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
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].