6%) demolished)
Green43%
Yellow31%
Red11%
Unknown15%
Green Yellow Red Unknown
Green1%
Yellow17%
Red73%
Demolished6%
Unknown3%
Green Yellow Red Demolished Unknown
Figure 4. Placard assignments for URM buildings in Christchurch CBD (Note: “Demolished”
refers to buildings dismantled by building contractors before the February earthquake).
6 MOON ET AL.
120 majority of people killed by URM building damage (35) were located outside rather than
121 within the building (see Table 2).
122 DAMAGE STATISTICS FOR STONE URM BUILDINGS
123 The damage assessment inspections that were undertaken in September 2010 and
124 again in April and May 2011 identified 96 unreinforced stone masonry buildings in
125 the wider Canterbury region (approx. half being located in the Christchurch CBD and
126 the remainder located in surrounding suburbs), many of which are included on the
0
20
40
60
80
100
0%
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25%
30%
35%
Insignificant 1 - 10%
Moderate 10 - 30%
Heavy 30 - 60%
Major 60 - 100%
Destroyed 100%
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eye
d
Pe
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nta
ge
of B
uild
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s S
urv
eye
d
Extent of Damage to Building
1 Storey High 2 Stories High 3+ Stories High
Figure 5. Correlation between extent of damage and story height for 340 URM buildings in CBD
(22 February 2011).
(a) Risk to building occupant (b) Risk to nearby pedestrians
0%
20%
40%
60%
80%
100%
Insignificant 1 - 10%
Moderate 10 - 30%
Heavy 30 - 60%
Major 60 - 100%
Destroyed 100%
Perc
enta
ge o
f Build
ings
Extent of Damage to Building
unlikely likely near certain
0%
20%
40%
60%
80%
100%
Insignificant 1 - 10%
Moderate 10 - 30%
Heavy 30 - 60%
Major 60 - 100%
Destroyed 100%
Perc
enta
ge o
f Build
ings
Extent of Damage to Building
unlikely likely near certain
Figure 6. Risk to building occupants and nearby pedestrians for different building damage
levels.
THE DEMISE OF THE URM BUILDING STOCK IN CHRISTCHURCH 7
127 New Zealand Historic Places Trust register of heritage buildings. Many of these stone
128 masonry buildings were designed by the same architects or architectural firms and
129 most were constructed between 1850 and 1930 and hence have similar characteristics
130 both in terms of architectural features and in the details of their construction. These build-
131 ings are typically 2 or 3 stories in height, with two-story buildings being most common,
132 and either standalone or row buildings. The vast majority of stone masonry buildings, and
133 in particular those constructed in the Gothic Revival style, are characterized by structural
134 peripheral masonry walls that may be connected, depending on the size of the building, to
135 an internal frame structure constituted of cast iron or steel columns and timber beams or to
136 internal masonry walls that support flexible timber floor diaphragms and timber roof
137 trusses. A variety of wall cross section types were identified, as indicated in Table 3
138 and illustrated in Figure 7.
139 The seismic performance of stone masonry buildings was partially identified by consid-
140 ering the safety assessment data that were collected following the earthquakes that occurred
141 in September 2010 and February 2011. From Figure 8 it can be seen that there was a sig-
142 nificant escalation of damage due to the continuing earthquake activity in the Christchurch
143 region. Figure 9 gives a further breakdown of this data following the February 2011
Table 3. Stone wall cross section types and number of buildings for each type
Types of stone wall cross section No. of buildings
Multi-leaf masonry walls (basalt or lava flow) 24
Three-leaf masonry walls with rubble lava flow façade, internal concrete core
and Oamaru stone facing
13
Three-leaf masonry walls in Oamaru stone and concrete core 6
Ashlar stone facing (basalt or bluestone) 24
Ashlar stone facing and brickwork backing 17
Ashlar stone facing, concrete core and brickwork backing 3
Others 6
Undetermined 3
Total 96
Table 2. Cause of death for 39 people killed by URM buildings (extracted from Cooper
et al. 2012)
Number Cause of death associated with URM building
35 (90%) Killed as a result of the facade or walls of URM buildings collapsing onto:
26 (67%) Pedestrians or persons in vehicles
6 (15%) People in a neighboring building
3 (8%) People who had run out of a building to escape
4 (10%)Killed inside a URM building
8 MOON ET AL.
144 earthquakes on the basis of building usage. From this data no clear trends in damage patterns
145 could be observed on the basis of building usage.
