Progressive collapses may be accidental, due to design deficiencies, fire, unintentional overload, material failure or natural phenomenon (e.g. erosion, wind or earthquakes). They can also be induced deliberately as a demolition method - specifically building implosion. They may also be caused by acts of terrorism or war.
Initiating factors behind notable examples
- On July 14, 1902, the 323 foot St Mark's Campanile in Venice, Italy collapsed after its northern load-bearing wall began to separate from the main structure. The cause of the separation was attributed to over 700 years of wear on the structure, including fires, earthquakes, and stress-redistribution, primarily from drying-induced shrinkage on the wooden support beams, the bells themselves swaying back and forth, and creep. No one was injured except for a cat belonging to a caretaker. The tower was a stonemasonry design.
- On November 1, 1966, the 7 story University of Aberdeen Zoology Department building in Aberdeen, Scotland suffered a total collapse while under construction. The collapse was attributed to poor girder welds that were weakened by metal fatigue. The metal fatigue was induced by oscillating lateral forces on the structure (primarily wind). Five people were killed and three others were injured. The building was a steel-frame design and the first known example of the total progressive collapse of a steel-framed building.
- On May 16, 1968, the 22 story Ronan Point apartment tower in West Ham, London suffered a fatal collapse of one of its corners due to a natural-gas explosion, which destroyed a load-bearing wall. 4 people were killed and 17 others were injured. The building was a large-panel system building.
- On March 2, 1973, the 26 story Skyline Towers Building in Fairfax County, Virginia, collapsed as a result of wooden shoring being removed too soon from an upper-story floor during construction. 14 people were killed and 34 others were injured. The tower was a steel-reinforced concrete design.
- On December 19, 1985, the 22 story commercial office building at 1000 Wilshire Blvd, Los Angeles, currently known as the Wedbush Building, experienced a partial collapse of the structure. Construction crews were offloading recently arrived steel girders from a flatbed truck on to the deck of the newly completed 5th floor via crane when a girder broke loose from the crane and fell down on to the current stockpile below, which was already loaded to twice the maximum designed load capacity of the floor. This initiated a progressive collapse of the overloaded floor, causing the floor section and girders to crash through the 4th, 3rd, 2nd, and 1st floors, finally coming to rest in the parking garage. Three people were killed. The building was a steel-frame design.
- On March 15, 1986, the 6 story Hotel New World in Little India, Singapore collapsed due to a design error when the structural engineer forgot to add the building's dead load (the weight of the building itself) to his calculations when determining how strong he needed to make the support pillars that held up the building during its construction in 1971. 33 people were killed and 17 others were injured. The building was a steel-reinforced concrete design.
- On April 23, 1987, the 16 story L'Ambiance Plaza in Bridgeport, Connecticut, collapsed during its construction phase due to various instances of inadequate shoring that were in use throughout the construction site. 28 people were killed. The building was a lift-slab design.
- On March 17, 1989, the 255 foot Pavia Civic Tower in Pavia, Italy collapsed due to 800 years of stress-redistribution on the structure, primarily from drying-induced shrinkage on the wooden support beams, the bells themselves swaying back and forth, and creep. 4 people were killed and 15 others were injured. The tower was a stonemasonry design.
- On May 10, 1993, the 4 story Kader Toy Factory in Nakhon Pathom, Thailand collapsed after fire broke out on the first floor and spread throughout the complex. The factory was in full production at the time and all fire exits were locked. 188 people were killed and over 500 others were injured. The building was a steel-frame design.
- On May 24, 1993, the ancient bell tower at the medieval church of St Maria Magdalena in Goch, Germany, collapsed. The cause was attributed to hundreds of years of stress-redistribution on the structure, primarily from drying-induced shrinkage on the wooden support beams, the bells themselves swaying back and forth in the centuries prior, creep, and weathering on the exposed and unmaintained masonry, including crack enlargement from water freezing and expanding between stones in the winter months, causing further expansion of cracks. No one was injured. The tower was a stonemasonry design.
- On April 19, 1995, the 9 story Alfred P. Murrah Federal Building in Oklahoma City, Oklahoma, collapsed due to a truck bomb that was detonated outside of the southern facade. The bomb's compression wave caused floors 4 and 5 to shear up and off their columns and collapse onto floor 3. Floor 3 was connected to the main transfer beam, and pulled it inwards when floors 4 and 5 fell on it. This caused all the vertical columns on the southern perimeter that were connected to the transfer beam to collapse as well, along with any floor sections that depended on those columns for vertical support. Similar to the collapse of the World Trade Center, the Oklahoma City Bombing was the first known example of a terrorist-initiated progressive collapse of a building on US soil. 168 people were killed and 680 others were injured. The building was a steel-reinforced concrete design.
