Progressive collapse

A building undergoes progressive collapse when a primary structural element fails, resulting in the failure of adjoining structural elements, which in turn causes further structural failure.

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). 5 people were killed and 3 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.

Ronan Point, following the gas explosion.

• 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. 3 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.

• In 1992, the ancient bell tower at the medieval church of St Maria Magdalena in Goch, Germany, collapsed. A precise collapse date is not known as the church had been abandoned for some time. 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 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 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 bombs compression wave caused floors 4 and 5 to shear up and off their columns and collapse on to 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 subsequent fires that followed. 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. This structural failure should not to be confused with a progressive failure of the floor systems, or so-called "pancake theory" which was ruled out as a cause.

• 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 melting 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 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.

Terminology

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.

See also

• Structural robustness
• Applied Element Method
• Extreme Loading for Structures

External links

• Progressive Collapse Analysis: Applied Element Method
• Progressive Collapse Software: Extreme Loading for Structures
• What 9/11 Taught Us About Designing Skyscrapers