Thermal weakening of structural steel is a crucial element of the official theory of the collapse of the Twin Towers and Building 7. Since there are no examples of steel-framed buildings totally collapsing due to fire stress (outside of these three alleged examples) defenders of the theory frequently cite the McCormick roof collapse incident.

McCormick Place is a warehouse-type building that housed a large exhibition hall. It had a long-span roof supported by web trusses. When a fire broke out in an exhibition with many flammable displays it rapidly spread, and a portion of the roof collapsed within 30 minutes.

Comparisons of the McCormick Place incident to the collapses of the Twin Towers are sometimes made because the floor diaphragms on that constituted most tenant-space floors in the Towers were also supported by web trusses.

The first fact that should be noted in regard to any such comparison is that the McCormick Place incident was not a total building collapse -- it was only a roof collapse. Much less was it the total collapse of a high-rise building. Any comparison of it to the Twin Towers is limited to the Towers' floor diaphragms. FEMA blamed the heat-induced failure of the Towers' floor diaphragms, but failed to provide a convincing explanation of how floor failures could have led to total building collapse. Moreover, the alleged failure of the Towers floor trusses has lost relevance with NIST's endorsing the column failure theory to the exclusion of the truss failure theory.

Furthermore, the comparisons of the roof trusses of McCormick Place to the floor trusses of the Twin Towers is limited by the following facts:

The floor trusses were insulated, unlike the roof trusses.
The floor trusses spanned at most 60 feet, apparently much shorter than the roof trusses.
The floor trusses had to support the floor loads of the concrete slabs and office furniture, whereas the roof trusses only had to support snow loading.

page last modified: 2006-12-23