The core and perimeter columns were connected by lightweight, weblike floor supports called trusses at each floor. The trusses held up corrugated metal decks on which the concrete floors were poured. The same trusses provided lateral support for all the vertical columns, preventing them from buckling under the tremendous force of gravity.
Wide plates called spandrels tied the exterior steel columns together, creating a rigid surface that could resist hurricane-force winds.
All of these structural elements would become fateful as the jets plowed into the north tower at 8:46am and the south tower at seconds before 9:03am.
The report -- assembled with data collected at ground zero, in scrapyards and laboratories, and by analyzing more than 100 hours of videotapes and by interviewing to witnesses -- turned up the greatest amount of detail on south tower attack.
The United Airlines Flight 175, its wings slightly canted, angled into the south facade of the south tower, slicing through about 30 of the 59 exterior columns on that face. The immediate damage, probably including unseen devastation to the steel core, stretched from the 78th to the 84th floor.
The impact of plane, which had been traveling as fast as 940kph, was so great that it gathered office material like a snowplow and apparently forced it toward the northeast corner of the building. Parts of the plane came to rest there and others punctured the far wall, soaring as far as six blocks to the north before hitting the ground near the intersection of Murray and Church streets.
A fuel-fed fireball emerged from three sides of the tower and consumed roughly one-third of the estimated 38,000 liters on the plane. Some of the rest flowed down the face of the building and into elevator shafts and stairwells. What remained burned ferociously, setting acres of office space afire as well as the luggage and other items that had been carried by the plane.
The incredible energy generated by this blaze was estimated at its peak to be between three and five gigawatts. A typical nuclear power plant generates about one gigawatt. All of that energy was converted to deadly heat that began weakening the steel.
But the tower did not fall immediately. Preliminary calculations by the engineering team have revealed that the tight spandrel connections, built to resist the wind, gave the building a remarkable ability to redistribute loads from severed columns to those that remained intact. A separate structure at the top, called a hat truss, also allowed loads to pass between the exterior and the core.
This rearrangement was so efficient, the calculations show, that stresses on columns no more than 6m from the hole punched in the tower's face were scarcely above what they carried before the damage occurred. Like a horse with a bum leg, the buildings, though wounded, still stood.
But the fires continued to burn. Black smoke poured from shattered windows on floor after floor, fresh oxygen sucked in from the gaping holes caused by the impacts. In the northeast corner of the building's 80th floor, where office furniture had been shoved by the plane, the fire burned so hot that a stream of molten metal began to pour over the side like a flaming waterfall.