The February 2011 Christchurch Earthquake: Incorporating Transect Methodology into Rapid Damage Assessment L.M.Moon1, D.T. Biggs, Dist.M.ASCE2, J.M. Ingham, M.ASCE3, M.C. Griffith4 1
PhD student, School of Civil, Environmental and Mining Engineering, University of Adelaide, South Australia, 5005, Australia; Phone: +61-8-8303-4323; email:
[email protected] 2 Principal, Biggs Consulting Engineering, 740 Hoosick Road, Troy, NY 12180, USA; Phone: 518-495-5739; email:
[email protected] 3 Associate Professor of Structural Engineering and Deputy Head (Research), Department of Civil and Environmental Engineering, University of Auckland, Private bag 92019, Auckland, New Zealand; Phone: +64-9-373-7599; email:
[email protected] 4 Professor, Structural Engineering, School of Civil, Environmental and Mining Engineering, University of Adelaide, South Australia, 5005, Australia; Phone: +61-88303-5451; Fax: +61-8-8303-4359;
[email protected] ABSTRACT Ingham and Biggs were in Christchurch during the M6.3, 22 February 2011 earthquake and Moon arrived the next day. They were enlisted by officials to provide rapid assessment of buildings within the Central Business District (CBD). In addition, they were asked to 1) provide a rapid assessment of the numbers and types of buildings that had been damaged, and 2) identify indicator buildings that represent classes of structures that can be used to monitor changing conditions for each class following continuing aftershocks and subsequent damage. This paper explains how transect methodology was incorporated into the rapid damage assessment that was performed 48 hours after the earthquake. Approximately 300 buildings were assessed using exterior Level 1 reporting techniques. That data was used to draw conclusions on the condition of the entire CBD of approximately 4400 buildings. In the context of a disaster investigation, a transect involves traveling a selected path assessing the condition of the buildings and documenting the class of each building, and using the results in conjunction with prior knowledge relating to the overall population of buildings affected in the area of the study. INTRODUCTION Following the 22 February 2011 Christchurch (New Zealand) earthquake, officials immediately evacuated and cordoned off the majority of the central business district (CBD). Simultaneously, volunteer engineers from the Christchurch area began arriving at the command center established at the Christchurch Art Gallery, which became the headquarters for operations and the media center. In subsequent days,
additional engineers arrived from throughout New Zealand as well as from other countries. Urban search and rescue teams also arrived and were immediately deployed throughout the city. Civil Defense officials requested that a realistic assessment of the CBD be undertaken so that they could deploy emergency resources and report to the media based upon hard facts. Officials also requested the identification of indicator buildings that could be used to monitor changing conditions from aftershocks. With these challenges, a transect was proposed as a survey tool to generate an estimate of the extent of building damage in the CBD. Given the size of the CBD, it was not practical to assess all buildings in such a short time. So, a transect survey was deemed appropriate. THE TRANSECT Background. In the scientific community, a transect is a sampling method widely used to assess the abundance of animals or plants, or to estimate the density of a population of a species in an area (Marques 2004; Wikipedia 2012). Transects take a number of forms, including a line transect, line-intercept transect, strip transect and point transect. For a line transect, an observer travels a pre-determined path along which the count of the phenomena of study is recorded, as is the distance from the line to each sighted phenomena. For a strip transect, only the phenomena occurring between two parallel line segments are counted. An analogy of a strip transect was proposed for the Christchurch CBD. The route chosen is shown in Figure 1.
Figure 1. Route of transect through Christchurch CBD. The CBD transect route includes a large proportion of unreinforced masonry (URM) buildings which were familiar from previous reconnaissance work following the
September 2010 earthquake. Fortuitously, Moon made additional observations following a route similar to the transect route just two days prior to the February 2011 earthquake and had a photographic record from which new damage could be ascertained. Although the building types recorded during the transect did not specifically reflect the overall distribution of building types within the city, the transect route did contain a sample of all building types. Process. For days after the earthquake, aftershocks were occurring at approximately 20 to 120 minute intervals which created significant falling hazards. Thus, no buildings were entered and the backs of many buildings were not accessible for the rapid damage assessment. The transect process consisted of an assessor describing the visible damage and the likely cause; one recorder documenting addresses, GPS coordinates, building types and damage levels; and the second recorder documenting all building damage via a photograph log. Damage levels were recorded as either ‘green’, ‘yellow’ or ‘red’, depending on the likelihood of building demolition. Buildings with no or minor structural damage were classified green; those with major structural damage, but not in imminent danger of collapse were classified yellow; and those on the verge of collapse or deemed unsafe for entry were classified red. In contrast to the Level 1 Rapid Damage Assessment process where risk to the public is considered, buildings were not classified red if the only danger was from adjacent buildings as the focus of the transect was on the condition of individual buildings and whether they were repairable or would require demolition. TRANSECT RESULTS Data. The results of the transect observations are shown in Figure 2. The graph shows the number of each building type surveyed, and the breakdown of each building type into the different placard colors. The building types are: PC – precast concrete; RC – reinforced concrete; RC+Masonry infill - reinforced concrete with masonry infill; RCM – reinforced concrete masonry; ST – structural steel frame; T – timber frame; URM - unreinforced masonry. The data clearly showed that URM buildings performed the worst during the earthquake. It was estimated that approximately one-third of all buildings in the CBD would need to be demolished assuming that all of the red tagged buildings and 50% of all the yellow tagged buildings would be uneconomic to repair or would need to be demolished urgently for safety reasons. These decisions were based upon the presumption that the transect was a good representation of the distribution of and damage to all the building types within the CBD.
