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Background Attack Aftermath Evidence Misinformation Analysis Memorial

Progressive Collapse

Domino, Pancake, Zipper, ... Choose Your Metaphor

A feature of the Twin Tower collapses that needed some explaining was how they pulverized themselves from the crash zones, staying centered all the way to the ground. Why didn't the tops topple, like any other vertical structure would topple from the fulcrum of the damaged area? Enter progressive collapse, a theory so versatile it applies to all of the accepted collapse theories. "Progressive collapse" would be the umbrella giving a legitimate sound to the domino, pancake, and zipper metaphors used by educators such as Thomas Eagar.

progressive collapse of Twin Towers
Illustration in the September 12, 2001 edition of the New York Times

Typing progressive collapse into a search engine reveals that structural engineers are "discovering new ways to deal with this deadly phenomenon" 1   If one researches the history of total progressive collapse of buildings one will find that, in regard to the structural failure of large steel-framed or steel-reinforced-concrete-framed buildings it is a recent phenomenon, apparently starting with the Oklahoma City Bombing, and then rearing its ugly head again in the 9/11/01 attack.

Ronan Point building following collapse
The Ronan Point apartment building following a progressive collapse of cantilever balcony structures

Outside of these events, the most commonly cited example of progressive collapse is the collapse of one corner of a 23-story block of flats in Newham in east London, known as Ronan Point. At 5:45 AM on May 16, 1968, an explosion in a corner flat of the building's 18th floor blew out sections of the outer wall, causing a chain-reaction collapse of a stack of floor and wall sections on the building's southeast corner. 2  

Any comparisons of the Ronan Point incident to the collapses of the Murrah Federal Building and the World Trade Center skyscrapers are constrained by the fact that the section of the Ronan Point building that collapsed was not part of the support structure of the building. Rather, the collapsed balconies were short cantilever sections supported by the building's main structure. This contrasts with the collapse of a large structural section of the Murrah Building, and the total collapses of the World Trade Center skyscrapers.

The problem with the progressive collapse theory is that it's very difficult to actually build something that will exhibit this behavior. To attempt to verify that this phenomenon can be reproduced outside of a terrorist incident we have introduced the progressive collapse challenge.


1. Progressive Collapse, csemag.com, 11/01/03 [cached]
2. Ronan Point, LalAmy.demon.co.uk,

page last modified: 2009-09-11