POST-EARTHQUAKE BUILDING MANAGEMENT RECOVERY PHASE

Transcription

1 POST-EARTHQUAKE BUILDING MANAGEMENT RECOVERY PHASE H.J. Hare 1, D.R. Brunsdon 2, M.C. Stannard 3, R.D. Jury 4, G.J. Beattie 5, N.J. Traylen 6, K.J. McManus 7, D.K. Bull 8 ABSTRACT The Canterbury earthquake sequence caused high levels of damage to buildings, due in part to the close proximity of the earthquakes to central Christchurch and to widespread liquefaction. Most buildings performed generally to expectations, but the issues with liquefaction under residential suburbs and the significant levels of damage to the remaining commercial buildings posed many challenges to building owners and assessors. The development of guidelines (by the Engineering Advisory Group working on behalf of the Ministry of Building, Innovation and Employment) for assessment of damaged buildings was conducted in two separate parts. The first stream of work, commencing in October 2010, involved the development of guidelines for assessment of residential structures, beginning with those founded on liquefiable ground. The second stream, commencing in April 2011, involved the development of guidelines for assessment of earthquake damaged commercial buildings. During the development of the guidelines, the Engineering Advisory Group has encountered a number of significant issues. This paper describes a number of such issues and some of the learnings that may be drawn from them. Many of these learnings are of broader relevance for other regions that have similar building types and geological conditions. 1 Director, Holmes Consulting Group, Christchurch, New Zealand 2 Director, Kestrel Group, Wellington, New Zealand 3 Chief Engineer, Ministry of Building, Innovation and Employment, Wellington, New Zealand 4 Technical Director, Beca,, Wellington, New Zealand 5 Principal Structural Engineer, BRANZ, Wellington, New Zealand 6 Director, Geotech Consulting Limited, Christchurch, New Zealand 7 Director, McManus Geotech Limited, Nelson, New Zealand 8 Technical Director, Holmes Consulting Group, Christchurch, New Zealand Hare HJ, Brunsdon DR, Stannard MC, Jury RD, Beattie GJ, Traylen NJ, McManus KJ, Bull DK. Post-earthquake building management recovery phase. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

2 Post-earthquake Building Management Recovery Phase H.J. Hare 9, D.R. Brunsdon 10, M.C. Stannard 11, R.D. Jury 12, G.J. Beattie 13, N.J. Traylen 14, K.J. McManus 15, D.K. Bull 16 ABSTRACT The Canterbury earthquake sequence caused high levels of damage to buildings, due in part to the close proximity of the earthquakes to central Christchurch and to widespread liquefaction. Most buildings performed generally to expectations, but the issues with liquefaction under residential suburbs and the significant levels of damage to the remaining commercial buildings posed many challenges to building owners and assessors. The development of guidelines (by the Engineering Advisory Group working on behalf of the Ministry of Building, Innovation and Employment) for assessment of damaged buildings was conducted in two separate parts. The first stream of work, commencing in October 2010, involved the development of guidelines for assessment of residential structures, beginning with those founded on liquefiable ground. The second stream, commencing in April 2011, involved the development of guidelines for assessment of earthquake damaged commercial buildings. During the development of the guidelines, the Engineering Advisory Group has encountered a number of significant issues. This paper describes a number of such issues and some of the learnings that may be drawn from them. Many of these learnings are of broader relevance for other regions that have similar building types and geological conditions. Introduction The Canterbury Earthquake sequence commenced at 4:35am on September 4 th, 2010, but the most violent and destructive shaking occurred during on February 22 nd The major events are considered to have been: September 4 th 2010, Mw 7.1 centered near Darfield, approximately 25 miles west of the CBD February 22 nd 2011, Mw 6.2, centered 6 miles southeast of the CBD at 3 miles depth June 13 th 2011, Mw 6.0, centered approximately 8 miles to the south east of the CBD December 23 rd 2011, Mw 6.0, centered approximately 10 miles to the east of the CBD Figure 1 below shows the overall spread of the earthquakes with the different colors representing 9 Director, Holmes Consulting Group, Christchurch, New Zealand 10 Director, Kestrel Group, Wellington, New Zealand 11 Chief Engineer, Ministry of Building, Innovation and Employment, Wellington, New Zealand 12 Technical Director, Beca,, Wellington, New Zealand 13 Principal Structural Engineer, BRANZ, Wellington, New Zealand 14 Director, Geotech Consulting Limited, Christchurch, New Zealand 15 Director, McManus Geotech Limited, Nelson, New Zealand 16 Technical Director, Holmes Consulting Group, Christchurch, New Zealand Hare HJ, Brunsdon DR, Stannard MC, Jury RD, Beattie GJ, Traylen NJ, McManus KJ, Bull DK. Post-earthquake building management recovery phase. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

