Performance of unreinforced masonry buildings during the 2010 Darfi eld (Christchurch, NZ) earthquake
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1 Performance of unreinforced masonry buildings during the 2010 Darfield (Christchurch, NZ) earthquake* J Ingham Department of Civil and Environmental Engineering, University of Auckland, New Zealand M Griffith† School of Civil, Environmental and Mining Engineering, University of Adelaide, South Australia, Australia ABSTRACT: The 2010 Darfield earthquake caused extensive damage to a number of unreinforced masonry buildings. While this damage to important heritage buildings was the largest natural disaster to occur in New Zealand since the 1931 Hawke’s Bay earthquake, the damage was consistent with projections for the scale of this earthquake, and indeed even greater damage might have been expected. In general, the nature of damage was consistent with observations previously made on the seismic performance of unreinforced masonry buildings in large earthquakes, with aspects such as toppled chimneys and parapets, failure of gables and poorly secured face-loaded walls, and in-plane damage to masonry frames all being extensively documented. This report on the performance of the unreinforced masonry buildings in the 2010 Darfield earthquake provides details on typical building characteristics, a review of damage statistics obtained by interrogating the building assessment database that was compiled in association with post-earthquake building inspections, and a review of the characteristic failure modes that were observed. It was observed that structures that had been seismically retrofitted appeared to perform well, with further study now required to better document the successful performance of these retrofit solutions. 1 INTRODUCTION have been expected, with associated fatalities and major casualties. Instead, the single most striking At 4.35 am on the morning of Saturday 4 September statistic was that there were no fatalities directly 2010, a magnitude 7.1 earthquake occurred associated with the earthquake (although there was approximately 40 km west of the city of Christchurch, one heart attack fatality and one person hospitalised New Zealand, at a depth of about 10 km (GNS, 2010), due to a falling chimney (NZ Herald, 2010)); and having an epicentre located near the town of Darfield. the overall impression is that damage in the central The ground motion had a peak ground acceleration business district (CBD) was reasonably contained, (PGA) of about 0.25g and a spectral acceleration in restricted primarily to unreinforced masonry (URM) the plateau region of about 0.75g, which corresponds buildings, and damage to windows in taller steel well with the design spectra for a site class D soil and concrete structures. The absence of fatalities site in Christchurch for spectral periods greater than and more extensive damage is attributed to the 0.2 s. In general, the earthquake represented 67% to comparatively high level of seismic design capability 100% of the design level event, depending upon the in New Zealand, and the fact that the CBD, which spectral period being considered (see figure 1), with is the region containing the highest density of URM most of Canterbury reporting damage consistent buildings, was almost completely unoccupied with MM8 on the Modified Mercalli intensity scale. at 4.35 am. Despite the number of buildings that Hence, significant damage to earthquake prone had received some form of seismic improvement, buildings and to non-structural components might a question currently remains as to why a greater number of URM buildings did not suffer significant * Reviewed and revised version of paper originally presented damage in the earthquake. at the 2009 Australian Earthquake Engineering Society Conference, 11-13 December 2009, Newcastle, NSW. The soil conditions in Christchurch have three † Corresponding author A/Prof Michael Griffith can be separate material types: river outwash gravels, contacted at mcgrif@civeng.adelaide.edu.au. sands, and marshy ground in former swamp © Institution of Engineers Australia, 2011 Australian Journal of Structural Engineering, Vol 11 No 3
2 “Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith 1.0 N26W component – Cathedral College site NZSEE Class D Deep or Soft Soil 0.30 Acceleration(g) Median of Soft Soil Sites N26W 5% damped Response Spectrum Botanical Gardens CBGS-H1 0.15 0.8 Botanical Gardens CBGS-H2 Cathedral Collage CCCC-H1 0 Cathedral Collage CCCC-H2 -0.15 Acceleration (g) Hospital CHHC-H1 0.6 Hospital CHHC-H2 -0.30 0 10 20 30 40 50 60 Time(s) 0.4 N64E component – Cathedral College site 0.30 Acceleration(g) N64E 0.15 0.2 0 -0.15 0 -0.30 0 1 2 3 4 5 0 10 20 30 40 50 60 (a) Period T(s) (b) Time(s) Figure 1: Details of earthquake ground motion – (a) acceleration spectra, and (b) acceleration time-history record (source: GeoNet). areas, with the central city built mainly on gravels, buildings most widely affected (Page, 1991) and although there are pockets of sand and soft soil in to date is the only earthquake to have caused any former marsh deposits. The earthquake ground fatalities in Australia (see figures 2(c) and 2(d)). motion characteristics shown in figure 1 reflect the • The M6.8 2007 Gisborne (New Zealand) underlying soil condition, with the long period earthquake caused damage to numerous URM nature of the