146 A comparison between the data reported in Figure 4b and in Figure 8b suggests that stone
147 URM buildings fared better than clay brick URM buildings. This finding is contrary to
148 expectation when accounting for the inherent vulnerability of the two forms of construction
149 and is attributed to a number of extensive earthquake strengthening interventions that had
150 been applied to iconic stone masonry buildings, such as the Christchurch Arts Centre (Bailey
151 et al. 2013). Further details on the performance of unreinforced stone masonry buildings are
152 reported by Senaldi et al. (2013).
(a) Time Ball Station with
multiple-wythe stone
masonry
(b) Cathedral of the Blessed
Sacrament in Oamaru stone
with poured concrete core
(c) St. Luke’s Anglican
Church characterized by
stone facings with clay
brick interior wythes
Figure 7. Representative examples of wall cross-sections for Christchurch stone masonry
buildings.
(a) After September 2010 (b) Data updated 07 June 2011
unknown
49%
demolished
0%
red
7%
yellow
26%
green
18%
green yellow red demolished unknown
green
16%
yellow
22%
red
58%
demolished
2%
unknown
2%
green yellow red demolished unknown
Figure 8. Distribution of safety evaluation placards applied to 96 stone masonry buildings (Note:
“Demolished” means buildings dismantled by building contractors).
THE DEMISE OF THE URM BUILDING STOCK IN CHRISTCHURCH 9
153 OBSERVED PERFORMANCE OF EARTHQUAKE
154 STRENGTHENED URM BUILDINGS
155 Following the February 2011 earthquake an exercise was undertaken to identify the seis-
156 mic improvements to URM buildings in Christchurch existing prior to the September 2010
157 earthquake. These improvements were divided into three categories for ease of assessment
158 and the interpretation of data: Parapet restraints; Type A seismic improvements; and Type B
159 seismic improvements. Definition of these categories is provided below.
160 PARAPET RESTRAINTS
161 Parapet restraints (considered structural) were looked at separately (i.e., they were not
162 part of A or Aþ B grouping which are discussed later). Recognizing the obvious hazard
163 that parapets pose to building occupants and nearby pedestrians, it follows that parapet
164 restraints are one of the most commonly encountered earthquake strengthening techniques
165 in URM buildings. Typically observed methods used to improve the earthquake response of
166 parapets include but are not limited to: (a) parapet removal; (b) bracing of the parapet back
167 into the roof structure via steel members connected to the parapet and secured into the
168 masonry using either adhesive anchors or through ties; and (c) addition of a concrete
169 ring beam (or cap beam) in order to tie a parapet to the end walls that run orthogonal to
170 the parapet.
171 A total of 435 records of parapets (predominantly solid two-wythe thick) were associated
172 with the surveyed buildings, with some buildings having multiple parapets, such as those on
173 street corners or for end-of-row or standalone buildings. As shown in Table 4, of these 435
174 parapets only 149 (34%) parapets could be positively identified as having parapet restraints
175 installed. Unfortunately, it was not possible to definitively identify a sufficient sample size of
176 specific types of parapet restraint systems from building inspections although steel bracing
177 was by far the most commonly observed form of restraint.
0
5
10
15
20
25
30
35
Cultu
ral/m
useu
m
Schoo
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umen
t
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lic O
ffice
Touris
tic F
acilit
y
Res
ident
ial
Com
mer
cial
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rch
Nu
mb
er
of
bu
ildin
gs
Green Yellow Red
Figure 9. Distribution of safety evaluation placards applied to stone masonry buildings, differ-
entiated by usage (data updated 7 June 2011).