Figure 2. Number of buildings and damage levels from the transect. Follow-up. The transect was conducted in one day and the results were collated, written, and published on the New Zealand Society for Earthquake Engineering Inc. (NZSEE) Clearing House blog the same evening (Blog 2011). The results were conveyed to the Civil Defense Emergency Management team the next morning (25 February 2011) and interviews were held with media that same morning to inform the general public of the condition of buildings within the CBD (TVNZ 2011), with the story then being circulated worldwide over the next 24 hours (BBC 2011). The first technical publication of the results was produced in the UK only 3 weeks after the earthquake (Ingham et al., 2011). Comparison of results to full survey. The transect results were the first overall study of the damage levels of buildings within the CBD. Figure 3 shows the overall distribution of damage classifications assigned during the transect (Figure 3a) and those assigned by the Civil Defense volunteers in the following month (Figure 3b). The Civil Defense data, published by the Christchurch City Council (2011), includes Level 1 assessment data for over 4000 buildings within the CBD. The results of the transect are generally consistent with these assessments. This may seem surprising when recognizing that the route was selected based upon engineering intuition, judgment and previous experience with many of the buildings. However, given the homogeneity of building types and age of construction throughout the CBD any path may have been expected to produce similar results.
STATISTICAL OVERVIEW OF THE TRANSECT DATA To further evaluate the results of the transect with respect to the full study, a subsequent statistical analysis was performed. The number of each type of building sampled during the transect and the percentage of the sample that each building type represents are shown in Table 1.
a) CBD data from transect – 24 February 2011
b) CBD Placard data from Civil Defense – 24 March 2011
Figure 3. Damage level data from Christchurch CBD, February & March 2011. While it has been determined that there are over 4000 buildings in the CBD, building type data only exists for 2,711 (Table 1). In general, the distribution of building types shown in Table 1 was extracted from Kam et al. (2011). This extracted data was supplemented by data on URM buildings that was taken from the reports prepared by Ingham and Griffith (2011a; 2011b) for the Canterbury Earthquakes Royal Commission of Inquiry and by data on RCM buildings taken from Dizhur et al. (2011). Table 1. CBD Building Population (Transect Values in Brackets) CBD Building Type Number of Buildings % of Total in CBD Precast Concrete 176 (29) 6(10) Reinforced concrete 448 (56) 17 (19) RC + Masonry infill 209 (20) 8 (7) RCM 342 (25) 13(8) Steel 138 (1) 5 (0.3) Timber 1028 (8) 38(3) Unreinforced masonry (clay brick and stone) 370 (155) 14 (52) TOTAL 2,711 (294) 100 Assuming that the data from Christchurch City Council is adequately representative of the building distribution within the CBD, then the distribution of buildings surveyed during the transect were close to the overall distribution of building types throughout the CBD, with an R2 value of 0.92.
