As the temperature of floor slabs and support framing increases, these elements can lose rigidity and sag into catenary action. As catenary action progresses, horizontal framing elements and floor slabs become tensile elements, which can cause failure of end connections (Figure 2-21) [right] and allow supported floors to collapse onto the floors below. The presence of large amounts of debris on some floors of WTC 1 would have made them even more susceptible to this behavior.

In addition to overloading the floors below, and potentially resulting in a pancake-type collapse of successive floors, local floor collapse would also immediately increase the laterally unsupported length of columns, permitting buckling to begin. As indicated in Appendix B, the propensity of exterior columns to buckle would have been governed by the relatively weak bolted column splices between the vertically stacked prefabricated exterior wall units. This effect would be even more likely to occur in a fire that involves several adjacent floor levels simultaneously, because the columns could effectively lose lateral support over several stories (Figure 2-22) [right].

site: www.fema.gov page: 911research.wtc7.net/mirrors/guardian2/wtc/WTC_ch2.htm

The pancaking theory was sold to the American public through a number of documentaries that purported to explain the collapses. The most widely publicized of these was entitled Why the Towers Fell. A companion website to this documentary hosted by PBS.org featured an interview with Materials Engineering Professor Thomas Eagar.

e x c e r p t

title: The Collapse: An Engineer's Perspective

authors: NOVA and Dr. Thomas Eagar, critiqued by Jim Hoffman

NOVA: The Twin Towers collapsed essentially straight down. Was there any chance they could have tipped over?
Eagar: It's really not possible in this case. In our normal experience, we deal with small things, say, a glass of water, that might tip over, and we don't realize how far something has to tip proportional to its base. The base of the World Trade Center was 208 feet on a side, and that means it would have had to have tipped at least 100 feet to one side in order to move its center of gravity from the center of the building out beyond its base. That would have been a tremendous amount of bending.

site: www.PBS.org page: 911research.wtc7.net/disinfo/experts/articles/eagar_nova/nova_eagar1.html

There are several problems with Eagar's argument.

The core of each tower was 87 by 133 feet wide. The South Tower's top started to lean in the direction of its shorter dimension, so it only had to lean about 44 feet, on average, before its center of gravity cleared the core structure. Photographs show that the top did indeed lean that much before it was engulfed in the exploding cloud of rubble.
The fact that the South Tower's top was already rotating meant that it should have continued to rotate and rotate faster, as any toppling object does. Yet instead of continuing to topple, it started to righted itself as it began to explode.
Regardless of whether the top of either tower was tipping, there is no explanation for the destruction of the cores, which would have survived the pancaking of the floor diaphragms.
Eagar's argument does not begin to explain the explosiveness of the collapses.

page last modified: 2006-08-18

Figures 2-20 through 2-22 from Chapter 2 of FEMA's Building Performance Study

Photograph from The Behaviour of Multi-storey Composite Steel Framed Structures in Response to Compartment Fires

This illustration from the NOVA interview with Professor Eagar replaces the columns of the Tower's core structure with a series of horizontal slabs.