On February 22, 2011 at 12:51pm, a 6.3 magnitude earthquake shook Christchurch, New Zealand and the surrounding area. This is the most devastating earthquake accompanied with high loss of life and building damage since the 1931 7.8 magnitude Hawke's Bay earthquake where 256 people were killed. The epicenter was located 3 miles South South-East of Christchurch, which has a population of approximately 350,000 people. The earthquake is blamed for 180 deaths due to widespread damage to many buildings weakened from the 2010 Canterbury Earthquake. Many reports are stating over 100,000 damaged buildings will need to be demolished in the city center and surrounding suburbs. The city is now facing between $11-15 billion in repairs. Aid groups have been working to help rebuild the broken community and learn from the structural design and procedural errors of the damaged buildings (USGS, 2011).
Keywords
Earthquake, Christchurch, New Zealand, Soil Liquefaction, Pyne Gould Guiness Building, Canterbury Television Building, Hotel Grand Chancellor, Eccentric Braced Frames, Lead Base Isolators Summary of Seismological Events Seismology
Figure 1: Diagram of fault slip following the 2011 Earthquake (Image courtesy of GNS Science)
Christchurch is located on the South Island of New Zealand. The regional plate boundary between the Pacific and Australian plates, which meet in the central South Island, together form the Alpine Fault line. The Alpine Fault is a fast moving fault in geological terms, moving approximately thirty meters per one thousand years (GNS Science, 2011). The fault does not continuously move, instead an oblique thrust is the resulting action when one plate suddenly pushes on top of the other plate. The close proximity of this earthquake to Christchurch and the Earth's surface caused more widespread damage than the 2010 Canterbury Earthquake, as will be discurssed in the next section. Christchurch recorded peak ground accelerations surpassing their historical record as well as many world records (USGS, 2011).
2010 Canterbury Earthquake
On September 4, 2010 a 7.1 magnitude earthquake hit the Canterbury region; more detailed information can be found on the event by following the link in the Introduction. Damage was experienced in Christchurch, but no complete building collapses or fatalities were recorded. As mentioned before, the 6.3 magnitude 2011 Christchurch Earthquake caused much more damage and casualties. The location of the oblique thrust was very close to the Earth's surface, so this magnified the effects of the earthquake and did not allow for much energy dissipation. Also, the geology surrounding the area guided the seismic generated shock waves to Christchurch. All of these conditions combined to produce ground accelerations exceeded the maximum considered event, explaining the widespread damage. Many initial reports speculated the 2010 had weakened many structures, so it was only a matter of time before a seismic event brought these buildings down (Institution, 2011). This hypothesis may be true, but in the case of Pyne Gould Guinness Building, the building collapsed solely due to the effects of the 2011 earthquake (Structural, 2011).
Soil Liquefaction
Soil liquefaction is a unique case where sandy or silty soils act as a liquid due to extreme ground shaking. This form of ground failure can cause even the most seismically designed superstructure to collapse because of a failed foundation. Most of the damage incurred to buildings in Christchurch was due to soil liquefaction. The New Zealand Building Code is planning to change the design maps for Christchurch to provide greater detail of the soil liquefaction experienced during the 2011 earthquake. Also, the code notes the design of foundations against soil liquefaction is outside the scope of the code (Institution, 2011). A seismic provision has been written outlining three different categories for residential foundation design. The first category is for areas not prone to liquefaction, so typical foundation design is still acceptable. Larger foundations that tie into the superstructure are expected for areas experiencing minor to moderate liquefaction, the second category. The final category requires a geotechnical investigation prior to foundation design for areas with moderate to significant liquefaction (Brownlee and Williamson, 2011).
Figure 2: Liquefaction in Christchurch suburb. (Photo courtesy of Malcolm Locke)
Figure 3: Sending a loose boulder down the hill. (Photo courtesy of GNS Science)
Rockfalls
A little outside of Christchurch, in Heathcote and Port Hills, another type of side effect of the earthquake had taken place. The hills surrounding these
towns are covered in jagged boulders and many large pieces had come loose during the earthquake. These rocks roll, or even bounce, down the hillside until they lodge into a solid mass. In one instance during the 2011 earthquake, a large boulder traveled 500 meters down a hill and cleared a 2 meter fence before finally landing in a residential garage. Teams are working on assessing the area and safely removing any boulders showing signs of eventually breaking free. Studies are being carried out to learn about the rocks' fall trajectories to eventually create a hazard map. This map will be able to show where rocks could hit a building in town so proper proactive action can be taken to prevent this (Thomson, 2011).
