THE MAGAZINE

The Challenge of Making Safer Structures

By Michael A. Gips

Pondering the Meaning of "Progressive Collapse"

This article discusses the term "progressive collapse" and how architects, designers, and engineers are trying to put up buildings that are more resistant to the phenomenon. Progressive collapse is defined by the U.S. General Services Administration as a situation in which "local failure of a primary structural component leads to the collapse of adjoining members which, in turn, leads to additional collapse."

But some experts say that building designers often confuse this phenomenon with "disproportionate collapse" (a situation in which huge damage is out of proportion to the minor triggering event) and that buildings should be designed with prevention of disproportionate collapse, not progressive collapse, in mind. In a paper on the topic, engineer R. Shankar Nair says that "progressive collapse" is almost a meaningless term, because virtually all collapses could be called progressive in that they follow a sequence or progression of events.

Although the events of September 11 have led to a flurry of work on preventing progressive collapse, Nair writes that "the engineering imperative should be not the prevention of progressive collapse but the prevention of disproportionate collapse (be it progressive or not)." Ironically, the Twin Towers collapse was not disproportionate, according to Nair, because each collapse was precipitated by a large crash and fire.

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