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.
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. 
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: