SESOC NZ Conference 2-3 November 2012
REPORT ON THE ‘PROJECT MASONRY’ RECOVERY PROJECT Dmytro Dizhur1, Jason Ingham2,
ABSTRACT: As part of the ‘Project Masonry’ Recovery Project funded by the New Zealand Natural Hazards Research Platform, commencing in March 2011, an international team of researchers was deployed to document and interpret the observed earthquake damage to masonry buildings and to churches as a result of the 22nd February 2011 Christchurch earthquake. The study focused on investigating commonly encountered failure patterns and collapse mechanisms. A brief summary of activities undertaken is presented, detailing the observations that were made on the performance of and the deficiencies that contributed to the damage to approximately 650 inspected unreinforced clay brick masonry (URM) buildings, to 90 unreinforced stone masonry buildings, to 342 reinforced concrete masonry (RCM) buildings, to 112 churches in the Canterbury region, and to just under 1100 residential dwellings having external masonry veneer cladding. Also, details are provided of retrofit techniques that were implemented within relevant Christchurch URM buildings prior to the 22nd February earthquake. In addition to presenting a summary of Project Masonry, the broader research activity at the University of Auckland pertaining to the seismic assessment and improvement of unreinforced masonry buildings is outlined. The purpose of this outline is to provide an overview and bibliography of published literature and to communicate on-going research activity that has not yet been reported in a complete form. KEYWORDS: Recovery Project, Masonry buildings, Canterbury earthquakes
1 INTRODUCTION 123 As part of the ‘Project Masonry’ Recovery Project funded by the New Zealand Natural Hazards Research Platform, commencing in March 2011, a team composed of local and international researchers was deployed to document and interpret the observed earthquake damage to masonry buildings and to churches as a result of the 22nd February 2011 Christchurch earthquake. The study focused on investigating commonly encountered failure patterns and collapse mechanisms. Observations and data were collected on the performance of and the deficiencies that contributed to the damage to approximately 650 inspected unreinforced clay brick masonry (URM) buildings, to 90 unreinforced stone masonry buildings, to 342 reinforced concrete masonry (RCM) buildings, to 112 churches in the Canterbury region, and to just under 1100 residential dwellings having external masonry veneer cladding. In addition, retrofit techniques that were implemented within
1
Dmytro Dizhur, Department of Civil and Environmental Engineering, The University of Auckland. Email:
[email protected] 2 Jason Ingham, Department of Civil and Environmental Engineering, The University of Auckland. Email:
[email protected]
relevant Christchurch URM buildings prior to the 22 nd February earthquake were assessed. Whilst in September 2010 most earthquake shaking damage was limited to URM buildings, in February 2011 all types of buildings sustained damage. Temporary shoring and strengthening techniques applied to buildings following the Darfield earthquake were tested in February 2011. In addition, two large aftershocks (magnitudes M5.7 and M6.2) occurred on 13 th June 2011, further damaging many already weakened structures.