10 MOON ET AL.
178 As expected (Figure 10), restrained parapets performed significantly better than parapets
179 having no restraint, with 84% (75∕89) of unrestrained parapets suffering full or partial col-
180 lapse while only 44% (65∕149) of restrained parapets suffered similar damage. Furthermore,
181 48% (71∕149) of restrained parapets suffered no or moderate damage while only 13%
182 (12∕89) of unrestrained parapets achieved such good response, such that 86% (71∕83) of
183 those parapets that performed satisfactorily were restrained.
184 Overall it may be concluded that unrestrained parapets were twice as likely to collapse
185 as were restrained parapets. It would seem that this is a disappointing finding as it would
186 have been assumed that the majority of restrained parapets would have performed satis-
187 factorily. However, it is emphasized that many of the restrained parapets had suffered
188 some damage in previous earthquakes and had not been repaired before the February
189 2011 earthquake. Nevertheless, only 13% of the unrestrained parapets performed satisfac-
190 torily, supporting the Royal Commission recommendation that all parapets and ornaments
191 require restraint. This finding suggests that further investigation is also required to better
192 understand why parapet restraints were not more uniformly successful. Ground motions
193 well in excess (six times higher in February 2011) of those assumed for the design of
Table 4. Distribution of parapet restraints (“cases”
refers to a single line of wall)
No. of cases % of parapets
Restrained 149 34%
Unrestrained 89 21%
Unknown 197 45%
Total 435 100%
0%
10%
20%
30%
40%
50%
60%
Insignificant 1 - 10%
Moderate 10 - 30%
Heavy 30 - 60%
Major 60 - 100%
Destroyed 100%
Pe
rce
nta
ge
of B
uild
ing
s S
urv
eyed
Extent of Damage to Building
Unrestrained parapets
Restrained parapets
Figure 10. Performance of unrestrained and restrained parapets.
THE DEMISE OF THE URM BUILDING STOCK IN CHRISTCHURCH 11
194 parapet restraints, low mortar strengths, the absence of superimposed load on parapets,
195 correspondingly low masonry shear and anchorage strengths, ineffective adhesive anchor
196 development and spacing, and damage from prior earthquakes, all clearly influenced the
197 performance of restrained parapets.
198 TYPE A SEISMIC IMPROVEMENT
199 Examples of Type A retrofits included restraints to gable-ended walls, the installation of
200 wall–diaphragm anchorages, and diaphragm improvements. As with parapets, because it is
201 widely known that gable end walls are prone to failure, it is not uncommon to find some form
202 of restraint provided to inhibit out-of-plane failure (see Figure 11). Where present, the type of
203 gable end wall restraint was identified and recorded. The Canterbury Earthquake Royal Com-
204 mission Report (2011) reports that for 185 gable end walls, 128 had some form of restraint
205 which were further broken down using three categories: (i) most (109∕128) used through ties
206 (Figure 11a); (ii) 5∕128 used adhesive anchors; and (iii) 14∕128 were restrained with some
207 form of tie at the time of original construction (Figure 11b).
208 In some cases connections between the walls and the roof and floor diaphragm systems of
209 the building had been installed to prevent walls responding as vertical cantilevers secured
210 only at their base. Connections were typically either older “rose” type through ties with exter-
211 nal plates (see Figure 12a), which were installed from 1940s onward, or more recent adhesive
212 anchors (see Figure 12b) which in many locations were seen to be ineffective. The out-of-
213 plane failure of URM walls was in many cases attributed to the low shear strength of
214 masonry, wide anchorage spacing and/or insufficient embedment depth of anchors. In
215 some cases the reasons for the adhesive anchor failures were apparent: anchor pull-out
216 due to insufficient embedment length or a lack of bonding between the anchor and the sub-
217 strate material. In other cases, the reasons for such failures were not evident from visual
218 observation. The construction quality of adhesive type anchorages was commonly observed
219 to be poor, due to insufficient anchorage depths and poor workmanship.
Figure 11. Examples of gable end wall restraints.