3 some of the major events and associated aftershocks, well in excess of 10,000 in total. More than 50 of these have exceeded Mw 5, the level at which damaging effects have been noted. Figure 1: Earthquakes recorded to December 17th 2012 (from GNS Science) There have already been numerous comprehensive papers published describing the immediate effects of the earthquake, including evidence [1] prepared by the Department of Building and Housing for the following Canterbury Earthquakes Royal Commission. This paper will not attempt to comprehensively describe the effects but a brief summary of the main impacts comprises: Significant liquefaction and related damage to buildings and services, typically to the east of the Christchurch city center, but including a number of other low-lying areas about the city and nearby towns. Severe shaking damage to the greater Christchurch area, concentrated typically in the CBD and to the southeast of the city, nearer to the main fault zone. Closure of parts of the CBD for a periods of up to two and a half years while demolition and make safe works were completed, with approximately 1,400 commercial buildings demolished, mostly after the emergency phase. Well over 100,000 homes (predominantly single-family housing) damaged, with approximately 30,000 of these requiring rebuilding. 7,000 homes bought back by the Government in the worst of the low-lying liquefiable areas, due to concerns over the future extent of severe liquefaction and lateral spread. The total cost of the earthquake sequence has progressively grown. It is currently estimated that the total cost to the crown is in the region of NZ$11.5B [2]. However estimates of the total loss are in some cases as high as NZ$40B, with insured losses in the region of NZ$32B. The high proportion of property insured for earthquake is in itself a significant factor in the overall loss.

4 The intention of this paper is to discuss aspects of building damage, assessment and repair that have necessitated considerable change or addition to New Zealand s approach to building evaluation procedures, and which may have application beyond New Zealand. Engineering Advisory Group Shortly following the Sept 4th earthquake, the Engineering Advisory Group (EAG) was assembled, initially by the Earthquake Commission (EQC). This group comprised leading consulting engineers (structural and geotechnical) and researchers drawn from industry, and with representation of the technical societies, comprising the NZ Society for Earthquake Engineering, the New Zealand Structural Engineering Society, and the New Zealand Geotechnical Society. Overall responsibility for the group was subsequently transferred to the Department of Building and Housing (since amalgamated into the Ministry of Building, Innovation and Employment (MBIE)). The main purpose of the group is to determine solutions for technical issues that emerged as a consequence of the earthquake. The EAG s initial brief was to develop practical measures for assessment and improvement of foundations for residential housing following the September 4th earthquake. Following the February 22 nd 2011 event, the scope and membership of the EAG was broadened to take into account the wider scope of damage, and emerging needs of the commercial sector. The group was split into two, and then later three streams: Residential, Commercial and Geotechnical. Once again, the additional members were drawn from industry, The balance of this paper focuses on the activities of the EAG. Background Residential Sector Guidance The residential building stock had generally performed well structurally during the Sept 4 th 2010 earthquake. Most homes suffered relatively little shaking damage, although there was significant shaking damage to the west, closer to the epicenter; and predictably, older homes were the most affected. Homes built since the advent of New Zealand s non-specific wood-framed design Standard [3] in the 1970 s generally performed to expectations although there were numerous instances of loss of masonry veneer and/or damage to heavy tile roofs and other masonry elements [4]. However, liquefaction and lateral spread had caused significant impairment to both homes and to in-ground services. The worst affected homes were generally in the low lying eastern suburbs although there was also significant damage in Kaiapoi, a smaller town to the north of the city. The worst damage tended to occur in those areas adjacent to waterways, where there was a combination of both liquefaction and lateral spread. The Feb 22 nd 2011 earthquake, although not as damaging throughout the region, caused more severe liquefaction damage in Christchurch than the September event. Figure 2 below indicates the extent of liquefaction observed across the city following that event. Ground conditions in the