2 UNREINFORCED MASONRY CONSTRUCTION A research effort considering the seismic assessment and strengthening of unreinforced masonry (URM) construction began at the University of Auckland in July 2004 and ran 6.25 years as part of the FRST-funded ‘Retrofit solutions’ project. Part of the program output included 8 doctoral dissertations [1-9], and the results were disseminated through the relevant publications in the Journal of the Structural Engineering Society of New Zealand [10-21], and in other structural engineering journals [22-44]. This project concluded in September 2010, aligning with the beginning of the Canterbury earthquake sequence. Following the February 2011 Christchurch earthquake a new phase of URM research activity began, with funding
provided by the New Zealand Natural Hazards Research Platform. The primary aim of Project Masonry was to document the characteristics of and damage to retrofitted and unretrofitted masonry buildings in and around Christchurch. Using the collected data [15, 45-51], a unique database will be created for the URM building stock in Christchurch, including building characteristics, damage levels and damage modes sustained in each major earthquake, any presence of seismic strengthening and the performance of such seismic strengthening techniques. 2.1 CLAY BRICK MASONRY When subjected to the higher forces generated by the earthquake on 22nd February 2011, Christchurch’s unreinforced masonry building stock sustained much greater and more widespread damage than in the 4 th September 2010 earthquake. The damage modes observed in September 2010 were again observed after February 2011, together with additional modes. Cavity construction was encountered in almost half of the URM buildings surveyed in Christchurch, with the remainder having solid interconnected multi-leaf walls. Chimney, parapet and gable failures were again observed, along with out-of-plane failures. Primary types of out-of-plane wall failures that were observed were: Cantilever type out-of-plane failure with the entire top section of a wall or building façade collapsing; One-way bending, which tended to occur in longer walls or walls without side supports; Two-way bending, which required support of at least one vertical edge of a wall. Damage patterns in URM walls that were widely observed to develop in-plane included: Diagonal shear cracking in piers, spandrels and walls; Shear sliding on mortar bed joints or between storeys; In-plane rocking and toe crushing of piers. For URM buildings damaged in the September 2010 earthquake, temporary shoring was commonly used to prevent further out-of-plane wall damage or collapses. Hence the performance of temporary shoring was assessed following the major aftershocks. Typically where implemented, shoring assisted in preventing collapse of URM buildings in February 2011. Temporary stabilisation of a significant heritage building was reported by Lester et al. [52]. Following the Darfield earthquake there were 389 nonresidential URM buildings in the Christchurch Central Business District (CBD). By the time of the 22 February earthquake 21 of these buildings had been demolished, leaving 368 URM buildings. Civil defence rapid damage assessments for these buildings are shown in Figure 1. It can be seen that in September 2010 only 10% of URM buildings were deemed too unsafe to enter (Red), while following 22 February this value had risen to nearly 80%; only 1% of URM buildings were assessed as safe to be entered after February (Green).
Green 1%
Unknown Red 10% 14%
Green 42%
Yellow 34%
(a) September 2010 (389 buildings)
Unknown 3% Yellow 17%
Red 79%
(b) February 2011 (368 buildings)
Figure 1: Rating of URM buildings in Christchurch CBD following September 2010 and February 2011 earthquakes 2.1.1 Cavity construction Cavity construction refers to a form of wall construction where an air gap is left between leaves or wythes of brick, and during post-earthquake inspections cavity construction was encountered in almost half of the URM buildings surveyed in Christchurch, with the remainder having solid interconnected multi-leaf walls. A single leaf of outer clay brick veneer is the most common type of cavity construction, with the inner section being two or more leaves thick. Double leaf construction on each side of the cavity was also observed. Leaves on either side of a cavity are typically held together by regularly spaced metal cavity ties but in the case of poor connection between the leaves the outer veneer layer can ‘peel’ separately. It was commonly observed that cavity ties in failed cavity walls had deteriorated and were in poor condition due to corrosion. Out-of-plane failure of the veneer was typically attributed to either the deteriorated condition of the metal ties or to pull-out of the ties from the mortar bed joints due to the use during construction of weak lime mortar. 2.1.2 Diaphragm deformations and pounding damage Following the 22nd February 2011 earthquake it became possible to inspect timber diaphragms within buildings from the safety of the street because of the large number of URM walls that had collapsed out-of-plane, and in many cases these diaphragms were observed to be in a deteriorated condition. As many of the URM buildings in Christchurch had timber floor and roof diaphragms, the performance of such buildings in the recent earthquakes presented a unique opportunity to develop an improved understanding of the role of flexible diaphragm on the overall seismic behaviour of URM buildings. Evidence of diaphragm movement was seen in many buildings, and the effect of diaphragm deformations on wall response varied from cracked plaster to complete wall collapse. Examples were observed where excessive lateral movement of the timber roof diaphragm had displaced the external perimeter walls beyond their outof-plane deflection capacity and resulted in wall collapse. Pounding damage was commonly seen in tightly spaced buildings in the CBD, and in many cases pounding appeared to have been the loading condition principally responsible for in-plane wall failures.