motion resulting from the soft soils buildings, including the collapse of 22 parapets upon which Christchurch is founded. These soft (Davey & Blaikie, 2010). Examples of damage to soils effectively act as a filter and remove high URM buildings are shown in figures 2(e) and 2(f). frequency ground motion (leading to smaller • The M8.8 2010 Maule (Chile) earthquake caused PGA values than on rock sites), but amplify long extensive damage to older houses, churches and period motion, resulting in significantly larger other buildings constructed of URM or adobe, as longer period motion at about 2.5 s on several of shown in figures 2(g) and 2(h) (EERI, 2010). the softer soil sites. As most URM buildings have a fundamental period of 0.2-0.3 s, the underlying • The M5.0 2010 Kalgoorlie-Boulder (Australia) ground conditions appear to have assisted in earthquake occurred near Kalgoorlie-Boulder, reducing the seismic demand in this period range causing damage to historic buildings in the city to approximately 70% of the current NZS 1170.5 (see figures 2(i) and 2(j)). There were no fatalities, (Standards New Zealand, 2004) design level loading but two people were treated at Kalgoorlie for a site class D soil site in Christchurch. Hospital for minor injuries resulting from the earthquake (Edwards et al, 2010). As can be seen in figure 1(b), strong ground shaking in the CBD had a duration of approximately 30 s, with An initial evaluation procedure (IEP) is provided similar amplitudes in the two orthogonal recording in NZSEE (2006) as a coarse screening method for directions. This lack of distinct directionality determining a building’s expected performance in probably explains why parapet failures were an earthquake. The purpose of the IEP is to make an observed in streets running in both the north-south initial assessment of the performance of an existing and east-west directions. building against the standard required for a new building, ie. to determine the “Percentage New Building Standard” (%NBS). A %NBS of 33 or less 2 BACKGROUND means that the building is assessed as potentially earthquake prone in terms of the Building Act (New It is well established that URM buildings perform Zealand Parliament, 2004) and a more detailed poorly in large magnitude earthquakes, with a brief evaluation will then typically be required. A %NBS selection of relevant prior earthquakes that have of greater than 33 means that the building is regarded caused major damage to URM buildings being as outside the requirements of the Act, and no further detailed below: action will be required by law, although it may still • The M7.8 1931 Hawke’s Bay (New Zealand) be considered as representing an unacceptable risk earthquake caused widespread devastation to and seismic improvement may still be recommended URM buildings in the city of Napier (see figures (defined by the New Zealand Society for Earthquake 2(a) and 2(b)), with 256 fatalities. This earthquake Engineering (NZSEE) as potentially “earthquake remains the worst disaster of any type to occur risk”). A %NBS of 67 or greater means that the on New Zealand soil (Dowrick, 1998). building is not considered to be a significant earthquake risk. NZSEE noted that: • The M5.6 1989 Newcastle (Australia) earthquake resulted in 13 fatalities and over 160 casualties. A %NBS of 33 or less should only be taken as The earthquake damaged approximately 50,000 an indication that the building is potentially buildings (80% of these were homes) with URM earthquake prone and a detailed assessment may Australian Journal of Structural Engineering Vol 11 No 3
“Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith 3 (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) Figure 2: Representative examples of damage to URM buildings in past earthquakes – (a) overlooking Napier at the buildings destroyed by the 1931 earthquake and fires (source: Alexander Turnbull Library); (b) view down Hastings Street, Napier, after the 1931 earthquake (source: Alexander Turnbull Library); (c) out-of-plane wall failure in the 1989 Newcastle earthquake; (d) parapet and cavity wall damage in the 1989 Newcastle earthquake; (e) toppled parapet in the 2007 Gisborne earthquake; (f) out-of-plane failure of a gable wall in the 2007 Gisborne earthquake; (g) 1940s URM house, Maipu Street, Concepcion, Chile (source: Andrea Garcia); (h) Barros Arana Institute, Barros Arana corner with Angol, Concepcion, Chile (source: Andrea Garcia); (i) damage to parapet and awning in Boulder; and (j) toppled parapet in Boulder. Australian Journal of Structural Engineering Vol 11 No 3