- On June 29, 1995, the 5 story Sampoong Department Store in Seoul, South Korea collapsed. The collapse was due to the removal of several support columns on the lower floors in order to make room for escalators. This lack of structural support was worsened years later by the addition of several heavy air conditioners on the roof above the area where support columns had been removed. This caused the support column that was closest to the air conditioners to fail and pass its load onto nearby columns, which led to complete failure and collapse within 24 hours of major cracks appearing around the failed column. 501 people were killed and 937 others were injured. The complex was a steel-reinforced concrete design.
- On September 11, 2001, World Trade Center buildings 1, 2 and 7 in New York City collapsed as a result of terrorist attacks and the resulting fires. After a 3-year investigation by the National Institute of Standards and Technology, it was concluded that fire weakened the steel structure until the long bridge-like floor sections (called trusses) began to progressively sag. This sagging converted the downwards pull of the trusses into an inwards pull. This intensifying inwards pull on the walls eventually caused the outer columns of Tower 2, and later the inner columns of Tower 1, to buckle and fold, thus initiating the collapses. 2,752 people died in the buildings, including 157 passengers and crew members who were aboard two hijacked airplanes that struck buildings 1 and 2, initiating fires in both, with debris initiating fires in building 7 upon the collapse of buildings 1 and 2. The buildings were a steel-frame design. Progressive failure of the floor systems, or so-called "pancake theory," was ruled out as the initiating factor of the structural failure, but was found by both FEMA and NIST scientists to be the primary mode of failure after collapse initiation.
- On February 12, 2005, the 28 story Windsor Tower in Madrid, Spain suffered the collapse of the upper 11 floors of the building. The tower had a reinforced concrete inner-core surrounded by a traditional webbed steel-frame outer-perimeter. Between floors 16 and 17 was a 7-foot thick, reinforced concrete transfer floor, designed to act as a bulkhead and to support the steel framework of the upper 11 stories. An office fire began on the 21st floor and after 5 hours, the concrete inner-core could no longer support the buckling steel outer-framework. The upper 11 stories collapsed down to street level with remnants of the upper 3 floors collapsing down on to the transfer floor. No one was killed. The building was a composite steel-frame and steel-reinforced concrete design.
- On August 1, 2007, the I-35W Mississippi River Bridge collapsed during afternoon rush hour, resulting in the deaths of 13 people. The collapse was attributed to the failure of a gusset plate connecting two members within one of the main arch ribs. Failure of this fracture-critical joint resulted in total collapse of the structure.
- On 24 April 2013, the 8 story Rana Plaza commercial office complex in Savar, Bangladesh, suffered a collapse to the majority of the structure. The building was originally designed to accommodate shops and offices with light foot traffic but had been converted into a factory with heavy garment manufacturing equipment on the upper floors. This equipment acted like a mild tamping rammer by inducing oscillating forces to the building's frame. The use of substandard construction materials along with the weight of the workers and machinery (which together exceeded the original designed load capacity of the floors), contributed to the weakening and eventual failure of key structural elements. The final collapse occurred one day after preliminary cracks began to appear throughout the building, suggesting that a key structural element had failed and was passing its load forces onto surrounding elements. 1,129 people died in the building and approximately 2,515 people were injured. The incident is considered to be the deadliest garment-factory accident in history, as well as the deadliest accidental structural failure in modern human history.
- On 19 January 2017, the Plasco Building, a high-rise building in Tehran, Iran, caught fire and collapsed. The fire started on the eighth floor and the progressive collapse occurred during rescue operations. According to fire department spokesperson, Jalaal Maleki, around 200 firefighters were on the scene. It was reported that Plasco building progressive collapse was a pancake type because the collapse occurred straight down  the collapse appeared similar to WTC tower collapse.  16 firefighters and 10 civilians died as a result of the fire and collapse
Since the resulting damage in a progressive collapse is disproportionate to the original cause, the term disproportionate collapse is frequently used in engineering to describe this collapse type.
The first date-recorded instance of the term pancake collapse being published in lieu of "progressive collapse" occurred in the August 10, 1980, edition of the New York Times. Fire Chief John Connelly of the 19th Battalion explained that the apartment building, which they responded to in the Bronx, had been weakened by fire to the point that all floors had begun to pancake down on one another. "'It was a pancake collapse.' said Chief John Connelly of the 19th Battalion. 'The entire building was flaming and it went down to the ground.'"
Model code changes
Based on recommendations from the Commerce Department's National Institute of Standards and Technology (NIST), a comprehensive set of building code changes were approved by the International Code Council (ICC). The recommendations were based on the findings of NIST's three-year investigation of the collapses of New York City's World Trade Center (WTC) towers on Sept. 11, 2001.
