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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. The resulting damage is disproportionate to the original cause, so the term disproportionate collapse is also used to describe this collapse type.

The most notable examples are the two World Trade Center towers in New York City which collapsed as the result of the terrorist attacks on September 11, 2001. Another example is the collapse of the Ronan Point apartment tower which suffered a fatal partial collapse due to a natural gas explosion on 16 May 1968.

In response to the WTC tragedy, the National Institute of Standards and Technology conducted a 3-year building and fire safety investigation to study the factors contributing to the probable cause (or causes) of post-impact collapse of the WTC Towers (WTC 1 and 2) and WTC 7. NIST subsequently expanded its research in areas of high-priority need such as prevention of progressive collapse, fire resistance design and retrofit of structures, and fire resistive coatings for structural steel.[1][2]

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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 currently required for buildings 75–420 feet in height and seven times greater for buildings more than 420 feet 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 420 feet 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.

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