SESOC NZ Conference 2-3 November 2012
(a) 1st of October2010
(b) 20th of February 2011
(c) 26th February 2011
Figure 2: Progressive damage to a URM building 2.1.3 Directionality and Shaking Duration In a large number of cases in the Christchurch CBD inplane wall damage was observed on the north and south facing walls, while out-of-plane damage to the eastern and western walls was observed. This damage pattern indicated that in the CBD the direction of shaking on 22nd February 2011 was predominantly east-west. Many walls were observed to be on the verge of collapse following the 22nd February 2011 earthquake. Most noticeable were walls where individual bricks were seen to be on the verge of dislodgement. Had the severe shaking lasted longer, these bricks may have become dislodged, resulting in further wall failures. The damage progression of a URM wall in the September 2010, February 2011 and June 2011 earthquakes is illustrated in . 2.1.4 Ground deformations The 4th September 2010 earthquake resulted in significant ground deformations in many locations in Christchurch, due to liquefaction located primarily in the eastern suburbs and lateral spreading occurring near river banks. The aftershocks on both 22 nd February and 13th June 2011 caused further damage in these areas. Therefore in addition to shaking damage some buildings sustained damage from lateral ground spreading and differential settlement due to liquefaction. 2.2 STONE MASONRY Following the 13th June 2011 earthquakes and successive aftershocks, the condition of damaged heritage stone masonry buildings continued to deteriorate, with more cases of partial or complete collapse. Hence, the importance of earthquake strengthening New Zealand’s heritage masonry architecture to preserve a key element of the nation’s history continues to be emphasised. Damage assessment inspections undertaken in April and May 2011 identified 90 unreinforced stone masonry buildings in Christchurch, a large number of which are included on the Historic Places Trust register of heritage buildings. Unreinforced stone masonry buildings in Christchurch tend to have similar characteristics, in terms of both architectural features and construction details. This observation derives primarily from the fact that most of these structures were built over a comparatively short time period and were designed by the same architects or architectural firms. The vast majority of these structures, and in particular those constructed in the Gothic Revival
style, are characterised by structural peripheral masonry walls that may be connected, depending on the size of the building, to an internal frame structure constituted of cast iron or steel columns and timber beams, or to internal masonry walls that support flexible timber floor diaphragms and timber roof trusses. However, there are a few commercial buildings in the Christchurch CBD that are characterised by slender stone masonry piers in the front façade, with the other perimeter walls constructed of multiple leaves of clay brick. These buildings are typically two or three stories in height, with two storey buildings being most common, and may be either standalone or row type buildings (see [22] for further details of URM building typology). The wall sections can be of different types:
Three leaf masonry walls, with dressed or undressed basalt or lava flow stone units on the outer leaves while the internal core consists of rubble masonry fill;
Three leaf masonry walls, with the outer layers in Oamaru sandstone and a poured concrete core, such as that in the Catholic Cathedral of the Blessed Sacrament;
Two leaf walls, with the front façade layer constructed in dressed stone, typically being either dressed basalt or bluestone blocks, or undressed lava flow units, and the back leaf being one or two layers of clay bricks, usually with a common bond pattern, with the possible presence of a cavity or poured concrete between the inner and outer leaves. The seismic performance of stone masonry buildings was initially identified by considering the safety assessment data collected during the building safety evaluation process, shown in Figure 3. The damage assessments performed following the procedures suggested by the Italian Civil Protection Department permitted identification of the most recurrent types of failure mechanism exhibited by the stone masonry buildings in Christchurch and enabled an estimation of the different damage levels reached, evaluated following the classification of the EMS-98 proposal. Because each stone masonry building in Christchurch was assessed following the same procedure, it is possible to correlate the rate of occurrence of a certain activated mechanism to the average damage level experienced and hence to
determine the vulnerability of the macroelement affected by the mechanism analysed, with the Fitness For Use (FFU) index derived from the damage assessment forms used by the Italian Civil Protection. Full details are reported by Senaldi et al. [53].
Red 5%
Yellow 9%
Demolished 7% Green 11%
Green 14% Unknown 72%
(a) after September 2010
retrofitting. Figure 4 shows the damage levels of buildings for different levels of retrofit and shows that only buildings strengthened to 67% of national building standard (NBS) or greater showed significantly lower levels of damage that unretrofitted buildings.
