Structural integrity and failure
Structural integrity and failure is an aspect of engineering which deals with the ability of a structure to support a designed load (weight, force, etc...) without breaking, tearing apart, or collapsing, and includes the study of breakage that has previously occurred in order to prevent failures in future designs.
Structural integrity is the term used for the performance characteristic applied to a component, a single structure, or a structure consisting of different components. Structural integrity is the ability of an item to hold together under a load, including its own weight, resisting breakage or bending. It assures that the construction will perform its designed function, during reasonable use, for as long as the designed life of the structure. Items are constructed with structural integrity to ensure that catastrophic failure does not occur, which can result in injuries, severe damage, death, and/or monetary losses.
Structural failure refers to the loss of structural integrity, which is the loss of the load-carrying capacity of a component or member within a structure, or of the structure itself. Structural failure is initiated when the material is stressed beyond its strength limit, thus causing fracture or excessive deformations. In a well-designed system, a localized failure should not cause immediate or even progressive collapse of the entire structure. Ultimate failure strength is one of the limit states that must be accounted for in structural engineering and structural design.
Structural integrity is the ability of a structure or a component to withstand a designed service load, resisting structural failure due to fracture, deformation, or fatigue. Structural integrity is a concept often used in engineering, to produce items that will not only function adequately for their designed purposes, but also to function for a desired service life.
To construct an item with structural integrity, an engineer must first consider the mechanical properties of a material, such as toughness, strength, weight, hardness, and elasticity, and then determine a suitable size, thickness, or shape that will withstand the desired load for a long life. A material with high strength may resist bending, but, without adequate toughness, it may have to be very large to support a load and prevent breaking. However, a material with low strength will likely bend under a load even though its high toughness prevents fracture. A material with low elasticity may be able to support a load with minimum deflection (flexing), but can be prone to fracture from fatigue, while a material with high elasitcity may be more resistant to fatigue, but may produce too much deflection unless the object is drastically oversized.
Structural integrity must always be considered in engineering when designing buildings, gears or transmissions, support structures, mechanical components, or any other item that may bear a load. The engineer must carefully balance the properties of a material with its size and the load it is intended to support. Bridge supports, for instance, need good yield strength, whereas the bolts that hold them need good shear and tensile strength. Springs need good elasticity, but lathe tooling needs high rigidity and minimal deflection. When applied to a structure, the integrity of each component must be carefully matched to its individual application, so that the entire structure can support its load without failure due to weak links. When a weak link breaks, it can put more stress on other parts of the structure, leading to cascading failures.
The need to build structure with integrity goes back as far as recorded history. Houses needed to be able to support their own weight, plus the weight of the inhabitants. Castles needed to be fortified to withstand assaults from invaders. Tools needed to be strong and tough enough to do their jobs. However, it was not until the 1920s that the science of fracture mechanics, namely the brittleness of glass, was described by Alan Arnold Griffith. Even so, a real need for the science did not present itself until World War II, when over 200 welded-steel ships broke in half due to brittle fracture, caused by a combination of the stresses created from the welding process, temperature changes, and the stress points created at the square corners of the bulkheads. The squared windows in the De Havilland Comet aircraft of the 1950s caused stress points which allowed cracks to form, causing the pressurized cabins to explode in mid-flight. Failures in pressurized boiler tanks leading to boiler explosion were a common problem during this era, causing severe damage. The growing sizes of bridges and buildings began to lead to even greater catastrophes and loss of life. The need to build constructions with structural integrity led to great advances in the fields of material sciences and fracture mechanics.
Types of failure
Failure of a structure can occur from many types of problems. Most of these problems are unique to the type of structure or to the various industries. However, most can be traced to one of five main causes.
- The first, whether due to size, shape, or the choice of material, is that the structure is not strong and tough enough to support the load. If the structure or component is not strong enough, catastrophic failure can occur when the overstressed construction reaches a critical stress level.
- The second is instability, whether due to geometry, design or material choice, causing the structure to fail from fatigue or corrosion. These types of failure often occur at stress points, such as squared corners or from bolt holes being too close to the material's edge, causing cracks to slowly form and then progress through cyclic loading. Failure generally occurs when the cracks reach a critical length, causing breakage to happen suddenly under normal loading conditions.
- The third type of failure is caused by manufacturing errors. This may be due to improper selection of materials, incorrect sizing, improper heat treating, failing to adhere to the design, or shoddy workmanship. These types of failure can occur at any time, and are usually unpredictable.
- The fourth is also unpredictable, from the use of defective materials. The material may have been improperly manufactured, or may have been damaged from prior use.
- The fifth cause of failure is from lack of consideration of unexpected problems. Vandalism, sabotage, and natural disasters can all overstress a structure to the point of failure. Improper training of those who use and maintain the construction can also overstress it, leading to potential failures.