Statistical analysis. In a population N with sample size n, for a 95% confidence interval, the margin of error (MOE) can be approximated as: MOE = 0.98/(n1/2)
(1)
For small populations such as in this case, a finite population correction (FPC) factor can be used to modify the margin of error, where FPC = [(N - n)/ (N - 1)] 1/2
(2)
The adjusted margins of error for a 95% confidence interval for the transect results are shown in Table 2. while the number of buildings of each construction type required to be sampled to ensure a margin of error of 5% are presented in Table 3. Table 2. Margin of Error for 95% Confidence Interval for Transect Sample CBD Building Population Transect Adjusted Type Sample margin of error Precast concrete 176 29 17% RC 448 56 12% RC + Masonry infill 209 20 21% RCM 342 25 19% Steel 138 1 98% Timber 1,028 8 35% URM 370 155 6% TOTAL 2,711 294 5% Table 3. Sample Size Required for 95% Confidence Interval and 5% Margin of Error CBD Building Type Population Sample size required (transect value in brackets) Precast concrete 176 114 (29) RC 448 187 (56) RC + Masonry infill 209 127 (20) RCM 342 165 (25) Steel 138 97 (1) Timber 1,028 243 (8) URM 370 172 (155) TOTAL 2,711 1,105 (294)
CONCLUSIONS The transect methodology proved to by a quick, useful tool for rapid damage assessment in Christchurch. Performed within 48 hours after the 22 February 2011 earthquake, the transect provided valuable data that was immediately used by Civil Defense officials. It also indicated that approximately one-third of buildings in the CBD would need to be demolished, and this was quickly communicated to the national and international media. This may have been the first application of a transect for rapid damage assessment following a disaster. The Christchurch transect was developed without statistical input. From Table 2 and Table 3, more of each building type would have to be sampled to achieve 95% level of confidence with 5% margin of error. Despite the lack of adequate statistical sampling, the homogeneity of the building stock and damage produced acceptable results in Christchurch as seen in Figure 3. It is suggested that the methodology can be applied to future events provided there is pre-planning to achieve the desired accuracy. There are inherent errors in the data because the assessments were exterior only, and were visual. In addition, the assessments were made by different individuals. Finally, determining the type of construction and level of damage is highly dependent upon the abilities and knowledge of the assessor. These concerns can be improved upon with pre-planning. RECOMMENDATIONS Transects can be used for future rapid damage assessments following any disaster, not just earthquakes. The methodology should include the following pre-planning by government officials: 1. Identify the characteristics of individual buildings and assess their current condition. 2. Develop a database for the data. 3. Plan several transect routes. There should be multiple routes to provide options to address accessibility at the time of a disaster. The routes must also include a sufficient range and number of building types for the required levels of statistical accuracy, usually 95% confidence interval with 5% error. 4. Identify possible indicator buildings. These should be checked for damage as part of a transect. 5. Have experienced engineers on call who are familiar with the structures in the transect routes, are trained in the use of the database, and can provide the rapid damage assessment as part of the transect team.
REFERENCES BBC (2011). “Third of Christchurch buildings ‘could face demolition’”, http://www.bbc.co.uk/news/world-asia-pacific-1258704. Accessed 9 April 2012. Blog (2011) http://db.nzsee.org.nz:8080/en/web/chch_2011/report-from-the-field//blogs/transect-of-christchurch-city-after-the-earthquake-of-tuesday-february22 Christchurch City Council. (2011). "CDEM: Public information update." Christchurch City Council, Christchurch, Media release. Dizhur, D., Ingham, J.M., Moon, L.M., Griffith, M.C., Schultz, A., Senaldi, I., Magenes, G., Dickie, J., Lissel, S., Centeno, J., Ventura, C., Leite, J. (2011). "Performance of masonry buildings and of churches in the 22 February Christchurch earthquake." New Zealand Society for Earthquake Engineering Bulletin, 44(04), 279-296. Ingham, J.M., Biggs, D.T., Moon, L.M. (2011), "How Did Unreinforced Masonry Buildings Perform in the February 2011 Christchurch Earthquake?", “Structural Engineer”, United Kingdom, March 15, 2011 Ingham, J. M., and Griffith, M. C. (2011a). "The performance of earthquake strengthened URM buildings in the Christchurch CBD in the 22 February 2011 earthquake." Canterbury Earthquakes Royal Commission of Inquiry, Christchurch. Ingham, J. M., and Griffith, M. C. (2011b). "The performance of unreinforced masonry buildings in the 2010/2011 Canterbury earthquake swarm." Canterbury Earthquakes Royal Commission of Inquiry, Christchurch. Kam, W. Y., Pampanin, S., Elwood, K. (2011). "Seismic performance of reinforced concrete buildings in the 22 February Christchurch (Lyttelton) earthquake." New Zealand Society for Earthquake Engineering Bulletin, 44(04), 239-278. Marques, T. A. (2004). "Predicting and correcting bias caused by measurement error in line transect sampling using multiplicative error models." Biometrics, 60(03), 757-763. TVNZ (2001). "Structural engineer says a third of CBD will need demolishing", http://tvnz.co.nz/national-news/video-structural-engineer-says-third-cbd-needdemolishing-4040932. Accessed 9 April 2012. Wikipedia. (2012). "Transect." http://en.wikipedia.org/wiki/Transect. Accessed 20 March 2012.