Building Performances
Over 100,000 buildings suffered significant damage from the 2011 earthquake,and many will need to be demolished and rebuilt. Christchurch is tasked with planning the expensive restoration of the city to a pre-seismic event state. In the meantime, New Zealand Building Code changes will be discussed and studies will be performed on buildings that had unexpected complete collapse during the earthquake. The two most prominent building collapses are the Pyne Gould Guinness Building (PGG), and the Canterbury Television Building (CTV). Both of these buildings were constructed in the 1970s under an adequate seismic code, yet they fared the worst in the earthquake. The Christchurch Cathedral also suffered significant damage, but this is expected because the building's construction, unreinforced masonry, does not respond favorably to seismic activity. Finally, the Hotel Grand Chancellor will become a demolition study because this will be the largest, most complicated demolition the country has ever seen. The building is unable to be imploded due to structural instability, so precautionary measures dictated a manual deconstruction process. With the exception of the Hotel Grand Chancellor, these buildings are responsible for eighty-nine percent of the total recorded fatalities due to the earthquake (USGS, 2011). Lessons need to be learned from these buildings to prevent these events from happening again.
Christchurch Cathedral
One of the many buildings severely damaged by the 2011 earthquake is the Christchurch Cathedral. The building, constructed of unreinforced masonry, had been retrofitted after the 2010 Canterbury Earthquake, but those retrofits were not sufficient to protect the building from another major earthquake. Christchurch implemented a retrofit code following the 2010 earthquake, which is discussed in the next section. When the earthquake occurred, unreinforced masonry buildings without retrofitting only had 1/10 of the code required design-level strength. This further stresses the importance of having adequate retrofitting to bring buildings to the design-level strength as stated in the code. For example, the Orion Local Substation, which was constructed using unreinforced masonry, withstood the 2011 earthquake because of its retrofitted steel perimeter (Institution, 2011). On November 9, 2011 the people of Christchurch witnessed their Cathedral's deconsecration ceremony to allow partial or complete demolition of the structure (Donnell, 2011). Code changes will hopefully prevent this from happening again.
Figure 4: Ariel view of Christchurch Cathedral (Photo courtesy of Ross Becker)
Figure 5: Hotel Grand Chancellor (Photo courtesy of Malcolm Locke)
Figure 6: Temporary repair of damaged shear wall in Hotel Grand Chancellor (Photo courtesy of Ross Becker)
Hotel Grand Chancellor
The Hotel Grand Chancellor has become a lasting symbol of the 2011 earthquake with its distinctive lean. The sixteen year old, 85 meter tall, concrete-framed building dropped one meter on the southeast corner of the building. A perimeter extending 90 meters from the damaged hotel has been established because the stability of the building is unknown (Post, February 25, 2011). On February 28, 2011 it was decided that the Hotel Grand Chancellor was stable enough to permit workers to perform temporary structural repairs. Concrete was sprayed on the damaged shear wall in the southeast corner of the building to strengthen it. Steel jackets were placed around failed columns to provide stability and allow concrete to be injected between the jacket and the column. The repair work allowed building inspectors and engineers to properly inspect the building, at which time they determined it needed to be demolished (Structural, 2011). The close proximity to neighboring buildings and structural instability has called for a $10 million demolition, the most complicated and expensive one New Zealand has ever seen. The Hotel Grand Chancellor will be demolished floor by floor starting November 7, 2011 and is scheduled to be completed mid-April 2012 (Donnell, 2011). As of November 26, 2011 the building's roof has been completely removed, allowing for the deconstruction of the 28 concrete floors to commence (Stuff.Co.NZ, 2011).
PGG & CTV
PGG and CTV both completely collapsed following the 2011 Christchurch Earthquake. Even though the MCE was reached and surpassed, these buildings should not have collapsed as quickly as they did. Buildings are designed in a seismic region to yield to a point where enough time is given for the occupants of the building to evacuate. Neither of these buildings allowed their occupants to evacuate, and thus were the locations of the majority of the casualties from the earthquake. Both buildings were relatively new, built in the last fifty years, so this is why two investigations have been launched to study why PGG and CTV collapsed.