4 “Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith well show that a higher level of performance is and Australia are evident by comparing the images achievable. The slight skewing of the IEP towards shown in figure 2. A common architectural feature is conservatism should give confidence that a building for passing pedestrians to be covered by an overhead assessed as having a %NBS greater than 33 by awning that is located at the height of the first floor, the IEP is unlikely to be shown, by later detailed which is typically braced back to the piers of the assessment, to be earthquake prone. second floor. Figure 3 shows this detail. Recent research has suggested that there are Post-earthquake inspection of building performance approximately 3750 URM buildings in New Zealand led to 595 URM buildings being assessed, out of the (Russell & Ingham, 2010), with their distribution 958 URM buildings reported to exist in Christchurch. throughout New Zealand being aligned with the It is believed that the majority of unassessed URM relative prosperity of communities during the buildings were undamaged and outside the primary period between approximately 1880 and 1930. inspection zone associated with the CBD and arterial Following the Darfield earthquake, it was reported routes extending from the central city. General that the Canterbury region had approximately features of the 595 assessed URM buildings are 7600 earthquake-prone buildings, with 958 of these reported in figure 4, indicating that the majority of buildings being constructed of URM (Christchurch buildings were either 1- or 2-storey, consistent with City Council, 2010; Wells, 2010). prior findings by Russell & Ingham (2010). Figure 4(c) shows that the most common occupancy type 3 ARCHITECTURAL CHARACTER was commercial or office buildings, and hence the majority of buildings were unoccupied at the time The architectural features of the URM building of the earthquake, significantly contributing to the stock in the Canterbury region of New Zealand are lack of direct earthquake fatalities. The survey forms consistent with the general form of URM buildings contained a field to record the estimated gross floor throughout both New Zealand and Australia area of the building, and hence the estimated building (Russell & Ingham, 2008; 2010). These buildings footprint could be determined once accounting for can be characterised as typically 2 or 3 storeys in the number of storeys (see figure 4(b)), but the data height, with 2-storey buildings being most common, is unfortunately incomplete as only 301 entries were and being either stand-alone or row buildings, recorded for the 595 separate buildings assessed. It is but with row buildings being most common (refer not possible to establish from the database whether figure 3). In particular, similarities in architectural individual entries belonged to a stand-alone or a character between URM buildings in New Zealand row building. (a) (b) (c) (d) Figure 3: Pre-earthquake examples of typical URM building architecture in Christchurch – (a) 188 Manchester Street; (b) corner of Sandyford and Colombo Street; (c) 118 Manchester Street; and (d) corner of Colombo and Tuam Street. Australian Journal of Structural Engineering Vol 11 No 3
“Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith 5 501ч ч50 13% 11% Other Public Religious 3+Storey residential 17% assembly 2% 7% 5% 1Storey 29% Other Industrial 3% 2% 51Ͳ100 Heritage 201Ͳ500 24% Listed 25% 3% Dwelling 11% Commercial 2Storey /Offices 101Ͳ200 (a) 54% (b) 27% (c) 67% Figure 4: Building characteristics derived from interrogation of the inspection database – (a) storey height (595 entries); (b) footprint area (m2; 301 entries); and (c) occupancy type (595 entries). (a) (b) Figure 5: Masonry rubble from collapsed wall – (a) masonry rubble showing “clean” bricks; and (b) weak mortar crumbles between fingers. (a) (b) Figure 6: Large sections of masonry intact after fall from buildings – (a) solid section of masonry gable; and (b) solid section of parapet. 4 MATERIAL PROPERTIES centre of the wall facing the street, as this segment of the collapsed parapet often remained intact as it The general observation from the debris of collapsed collapsed (refer figure 6). URM walls was that the kiln-fired clay bricks were generally of sound condition, but that the mortar was in poor condition. In most cases the fallen debris had 5 BUILDING DAMAGE STATISTICS collapsed into individual bricks, rather than as larger chunks of masonry debris (refer to figure 5(a)). When In general, the observed damage to URM buildings rubbing the mortar that was adhered to bricks it was in the 2010 Darfield Earthquake was consistent with routinely found that the mortar readily crumbled the expected seismic performance of this building when subjected to finger pressure (refer figure 5(b)), form, and consistent with observed damage to URM suggesting that the mortar compression strength was buildings both in past New Zealand and Australian very low. However, it appears that superior mortar earthquakes and in numerous earthquakes from other was often used in the ornate parapet above the countries (see figure 2). As part of the emergency Australian Journal of Structural Engineering Vol 11 No 3
6 “Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith response to this earthquake, the authors spent 72 than 10% damage. It was also possible to study the hours assisting Christchurch City Council with distribution of damage dependent on storey height building damage assessments, tagging buildings (figure 8(c)), with the data indicating no definitive with either a green, yellow or red placard depending, trend and a comparatively uniform level of damage respectively, upon whether a building was safe for assigned to buildings in each height category. public use, had limited accessibility for tenants/ occupants, or was not accessible (refer to figure 6 CHIMNEYS 7). Many examples of earthquake damage were observed during this exercise, as well as many Unsupported or unreinforced brick chimneys examples of seismic retrofits to URM buildings that performed poorly in the earthquake (figure 9), had performed well. with numerous