12 MOON ET AL.
220 Diaphragm improvement techniques typically consisted of an addition of: (i) steel bra-
221 cing (see Figure 13a); (ii) steel brace frames (see Figure 13b); (iii) plywood overlays (see
222 Figure 13c); (iv) concrete slab topping; or (v) combinations of these (see Figure 13d).
223 TYPE B SEISMIC IMPROVEMENT
224 Examples of Type B retrofits generally include: (i) out-of-plane strengthening of existing
225 masonry walls for response perpendicular to wall planes (i.e., addition of strong-backs);
Figure 12. Wall-to-diaphragm connections.
Figure 13. Example of diaphragm improvement techniques.
THE DEMISE OF THE URM BUILDING STOCK IN CHRISTCHURCH 13
226 (ii) in-plane shear strengthening of masonry walls; and (iii) introduction of higher-level inter-
227 ventions to supplement or take over the seismic resisting role from the original unreinforced
228 masonry structure. Techniques to achieve these improvements include the use of: internal/
229 external strong backs; steel and concrete (moment and braced) frames, additional cross walls,
230 and strengthening of existing masonry walls with shotcrete, surface bonded Fiber Reinforced
231 Polymer (FRP), or post-tensioning. In practice, Type B retrofits are seen only in conjunction
232 with Type A retrofits. Two case study buildings with TypeAþ B retrofits are discussed later.
233 RETROFIT LEVEL (%NBS)
234 The level of seismic improvement was expressed as %NBS (Percentage of New Building
235 Standard), where a building having a score of %NBS ≤ 33 corresponds to the definition
236 given in Section 122 of the New Zealand Building Act (2004) (New Zealand Parliament
237 2004) as “earthquake-prone” and a building having a score of 33 ≤ %NBS ≤ 67 is defined
238 by NZSEE (2006) as “earthquake risk.” The value of %NBS was established either from
239 Christchurch City Council (CCC) records, personal communication with building owners,
240 engineers and heritage personnel, or in some cases where sufficient seismic details about the
241 building were known, was based upon estimation.
242 Accessed CCC records showed that most engineering reports provided the level of
243 strengthening in terms of design PGA values, which were correlated to the then-current
244 (2011) %NBS values using the seismic zone factor of Z ¼ 0.22 for Christchurch and the
245 procedures provided in NZS 1170.5 (Standards New Zealand 2004). Note that the seismic
246 zone factor for Christchurch was later elevated to Z ¼ 0.3.
247 COMPARISON BETWEEN %NBS AND ASSESSED DAMAGE LEVEL
248 Earthquake strengthening levels in terms of %NBS were identified for 94 URM buildings
249 using the pre-earthquake value for Z of 0.22. The distribution of %NBS data is reproduced in
250 Table 5 where it is shown that 61 buildings had been earthquake strengthened to at least 67%
251 NBS.With respect to Type A and B retrofit techniques, it can be generalized that TypeAþ B
252 retrofit was needed to achieve a%NBS > 67% whereas Type A retrofit on its own was most
253 likely to correspond to a %NBS classification of between 33% and 67%.
254 The performance of these earthquake strengthened buildings was analyzed by determin-
255 ing the damage distribution for each category of %NBS. From Table 6 and Figure 14 it can be
256 seen that URM buildings having retrofits with less than 33%NBS did not perform appreci-
257 ably better than URM building having no retrofits at all. This finding is consistent with
Table 5. Distribution of %NBS classifications for 94
earthquake strengthened URM buildings
NBS Retrofit level No. of buildings % of buildings
%NBS < 33 15 16%
33 ≥ %NBS < 67 18 19%
67 ≥ %NBS < 100 50 53%
%NBS ≥ 100 11 12%
14 MOON ET AL.
258 observations from past earthquakes (ATC-31 1992). In addition, incomplete retrofits can
259 only be expected to provide, at best, satisfactory seismic behavior of only the properly
260 strengthened parts of the building (Bruneau 1995). In contrast, there was a significant reduc-
261 tion in the “Major” and “Destroyed” levels of damage for URM buildings when strengthened
262 to 33% < %NBS < 67% and an even more dramatic reduction in “Heavy” damage when
263 URM buildings were retrofitted to a level of %NBS > 67%. Thus, it can be concluded
264 that the applications of seismic retrofits to URM buildings in the CBD were successful
265 in reducing building damage and improving life safety, especially when considering that
266 the intensity of the February 2011 earthquake was well in excess of the code level specified
267 for Christchurch.