Red 56%
Yellow 26%
(b) after February 2011 Figure 4: Effects of retrofit level on damage level
Figure 3: Distribution of safety evaluation placards applied to stone masonry buildings 2.3 EARTHQUAKE STRENGTHENING Generally retrofit and temporary shoring techniques prevented entire building collapse. Common types of seismic retrofits observed in Christchurch URM buildings were: Steel moment frames, which increased the lateral capacity of a building; Steel strong-backs, which helped prevent out-ofplane failure of URM walls; Application of shotcrete, which increased the inplane and out-of-plane wall strength. In general terms retrofitted clay brick masonry buildings that performed well were those that had well-conceived designs which aimed to reduce torsional effects and tied the various structural masonry building elements together. The addition of concrete and shotcreted walls, steel strong backs and steel moment frames all provided significantly improved building response in the earthquakes when compared to the response of similar buildings that had not been retrofitted. Field observations revealed numerous cases where anchor connections joining masonry walls or parapets with roof or floor diaphragms appeared to have failed prematurely, particularly for the case of adhesive anchors. In many cases these failures were attributed to the low shear strength of masonry, wide anchorage spacing, insufficient embedment depth of anchors, and/or poor workmanship. It was concluded that the successful performance of anchors does not necessarily prevent out-of-plane wall failure, as the potential for one or two way out-of-plane wall bending failure is not necessarily precluded. Buildings that were well maintained over their life generally performed better than their less well maintained equivalent as a weather-tight building envelope reduces the rate of progressive deterioration due to water ingress to masonry and to timber diaphragms. Of the 368 URM buildings that remained in the Christchurch CBD in February 2011, 62% were confirmed to have received some form of seismic
62% of all URM buildings in the Christchurch CBD had received some form of earthquake strengthening and 97% of URM buildings that had received no prior earthquake strengthening were either seriously damaged (i.e. suffered heavy or major damage) or collapsed. With respect to URM buildings that had received some form of earthquake strengthening: Of those URM buildings that had been earthquake strengthened to less than 33%NBS, 60% were seriously damaged (i.e., suffered heavy or major damage) or collapsed; Of those URM buildings that had been earthquake strengthened to 34-67%NBS, 72% were seriously damaged or collapsed; Of those URM buildings that had been earthquake strengthened to 67-100%NBS, only 24% were seriously damaged or collapsed; Of those URM buildings that had been earthquake strengthened to 100%NBS or greater, none were seriously damaged or collapsed. With respect to earthquake strengthening methods: 44% of restrained parapets failed, compared with failure of 84% of unrestrained parapets. Whilst parapet restraint generally improved earthquake performance, it is clear that many parapet restraints failed to perform as intended. Clearly, parapet retrofits provided some earthquake resistance but probably less than would be expected. This somewhat surprising result was partly attributable to the observed poor performance of adhesive anchorage systems and may have also been due to parapets being secured to roof systems having diaphragms that were too flexible. 57% of restrained gable end walls failed, compared with 88% of unrestrained gable end walls. Similar to parapets, whilst gable restraint also generally improved earthquake performance, it is clear that many gable restraints also failed to perform as intended. There is clear evidence that installed earthquake strengthening techniques reduced damage levels, that retrofits considering the entire building structure were significantly more effective at
reducing overall structural damage, and that shotcrete strengthened wall retrofits and added cross wall retrofits appeared to be more effective than steel strongback retrofits, again probably due to better material deformation compatibility with masonry. Full details can be found in [51, 54]. 2.3.1 Timber diaphragm to wall connection testing The connections between flexible timber diaphragms and URM buildings are critical building components that must perform adequately before the desirable seismic response of URM buildings may be achieved. Field observations made during the initial reconnaissance and the subsequent damage surveys of clay brick URM buildings in Christchurch following the 2010/2011 Canterbury earthquakes revealed numerous cases where anchor connections joining masonry walls or parapets with roof or floor diaphragms appeared to have failed prematurely. These observations were more frequent for the case of adhesive anchors than for the case of through-bolt connections (i.e. anchorages having plates on the exterior façade of the masonry walls). An in-field test program was undertaken in an attempt to evaluate the performance of adhesive anchor connections between roof or floor diaphragm and unreinforced clay brick masonry walls. The test program consisted of over 300 test specimens in eight existing URM buildings located in Christchurch, Whanganui and Auckland and was conducted in order to obtain accurate data on the pull-out strength of adhesive type anchors in existing clay brick masonry walls. Specific objectives of the field test program included the identification of failure modes of adhesive anchors in existing masonry and determination of the influence of the following variables on anchor load-displacement response: type of adhesive, the strength of the masonry materials (brick and mortar), anchor embedment depth, anchor diameter, and use of metal foil sleeve. In addition, the comparative performance of bent anchors (installed at an angle of minimum 22.5o to the perpendicular projection from the wall surface) and anchors positioned horizontally was investigated, as well as the performance of through-bolt anchors with end plate connections.