5 flat areas of Christchurch consist of interbedded alluvial, estuarine and beach deposits, with a relatively shallow water table (to the west this generally grades to more gravel-dominated alluvial deposits and a deeper water table). This indicates that there is potentially liquefiable material under much of the city. Figure 2: Liquefaction observations from the Feb 22nd earthquake (from EQC report [5]) It was immediately clear following the Sept 4th earthquake that the residential housing stock had been significantly impacted by the liquefaction and lateral spread. Further, that a means of assessing and repairing the foundations of these homes would need to be developed quickly, due to there being no existing guidance in place for such a circumstance. Hence the EAG was initiated, for the purposes of developing such guidance. Rockfall issues were common in the hillside suburbs to the south and southeast of the city following the February 2011 earthquake. Initially these areas were not included in the EAG brief as they were well understood and dealt with under the existing regulatory framework. More recently, mass movement zones (both soil and rock) have been identified which have required the EAG s input, developing guidance for construction in some of these zones. This is a work in progress at the time of writing and so not dealt with here. Residential Foundation Guidance The most critical part of the residential workstream s activities has been in the development of guidance for repair or replacement of damaged foundations [6]. The majority of New Zealand s residential building stock is in lightweight single family homes, the majority of which are designed and constructed largely using non-specific design [7]. However the non-specific design standards do not specifically address the potential for liquefaction, which is potentially more widespread than just for the Christchurch area.

6 Technical Categories A necessary first step of the residential guidance was to develop a means part of classifying the ground conditions. Three technical categories were developed, according to the likely outcome of future shaking, as follows: TC1: Future land damage from liquefaction is unlikely, and ground settlements are expected to be within normally accepted tolerances. TC2: Minor to moderate land damage from liquefaction is possible in future large earthquakes. TC3: Significant land damage from liquefaction is possible in future large earthquakes. Foundation solutions Recommended solutions have been developed for foundations according to land classification, with the emphasis on using the existing non-specific guidance to the extent possible. This has generally resulted in the following outcomes: Table 1: General repair or re-build strategies (Leeves et al [8]) Building type TC1 TC2 TC3 Type A (suspended timber floor, on short piles only) Re-level and re-pile if required. No enhancement required. As for TC1. New build. Type B (suspended timber floor with perimeter foundation wall system) Type C (concrete slab on grade) Re-construct damaged portions of perimeter foundation in accordance with current code, re-pile as required under house. Repair slab where practical. For new dwellings concrete slab with nominal (ductile) mesh reinforcement sufficient. As for TC1 but with enhancement to perimeter foundation wall throughout. Localized repair of floor with enhanced tensile and flexural strength permissible. For new dwellings stiff reinforced concrete slab required. Special engineering measures required Special engineering measures required Special engineering measures required In general, the outcome is that for TC1, only minor alterations are required to allow any foundation type to achieve a suitable outcome. For TC2, there are some limitations placed on foundation type, and for TC3, specifically designed solutions have been required. Standard solution options are however provided for TC2 and TC3 corresponding to the outcomes of appropriate geotechnical investigations, with the aim minimizing the level of engineering inputs required.

7 Commercial Sector Guidance The Commercial workstream of the EAG was instigated following the Feb 22 nd 2011 earthquake in response to the widespread significant damage to commercial buildings, particularly in the CBD. This group has concentrated primarily on the development of guidance for the assessment of damaged buildings. Detailed Engineering Evaluations Following the September 4th 2010 earthquake, many building owners and users had largely ignored (or at least downplayed) the need to have further detailed evaluation of their buildings completed, despite warnings on the placards that such evaluations were needed. Compounding this, there were no effective guidelines readily available for engineers to determine the appropriate level of evaluation required. Although most engineers had been very busy in the interim, documenting and overseeing repairs, many buildings had consequently not had thorough reviews by the time of the February 22nd event. The first activity of the Commercial workstream of the EAG was to develop guidelines for the assessment of buildings, known as the Detailed Engineering Evaluation (DEE) Guidelines (EAG 2011). This required a review of existing guidelines and Standards, and consideration of the building failures that had been observed in the earthquakes. The documents are presented in three parts, as follows: Part 1: Background (May 1 st 2012) [9]. Not released for public use, this document contains briefing material and discussion of background issues such as seismicity and risk. Part 2: Procedures [10]. This was the first part published and describes a consistent evaluation process, described in more detail below Part 3: Technical Guidance [11]. This is being published gradually in sections, as it is written. This series of documents is intended to provide detailed guidance on the evaluation of damaged buildings, by type, with reference to existing documents, where possible. The DEE guidelines have been published initially in draft form through the Structural Engineering Society (SESOC), in the interests of expediency and in order to provide the advice to engineers as soon as possible. Because Christchurch had only limited numbers of some forms of building (for example multi-story steel framed structures), only selected sections of Part 3 have been completed at this time. In the longer term, the intention is to have a full suite of the guidance documents updated and approved for use over the whole country, fully endorsed (and published) by MBIE. The emphasis of the guidance has been to use available existing knowledge, even though it is recognized that there is significant learning to be taken from the observations of building performance form this earthquake series.