On 24 May 1847 the new railway bridge over the river Dee collapsed as a train passed over it, with the loss of 5 lives. It was designed by Robert Stephenson, using cast iron girders reinforced with wrought iron struts. The bridge collapse was the subject of one of the first formal inquiries into a structural failure. The result of the inquiry was that the design of the structure was fundamentally flawed, as the wrought iron did not reinforce the cast iron at all, and that, owing to repeated flexing, the casting had suffered a brittle failure due to fatigue.
First Tay Rail Bridge
The Dee bridge disaster was followed by a number of cast iron bridge collapses, including the collapse of the first Tay Rail Bridge on 28 December 1879. Like the Dee bridge, the Tay collapsed when a train passed over it causing 75 people to lose their lives. The bridge failed because of poorly made cast iron, and the failure of the designer Thomas Bouch to consider wind loading on the bridge. The collapse resulted in cast iron largely being replaced by steel construction, and a complete redesign in 1890 of the Forth Railway Bridge. As a result, the Forth Bridge was the first entirely steel bridge in the world.
First Tacoma Narrows Bridge
The 1940 collapse of the original Tacoma Narrows Bridge is sometimes characterized in physics textbooks as a classic example of resonance, although this description is misleading. The catastrophic vibrations that destroyed the bridge were not due to simple mechanical resonance, but to a more complicated oscillation between the bridge and winds passing through it, known as aeroelastic flutter. Robert H. Scanlan, father of the field of bridge aerodynamics, wrote an article about this misunderstanding. This collapse, and the research that followed, led to an increased understanding of wind/structure interactions. Several bridges were altered following the collapse to prevent a similar event occurring again. The only fatality was a dog named Tubby.
The I-35W Mississippi River bridge (officially known simply as Bridge 9340) was an eight-lane steel truss arch bridge that carried Interstate 35W across the Mississippi River in Minneapolis, Minnesota, United States. The bridge was completed in 1967, and its maintenance was performed by the Minnesota Department of Transportation. The bridge was Minnesota's fifth–busiest, carrying 140,000 vehicles daily. The bridge catastrophically failed during the evening rush hour on 1 August 2007, collapsing to the river and riverbanks beneath. Thirteen people were killed and 145 were injured. Following the collapse, the Federal Highway Administration advised states to inspect the 700 U.S. bridges of similar construction after a possible design flaw in the bridge was discovered, related to large steel sheets called gusset plates which were used to connect girders together in the truss structure. Officials expressed concern about many other bridges in the United States sharing the same design and raised questions as to why such a flaw would not have been discovered in over 40 years of inspections.
Thane building collapse
On 4 April 2013, a building collapsed on tribal land in Mumbra, a suburb of Thane in Maharashtra, India. It has been called the worst building collapse in the area. 74 people died, including 18 children, 23 women, and 33 men, while more than 100 people survived. The search for additional survivors ended on 6 April 2013.
The building was under construction and did not have an occupancy certificate for its 100 to 150 low- to middle-income residents. Living in the building were the site construction workers and families. It was reported that the building was illegally constructed because standard practices were not followed for safe, lawful construction, land acquisition and resident occupancy.
By 11 April, a total of 15 suspects were arrested including builders, engineers, municipal officials and other responsible parties. Governmental records indicate that there were two orders to manage the number of illegal buildings in the area: a 2005 Maharashtra state order to use remote sensing and a 2010 Bombay High Court order. There were also complaints made to state and municipal officials.
On 9 April, a campaign began by the Thane Municipal Corporation to demolish area illegal buildings, focusing first on "dangerous" buildings. The forest department said that it will address encroachment of forest land in the Thane district. A call centre was established by the Thane Municipal Corporation to accept and track resolution of caller complaints about illegal buildings.
Savar building collapse
On 24 April 2013, Rana Plaza, an eight-storey commercial building, collapsed in Savar, a sub-district in the Greater Dhaka Area, the capital of Bangladesh. The search for the dead ended on 13 May with the death toll of 1,129. Approximately 2,515 injured people were rescued from the building alive.
The building contained clothing factories, a bank, apartments, and several other shops. The shops and the bank on the lower floors immediately closed after cracks were discovered in the building. Warnings to avoid using the building after cracks appeared the day before had been ignored. Garment workers were ordered to return the following day and the building collapsed during the morning rush-hour.