Table 6. Damage levels for different %NBS categories (after February 2011)
Damage level %NBS < 33
33 ≥ %NBS
< 67
67 ≥ %NBS
< 100
%NBS
≥ 100 Total
Insignificant
1–10%
1 7% 1 6% 10 20% 8 73% 20
Moderate
10–30%
5 33% 4 22% 28 56% 3 27% 40
Heavy
30–60%
5 33% 9 50% 10 20% 0 0% 24
Major
60–100%
1 7% 4 22% 2 4% 0 0% 7
Destroyed
100%
3 20% 0 0% 0 0% 0 0% 3
Total 15 18 50 11 94
0%
20%
40%
60%
80%
Level of retrofit
Pe
rce
nta
ge
of b
uil
din
gs
Damage level
%NBS ≥ 100 67 ≤ %NBS < 100 33 ≤ %NBS < 67
%NBS < 33 All buildings No retrofit
Figure 14. Damage levels for different levels of %NBS earthquake strengthening.
THE DEMISE OF THE URM BUILDING STOCK IN CHRISTCHURCH 15
268 CASE STUDY BUILDINGS
269 Four case study buildings are considered here; two with Type A retrofit and two with
270 Type Aþ B retrofit. The location of the four buildings is shown in Figure 15, where it can be
271 seen that two of the buildings, A1 and B2, lie within the Christchurch CBD. Buildings B1 and
272 A2 were included for illustrative purposes because they demonstrate excellent performance
273 of a seismic retrofit (B1) and the effect of cumulative damage (A2). A description of each
274 building and its performance is given below.
275 TYPE A RETROFIT
276 Example A1: Mayfair Building, 140–142 Lichfield Street
277 This four-story clay brick URM building had no evidence of parapet restraints but
278 did have through-bolts for wall-diaphragm (floor and roof) anchors which initially restricted
279 damage to just the top story. However, in subsequent earthquakes the out-of-plane wall
280 damage spread down and to the corners and eventually around into the side walls (see
281 Figure 16). This building has since been demolished.
282 Example A2: Sumner Borough Council Building, 16 Wakefield Street
283 This two-story clay brick URM building used through-bolts for wall-diaphragm (all
284 floors and roof) anchorage as well as having all parapets restrained (see Figure 17). The
285 September 2010 earthquake caused cracking in the side and front walls but no major cracking
286 was observed until after the February 2011 earthquake, and by June 2011 entire sections of
287 the side walls had fallen away. The building has since been demolished.
Figure 15. Locations of case study buildings and strong motion seismographs (active
22 February 2011).
16 MOON ET AL.
288 TYPE B RETROFIT
289 Example B1: The Smokehouse, 650 Ferry Road
290 The Smokehouse is a two-story freestanding clay brick URM building as shown in
291 Figure 18. The building’s exact construction date is unknown but can be confirmed as
292 pre-1930s, and the building has been categorized as a heritage building by the Christchurch
293 City Council. The building’s footprint has dimensions of 13 m along Ferry Road and 10 m
Figure 16. Progressive damage to Mayfair building—Level A retrofit.
Figure 17. Progressive damage in Sumner Council building—Level A retrofit.
Figure 18. Progression of damage in Smokehouse—Level Aþ B retrofit.
THE DEMISE OF THE URM BUILDING STOCK IN CHRISTCHURCH 17
294 along Catherine Street. The original mortar is a weak lime/cement mortar with large grain
295 size sand. In places the original mortar was re-pointed with strong cement mortar.