chimney collapses occurring in The results from the interrogation of damage domestic as well as small commercial buildings and assessment information are reported in figure 8. some churches. Many examples of badly damaged Figure 8(a) reports the “useability” assignment of the chimneys that were precariously balanced on rooftops 595 URM buildings assessed. In consultation with were also seen (figure 9(b)) and it was reported that staff of Christchurch City Council it is believed that one week after the earthquake, 14,000 insurance the complement of the 958 URM buildings thought claims involving chimney damage had been received, to exist in Christchurch probably had a green tag from a total of 50,000 claims (NewstalkZB, 2010). usability rating, and so this theorised damage Emergency services personnel were in significant distribution for the entire URM building stock is demand, being deployed to remove damaged shown in figure 8(b). chimneys in order to minimise further risk and eliminate these “falling hazards” (figure 9(c)). In Figure 8(c) reports the level of damage for the contrast, figure 9(d) shows an example of a braced 595 buildings that were surveyed by the Rapid chimney that performed well. Note that figure 9(b) Building Assessment teams. No explicit definitions shows further evidence of the poor performance of were provided to the assessment teams for what mortar during the earthquake. represented and range (%) damage. The values recorded by the teams for each building surveyed were simply estimates (excluding contents damage). 7 GABLE END WALL FAILURES Despite the known vulnerability of URM buildings to earthquake loading, 395 of the 595 buildings (66%) Many gable end failures were observed, often were rated as having 10% damage or less, with only collapsing onto or through the roof of an adjacent 162 (34%) of the buildings assessed as having more building (refer figures 9(a) and 10). However, there (b) (c) (a) (d) Figure 7: Building assessment notices – (a) Urban Search And Rescue personnel applying an assessment notice; (b) green tag; (c) yellow tag; and (d) red tag. Australian Journal of Structural Engineering Vol 11 No 3
“Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith 7 Red Red 13% 21% Yellow Green 17% 47% Green Yellow 70% (a) 32% (b) 30% 180 Unknown 3+StoreysHigh PercentageofBuildingsSurveyed NumberofBuildingsSurveyed 24% 2StoreysHigh 144 1StoreyHigh 18% 108 12% 72 6% 36 0% 0 none 0Ͳ1% 2Ͳ10% 11Ͳ30% 31Ͳ60% 61Ͳ99% notspec (c) ExtentofDamagetoBuilding Figure 8: Damage statistics – (a) distribution of placard assignments (595 entries); (b) theorised damage distribution for entire building stock (958 entries); and (c) extent of building damage. (a) (b) (c) (d) Figure 9: Examples of chimney performance during the Darfield earthquake – (a) two damaged chimneys and gable wall; (b) unstable damaged chimney; (c) emergency service workers remove chimney; and (d) braced chimney performed well. Australian Journal of Structural Engineering Vol 11 No 3
8 “Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith (a) (b) (c) (d) Figure 10: Examples of gable end wall failures – (a) 93 Manchester St; (b) 816 Colombo St; (c) Montreal-Armagh street corner; and (d) Kilmore-Montreal street corner. were also many gable ends that survived; many more at the wall end that is about 50 mm wide × 450 mm than might have been expected, with the majority long, and fastened to the rod and positioned either having some form of visible restraints that tied back inside the brick wall or in the centre of a masonry pier to the roof structure. These examples are shown and or wall. In most cases the force on the rod exceeded discussed later (refer figures 17-18). the capacity of the masonry wall anchorage, causing a punching shear failure in the masonry wall identified by a crater in the masonry (refer figure 12(a)). 8 PARAPET FAILURES Numerous parapet failures were observed along 10 WALL FAILURES both the building frontage and along their side walls, and for several URM buildings located on the Out-of-plane wall failures were the first images to corners of intersections, the parapets collapsed on appear on television directly after the earthquake. both perpendicular walls (refer figure 11). Restraint Inspection of this damage typically indicated poor of URM parapets against lateral loads has routinely or no anchorage of the wall to its supporting timber been implemented since the 1940s, so while it is diaphragm. Several examples of wall failure are difficult to see these restraints unless roof access is shown in figure 13. Figure 13(a) shows a corner available, it is believed that the majority of parapets building that had walls fail in the out-of-plane that exhibited no damage in the earthquake were direction along both directions. Figure 13(b) shows provided with suitable lateral restraint. In several a 3-storey building where walls in the upper two cases, it appears that parapets were braced back to the storeys suffered out-of-plane failures and figure 13(c) perpendicular parapet, which proved unsuccessful. shows similar damage for a 2-storey building. In all three of these instances, it appears that the walls were not carrying significant vertical gravity loads, 9 ANCHORAGE FAILURES other than their self weight, due to the fact that the remaining roof structures appear to be primarily Falling parapets typically landed on awnings, undamaged. In contrast, figure 13(d) shows an out- resulting in an overloading of the braces that of-plane failure of a side wall that was supporting supported these awnings and leading to collapse. the roof trusses prior to failure. Most awning supports in Christchurch involved a tension rod tied back into the building through the As shown in figure 14, several examples of face load front wall of the building. Many of these connections wall failure closely resembled observed damage in appear to consist of a long, roughly 25 mm diameter dry stack masonry experiments (Restrepo-Velez & rod, with a rectangular steel plate (about 5 mm thick) Magenes, 2009), providing further support to the Australian Journal of Structural Engineering Vol 11 No 3
“Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith 9 (a) (b) (c) (d) Figure 11: Examples of typical parapet failures – (a) multiple front wall parapet failures; (b) corner of Sandyford and Colombo Street (see also figure 3(b)); (c) side wall parapet collapse onto roof; and (d) corner Columbo and Tuam Street (see also figure 3(d)). (a) (b) Figure 12: Anchorage failure of awning brace due to parapet collapse – (a) anchorage failure; and (b) close-up of failed anchorage detail. (a) (b) (c) (d) Figure 13: Examples of out-of-plane failures in solid masonry walls – (a) corner Worcester and Manchester streets (see also figure 3(a)); (b) 118 Manchester Street (see also figure 3(c)); (c) 179 Victoria Street; and (d) out-of-plane failure of a side wall that was supporting the roof trusses prior to failure. Australian Journal of Structural Engineering Vol 11 No 3
10 “Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith (a) (b) Figure 14: Failure mechanism comparisons (observed earthquake damage versus experimental simulation) – (a) wall damage at 140 Linchfield Street; and (b) high speed photograph of a dry-stacked masonry wall failing during a tilt test. (a) (b) (c) (d) Figure 15: Examples of out-of-plane failures in cavity walls – (a) cavity wall failure in a residential building; (b) 832 Columbo Street; (c) butterfly wall ties still intact; and (d) metal wall ties badly deformed. supposition that many of the wall failures were partly In some cases wall-diaphragm anchors remained attributable to poor mortar strength. visible in the diaphragm after the wall had failed, again indicating that failure had occurred due to Cavity wall construction is generally believed to be bed joint shear in the masonry (refer figure 16(a)). much less common in New Zealand than is solid Figure 16(b) shows a situation where a diaphragm multi-leaf (or multi-wythe) construction. However, anchor had been embedded within the wall. It can cavity wall construction can be extremely vulnerable be seen that the anchor successfully prevented the to out of plane failure in earthquakes in situations restrained wall from failing, but was not able to where the cavity ties were poorly installed, or more prevent toppling of the parapet that was located commonly have corroded over time, as the wall is above the anchor. then comparatively slender and less stable than for solid construction. Figures 15(a) and 15(b) show examples of cavity wall buildings that suffered out- 11 SUCCESSFUL WALL ANCHORAGE of-plane wall failures. Figures 15(c) and 15(d) show that cavity ties were present but were insufficient to A significant feature of the earthquake was the prevent the outer leaf from failing. number of occasions where anchored walls performed Australian Journal of Structural Engineering Vol 11 No 3
“Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith 11 (a) (b) Figure 16: Wall-to-diaphragm anchor details – (a) gable end wall failure despite anchor (see also figure 13(a)); and (b) wall anchor still intact (see also figure 6(a)). Figure 17: Successful gable end wall and side wall anchorages on an arts centre building. (a) (b) Figure 18: Successful wall-floor and wall-roof diaphragm anchorages – (a) front elevation; and (b) side elevation. well during the earthquake. Photographs showing 13 PARTIAL WALL FAILURES this are presented in figures 17-18. A typical wall-to- diaphragm (roof or floor) anchor typically consists of Another interesting feature of this earthquake is a long 20 mm bolt with a large circular disc of about the observation of walls that only partly failed, 150-200 mm diameter between the wall exterior and allowing for identification of the specific failure nut that clamped the disc to the wall. This detail is mode at its onset. Several excellent examples are shown quite clearly in figure 16(a). described below. The first of these is a 2-storey URM building on Ferry Road (see figure 20), where the 12 IN-PLANE WALL FAILURES front, street facing, wall of the building had started to fail out-of-plane despite the presence of wall-roof Where walls exhibited some damage to in-plane diaphragm anchors. As is shown, the anchors were deformation the cracks were mostly seen to pass on the verge of pulling through the masonry wall. vertically through the lintels over door or window Internal inspection of the building revealed that the openings. Examples are shown in figure 19. However, front wall had separated from the long side walls this type of damage was not widely observed. of the building and moved approximately 50 mm Australian Journal of Structural Engineering Vol 11 No 3
