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– “why has there been not a single example of such failure anywhere , ever?”
Actually, numerous steel structures have collapsed; Crystal Palace springs to mind. The Twin Tower collapses were highly unusual, but so was the extent of the initiating damage and fire.
– “These fragile structures you mention seem to be remarkably robust”
Actually, the Twin Towers had about one third of the overall density of skyscrapers built in the 1930s, such as the Empire State building. Such earlier buildings also had internal support more evenly distributed, whereas the Twin Towers incorporated 22 metre floor spans for “open plan” office design. It was these stacked floor structures that underwent catastrophic runaway destruction. But we all know that things were built more solidly in the ’30s than the ’70s; just think of your old Ford Cortina.
The Twin Towers were built within the jurisdiction of the Port Authority of New York and New Jersey, and as such were not subject to the usual building regulations; that design would not have been permitted elsewhere. Some engineers such as Charlie Thornton were horrified by the design.