296 The Smokehouse was seismically retrofitted in 2007 with adhesive anchors, steel frame
297 stiffening of the roof, alterations to the internal layout which involved replacement of some
298 original external walls with moment resisting steel frames (see Figure 18b, 18d) and infilling
299 of one window at the second floor level. The retrofit design of the building won the
300 New Zealand Architectural Award in 2008 for initiative in retention, restoration and exten-
301 sion of a significant building and its adaption to new uses (Smokehouse Restaurant 2009).
302 There were no signs of damage from the September 2010 earthquake on the exterior face
303 of the building. However some minor vertical cracks could be seen in the interior at the wall
304 corners, with minor cracks around the perimeter of the in-filled window and one small hor-
305 izontal crack slightly above the base of one pier at the second floor level. The scaffolding
306 shown in Figure 18 was erected to repair damage prior to the February 2011 earthquake. The
307 February event caused only slight cracking as well as some minor damage to the exterior
308 brickwork, which required repointing with minimal interruption to business. Hence, the retro-
309 fit was seen by the owner to have been very successful and a good investment.
310 Example B2: Red Cross Shop, 223–227 High Street
311 This building was constructed in 1890 with timber truss roof and floors and was later
312 retrofitted (2006) with the addition of concrete floor diaphragms, internal steel strongbacks
313 and additional cross-walls. The building has survived all the earthquakes and has been
314 repaired with the expectation that the building will not be demolished. The progressive con-
315 dition of the building (see Figure 19) shows little damage to the façade, with the only major
316 damage caused by brickwork from the neighboring building falling through its roof and awn-
317 ing. However, minor visible damage occurred in a rear wall (not shown).
318 RESPONSE OF URM BUILDINGS TO CUMULATIVE GROUND SHAKING
319 Cabanas et al. (1997) presented the results of a comparative study aimed at identifying a
320 ground motion intensity index that best describes URM building damage based on post-
321 earthquake observations. Six scales of building damage were considered, ranging from
Figure 19. Progression of damage to Red Cross building.
18 MOON ET AL.
322 0 (slight plaster cracking) to 5 (building collapse), and two indices were described, being the
323 Arias Intensity (Arias 1970) and the Cumulative Absolute Velocity with an acceleration
324 threshold of 20 cm∕s2, CAV20. A threshold of CAV20 ¼ 1.5 g-s or of AI ¼ 1.5 m∕s was
325 suggested as appropriate for the onset of the “Destruction” damage level (Level 4, partial
326 building collapse, interior wall collapse), and it was suggested that the “Heavy” damage
327 level (Level 3, falling chimneys and deep and wide cracks in walls) could occur at intensities
328 as low as CAV20 ¼ 0.6 g-s or AI ¼ 0.45 m∕s.
329 Ground accelerations at the Christchurch Cathedral College (CCCC) site (refer Figure 1a)
330 within the Christchurch CBD were investigated using the two earthquake intensity measures
331 described above, and the results were correlated with the observed building damage. Instead
332 of CAV20, a similar but more recently proposed index, CAVDP, was calculated using the
333 procedure suggested by Campbell and Bozorgnia (2012a, b). The building damage incurred
334 in the 4 September 2010 earthquake was largely in the form of extensive wall cracking and
335 falling chimneys and parapets, which is best described as damage Level 3, whereas the exten-
336 sive URM building damage during the 22 February 2011 earthquake is consistent with
337 damage Levels 4 or 5 using the scale suggested by Cabanas et al. (1997). Figure 20a
338 shows that the cumulative AIs of both N64E and N26W components of the Canterbury
339 earthquake sequence first exceeded the 1.5 m∕s threshold for damage Level 4 during the
340 22 February 2011 earthquake.
341 Similar to the finding reported above, Figure 20b shows that the CAVDP value for the
342 4 September 2010 earthquake was between the proposed limits for damage Level 3 and
343 Level 4, but that the cumulative values exceeded the damage Level 4 threshold during
344 the 22 February 2011 earthquake. Consequently it is concluded that the correlation between
345 the CAVDP parameter and the observed building damage in the 4 September 2010 earthquake
346 is consistent with the recommendations of the Cabanas et al. (1997) study. The large cumu-
347 lative CAVDP values for the 22 February 2011 earthquake are also consistent with the wide-
348 spread observation of extensive damage to URM building during the 22 February 2011
349 earthquake.