The proposals addressed areas such as increased resistance to building collapse from fire and other incidents, use of sprayed fire-resistive materials (commonly known as "fireproofing"), performance and redundancy of fire protection systems (i.e., automatic sprinklers), fuel oil storage/piping, elevators for use by first responders and evacuating occupants, the number and location of stairwells, and exit path markings.
The model code changes consistent with the NIST WTC investigation recommendations that are now required by the IBC include:
- Increased bond strength for fireproofing (nearly three times greater than previously required for buildings 25 to 130 metres (75 to 420 ft) in height and seven times greater for buildings more than 130 metres (420 ft) in height).
- Field installation requirements for fireproofing to ensure that:
- installation complies with the manufacturer's instructions;
- the substrates (surfaces being fireproofed) are clean and free of any condition that prevents adhesion;
- testing is conducted to demonstrate that required adhesion is maintained for primed, painted or encapsulated steel surfaces; and
- the finished condition of the installed fireproofing, upon complete drying or curing, does not exhibit cracks, voids, spalls, delamination or any exposure of the substrate.
- Special field inspections of fireproofing to ensure that its as-installed thickness, density and bond strength meet specified requirements, and that a bonding agent is applied when the bond strength is less than required due to the effect of a primed, painted or encapsulated steel surface. The inspections are to be performed after the rough installation of mechanical, electrical, plumbing, sprinkler and ceiling systems.
- Increasing by one hour the fire-resistance rating of structural components and assemblies in buildings 130 metres (420 ft) and higher. (This change was approved in a prior edition of the code.)
- Explicit adoption of the "structural frame" approach to fire resistance ratings that requires all members of the primary structural frame to have the higher fire resistance rating commonly required for columns. The primary structural frame includes the columns, other structural members including the girders, beams, trusses, and spandrels having direct connections to the columns, and bracing members designed to carry gravity loads.
- Ellingwood, B. R.; Leyendecker, E. V. (1978). "Approaches for design against progressive collapse". Journal of the Structural Division. 104 (3): 413–423.
A progressive collapse is a chain reaction type of failure which follows damage to a relatively small portion of a structure.
- A study on St. Mark's Campanile can be found in Northwestern Universities civil engineering archive and a study about the effects of aging on ancient medieval bell towers was released by the University of Pisa in 2001.
- An engineering overview on the collapse with articles from The Scotsman is available. An article on the collapse can also be found in The St. Petersburg Times as well as in the Hansard Archives HERE and HERE
- Although certain articles claim the building was a 21-story building and others claim it to be a 22-story building, the ultimate height of the building was attained by looking up renter addresses for "1000 Wilshire Blvd 22nd Floor" There are renter addresses listed up to the 22nd floor but not higher
- 1000 Wilshire Blvd. Los Angeles, CA 90017 was named the Wedbush Building in January 2002
- The location of the office building was at 1000 Wilshire Blvd. in Los Angeles and consisted of a 5 floor collapse
- A documentary about the Hotel New World collapse is available on YouTube
- The L'Ambiance Plaza case can be found on Engineering.com
- A study on Pavia Civic Tower can be found in Northwestern Universities civil engineering archive and a study about the effects of aging on ancient medieval bell towers was released by the University of Pisa in 2001.
- A case study on the Kader Toy Factory fire is available.
- From a brief history of the organs at St. Maria Magdalena upon the dedication of the new organ in 2015. http://orgelbauverein.st-arnold-janssen.de/unsere-orgel/index.html
- Reference to the church of St Maria Magdalena can be found in a study about the effects of aging on ancient medieval bell towers, which was released by the University of Pisa in 2001.
- A documentary about the Murrah Federal Building collapse is available on YouTube. Note that the building floors in this documentary are referred to by British Convention and will thus be one floor less than the American name for each floor.
- The NIST has released a summary and a report on the bombing of the Alfred P. Murrah Federal Building and its collapse.
- A documentary about the Sampoong Department Store collapse is available on YouTube
- NIST has published a final report on the causes of the World Trade Center collapse.
- "Questions and Answers about the NIST WTC Towers Investigation". NIST. p. FAQ #8. Retrieved 3 May 2014.
NIST's findings do not support the "pancake theory" of collapse, which is premised on a progressive failure of the floor systems in the WTC towers Instead, the NIST investigation showed conclusively that the failure of the inwardly bowed perimeter columns initiated collapse and that the occurrence of this inward bowing required the sagging floors to remain connected to the columns and pull the columns inwards. Thus, the floors did not fail progressively to cause a pancaking phenomenon.CS1 maint: discouraged parameter (link)
- An engineering case study on the Windsor Tower Fire is available.
- David Blair, David Bergman (3 May 2013). "Bangladesh: Rana Plaza architect says building was never meant for factories". The Telegraph. London. Retrieved 8 May 2013. CS1 maint: discouraged parameter (link)
- The full Pay-Per-View article on the Bronx fire is available through the New York Times