PGG was built in 1963 and current reports state the building met the design codes at the time of construction. In 1998, steel props were added to the perimeter columns as an effort to reinforce the building. A prior inspection had also called for installation of horizontal reinforcement, but no additional horizontal bracing was added during this renovation. A 12 meter tall steel communications mast was added to the building in 2008 directly above the central core walls. A building inspection following the 2010 Canterbury Earthquake found only minor structural damage. The issued report stated damage had in no way weakened the building to a point where it was near collapse, making the 2011 collapse that much more unexpected. During the 2011 earthquake, PGG's collapse seemed to have been started by the failure of the shear wall between levels 1 and 2. The structure between the ground level and level 1 was much stiffer than the structure between levels 1 and 2, thus triggering the failure in the wall between levels 1 and 2. Subsequently, following the shear wall failure, the perimeter columns and the connections between the floor slabs and shear walls failed because the horizontal deflections had rapidly and significantly increased. The floors pancaked on top of each other, finalizing the complete collapse of the building. Analysts have compared PGG to the current code standards and found the building would have only scored between 30-40%. So, even though the building was code compliant for the time it was built, it was not adequate for the current seismic provisions. The Department of Housing and Building is now discussing how to plan to take a more active role in assessing existing buildings for current code compliance and structural weakness to avoid a situation like PGG again (Structural, 2011).
Two engineers have presented their testimonies concerning the inspections of PGG during the hearing that is taking place concerning the PGG collapse. Evidence has been presented stating the building had been inspected 5 times in the months between the 2010 Canterbury Earthquake and the 2011 Christchurch Earthquake. The first inspection was to determine the amount of damage incurred by the 2010 earthquake, but the following four inspections were by owner's request. The building tenants had noticed cracks forming and expanding in the building and wanted a structural engineer to reevaluate the building's integrity. Each inspection lasted for approximately an hour, and only superficial observations of the buildings were made. The engineers state they would not have inspected the building any differently because they cracks did not seem to effect the structural capacity of the building. The hearing is to continue into next week (New Zealand, 2011).
CTV was built in 1986 and was constructed out of reinforced concrete. For the twenty-five years the building was in use, the occupants complained of the building creaking and groaning. Some employees who worked in CTV expressed their concerns that the building experienced excessive vibrations regularly, but these comments were never acted upon. During the 2011 earthquake, CTV collapsed killing well over 100 people as the structure fell. The elevator shaft still remained standing after the collapse, leading many engineers to question how CTV failed. The investigation of CTV's collapse is currently still on going because it is much more complex than the PGG collapse. The investigators have stated they hope to have a report released by 2012 (Structural, 2011).
New Zealand Building Code
Code Overview
The New Zealand Building Code (NZBC) is written in accordance with the Building Act 2004 of New Zealand (Compliance, 2011). NZBC is known to be one of the most stringent on seismic design in the world. Many people questioned this fact after seeing the damage in Christchurch, but the questions are explained because this earthquake topped the maximum considered event, MCE. The MCE is a worst case scenario seismic event standards are set to in the code for seismic building design. Actually, most engineers were surprised to not see more severe building damage in Christchurch after learning this earthquake surpassed the MCE (Post, February 23, 2011).
Code Revisions
As for the code revisions, many were decided upon following the 2010 Canterbury Earthquake. The NZBC addresses the issue of widespread liquefaction experienced during the earthquake. Liquefaction is when the shaking of ground comprised of sands and silts, with a high water content, results in the ground behaving like a liquid for a short time. Canterbury, Christchurch, and Lyttelton were effected by the soil turning into a silty mud resulting in building settlements and failures. NZBC responded by stating foundation designs for liquefaction are outside the scope of NZBC. The code council agreed on changing the seismic maps to increase the Canterbury region from Zone B to Zone A and increase the seismic bracing design requirements from Zone 1 to Zone 2 (Compliance, 2011).
The code revisions will help increase seismic stability for new construction following the 2010 earthquake, but these code changes were unable to prevent the damage that occurred during the 2011 Christchurch Earthquake. This earthquake caused more widespread liquefaction than its 2010 counterpart. As stated earlier, the earthquake surpassed the maximum considered event that every building is designed for in Christchurch. NZBC held an emergency meeting in May 2011 to discuss the code revisions needed following the 2011 earthquake. Seismic retrofitting was increased from 33% design-level strength to 67% design-level strength after the 2010 earthquake, therefore NZBC is considering increasing that standard seeing how some retrofitted buildings responded poorly during the 2011 earthquake. Also, provisions discussing foundation design to withstand liquefaction are in the preliminary stage, as will be discussed in the Liquefaction section of this wiki (Institution, 2011).
