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All of New Zealand has been shocked and sadden by the earthquake of 22nd February 2011. It saw the loss of 182 lives, severe injured and considerable distress to many. CTV building failure was one of them. 115 people killed in its catastrophic collapse. This building experienced significant failure of building components including stairs, columns and walls.
This report identifies the causes of this failure and sets out the recommendation for each of the cause. CTV building's lead engineer and his employer were the primary cause of the failure. Poor design, lack of reinforcement, poor construction also led to this failure. Recommendation part clearly illustrates roles of people, the regulation and the procedures that should be followed.
The CTV building was the head quarters of Canterbury television and other companies. It was designed and constructed in about 1986. This building collapsed in the 22nd February 2011. This report analyses the factors that contributed to the catastrophic failure of the building including people qualification, building regulation, building materials and building inspection. Ishikawa methodology is used by me to identify the specific causes of this failure. Finally, find the ways to overcome this failure.
Look at the process after earth quake
The number of possible collapse scenarios was identified by the investigators based on examination of building remnants, eyewitness reports and various structural analyses. These ranged from collapse initiated by column failure on the east or south faces at mid to high level to collapse initiated by failure of a more heavily loaded internal column at mid low- level. The basic initiator in all scenarios was the failure of one or more non ductile columns due to the forces induced as a result of horizontal movement between on floor and the next. Additional inter storey movement due to possible failure of the connection between the floor slabs and the north core may have compounded the situation.
The most studied scenario, which was consistent with eye- witness reports of an initial tilt of the building to the east, involved initiation by failure of a column on the mid to upper levels on the east face. Inter storey displacements along this line were higher than most other sites and there was the prospect of premature failure due to contact with the spandrel panels. Loss of one of these columns on the east face would have caused load to shift to the adjacent interior columns. The low amount of confinement steel in the columns and the relatively large proportion of cover concrete gate the columns little capacity to sustain load and displacement once strains in the cover concrete reached their limit. As a result collapse was sudden.
It measured in the 22 February 2011 after shock were exceptionally high and may have contributed much to vertical forces in columns and walls. CTV building indicated that vertical accelerations could have reduced capacity of critical columns to sustain lateral displacements by around 15 to 32% depends on concrete strength.
Structural integrity: The failure of the CTV building in particular has highlighted the need for a high degree of integrity of building subject to earth quake actions. Requirements for the design of structures, particularly detailed rules and calculations at the cost of attention to the basic fundamental of structural mechanics that is essential to do structural integrity and robust load paths
The CTV building highlighted the lack of alternative load-paths or back-up mechanisms in the seismic response. Redundancy with in seismic and gravity load paths should be provided wherever possible.
Critical vulnerability factory were due to the era in which they built. Previous design codes and philosophies involved differing structural systems and detailing, differing connection systems between elements and differing seismic resisting systems to those that are applicable today. These vulnerabilities resulted in potential structural weaknesses which could have contributed to the collapse of the building. Some examples of these vulnerabilities include a lack of capacity design, poor anchorage details, lack of stirrups in the joint region, inadequate confinement and reinforcement in columns and walls.
Limit on axial load levels: CTV building underline the vital importance of the load-carrying capacity of columns and walls.
Construction quality and compliance
Building is usually "one-off" and special attention is needed to see that design intention is followed. The investigation highlighted the need for more attention to be paid to the quality of construction, particularly in the areas of quality control, quality assurance, construction monitory and design revenue. One specific aspect indentified in the investigations was the need to check the strength and quality of concrete used in building.
Concrete strength was an important factor in the investigation of the CTV building, with lower than expected strengths found in several columns.
Five Whys methods of tracking down the causes
This questioning technique can be used to find out the root causes that underlay in this particular problem
The problem: CTV building failure during the Christchurch earth quake 22nd February 2011
WHY: Design decisions made by the CTV building's lead engineer and his employer were the primary cause of the collapse
WHY: Failing and weakness at a number of levels including the Christchurch city council's regulatory process and the inadequacies of the post quake assessment process carried out by the council
WHY: Lack of construction
WHY: Lack of reinforcement
WHY: Intensity of the horizontal ground shaking
Isikawa Fishbone diagram for this failure
Poor design Lack of reinforcement
Design short of 1980's standards Lack of concrete reinforcement
Lack of awareness Dray-bar remedy was installed only on top 3 levels
Lack of alternative load paths
Asymmetrical layout of shear walls
Column ductility Column shear strength Lack of public understanding
Inadequacies of assessment process
Lack of construction Lack of post earth quake inspection
Summary of identified factors contributed to the failure
According to technical investigation in the structural performance of building in Christchurch- final report, Design decisions made by the CTV building's lead engineer and his employer were the primary cause of the collapse. Failing and weakness at a number of levels including the Christchurch city council's regulatory process and the inadequacies of the post quake assessment process carried out by the council. The building was sufficiently robust to resist the effects of the 4th September 2010 earthquake and the 26th December 2010 after shock without significant damage. However, the demands placed on the CTV building by the aftershock of 22nd February 2011 greatly exceeded those anticipated in the structural design of the building. The actual mechanism of the collapse cannot be determined in every detailed and a range of factors contributed to the collapse. Three critical factors were identified. These were the;
Intensity of the horizontal ground shaking
Lack of ductility in the columns
Asymmetrical shear wall layout
The following factors added to the effects of critical factors;
These were the,
Low concrete strengths
Vertical ground accelerations
Interaction of columns and spandrels
Separation of floor slabs from the north core
Structural influence of masonry walls
There were three aspects of design and construction for which the standards of the day (1980) were not met. These were,
Asymmetrical layout of shear walls
Column shear strength
Based on the above analysis, I found some factors that led to this major failure. Poor design, lack of reinforcement, poor quality of construction also is some of them. Recommendation part clearly illustrates the actions that should be taken to prevent similar failure in future.
Key roles of people, the regulations and procedures
Following structural design issues should be followed;
Improving structural integrity and resilience
Encouraging capacity design
Identifying and removing critical vulnerabilities
Introducing tighter controls to trigger requirements for earthquake strengthening when building are altered
Strength and ductility of walls and columns
Following construction quality and compliance should be followed;
Review quality assurance process in all phases of building design and construction, especially in light of the findings of these building investigations. Implement tighter controls and promote more designer's involvement to ensure that design intentions are being achieved and that the work complies with the requirements of the approved design documents.
Following material quality requirement should be taken in to consideration;
Specified concrete strengths have been and will be achieved. Measures considered should include further strength testing of in situ concrete in existing building and reasons to standard and procedures covering the manufacture, delivering, and placement and caring of concrete in new building.
The following regulations should be followed;
Effective and economic retrofit strategies that improve the earth quake safety of building
Adoption by territorial authorities of strongly active policies to reduce the risks posed by building of low earth quake resistance
Improve definitions earth quake- prone building and more effective implementation of strengthening measures, particularly for building likely to fail in a brittle manner
The need for legislation covering the structural assessment and rehabilitation of building affected by earth quake
Legislative requirements for the documentation of post- earth quake inspection information and public accessibility to such information
Limitation on eccentricity should be reviewed, limits tightened and the concerns brought to the attention of structural engineers and territorial authorities
Particular attention should be given to the evaluation of the actual displacement capacity of gravity- load bearing columns designed according to pre-1995 code provisions
Adequate attachment of floor to shear walls must be achieved
There is a need for improved confidence in design and construction quality
There is a need to assess minimum clearance requirement to non-structural components that may detrimentally affect structural performance.