8 Implementation The need for the guidelines was evident almost immediately after the Feb 22 nd 2011 earthquake. There were several contributing factors in this: Firstly, the severe shaking and liquefaction had generated structural actions in buildings in much of central and southeast Christchurch that was well beyond current Building Code loading requirements. At the time, there was no regulatory framework to require building owners either to assess or even to repair the damage to their buildings. It was not clear in many cases whether the forms of damage observed were such that effective repairs could be made, Finally, the public s concern for building safety in the aftermath of what many saw as a failure of engineering process following the September 4 th 2010 earthquake was extreme. In order to restore faith in the remaining building stock, it was necessary that a consistent set of procedures be developed and implemented as quickly as possible. Given the extent of the damage and the limited capacity of the Christchurch City Council to deal with the issues created by the earthquakes, the Government established the Canterbury Earthquake Recovery Authority (CERA). Included in the enabling legislation [12] were provisions enabling the Chief Executive to require building owners to complete assessments, and to close buildings found to be unsafe. From mid-2011, working with input from the EAG, CERA began to require owners of nonresidential buildings to complete assessments. At the time, there were an estimated 12,000 potentially affected buildings so the work was initially prioritized according to building use, location and assessed vulnerability. Major considerations included whether the imposition of this requirement in such widespread fashion was warranted and whether the consulting engineering community would be able to cope with the workload. However, early indications of the extent of hidden damage and the variability of review provided some vindication. At the time of writing (approximately two years through an estimated five year process), approximately 3,500 DEE reports had been approved. Of these approximately 15% have resulted in the issue of a section 45 (building closure) notice or demolition order. It is estimated that there are a further 5,000 underway or close to completion in the ownership of several large institutional owners. The enforced application (by CERA) of the guidelines to all commercial buildings in Christchurch is an outcome of the almost unique aspects of the Canterbury earthquakes, with a high concentration of extensively damaged buildings over a relatively small area. However, it is anticipated that this level of enforcement will not generally be required in future earthquakes. Further consideration of the triggering circumstances for enforced application and the means for determining how widely to apply it will be required. A review of the DEE implementation procedure is underway, considering a reduction in the extent of review for selected building types. In particular, timber-framed buildings have been

9 reviewed and tested and it is considered that these buildings in most instances offer little risk to occupants, even in cases where it is difficult to justify their capacity by conventional engineering analysis. Technical Issues The following are particular issues that have arisen over the last two years, since the first guidance was drafted. Critical Structural Weaknesses In reviewing the buildings that collapsed during the earthquakes, the failures generally occurred where one or more key vulnerabilities aligned to cause a full system failure. In almost all instances, the failures were in brittle (non-ductile) elements. This observation was equally valid when looking at the more general case of building failures not leading to total collapse. Section 124 of the Building Act [13] confers certain powers and responsibilities to Territorial Authorities to reduce earthquake risk by identifying (and ultimately requiring to be strengthened) buildings that are earthquake prone, that is, which have capacity less than 34% of that required for an equivalent new building, and which may fail leading to loss of life. Although this may imply that brittle systems must be addressed, this does not necessarily create the same margin between ultimate limit state (ULS) capacity and the possible onset of collapse that is deemed to be provided by buildings designed and detailed to current code. One of the challenges confronting the EAG was to find a way to capture this requirement as part of the assessment process. The approach taken was to introduce a simple modification factor, K d, to apply to the calculation of the capacity of affected elements: Where: capacity % NBS element = K d demand, %NBS is the capacity of an element, compared to current code demands, and Kd = a factor reflecting the required margin from ULS to collapse, generally 2. The overall reported capacity of the building is then the lesser of the overall building capacity, or the lowest modified capacity of any CSWs. Concrete crack development and Strain Hardening Like most of New Zealand, Christchurch commercial buildings up to the time of the earthquakes tended to be constructed of reinforced concrete. Early reinforced concrete buildings were not specifically designed for ductility but in similar fashion to the US, specific ductility detailing requirements were introduced in the 70 s. Since then both seismic loading and design requirements have been progressively upgraded. A significant assumption in ductile reinforced concrete design is that well-distributed energy dissipation through yielding of reinforcement occurs in earthquakes which exceed the design