12 “Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith (a) (b) (c) (d) Figure 19: Examples of in-plane wall damage above window openings – (a) extensive vertical cracking above window openings; (b) vertical crack above window opening; (c) vertical crack through spandrel; and (d) diagonal crack extending from window opening. (a) (b) (c) (d) Figure 20: Wall-roof anchorage failure and partial wall failure – (a) building overview; (b) detail of partial anchorage failure; (c) onset of anchorage failure; and (d) internal view showing wall separation. towards the road with respect to the ceiling/roof earthquake not imposing sufficient displacement on diaphragm (figure 20(d)). It is believed that due to the the wall after the anchorage failure. nature of strength degradation of the brickwork at the A similar style of partial failure was observed in onset of a punching shear failure, the anchorage has another building on Ferry Road (figure 21(a)) but effectively failed and offers little residual resistance the authors were only able to observe the building against further shaking. The only reason the wall externally. It should be noted that the buildings were did not completely collapse is probably due to the not in close proximity to each other. An example Australian Journal of Structural Engineering Vol 11 No 3
“Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith 13 (a) (b) Figure 21: Partial bed joint shear failure surrounding anchorage detail – (a) wall-roof anchorage failure; and (b) gable end anchorage failure. (a) (b) Figure 22: Examples of yielded wall anchors – (a) overview of wall anchors; and (b) close-up view of yielded anchor. of a gable end partial failure is shown in figure damage is not necessarily a catastrophic problem if 21(b), which can be compared to the comparable stiff horizontal diaphragms are well connected to anchorage detail shown in figure 16(a) that resulted the walls in both directions, but where there is not in complete failure. good diaphragm connectivity, there is the potential There were frequent examples of wall-diaphragm for complete out-of-plane collapse of one or both anchors that had deformed plastically. In these walls. Figure 24 shows some examples where major photographs (figure 22), the circular plate can be seen cracking was observed between the side return walls to be slack due to plastic stretching of the anchor rod. and the front parapet and wall. 14 DIAPHRAGM DEFORMATIONS 16 POUNDING There was one instance where it was clear that Several instances of damage due to buildings diaphragm deformation, relative to the in-plane pounding against each other during the earthquake walls, contributed to partial failure of an out-of-plane were observed. Figure 25 shows how the shorter wall. Figure 23 shows several views of a building that building in the centre, which has different floor suffered out-of-plane parapet failure along its long, heights than the building to the left, damaged the side walls. In figure 23(b) it can be seen that the roof column of the taller building at its top storey. joists have tilted towards the front of the building. This suggested that the front wall of the building 17 SPECIAL BUILDINGS was driven forward at its top. Careful inspection of the front wall (figure 23(c)) revealed a substantial 160 Manchester Street is a 7-storey office building outwards curvature that was most pronounced at that is reported to consist of load bearing masonry the top. and is the most significant masonry building, at least in terms of height, in Christchurch (figure 26). It is 15 RETURN WALL SEPARATION a registered heritage building and is a significant part of the fabric of the Christchurch city landscape. Many buildings exhibited substantial cracking Unfortunately, the building suffered significant between their front wall and side (return) walls. This damage in the earthquake. The bottom two storeys Australian Journal of Structural Engineering Vol 11 No 3
14 “Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith (a) (b) (c) Figure 23: Example of diaphragm deformation causing out-of-plane wall failure – (a) overview of building; (b) side-view of tilted joists; and (c) front wall curvature. (a) (b) (c) Figure 24: Examples of wall separation at corners of buildings. (a) (b) (c) Figure 25: Example of building pounding damage – (a) building overview; and (b) and (c) close-up of column. Australian Journal of Structural Engineering Vol 11 No 3
“Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith 15 are reported to be reinforced concrete, while the pier on the western face of the building to reveal that top five storeys are reported to have load bearing the external cracking continued through the entire URM piers around the exterior of the building and pier thickness. a steel frame internally (columns spaced roughly at Two days after the main earthquake, structural 5 m) with timber floors throughout (New Zealand engineers met with Urban Search and Rescue Team Historic Places Trust, 2010). The masonry piers, leaders and city officials to determine a strategy having dimensions of approximately 1200 × 900 mm, for making the structure safe enough for building were badly cracked at levels 3 and 4 (figure 27). contractors and engineers to enter to determine more This damage was most likely due to the transition fully the extent of damage and the viability of repair. from concrete to masonry at level 3 and the fact Four days