After the 2011 Christchurch Earthquake, every qualified engineer was called on to perform structural integrity assessments of the buildings in the surrounding area. David T. Biggs, P.E. (Biggs), formerly of Ryan-Biggs Associates, was in Christchurch giving a lecture when the earthquake struck. His online diary outlines how he performed Level 1 and Level 2 assessments of various buildings in the Christchurch Building District. Level 1 rapid damage assessment is exactly what the name states, a quick overview of the building's structural integrity. In this assessment, a qualified structural engineer will quickly investigate a building without entering it and assign a colored placard to represent their conclusion. A green placard means there is no apparent damage, yellow placard represents safety concerns with limited access permitted, and a red placard shows the structure is unsafe to enter. Level 2 assessments involve entering the building and performing more detailed structural evaluations, which can result in changing the color of the placard determined in a Level 1 assessment. Biggs was a part of team who would be tasked with providing basic analysis of the structural integrity of buildings and then provide a report to the Christchurch building officials. On the last day of his inspections, the building officials asked Biggs' team to find indicator buildings for each type of construction. This would allow the building officials to observe one building per type of construction to provide a general idea as how the rest of those similarly constructed buildings are faring (Biggs, 2011).
Claims have arisen that some Level 1 rapid damage assessments following the 2010 Canterbury Earthquake were not conducted by engineers. This is a breach of the New Zealand's government's building safety guidelines for their protocol on past earthquake damage assessments. Their law states a qualified structural engineer must be present, however municipal building officers, not engineers, were sent to perform building inspections. These checks were supposed to have been verified by engineers, but building owners are not required to provide documentation to the authorities justifying an engineer's assessment has been performed (Jiji Press, 2011). For example, CTV had been given a green placard following the 2010 earthquake, even though a complete collapse occurred during the 2011 earthquake. The New Zealand Department of Building and Housing has launched an investigation to determine if an engineer had cleared the building in 2010. The investigation is still ongoing and no conclusions have been reached yet concerning CTV (Jiji Press, 2011).
Lessons Learned
Eccentric Braced Frames
An eccentric braced frame (EBF) is a type of braced frame that dissipates energy by controlled yielding of the shear link. The link element is the piece of frame between the points where the braces frame into the top member. The link element is the key to a successful performance of an EBF because the link is the portion of the frame that controls the inelastic deformation of the system. Christchurch predominately uses concrete for construction, but the city does have six buildings with EBFs. All but one building performed favorably during the 2011 earthquake, which has led to a study of the two failed EBFs in that single building. This study interests many people because this is the first example of an EBF failing due to a seismic event (Post, March 3, 2011). The one EBF fractured at the panel zone, which analysts speculate is due to overloading at the joint. The fracture is proportional to the eccentricity between the braced flange and the active link end stiffener, which is one of the justifications to their conclusion. The other EBF had substantial link inelastic distortions because of extreme torsional response and shear failure. Many of these researchers are studying ways to repair and replace EBFs, which will be published in the near future (Bruneau, Clifton, et.al., 2011).
Lead Base Isolators
Construction started in 2003 of Christchurch's Women's Hospital and Day Surgery Unit for the Canterbury District Health Board. The unique feature of this building is the use of lead base isolators, the first building in the South Island to incorporate this. The purpose of a base isolator is to separate the building from the ground and allow it to displace without causing major damage. The base isolators on the Women's Hospital allow the building to move 420 millimeters in any direction (Chow, 2003). The Women's Hospital and Parliament, another buildingconstructed using base isolation, fared well during the 2011 earthquake due to the base isolators.
Building Responses to the Earthquake
Other than PGG and CTV, no specific lessons can be learned from the building responses to an earthquake of this magnitude. Christchurch is studying upgrading the code standards for retrofitting unreinforced masonry structures, seeing as some of the current retrofitting failed during this earthquake.
General Lessons Learned
Overall, the widespread damage following the earthquake was less than expected seeing as the MCE was reached and surpassed. Code changes have been discussed to address the issues of liquefaction and resulting foundation designs. More changes will be following seeing as more studies are still being conducted to thoroughly learn from the earthquake. The PGG and CTV Buildings will serve as benchmarks on areas the Building Code needs to be strengthened to try to prevent complete building collapses. For those buildings needing to be rebuilt, attention to seismic performance and design must be paid. Further research will be conducted on EBFs and lead base isolators to study their response to this extreme seismic event. In the years to come, Christchurch will be in the process of rebuilding their city to prevent the structural problems and loss of life experienced on February 22, 2011.
Following studies on the damage due to liquefaction, the NZ Government decided to provide foundation design guidelines that have been strengthened from previous guidelines.
This article gives a brief introduction to how base isolators are beneficial to the design of the Women's Hospital in Christchurch.
Compliance Document for New Zealand Building Code. Wellington: Department of Building and Housing, 2011.
The New Zealand Code, specifically the Seismic Performance of Engineering Systems in Buildings section, was updated after the 2010 Canterbury, New Zealand Earthquake. Christchurch saw direct changes to seismic design values for their region.
GNS Science provides information on the fault lines and resulting seismic geography in New Zealand. The website also provides videos, maps, and reports concerning the 2011 earthquake.