3 REINFORCED CONCRETE MASONRY CONSTRUCTION Following the 22nd February 2011 earthquake, many cases of existing midrise buildings of RCM construction achieved life safety performance for a level of shaking beyond that specified in the current New Zealand Loadings Standard (NZS-1170.5:2004). In the Christchurch CBD 342 RCM buildings (including RCM buildings having veneer construction) were inspected and evaluated. The majority of these buildings suffered little or no damage, and in the remainder shear failure was the predominant damage mode observed. Cases of severe structural damage to RCM buildings were found in the vicinity of the CBD. Structural damage to these buildings has been documented and is currently being studied to establish the lessons which can be learned from this earthquake and how to incorporate these lessons into future RCM design and construction.
The majority (83%) of RCM buildings had little or no damage following the earthquake, as shown in Figure 5. Nevertheless, exceptions to this good behaviour were observed. The concrete masonry buildings damaged in the earthquake exhibited diagonal in-plane shear cracking as the primary failure mode. Diagonal in-plane shear cracking included both step pattern cracking along the head and bed mortar joints, and diagonal cracking through masonry blocks. Vertical cracking of the block was also commonly observed, followed less frequently by horizontal cracking along bed joints and spalling of the block, typically due to face shell blowouts. Spalling 5%
Severe 5% Moderate 12%
Horizontal 6% None 48%
Low 35%
Other 5%
Vertical 20%
Diagonal shear 64%
(a) Damage Level of (b) Failure Types of RCM RCM Figure 5: RCM inspected buildings Severe diagonal cracking of RCM shear walls in a multistorey commercial building in the central CBD was observed. This building had a glass store front with RCM shear walls in the N-S and E-W directions, as well as an RCM elevator shaft. The reinforcement had insufficient cover in places, causing face shell spalling of some concrete masonry units. Similar diagonal shear cracking was observed in a fivestorey apartment complex having RCM cavity wall construction. The diagonal crack pattern of the 100 mm external leaf was mirrored, although less extensively, on the RCM internal leaf. This form of cavity construction was somewhat unconventional as the external leaf was also reinforced, although provided insufficient cover to the reinforcement due to the narrow width of the concrete masonry units (CMUs) used in the exterior leaf. Because of the small void size in the exterior CMUs, in several locations grout was observed to be discontinuous or honeycombed, providing inadequate bond to the reinforcement. The damage to this building was so severe that the building was scheduled to be demolished. Similarly, of the severely damaged RCM buildings where the grout and reinforcement were visible, approximately fifty per cent had improper fill or incorrect reinforcement placement. During the Christchurch earthquake of February 2011, several existing midrise buildings of RCM construction achieved life safety to near collapse performance levels for seismic demands beyond that specified in the current New Zealand Loadings Standard. Structural damage to these buildings was documented and is currently being studied to establish lessons to be learned from this earthquake and how to incorporate these lessons into future RCM design and construction practices. [55] presents the case of a six story RCM building deemed to have reached the near collapse performance level. Results of nonlinear dynamic analysis by 2D and 3D
models are used to assess the governing failure mechanism based on similarity between the predicted and observed levels of damage with design implications being discussed.