after the main earthquake, the building that the adjoining 2-storey building located along had survived one M5.4 and three M5.1 aftershocks. the southern wall side stopped providing lateral At the time of writing, no decision had been made support at that level. It appears that the lift core had as to whether the building will be demolished or received some strengthening previously, as well as the roof, perhaps in the late 1980s as reported by the repaired. Currently the building poses a significant New Zealand Historic Places Trust (2010). Close up falling hazard for people, traffic and buildings photographs of the masonry piers at levels 3 and located within 70 m in all four directions. This falling 4 show the primary damage that concerned the hazard has resulted in all the buildings within this assessment teams (figure 27). Further inspection by distance being closed to the public (red-tagged) as the assessment team exposed the internal face of one well as both main streets (Manchester and Hereford) that run adjacent to the building. St Elmo Chambers is also a 7-storey building, reported to be a reinforced concrete frame building with external clay brick masonry piers. Owing to the absence of control joints between the masonry and concrete frame, it appears that the masonry piers attracted sufficient seismic in-plane forces to cause shear failure (refer figure 28). However, once the masonry cracked the seismic loads were transferred to the concrete frame. Judging by the extent of cracking in the brickwork, it appears that the storey drifts were less than 1%, implying that the concrete frame was not pushed to its maximum capacity (strength or drift). The authors were not able to inspect the building from inside. From external inspection, the building was judged to pose a falling hazard to pedestrians near the building, but was in little danger of the building itself collapsing in an aftershock. 18 BUILDING DAMAGE DUE TO GROUND DEFORMATION Figure 26: Manchester Courts Building (view Perhaps the most striking aspect of this earthquake from northwest). overall was the extensive amount of liquefaction (a) (b) Figure 27: Damage to masonry piers at levels 3-5 – (a) north wall piers, levels 3-4; and (b) west wall piers, levels 4-5. Australian Journal of Structural Engineering Vol 11 No 3
16 “Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith (a) (b) Figure 28: Views of St Elmo Chambers building in Montreal Street – (a) overview of building; and (b) close-up of damage to brickwork. (a) (b) (c) (d) Figure 29: Damage to buildings having masonry veneer over timber frame, due to ground deformation and liquefaction – (a) damage to masonry veneer due to ground deformation; (b) wide cracks due to ground deformation; (c) masonry veneer damage to recently built residence; and (d) damage due to liquefaction. and ground deformation that occurred. These 19 CLOSING REMARKS phenomena were not seen to a significant extent in the Christchurch CBD region containing the highest On the few occasions that building owners or density of URM buildings, but did impact on a occupants were in attendance it was possible to number of timber-framed structures with masonry gain access to the interior of URM buildings and veneer. As shown in figure 29, several cases of often observe that some separation had occurred extreme ground deformation were observed, and between the floor and/or roof diaphragms and the there were numerous cases where large crack widths masonry walls (in the out-of-plane direction). This formed in residential timber framed structures damage was not easy to detect from the outside of a having a masonry veneer (figure 29(c)). There were building, so that the damage reported from building also cases where ground liquefaction had resulted surveys in the first 72 hours can safely be assumed in masonry structures having sunk into the ground to be a lower bound estimate of structural damage (figure 29(d)). to URM buildings. Australian Journal of Structural Engineering Vol 11 No 3
“Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith 17 There were many instances of buildings that were images/eeri_newsletter/2010_pdf/Chile10_insert. structurally sound themselves, but had suffered pdf, retrieved 16 September 2010. damage or were yellow or red-tagged owing to “falling hazards” from neighbouring buildings. GNS, 2010, “Darfield earthquake damages In some instances it was clear that a parapet or Canterbury”, www.geonet.org.nz/news/article-sep- chimney from a neighbouring building had fallen 4-2010-christchurch-earthquake.html, 16 September, onto or through the roof, being the only damage to retrieved 16 September 2010. the structure. In other instances, a building abutting New Zealand Historic Places Trust, 2010, “Manchester a taller building with damaged parapet or gable Courts”, www.historic.org.nz/TheRegister/ side walls or chimney was given a yellow card RegisterSearch/RegisterResults.aspx?RID=5307; (no public access) due only to the falling hazard retrieved 9 September 2010. posed by the structure next door. These examples of “collateral damage and risk”, such as that posed by New Zealand Parliament, 2004, Building Act 2004, 160 Manchester Street, and the associated business Department of Building and Housing – Te Tari interruption costs, will surely make the financial Kaupapa Whare, Ministry of Economic Development, impact of this