This report is a culmination of all of initial findings from the earthquake. It focuses on the failure of unreinforced masonry structures and soil liquefaction.
Jiji Press English News Service (Tokyo), “Exclusive: Post-Quake Checks of Christchurch Buildings Breached National Guidelines,” March 4, 2011. http://search.proquest.com/docview/854996912 (accessed September 15, 2011).
In this article, the role of the building inspector officers' inspections of buildings after the 2010 earthquake that have since collapsed is described.
Jiji Press English News Source (Tokyo), “NZ Government Unsure about CTV Building Check,” March 5, 2011. http://search.proquest.com/docview/855084809 (accessed September 15, 2011).
This article discusses the pre-quake analysis of the CTV Building.
This article opens by explaining how most eccentric braced frames performed well in the 2011 earthquake. The engineers are perplexed by two eccentric-braced frames that did fail, which the article outlines their observations.
In this article, damage to newly constructed buildings is analyzed. The design of these buildings are under scrutiny to ensure the buildings met the New Zealand construction code.
This article discusses how New Zealand’s seismic retrofit code and/or construction methods need to be strengthened based on the damage incurred from the 2011 earthquake. Soil liquefaction is one of the main reasons why so many buildings were damaged, and that needs to be taken in account when these codes are changed.
Julian Thomson discusses his findings outside of Christchurch after following a team who is researching the damaging rockfalls resulting from the earthquake.
2011 Christchurch, New Zealand Earthquake (February 22, 2011)
Caitlin Behm BAE/MAE Penn State 2012Introduction
Table of Contents
Keywords
Earthquake, Christchurch, New Zealand, Soil Liquefaction, Pyne Gould Guiness Building, Canterbury Television Building, Hotel Grand Chancellor, Eccentric Braced Frames, Lead Base IsolatorsSummary of Seismological Events
Seismology
Christchurch is located on the South Island of New Zealand. The regional plate boundary between the Pacific and Australian plates, which meet in the central South Island, together form the Alpine Fault line. The Alpine Fault is a fast moving fault in geological terms, moving approximately thirty meters per one thousand years (GNS Science, 2011). The fault does not continuously move, instead an oblique thrust is the resulting action when one plate suddenly pushes on top of the other plate. The close proximity of this earthquake to Christchurch and the Earth's surface caused more widespread damage than the 2010 Canterbury Earthquake, as will be discurssed in the next section. Christchurch recorded peak ground accelerations surpassing their historical record as well as many world records (USGS, 2011).
2010 Canterbury Earthquake
On September 4, 2010 a 7.1 magnitude earthquake hit the Canterbury region; more detailed information can be found on the event by following the link in the Introduction. Damage was experienced in Christchurch, but no complete building collapses or fatalities were recorded. As mentioned before, the 6.3 magnitude 2011 Christchurch Earthquake caused much more damage and casualties. The location of the oblique thrust was very close to the Earth's surface, so this magnified the effects of the earthquake and did not allow for much energy dissipation. Also, the geology surrounding the area guided the seismic generated shock waves to Christchurch. All of these conditions combined to produce ground accelerations exceeded the maximum considered event, explaining the widespread damage. Many initial reports speculated the 2010 had weakened many structures, so it was only a matter of time before a seismic event brought these buildings down (Institution, 2011). This hypothesis may be true, but in the case of Pyne Gould Guinness Building, the building collapsed solely due to the effects of the 2011 earthquake (Structural, 2011).
Soil Liquefaction
Soil liquefaction is a unique case where sandy or silty soils act as a liquid due to extreme ground shaking. This form of ground failure can cause even the most seismically designed superstructure to collapse because of a failed foundation. Most of the damage incurred to buildings in Christchurch was due to soil liquefaction. The New Zealand Building Code is planning to change the design maps for Christchurch to provide greater detail of the soil liquefaction experienced during the 2011 earthquake. Also, the code notes the design of foundations against soil liquefaction is outside the scope of the code (Institution, 2011). A seismic provision has been written outlining three different categories for residential foundation design. The first category is for areas not prone to liquefaction, so typical foundation design is still acceptable. Larger foundations that tie into the superstructure are expected for areas experiencing minor to moderate liquefaction, the second category. The final category requires a geotechnical investigation prior to foundation design for areas with moderate to significant liquefaction (Brownlee and Williamson, 2011).