10 level, expected to be reached, in some cases, well before the nominal 500 year return period design earthquake (for normal use buildings). It is expected that this damage should be reasonably easily repaired. However, the Christchurch experience, possibly fuelled by the high levels of insurance cover (and hence entitlement, in the eyes of many claimants), has been that repairs have not been attempted. One of the main areas of contention has been in assessing the remaining life of yielded steel. This arises from several concerns: Instead of many fine cracks (as routinely observed in lab testing), many concrete elements have developed only one or two wide cracks. Testing of reinforcement has shown elevated levels of strain hardening over very short lengths adjacent to the cracks, sometimes as little as 1.5 bar diameters. The implications of this may be far-reaching. Conventional ductile design assumes that yield penetration may occur over a length of 6-11 bar diameters either side of a crack. Strain hardening is a necessary phenomenon that helps to progress crack development but if the plastic hinge formation does not follow normal patterns and the yield penetration does not confirm to expectation, reinforcement may fracture at relatively low rotations. Possible explanations have been put forward as follows: The influence of age on concrete strength and behavior ( labcrete vs. realcrete ) The impact of rates of strain and the loading sequence (a real earthquake, not cycles of increasing load up to the maximum ductility demand). Minimum reinforcing contents are not high enough. This is a matter requiring significant research, but in the interim, a conservative approach has been adopted by some of the industry. Light Industrial Buildings Light industrial buildings (LIBs) are typically single story (sometimes with partial mezzanine floors) and frequently constructed of steel portal frames with precast tilt panel perimeter walls. A significant feature of these buildings has been the performance of the concrete ground floor slabs on grade, which have been badly affected by ground movement, particularly liquefaction. Many of these buildings are located in the east of the city on poor ground (analogous to TC3 as described in the residential sections above). Many are uninsured or under-insured and in most cases, the value of the contents or processes within the buildings far outweighs the value of the building itself. This is in stark contrast to most commercial buildings where the building is the business. If the slabs on grade in these buildings were required to meet the full serviceability requirements of the Building Code in all respects, deep piles would be required. However, the earthquakes have demonstrated that provided there is no life safety hazard as a result of the slab movement, the movement may be tolerated by the users.

11 The EAG is currently working on guidance for the assessment, repair and rebuilding of LIBs, focused on finding ways to enable continued occupation while repairs are being completed; and to establish a compliance path for repair and rebuilding that is not too onerous. Without this, the cost of likely foundations or ground improvement may be prohibitive and could force the abandonment of some existing industrial areas. Conclusions The Detailed Engineering Evaluation guidelines that have been developed and implemented in Christchurch since the earthquakes have imposed a degree of consistency on the evaluation of damaged commercial and residential apartment buildings. Whether this has contributed to public confidence in buildings, or whether this will be a simple consequence of the passage of time may never be known. These guidelines are currently being developed into national document, and are considered to have international application. The resulting DEE reports that have been submitted for review have been mixed in quality. This has illustrated the need for training of engineers in the assessment of both damaged and undamaged buildings. This is a topic that is currently receiving particular attention in New Zealand, and is of relevance in other countries of comparable seismicity. References 1. Structural Performance of Christchurch CBD Buildings in the 22 February 2011 Aftershock, Expert Panel Report appointed by the Department of Building and Housing, New Zealand Treasury. Financial Statements of the Government of New Zealand for the year ended 30 June 2013, NZS3604:1978 Code of practice for light timber framed buildings not requiring specific design. The Standards Association of New Zealand, Wellington, New Zealand. 4. Beattie GJ. The Response of Houses to the Canterbury Earthquake Series Proceedings of the 15 th World Conference of Earthquake Engineering, Lisbon Paper Tonkin & Taylor. Liquefaction Vulnerability Study. Report to the Earthquake Commission, February Engineering Advisory Group. Repairing and rebuilding houses affected by the Canterbury earthquakes, Technical Guidance Document, Ministry of Building Innovation and Employment, Version 3, December NZS3604:2011 Timber-framed buildings. Standards New Zealand, Wellington, New Zealand. 8. Leeves, JR, Sinclair TJE, Stannard, MC, Brunsdon DR, Traylen NJ and Beattie GJ, Building in Resilience for Remediated Residential Housing, Proceedings of the 15th World Conference of Earthquake Engineering, Lisbon Paper Engineering Advisory Group. Engineering Advisory Group. Guidance on Detailed Engineering Evaluation of Earthquake Affected Non-residential Buildings in Canterbury Part 1 Background, Internal to EAG. 10. Engineering Advisory Group (2011) Guidance on Detailed Engineering Evaluation of Earthquake Affected Non-residential Buildings in Canterbury Part 2 Evaluation Procedure, Rev 7, May 2012, SESOC. 11. Engineering Advisory Group (2011) Guidance on Detailed Engineering Evaluation of Earthquake Affected Non-residential Buildings in Canterbury Part 3 Technical Guidance, Various dates, SESOC. 12. Canterbury Earthquake Recovery Act, New Zealand Government, Wellington, New Zealand. 13. Building Act, New Zealand Government, Wellington, New Zealand.