earthquake much greater than just the New Zealand Government, Wellington, New cost of rebuilding. Zealand, date of assent 24 August 2004. NewstalkZB, 2010, “Most damage claims over ACKNOWLEDGEMENTS $10k”, www.newstalkzb.co.nz/newsdetail1. asp?storyid=182291, 13 September, retrieved 13 Rodney Henderson-Fitzgerald, a data analyst September 2010. for Christchurch City Council, is thanked for his assistance with securing the building inspection NZ Herald, 2010, “Christchurch earthquake victim: ‘I data reported herein. Lisa Moon and Najif Ismail was pinned to the ground’”, www.nzherald.co.nz/ are thanked for their analysis and graphing of the nz/news/article.cfm?c_id=1&objectid=10673369; 14 building damage statistics. Dmytro Dizhur, Yi-Wei September, retrieved 20 September 2010. Lin, Alistair Russell, Charlotte Knox and Claudio NZSEE, 2006, “Assessment and Improvement of the Oyarzo-Vera are thanked for supplying some of the Structural Performance of Buildings in Earthquakes”, photographs that have been used herein. Recommendations of a NZSEE Study Group on Earthquake Risk Buildings, New Zealand Society REFERENCES for Earthquake Engineering. Christchurch City Council, 2010, “Review of Page, A. 1991, “The Newcastle earthquake – behaviour earthquake-prone, dangerous and insanitary buildings of masonry structures”, Masonry International, Journal policy”, www1.ccc.govt.nz/Council/agendas/2010/ of the British Masonry Society, Vol. 5, No. 1, pp. 11-18. March/RegulatoryPlanning4th/4.Earthquake.pdf, 4 Restrepo-Velez, L. F. & Magenes, G. 2009, “Static tests March, retrieved 20 September 2010. on dry stone masonry and evaluation of static collapse multipliers”, Research Report No. ROSE-2009/02, Davey, R. A. & Blaikie, E. L. 2010, “Predicted and IUSS Press, Fondazione EUCENTRE, Pavia, 72 pp. observed performance of masonry parapets in the 2007 Gisborne earthquake”, 2010 Annual Conference Russell, A. P. & Ingham, J. M. 2008, “Architectural of the New Zealand Society for Earthquake Trends in the Characterisation of Unreinforced Engineering, Wellington, 26-28 March, http:// Masonry in New Zealand”, 14th International Brick db.nzsee.org.nz/2010/Paper07.pdf. and Block Masonry Conference (14IBMAC), Sydney, Australia, 17-20 February. Dowrick, D. 1998, “Damage and Intensities in the Magnitude 7.8 1931 Hawke’s Bay, New Zealand, Russell, A. P. & Ingham, J. M. 2010, “Prevalence of Earthquake”, Bulletin of the New Zealand Society for New Zealand’s Unreinforced Masonry Buildings”, Earthquake Engineering, Vol. 30, No. 2, pp. 133-158. Bulletin of the New Zealand Society for Earthquake Engineering, Vol. 43, No. 3, pp. 182-201. Edwards, M., Griffith, M., Wehner, M., Lam, N., Corby, N., Jakab, M. & Habili, N. 2010, “The Standards New Zealand, 2004, NZS 1170.5:2004, Kalgoorlie Earthquake of the 20 th April 2010: Structural Design Actions Part 5:Earthquake actions – Preliminary Damage Survey Outcomes”, Proceedings New Zealand, Wellington, New Zealand. of the Australian Earthquake Engineering Society Conference, Perth. Wells, S. 2010, “Council fast tracks earthquake policy”, www.stuff.co.nz/the-press/news/ EERI, 2010, “The Mw 8.8 Chile Earthquake of christchurch-earthquake/4114503/Council-fast- February 27, 2010”, Earthquake Engineering Research tracks-earthquake-policy, 15 September, retrieved Institute Newsletter, June, www.eeri.org/site/ 15 September 2010. Australian Journal of Structural Engineering Vol 11 No 3
18 “Performance of unreinforced masonry buildings during the 2010 Darfield ...” – Ingham & Griffith JASON INGHAM Jason Ingham is Associate Professor in the Department of Civil and Environmental Engineering at the University of Auckland. Jason obtained his PhD from the University of California at San Diego in 1995, having previously completed his BE (Hons) and ME (Dist) from the University of Auckland. Jason also has an MBA from the University of Auckland. Jason is currently on the management committees of the New Zealand Society of Earthquake Engineering and the Structural Engineering Society of New Zealand, and is currently vice-president of the New Zealand Concrete Society. Jason’s primary research interests are associated with seismic assessment and retrofit of concrete and masonry structures, and sustainable concrete technology. Over the last 6 years Jason has led a project considering the seismic assessment and retrofit of unreinforced masonry buildings, with further details available at www. retrofitsolutions.org.nz. MICHAEL GRIFFITH Michael Griffith is Professor in the School of Civil, Environmental and Mining Engineering at the University of Adelaide. He obtained his PhD in Structural Engineering from the University of California at Berkeley (1988) after completing his BSc (Eng) and MSc (Eng) degrees in Civil Engineering at Washington State University in the US. Michael is a member of the Standards Australia Australian Earthquake Loading Code committee, and he is also a member of Engineers Australia, state committee member of the SA Structural College and Past-President of the Australian Earthquake Engineering Society. His main professional and research interests are in the field of earthquake engineering and structural dynamics. He has co-authored two book chapters and over 100 research papers in the field of structural engineering. Australian Journal of Structural Engineering Vol 11 No 3
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