(a) Cumulative Arias intensity values (b) Cumulative CAVDP intensity values
0
2
4
6
Cu
mula
tive
Arias In
tensity,
AI
(m/s
)
N64E
N26W
04Sep10
26Dec10
22Feb11
23Dec11
23Dec11
Lvl. 4 damage
Lvl. 3 damage
0
1
2
3
4
Cum
ula
tive C
AV
DP
(g-s
)
N64E
N26WLvl. 4 damage
Lvl. 3 damage
04Sep10
26Dec10
22Feb11
23Dec11
23Dec11
Figure 20. Earthquake intensity of the main events of the Canterbury earthquake sequence based
on recordings from CCCC site.
THE DEMISE OF THE URM BUILDING STOCK IN CHRISTCHURCH 19
350 In summary, both Arias Intensity and the CAVDP indices calculated for ground motions
351 in the Christchurch CBD greatly exceeded values previously proposed as resulting in
352 widespread and severe damage to URM buildings. Consequently, observations from the
353 Canterbury earthquake sequence support earlier recommendations that ground motion
354 metrics that account for both intensity and duration have the potential to effectively char-
355 acterize the damage to URM buildings associated with earthquake ground motions.
356 CHRISTCHURCH BUILDING DEMOLITION STATISTICS
357 The high percentages of red-tagged (73%) and yellow-tagged (17%) URM buildings after
358 the February 2011 earthquake has had a significant impact on the Christchurch CBD, leading
359 to the majority of these building having been subsequently demolished as indicated in
360 Figure 21. The large jump in the plots during August 2011 is due to a combination of (i) dis-
361 continuity in the data collection process; (ii) the fact that it took a long time for demolitions to
362 get started in earnest; and (iii) until the June 2011 earthquakes it was hoped that damaged
363 URM buildings could be saved. In hindsight, the difficult decision to demolish the seven-
364 story heritage listed Manchester Courts building, which was damaged in the September 2010
365 event probably saved lives in the February 2011 event.
(a) No confirmed retrofit (b) Type A retrofit only
(c) Type A and B retrofit (d) Demolished URM buildings as at 27/06/12
0
20
40
60
80
100
120
0%
20%
40%
60%
80%
100%
earlie
r
27
-Jul-11
3-A
ug- 1
1
30
-Aug-1
1
8-S
ep-1
1
20
-Sep-1
1
4-O
ct-
11
28-N
ov-1
1
11-J
an-1
2
7-F
eb-1
2
5-M
ar-
12
20-M
ar -
12
11- A
pr-
12
9- M
ay-1
2
30-M
ay-1
2
26-S
ep
-12
Num
be
r of B
uild
ing
s D
em
olis
hed
Perc
en
tage o
f B
uild
ings D
em
olis
he
d
Date of Demolition
0
20
40
60
80
100
120
0%
20%
40%
60%
80%
100%
earlie
r
27
-Jul -11
3-A
ug-1
1
30
-Aug-1
1
8- S
ep-1
1
20
-Sep-1
1
4-O
ct-
11
28
-No
v-1
1
11-J
an-1
2
7-F
eb-1
2
5-M
ar-
12
20-M
ar -
12
11- A
pr-
12
9-M
ay- 1
2
30-M
ay-1
2
26-S
ep-1
2
Num
ber o
f B
uild
ings D
em
olis
he
d
Perc
enta
ge o
f Build
ings D
em
olis
hed
Date of Demolition
0
10
20
30
40
50
60
70
0%
20%
40%
60%
80%
100%
ea
rlie
r
27
- Jul-11
3-A
ug
-11
30
-Aug
-11
8-S
ep
-11
20
-Sep
- 11
4-O
ct -
11
28
-Nov-1
1
11-J
an
- 12
7-F
eb
- 12
5-M
ar-
12
20- M
ar-
12
11-A
pr-
12
9- M
ay-1
2
30- M
ay-1
2
26-S
ep-1
2
Num
ber of B
uild
ing
s D
em
olis
hed
Perc
enta
ge o
f B
uild
ings D
em
olis
he
d
Date of Demolition
Figure 21. Distribution of retrofit types for demolished buildings.