A little outside of Christchurch, in Heathcote and Port Hills, another type of side effect of the earthquake had taken place. The hills surrounding these
towns are covered in jagged boulders and many large pieces had come loose during the earthquake. These rocks roll, or even bounce, down the hillside until they lodge into a solid mass. In one instance during the 2011 earthquake, a large boulder traveled 500 meters down a hill and cleared a 2 meter fence before finally landing in a residential garage. Teams are working on assessing the area and safely removing any boulders showing signs of eventually breaking free. Studies are being carried out to learn about the rocks' fall trajectories to eventually create a hazard map. This map will be able to show where rocks could hit a building in town so proper proactive action can be taken to prevent this (Thomson, 2011).
Building Performances
Over 100,000 buildings suffered significant damage from the 2011 earthquake,and many will need to be demolished and rebuilt. Christchurch is tasked with planning the expensive restoration of the city to a pre-seismic event state. In the meantime, New Zealand Building Code changes will be discussed and studies will be performed on buildings that had unexpected complete collapse during the earthquake. The two most prominent building collapses are the Pyne Gould Guinness Building (PGG), and the Canterbury Television Building (CTV). Both of these buildings were constructed in the 1970s under an adequate seismic code, yet they fared the worst in the earthquake. The Christchurch Cathedral also suffered significant damage, but this is expected because the building's construction, unreinforced masonry, does not respond favorably to seismic activity. Finally, the Hotel Grand Chancellor will become a demolition study because this will be the largest, most complicated demolition the country has ever seen. The building is unable to be imploded due to structural instability, so precautionary measures dictated a manual deconstruction process. With the exception of the Hotel Grand Chancellor, these buildings are responsible for eighty-nine percent of the total recorded fatalities due to the earthquake (USGS, 2011). Lessons need to be learned from these buildings to prevent these events from happening again.Christchurch Cathedral
One of the many buildings severely damaged by the 2011 earthquake is the Christchurch Cathedral. The building, constructed of unreinforced masonry, had been retrofitted after the 2010 Canterbury Earthquake, but those retrofits were not sufficient to protect the building from another major earthquake. Christchurch implemented a retrofit code following the 2010 earthquake, which is discussed in the next section. When the earthquake occurred, unreinforced masonry buildings without retrofitting only had 1/10 of the code required design-level strength. This further stresses the importance of having adequate retrofitting to bring buildings to the design-level strength as stated in the code. For example, the Orion Local Substation, which was constructed using unreinforced masonry, withstood the 2011 earthquake because of its retrofitted steel perimeter (Institution, 2011). On November 9, 2011 the people of Christchurch witnessed their Cathedral's deconsecration ceremony to allow partial or complete demolition of the structure (Donnell, 2011). Code changes will hopefully prevent this from happening again.
Hotel Grand Chancellor
The Hotel Grand Chancellor has become a lasting symbol of the 2011 earthquake with its distinctive lean. The sixteen year old, 85 meter tall, concrete-framed building dropped one meter on the southeast corner of the building. A perimeter extending 90 meters from the damaged hotel has been established because the stability of the building is unknown (Post, February 25, 2011). On February 28, 2011 it was decided that the Hotel Grand Chancellor was stable enough to permit workers to perform temporary structural repairs. Concrete was sprayed on the damaged shear wall in the southeast corner of the building to strengthen it. Steel jackets were placed around failed columns to provide stability and allow concrete to be injected between the jacket and the column. The repair work allowed building inspectors and engineers to properly inspect the building, at which time they determined it needed to be demolished (Structural, 2011). The close proximity to neighboring buildings and structural instability has called for a $10 million demolition, the most complicated and expensive one New Zealand has ever seen. The Hotel Grand Chancellor will be demolished floor by floor starting November 7, 2011 and is scheduled to be completed mid-April 2012 (Donnell, 2011). As of November 26, 2011 the building's roof has been completely removed, allowing for the deconstruction of the 28 concrete floors to commence (Stuff.Co.NZ, 2011).
PGG & CTV
PGG and CTV both completely collapsed following the 2011 Christchurch Earthquake. Even though the MCE was reached and surpassed, these buildings should not have collapsed as quickly as they did. Buildings are designed in a seismic region to yield to a point where enough time is given for the occupants of the building to evacuate. Neither of these buildings allowed their occupants to evacuate, and thus were the locations of the majority of the casualties from the earthquake. Both buildings were relatively new, built in the last fifty years, so this is why two investigations have been launched to study why PGG and CTV collapsed.