Beginning in May 2011, earthquake damage of 112 churches in the Canterbury region was inspected and assessed. The assessed churches were classified into three main categories according to the original construction material: stone (28%), brick (19%), wood (42%) and other (11%). Figure 6(a) shows that the assessed churches can be classified into three main categories according to the original construction material: stone (28%), brick (19%), wood (42%), and other (11%). Given the potentially different seismic behaviour of the three construction types, a general analysis of the placard classification (shown in Figure 6(b)) as undertaken by the NZ Fire Service, Urban Search and Rescue and volunteer engineers is potentially misleading. Other 4% Red 22% Wood 42%
Stone 28% Clay brick 19%
Green 16%
Red 38%
Green 57%
Yellow 21%
(a) Church construction (b) Overall church placard material breakdown classification Figure 6: Construction type and damage for inspected churches Figure 7(b) shows the distribution of the placard classifications for the stone masonry churches, with over half (52%) assigned a red placard. Also, the percentage of green placards received for the stone masonry churches was the smallest of the three church classifications. Clay brick churches, as shown in Figure 7(a), had better seismic performance than did stone masonry churches, with red placards assigned to 38% of the churches and yellow placards assigned to 43% of the total. The percentage of red placards for this construction type is smaller than the percentage for the stone masonry churches. However, the sum of red and yellow placards is similar for the two construction types and was over 80%. Timber churches had the best overall performance, with none of the assessed churches showing any type of structural damage and, as can be seen in Figure 7(c), 94% were assigned green placards. The only damage recorded to timber churches was to non-structural elements. Internal plaster damage is an example of damage that might limit the use of a timber church and result in a yellow placard. All red placards assigned (2%) were due to external causes (i.e. risk from a neighbouring building), where the churches were structurally undamaged.
Red 52%
Yellow 32%
Yellow 43%
4 CHURCHES
RC 7%
Green 19%
(a) Clay brick masonry churches Red 2%
(b) Stone masonry churches Yellow 4%
Green 94%
(c) Timber framed churches Figure 7: Distribution of the placard classification for each construction type The inspected churches, irrespective of the construction material, followed a similar architectural style and consequently presented similar possible collapse mechanisms. Certain elements such as domes, vaults and chapels were infrequently present, just as bell towers and presbyteries were only found in a limited number of churches. Normally these elements present a higher seismic vulnerability. Given this finding, the dominant activated and most vulnerable collapse mechanisms for the stone and clay brick masonry churches were shear cracks along the longitudinal walls, and the overturning of façades and apses. Full details are reported by Leite et al. [56]
5 RESIDENTIAL MASONRY VENEER CONSTRUCTION In total just under 1100 residential dwellings were inspected throughout the wider Christchurch area, of which 24% were constructed using the older nail-on veneer tie system (before 1996) and 76% were constructed using screw fixed ties to comply with the new 1996 standards revision, and where 30% of all inspected houses were of two storey construction. Of the inspected dwellings 27% had some evidence of liquefaction, ground settlement or lateral spreading. In areas where some form of liquefaction or lateral spreading had occurred, the cause of damage for 40% of the dwellings was attributed to ground movement only and 28% of dwellings had damage that was attributed to shaking damage only. Following the 2010/2011 Canterbury earthquakes a comprehensive literature review and detailed door to door assessments of residential masonry veneer dwellings were conducted in a variety of areas of Christchurch. Specifically, care was taken to include survey locations that had experienced different levels of earthquake shaking, in order to allow comparison between different system performances and different
shaking intensities. Following the 4th September 2010 Darfield earthquake little shaking damage was observed to residential masonry veneers and observed damage was instead due to foundation settlement, soil liquefaction and lateral spreading. However, it was noted that newer, lighter veneer systems appeared to perform better than older, heavier systems. 5.1 INSPECTION SURVEY In total just under 1100 residential dwellings were inspected throughout the wider Christchurch area of which 24% were constructed using the older nail-on veneer tie system (before 1996) and 76% were constructed using screw fixed ties to comply with the new 1996 standards revision (post-1996). 