Sampoong Department Store collapse
On 29 June 1995, the 5-story Sampoong Department Store in the Seocho District of Seoul, South Korea collapsed resulting in the deaths of 502 people. In April 1995, cracks began to appear in the ceiling of the fifth floor of the store's south wing due to the presence of an air-conditioning unit on the weakened roof of the poorly built structure. On the morning of 29 June, as the number of cracks in the ceiling increased dramatically, the top floor was closed and managers shut the air conditioning off. The store management failed to shut the building down or issue formal evacuation orders; however, the executives themselves left the premises as a precaution. Five hours before the collapse, the first of several loud bangs was heard emanating from the top floors, as the vibration of the air conditioning caused the cracks in the slabs to widen further. Amid customer reports of vibration, the air conditioning was turned off but the cracks in the floors had already grown to 10 cm. At about 5:00 p.m. local time, the fifth floor ceiling began to sink; by 5:57 p.m., the roof gave way, and the air conditioning unit crashed through into the already-overloaded fifth floor, trapping more than 1,500 people and killing 502.
On 16 May 1968, the 22-story residential tower Ronan Point in the London Borough of Newham collapsed when a relatively small gas explosion on the 18th floor caused a structural wall panel to be blown away from the building. The tower was constructed of precast concrete, and the failure of the single panel caused one entire corner of the building to collapse. The panel was able to be blown out because there was insufficient reinforcement steel passing between the panels. This also meant that the loads carried by the panel could not be redistributed to other adjacent panels, because there was no route for the forces to follow. As a result of the collapse, building regulations were overhauled to prevent disproportionate collapse and the understanding of precast concrete detailing was greatly advanced. Many similar buildings were altered or demolished as a result of the collapse.
Oklahoma City bombing
On 19 April 1995, the nine-story concrete framed Alfred P. Murrah Federal Building in Oklahoma was struck by a huge car bomb causing partial collapse, resulting in the deaths of 168 people. The bomb, though large, caused a significantly disproportionate collapse of the structure. The bomb blew all the glass off the front of the building and completely shattered a ground floor reinforced concrete column (see brisance). At second story level a wider column spacing existed, and loads from upper story columns were transferred into fewer columns below by girders at second floor level. The removal of one of the lower story columns caused neighbouring columns to fail due to the extra load, eventually leading to the complete collapse of the central portion of the building. The bombing was one of the first to highlight the extreme forces that blast loading from terrorism can exert on buildings, and led to increased consideration of terrorism in structural design of buildings.
Versailles wedding hall
The Versailles wedding hall (Hebrew: אולמי ורסאי), located in Talpiot, Jerusalem, is the site of the worst civil disaster in Israel's history. At 22:43 on Thursday night, 24 May 2001 during the wedding of Keren and Asaf Dror, a large portion of the third floor of the four-story building collapsed.
World Trade Center Towers 1, 2, and 7
In the September 11 attacks, two commercial airliners were deliberately crashed into the Twin Towers of the World Trade Center in New York City. The impact and resulting fires caused both towers to collapse within two hours. After the impacts had severed exterior columns and damaged core columns, the loads on these columns were redistributed. The hat trusses at the top of each building played a significant role in this redistribution of the loads in the structure. The impacts dislodged some of the fireproofing from the steel, increasing its exposure to the heat of the fires. Temperatures became high enough to weaken the core columns to the point of creep and plastic deformation under the weight of higher floors. Perimeter columns and floors were also weakened by the heat of the fires, causing the floors to sag and exerting an inward force on exterior walls of the building. WTC Building 7 also collapsed later that day. According to the official report, the 47 story skyscraper collapsed within seconds due to a combination of a large fire inside the building and heavy structural damage from the collapse of the north tower.
Repeated structural failures of aircraft types occurred in 1954, when two de Havilland Comet C1 jet airliners crashed due to decompression caused by metal fatigue, and in 1963-64, when the vertical stabilizer on four Boeing B-52 bombers broke off in mid-air.
Warsaw Radio Mast
On 8 August 1991 at 16:00 UTC Warsaw radio mast, the tallest man-made object ever built before the erection of Burj Khalifa collapsed as consequence of an error in exchanging the guy-wires on the highest stock. The mast first bent and then snapped at roughly half its height. It destroyed at its collapse a small mobile crane of Mostostal Zabrze. As all workers left the mast before the exchange procedures, there were no fatalities, in contrast to the similar collapse of WLBT Tower in 1997.
Hyatt Regency walkway
On 17 July 1981, two suspended walkways through the lobby of the Hyatt Regency in Kansas City, Missouri, collapsed, killing 114 and injuring more than 200 people at a tea dance. The collapse was due to a late change in design, altering the method in which the rods supporting the walkways were connected to them, and inadvertently doubling the forces on the connection. The failure highlighted the need for good communication between design engineers and contractors, and rigorous checks on designs and especially on contractor-proposed design changes. The failure is a standard case study on engineering courses around the world, and is used to teach the importance of ethics in engineering.
- Structural analysis
- Structural robustness
- Catastrophic failure
- Earthquake engineering
- Porch collapse
- Forensic engineering
- Progressive collapse
- Seismic performance
- Structural Fracture Mechanics
- Collapse zone
- Engineering disasters
- Tofu-dreg project
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