PGG was built in 1963 and current reports state the building met the design codes at the time of construction. In 1998, steel props were added to the perimeter columns as an effort to reinforce the building. A prior inspection had also called for installation of horizontal reinforcement, but no additional horizontal bracing was added during this renovation. A 12 meter tall steel communications mast was added to the building in 2008 directly above the central core walls. A building inspection following the 2010 Canterbury Earthquake found only minor structural damage. The issued report stated damage had in no way weakened the building to a point where it was near collapse, making the 2011 collapse that much more unexpected. During the 2011 earthquake, PGG's collapse seemed to have been started by the failure of the shear wall between levels 1 and 2. The structure between the ground level and level 1 was much stiffer than the structure between levels 1 and 2, thus triggering the failure in the wall between levels 1 and 2. Subsequently, following the shear wall failure, the perimeter columns and the connections between the floor slabs and shear walls failed because the horizontal deflections had rapidly and significantly increased. The floors pancaked on top of each other, finalizing the complete collapse of the building. Analysts have compared PGG to the current code standards and found the building would have only scored between 30-40%. So, even though the building was code compliant for the time it was built, it was not adequate for the current seismic provisions. The Department of Housing and Building is now discussing how to plan to take a more active role in assessing existing buildings for current code compliance and structural weakness to avoid a situation like PGG again (Structural, 2011).
Two engineers have presented their testimonies concerning the inspections of PGG during the hearing that is taking place concerning the PGG collapse. Evidence has been presented stating the building had been inspected 5 times in the months between the 2010 Canterbury Earthquake and the 2011 Christchurch Earthquake. The first inspection was to determine the amount of damage incurred by the 2010 earthquake, but the following four inspections were by owner's request. The building tenants had noticed cracks forming and expanding in the building and wanted a structural engineer to reevaluate the building's integrity. Each inspection lasted for approximately an hour, and only superficial observations of the buildings were made. The engineers state they would not have inspected the building any differently because they cracks did not seem to effect the structural capacity of the building. The hearing is to continue into next week (New Zealand, 2011).
CTV was built in 1986 and was constructed out of reinforced concrete. For the twenty-five years the building was in use, the occupants complained of the building creaking and groaning. Some employees who worked in CTV expressed their concerns that the building experienced excessive vibrations regularly, but these comments were never acted upon. During the 2011 earthquake, CTV collapsed killing well over 100 people as the structure fell. The elevator shaft still remained standing after the collapse, leading many engineers to question how CTV failed. The investigation of CTV's collapse is currently still on going because it is much more complex than the PGG collapse. The investigators have stated they hope to have a report released by 2012 (Structural, 2011).
New Zealand Building Code
Code OverviewThe New Zealand Building Code (NZBC) is written in accordance with the Building Act 2004 of New Zealand (Compliance, 2011). NZBC is known to be one of the most stringent on seismic design in the world. Many people questioned this fact after seeing the damage in Christchurch, but the questions are explained because this earthquake topped the maximum considered event, MCE. The MCE is a worst case scenario seismic event standards are set to in the code for seismic building design. Actually, most engineers were surprised to not see more severe building damage in Christchurch after learning this earthquake surpassed the MCE (Post, February 23, 2011).
Code Revisions
As for the code revisions, many were decided upon following the 2010 Canterbury Earthquake. The NZBC addresses the issue of widespread liquefaction experienced during the earthquake. Liquefaction is when the shaking of ground comprised of sands and silts, with a high water content, results in the ground behaving like a liquid for a short time. Canterbury, Christchurch, and Lyttelton were effected by the soil turning into a silty mud resulting in building settlements and failures. NZBC responded by stating foundation designs for liquefaction are outside the scope of NZBC. The code council agreed on changing the seismic maps to increase the Canterbury region from Zone B to Zone A and increase the seismic bracing design requirements from Zone 1 to Zone 2 (Compliance, 2011).
The code revisions will help increase seismic stability for new construction following the 2010 earthquake, but these code changes were unable to prevent the damage that occurred during the 2011 Christchurch Earthquake. This earthquake caused more widespread liquefaction than its 2010 counterpart. As stated earlier, the earthquake surpassed the maximum considered event that every building is designed for in Christchurch. NZBC held an emergency meeting in May 2011 to discuss the code revisions needed following the 2011 earthquake. Seismic retrofitting was increased from 33% design-level strength to 67% design-level strength after the 2010 earthquake, therefore NZBC is considering increasing that standard seeing how some retrofitted buildings responded poorly during the 2011 earthquake. Also, provisions discussing foundation design to withstand liquefaction are in the preliminary stage, as will be discussed in the Liquefaction section of this wiki (Institution, 2011).