20 MOON ET AL.
366 Despite the significantly reduced damage ratio for URM buildings strengthened using
367 Level Aþ B retrofits to > 67%NBS, a relatively large number of these buildings were
368 demolished as indicated in Figure 21c. Indeed, Figure 21 shows that while more than
369 90% of the non-retrofitted URM buildings were demolished, roughly 80% of the Level
370 A retrofitted and 70% of the Level Aþ B retrofitted URM buildings were also demolished.
371 The reasons for the relatively high demolition rate for strengthened URM buildings has not
372 yet been fully reported but it is thought that the cumulative effects of damage during the
373 sequence of earthquakes, the high level of insurance coverage, and uncertainties associated
374 with assessment of partly damaged buildings were all significant contributors. The location
375 of the demolished URM buildings is indicated on a map in Figure 21d, providing greater
376 detail of the former location of these buildings within the Christchurch CBD. The fact
377 that so many demolitions have been located in a relatively small area explains why businesses
378 in the Christchurch CBD have been so badly impacted (Cousins et al. 2012).
379 CLOSING REMARKS380
381 1. Material properties for Christchurch clay brick URM buildings were consistent with
382 earlier findings from throughout New Zealand, characterized by good quality clay
383 bricks and weak mortar, such that failure modes tended to be associated with mortar
384 joint failures rather than brick splitting.
385 2. Observed damage patterns to URM buildings were consistent with failure modes
386 seen in previous New Zealand and Australian earthquakes, and documented in
387 many other earthquake prone cities worldwide.
388 3. The majority (90%) of the 39 fatalities caused by earthquake damage to URM build-
389 ings was to pedestrians, persons in vehicles or neighboring buildings, or people who
390 ran from URM buildings in an attempt to escape.
391 4. Earthquake strengthened buildings generally sustained less damage than buildings
392 that had not been seismically upgraded or had been upgraded to lower levels of
393 %NBS:
394 • 73% of URM buildings that had been earthquake strengthened to a level
395 greater than 100%NBS sustained insignificant levels of overall building
396 damage.
397 • 76% of URM buildings that had been earthquake strengthened to a level of
398 between 67% and 100%NBS sustained moderate or lower levels of overall
399 building damage.
400 • 72% of URM buildings that had been earthquake strengthened to a level of
401 between 33% and 67%NBS sustained heavy or major levels of overall building
402 damage.
403 • 60% of URM buildings that had not been retrofit or earthquake strengthened to
404 a level below 33%NBS sustained heavy or major damage or were destroyed.
405 5. A surprising number of strengthened parapets failed after multiple damaging earth-
406 quakes. Further investigation is required into why and, if possible, how these
407 strengthening techniques can be improved.
408 6. Standard earthquake strengthening procedures for URM buildings do not prevent
409 earthquake damage to these buildings. At best these procedures significantly reduce
THE DEMISE OF THE URM BUILDING STOCK IN CHRISTCHURCH 21
410 the chance of partial or total building collapse (Ingham and Griffith 2011d, EERI
411 1994). Many earthquake strengthened URM buildings required extensive repair, or
412 even demolition.
413 7. Buildings with Type Aþ B earthquake strengthening techniques such as shotcrete
414 strengthened walls, additional cross-walls and reinforced concrete or steel strong-
415 backs generally performed reasonably well.
416 8. The five largest earthquakes in the Canterbury earthquake sequence had cumulative
417 absolute velocities and Arias intensities for recording sited in the Christchurch CBD
418 that were well in excess of previously recommended thresholds for partial building
419 collapse, and provide insight regarding the progressive damage to URM buildings
420 when subjected to an extended duration of moderate to severe intensity of ground
421 shaking.
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467 activity, The Structural Engineer 89(3), 14–15.
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THE DEMISE OF THE URM BUILDING STOCK IN CHRISTCHURCH 23
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24 MOON ET AL.
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