30% of all inspected houses were of two storey construction. Of the inspected dwellings 27% had some evidence of liquefaction, ground settlement or lateral spreading. In areas where some form of liquefaction or lateral spreading had occurred, the cause of damage for 40% of the dwellings was attributed to ground movement only and 28% of dwellings had damage that was attributed to shaking damage only. Severe and extreme damage to veneer dwellings was concentrated in the Port Hills and foothills suburbs (13% of inspected dwellings) due to the proximity to the epicentre as well as typographical amplification of ground motions. It was also evident that the majority of cases of severe and moderate damage were concentrated close to the river banks (typically the residential ‘Red Zone’), mainly due to substantial liquefaction and lateral spreading. As expected, the level of damage increased with an increasing level of ground acceleration. It was evident that severe and extreme damage only occurred to veneers in areas of severe peak ground acceleration (PGA) (0.62 – 1.3 g) or extreme (>1.3 g) shaking, see Figure 8. Of all inspected damaged dwellings, 60% sustained in-plane damage only, with dwellings constructed prior to 1996 being more likely to sustain out-of-plane damage in comparison to dwellings constructed after 1996. Of all the inspected dwellings which sustained some damage, 33% of these dwellings had problems with corner separation. From the survey it is evident that generally houses constructed since the 1990s tended to suffer lower levels of damage than those built earlier. It is evident that overall screw-fixed ties performed better, with the majority of the dwellings where this type of tie was used showing no visible or minor damage only.
It is apparent that damaged dwellings with nail-on ties featured more predominantly in the moderate to extreme damage categories, and it appears that wire ties performed the worst as a higher proportion of inspected dwellings that had wire type veneer ties sustained severe to extreme damage. From the survey, it is evident that the use of Oamaru stone veneer (and solid veneer units in general) performed the worst. Full details on the performance of residential masonry veneer construction can be found in [57].
6 CONCLUSIONS A brief summary of activities undertaken as part of Project Masonry was presented, detailing the observations that were made on the performance and the deficiencies that contributed to the damage to unreinforced clay brick masonry buildings, to unreinforced stone masonry buildings, to reinforced concrete masonry buildings, to churches in the Canterbury region, and to residential dwellings having external masonry veneer cladding. It was concluded that when subjected to the higher forces generated by the earthquake on 22 nd February 2011, Christchurch’s unreinforced masonry building stock sustained much greater and more widespread damage than in the 4th September 2010 earthquake. The damage modes observed in September 2010 were again observed after February 2011, together with additional modes. Chimney, parapet and gable failures were again observed, along with out-of-plane failures. Ground deformations were also observed to contribute to building damage. Generally retrofit and temporary shoring techniques prevented entire building collapse. Cavity construction was encountered in almost half of the URM buildings surveyed in Christchurch, with the remainder having solid interconnected multi-leaf walls. It was concluded that retrofits that generally performed well were: Well-conceived designs which aimed to reduce torsional effects and tied the masonry together; Well-connected steel strong backs and steel moment frames.
Damage to veneer (%)
Field observations revealed numerous cases where anchor connections joining masonry walls or parapets with roof or floor diaphragms appeared to have failed prematurely, particularly for the case of adhesive anchors. In many cases these failures were attributed to 100 the low shear strength of masonry, wide anchorage - 100 dwellings spacing, insufficient embedment depth of anchors, 80 and/or poor workmanship. 60 It was concluded that the successful performance of 40 anchors does not necessarily prevent out-of-plane wall 20 failure, as the potential for one or two way out-of-plane 0 wall bending failure is not necessarily precluded. A total of over 300 adhesive anchors were installed and -20 0.126 – 0.28g 0.28 – 0.62g 0.62 – 1.3g > 1.3g tested to identify the failure modes in existing masonry -40 PGA and to determine the influence on anchor loaddisplacement response the following variables: type Figure 8: % of veneer damage level vs PGA. Each circle (b) % of veneer damage level vs PGA. Each circle size corresponds to the for number of of adhesive, the strength of the masonry materials (brick size corresponds dwellings to the number of dwellings