Code Inspections of Christchurch
After the 2011 Christchurch Earthquake, every qualified engineer was called on to perform structural integrity assessments of the buildings in the surrounding area. David T. Biggs, P.E. (Biggs), formerly of Ryan-Biggs Associates, was in Christchurch giving a lecture when the earthquake struck. His online diary outlines how he performed Level 1 and Level 2 assessments of various buildings in the Christchurch Building District. Level 1 rapid damage assessment is exactly what the name states, a quick overview of the building's structural integrity. In this assessment, a qualified structural engineer will quickly investigate a building without entering it and assign a colored placard to represent their conclusion. A green placard means there is no apparent damage, yellow placard represents safety concerns with limited access permitted, and a red placard shows the structure is unsafe to enter. Level 2 assessments involve entering the building and performing more detailed structural evaluations, which can result in changing the color of the placard determined in a Level 1 assessment. Biggs was a part of team who would be tasked with providing basic analysis of the structural integrity of buildings and then provide a report to the Christchurch building officials. On the last day of his inspections, the building officials asked Biggs' team to find indicator buildings for each type of construction. This would allow the building officials to observe one building per type of construction to provide a general idea as how the rest of those similarly constructed buildings are faring (Biggs, 2011).
Claims have arisen that some Level 1 rapid damage assessments following the 2010 Canterbury Earthquake were not conducted by engineers. This is a breach of the New Zealand's government's building safety guidelines for their protocol on past earthquake damage assessments. Their law states a qualified structural engineer must be present, however municipal building officers, not engineers, were sent to perform building inspections. These checks were supposed to have been verified by engineers, but building owners are not required to provide documentation to the authorities justifying an engineer's assessment has been performed (Jiji Press, 2011). For example, CTV had been given a green placard following the 2010 earthquake, even though a complete collapse occurred during the 2011 earthquake. The New Zealand Department of Building and Housing has launched an investigation to determine if an engineer had cleared the building in 2010. The investigation is still ongoing and no conclusions have been reached yet concerning CTV (Jiji Press, 2011).
Lessons Learned
Eccentric Braced FramesAn eccentric braced frame (EBF) is a type of braced frame that dissipates energy by controlled yielding of the shear link. The link element is the piece of frame between the points where the braces frame into the top member. The link element is the key to a successful performance of an EBF because the link is the portion of the frame that controls the inelastic deformation of the system. Christchurch predominately uses concrete for construction, but the city does have six buildings with EBFs. All but one building performed favorably during the 2011 earthquake, which has led to a study of the two failed EBFs in that single building. This study interests many people because this is the first example of an EBF failing due to a seismic event (Post, March 3, 2011). The one EBF fractured at the panel zone, which analysts speculate is due to overloading at the joint. The fracture is proportional to the eccentricity between the braced flange and the active link end stiffener, which is one of the justifications to their conclusion. The other EBF had substantial link inelastic distortions because of extreme torsional response and shear failure. Many of these researchers are studying ways to repair and replace EBFs, which will be published in the near future (Bruneau, Clifton, et.al., 2011).
Lead Base Isolators
Construction started in 2003 of Christchurch's Women's Hospital and Day Surgery Unit for the Canterbury District Health Board. The unique feature of this building is the use of lead base isolators, the first building in the South Island to incorporate this. The purpose of a base isolator is to separate the building from the ground and allow it to displace without causing major damage. The base isolators on the Women's Hospital allow the building to move 420 millimeters in any direction (Chow, 2003). The Women's Hospital and Parliament, another buildingconstructed using base isolation, fared well during the 2011 earthquake due to the base isolators.
Building Responses to the Earthquake
Other than PGG and CTV, no specific lessons can be learned from the building responses to an earthquake of this magnitude. Christchurch is studying upgrading the code standards for retrofitting unreinforced masonry structures, seeing as some of the current retrofitting failed during this earthquake.
General Lessons Learned
Overall, the widespread damage following the earthquake was less than expected seeing as the MCE was reached and surpassed. Code changes have been discussed to address the issues of liquefaction and resulting foundation designs. More changes will be following seeing as more studies are still being conducted to thoroughly learn from the earthquake. The PGG and CTV Buildings will serve as benchmarks on areas the Building Code needs to be strengthened to try to prevent complete building collapses. For those buildings needing to be rebuilt, attention to seismic performance and design must be paid. Further research will be conducted on EBFs and lead base isolators to study their response to this extreme seismic event. In the years to come, Christchurch will be in the process of rebuilding their city to prevent the structural problems and loss of life experienced on February 22, 2011.
Bibliography
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Additional Resources
Parfitt, Kevin. "Buildings Collapse as New Zealand Hit by Magnitude 6.3 Earthquake." February 22, 2011. https://web.archive.org/web/20180825104122/https://buildingfailures.com/2011/02/22/buildings-collapse-as-new-zealand-hit-by-magnitude-6.3-earthquake/ (accessed October 13, 2011).