Structural integrity and failure

From Wikipedia, the free encyclopedia

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 a performance characteristic which is applied to a component, a single structure, or a structure consisting of different components. Structural integrity is the quality 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, 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 to 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.

Introduction

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.[1][2]

History

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 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.[3][4]

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 general 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.[5][6]

Notable failures

Further information: List of structural failures and collapses

Bridges

Dee bridge

The Dee bridge after its collapse

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.[7]

First Tay Rail Bridge

Main article: Tay Bridge disaster

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.[8]

First Tacoma Narrows Bridge

Main article: Tacoma Narrows Bridge (1940)

The 1940 collapse of the original Tacoma Narrows Bridge is sometimes characterized in physics textbooks as a classical 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.[9] 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 'Tubby' the dog.[8]

I-35W Bridge

Main article: I-35W Mississippi River bridge
Security camera images show the I-35W collapse in animation, looking north.

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,[10][11] carrying 140,000 vehicles daily.[12] 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 (FHWA) advised states to inspect the 700 U.S. bridges of similar construction[13] 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.[14][15] 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.[15]

Buildings

Sampoong Department Store collapse

Main article: 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.

Ronan Point

Main article: Ronan Point

On 16 May 1968, the 22-storey 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.[16]

Oklahoma City bombing

Main article: 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.[17]

Versailles wedding hall

Main article: Versailles wedding hall disaster

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, May 24, 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 Tower 1, 2, and WTC Building 7

Main article: Collapse of the World Trade Center

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.[18] 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 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. [19][20]

Aircraft

A 1964 B-52 Stratofortress test demonstrated the same failure that caused the 1963 Elephant Mountain & 1964 Savage Mountain crashes.

Repeated structural failures of aircraft types occurred in 1954, when 2 de Havilland Comet C1 jet airliners crashed due to decompression caused by metal fatigue, and in 1963-4, when the vertical stabilizer on 4 Boeing B-52 bombers broke off in mid-air.

Other

Warsaw Radio Mast

Main article: 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

Main article: Hyatt Regency walkway collapse
Design change on the Hyatt Regency walkways.
On 17 July 1981, two suspended walkways through the lobby of the Hyatt Regency in Kansas City, Missouri, collapsed, killing 114 and injuring 200 people[21] 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.[22][23]

See also

References

  1. ↑ Introduction to Engineering Design: Modelling, Synthesis and Problem Solving Strategies By Andrew E. Samuel, John Weir -- Elsevier 1999 Page 3--5
  2. ↑ Structural Integrity of Fasteners, Volume 2 Edited by Pir M. Toor -- ASTM 2000
  3. ↑ Assuring structural integrity in army systems By National Research Council (U.S.). National Materials Advisory Board, National Research Council (U.S.). Commission on Engineering and Technical Systems, National Research Council (U.S.). Committee on Assurance of Structural Integrity -- 1985 Page 1--19
  4. ↑ Structural Integrity Monitoring By R.A. Collacott -- Chapman and Hall 1985 Page 1--5
  5. ↑ Assuring structural integrity in army systems By National Research Council (U.S.). National Materials Advisory Board, National Research Council (U.S.). Commission on Engineering and Technical Systems, National Research Council (U.S.). Committee on Assurance of Structural Integrity -- 1985 Page 1--19
  6. ↑ Structural Integrity Monitoring By R.A. Collacott -- Chapman and Hall 1985 Page 1--5
  7. ↑ Petroski, H. (1994) p.81
  8. ↑ 8.0 8.1 Scott, Richard (2001). In the Wake of Tacoma: Suspension Bridges and the Quest for Aerodynamic Stabilitya. ASCE Publications. p. 139. ISBN 0-7844-0542-5. 
  9. ↑ K. Billah and R. Scanlan (1991), Resonance, Tacoma Narrows Bridge Failure, and Undergraduate Physics Textbooks, American Journal of Physics, 59(2), 118--124 (PDF)
  10. ↑ "2006 Metro Area Traffic Volume Index Map" (pdf). Mn/DOT. 2006. Retrieved 9 August 2007.  Index map for Mn/DOT's 2006 traffic volumes; relevant maps showing the highest river bridge traffic volumes are Maps 2E, 3E, and 3F.
  11. ↑ Weeks, John A. III (2007). "I-35W Bridge Collapse Myths And Conspiracies". John A. Weeks III. Retrieved 6 August 2007. 
  12. ↑ "2006 Downtown Minneapolis Traffic Volumes" (PDF). Minnesota Department of Transportation. 2006. Retrieved 7 August 2007.  This map shows average daily traffic volumes for downtown Minneapolis. Trunk highway and Interstate volumes are from 2006.
  13. ↑ "U.S. Secretary of Transportation Mary E. Peters Calls on States to Immediately Inspect All Steel Arch Truss Bridges" (Press release). 
  14. ↑ "Update on NTSB Investigation of Collapse of I-35W Bride in Minneapolis" (Press release). National Transportation Safety Board. 8 August 2007. Retrieved 1 December 2007. 
  15. ↑ 15.0 15.1 Davey, Monica; Wald, Matthew L. (8 August 2007). Potential Flaw Is Found in Design of Fallen Bridge. The New York Times. Retrieved 9 August 2007. 
  16. ↑ Feld, Jacob; Carper, Kenneth L. (1997). Construction Failure. John Wiley & Sons. p. 8. ISBN 0-471-57477-5. 
  17. ↑ Virdi, K.S. (2000). Abnormal Loading on Structures: Experimental and Numerical Modelling. Taylor & Francis. p. 108. ISBN 0-419-25960-0. 
  18. ↑ "NIST's Responsibilities Under the National Construction Safety Team Act". Retrieved 23 April 2008. 
  19. ↑ Bažant, Zdeněk P.; Jia-Liang Le, Frank R. Greening and David B. Benson (27 May 2007). Collapse of World Trade Center Towers: What Did and Did Not Cause It? (PDF). 22 June 2007. Department of Civil and Environmental Engineering, Northwestern University, Evanston, Illinois 60208, USA. Structural Engineering Report No. 07-05/C605c (page 12). Archived from the original on 9 August 2007. Retrieved 17 September 2007. 
  20. ↑ Bažant, Zdeněk P.; Yong Zhou (1 January 2002). "Why Did the World Trade Center Collapse?—Simple Analysis" (PDF). Journal of Engineering Mechanics 128 (1): 2–6. doi:10.1061/(ASCE)0733-9399(2002)128:1(2). Retrieved 23 August 2007. 
  21. ↑ M. Levy, M. Salvadori (1992). Why Buildings Fall Down. Norton & Co. 
  22. ↑ Feld, J.; Carper, K.L. (1997) p.214
  23. ↑ Whitbeck, C. (1998) p.115
    • Feld, Jacob; Carper, Kenneth L. (1997). Construction Failure. John Wiley & Sons. ISBN 0-471-57477-5.
    • Lewis, Peter R. (2007). Disaster on the Dee. Tempus.
    • Petroski, Henry (1994). Design Paradigms: Case Histories of Error and Judgment in Engineering. Cambridge University Press. ISBN 0-521-46649-0.
    • Scott, Richard (2001). In the Wake of Tacoma: Suspension Bridges and the Quest for Aerodynamic Stability. ASCE Publications. ISBN 0-7844-0542-5.

    External links

    This article is issued from Wikipedia. The text is available under the Creative Commons Attribution/Share Alike; additional terms may apply for the media files.