and mortar), anchor embedment depth, anchor diameter, and use of metal foil sleeve. Damage assessment of unreinforced stone masonry buildings in Christchurch was conducted in April and May 2011 and consequently the presented description of their seismic response is based on observations made at that time. Following the 13th June 2011 earthquakes and successive aftershocks, the conditions of damaged heritage stone masonry buildings continued to deteriorate, with more cases of partial or complete collapse. Hence, the importance of earthquake strengthening New Zealand’s heritage masonry architecture to preserve a key element of the nation’s history continues to be emphasised. Following the 22nd February 2011 earthquake, many cases of existing midrise buildings of RCM construction achieved life safety performance for a level of shaking beyond that specified in the current New Zealand Loadings Standard. In the Christchurch CBD 342 RCM buildings (including RCM buildings having veneer construction) were inspected and evaluated. The majority of these buildings suffered little or no damage, and in the remainder shear failure was the predominant damage mode observed. Cases of severe structural damage to RCM buildings were found in the vicinity of the CBD. Structural damage to these buildings has been documented and is currently being studied to establish the lessons which can be learned from this earthquake and how to incorporate these lessons into future RCM design and construction. Beginning in May 2011, earthquake damage of 112 churches in the Canterbury region was inspected and assessed. The assessed churches were classified into three main categories according to the original construction material: stone (28%), brick (19%), wood (42%) and other (11%). Given the potentially different seismic behaviour of the three construction types, a general analysis of the placard classification undertaken by the NZ Fire Service, Urban Search and Rescue and volunteer engineers was presented. In total just under 1100 residential dwellings were inspected throughout the wider Christchurch area, of which 24% were constructed using the older nail-on veneer tie system (before 1996) and 76% were constructed using screw fixed ties to comply with the new 1996 standards revision (post-1996), and where 30% of all inspected houses were of two storey construction. Of the inspected dwellings 27% had some evidence of liquefaction, ground settlement or lateral spreading. In areas where some form of liquefaction or lateral spreading had occurred, the cause of damage for 40% of the dwellings was attributed to ground movement only and 28% of dwellings had damage that was attributed to shaking damage only. Whilst it may be too late to save many of Christchurch’s historic clay brick and stone URM buildings, lessons learnt from damage observations made during and after the Canterbury earthquake swarm of 2010/2011 and future detailed analysis of the collected data can be applied to masonry buildings throughout the rest of New Zealand, Australia, and around the world.
7 CURRENT RESEARCH ACTIVITY The research team at the University of Auckland is continuing study on unreinforced masonry as part of the GNS Science - funded ‘Retrofit solutions for heritage URM buildings’ project. In response to the recommendations of the Canterbury Earthquakes Royal Commission, the objective of the project is to make available experimentally validated and industry-accepted best practice structural details for the securing of URM parapets and solutions for the securing of flexible timber diaphragms to URM walls. Solutions are to be Other objectives of the GNS Science - funded project are to characterise the stone masonry building stock of New Zealand, detailing location and number of buildings. Also, to undertake core sampling of selected stone masonry buildings to determine characteristic wall crosssection details and material properties and identify proven techniques to address the failure modes observed in New Zealand stone masonry buildings. It is envisioned that in the near future the research team will be returning to Christchurch in order to acquire council property records and data held with practicing engineering consultancies to develop a full comprehensive database on the performance of URM building in the Canterbury 2010/2011 earthquakes and fill in any knowledge gaps that currently exist on the remaining buildings in and areas outside of the CBD zone. In-situ testing of existing timber diaphragms found in vintage URM buildings is also planned to be undertaken.
ACKNOWLEDGEMENT The authors wish to thank the numerous professional structural engineers and building owners who have provided valuable data, opinions, expertise and their experiences. The authors acknowledge the financial support for Project Masonry from the New Zealand Natural Hazards Research Platform.
REFERENCES [1]. Dizhur, D, Performance of masonry buildings in the Canterbury earthquakes and corresponding strengthening solution using NSM CFRP, in Department of Civil and Environmental Engineering. 2012, The University of Auckland: Auckland. [2]. Knox, C, Assessment of perforated unreinforced masonry walls responding in-plane, in Department of Civil and Environmental Engineering. 2012, The University of Auckland: Auckland. p. https://researchspace.auckland.ac.nz/handle/2292/1 9422.
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