The fact is we'll never really know. It's fun indeed to speculate sure, but to claim you know what happened like people here are doing is just asinine.
Oh look Brink the coward is back to give us his knowledge about facts like he did here:
Engineers don't design structures that size to be 'just strong enough'. That's ridiculous. And it's not the steel that protects the building from gravity, it's the concrete columns which have tremendous compressive strength. Steel is merely meant to keep the concrete from twisting apart and to protect its integrity from vector type forces. Any engineer will tell you that.
Physics!!!!!
Again you are wrong. You are drawing conclusions from incorrect assumptions and inaccurate information. If you don't like the wiki proof there is more i can post from actual engineers to show how wrong you are.
https://en.wikipedia.org/wiki/Construction_of_the_World_Trade_CenterThe World Trade Center towers included many structural engineering innovations in skyscraper design and construction, which allowed the buildings to reach new heights and become the tallest in the world. Traditionally, skyscrapers used a skeleton of columns distributed throughout the interior to support building loads, with interior columns disrupting the floor space. The tube-frame concept, earlier introduced by Fazlur Khan, was a major innovation, allowing open floor plans and more space to rent.
The buildings used high-strength, load-bearing perimeter steel columns called Vierendeel trusses that were spaced closely together to form a strong, rigid wall structure. There were 60 perimeter columns, narrowly spaced, on each side of the buildings. In all, the perimeter walls of the towers were 210 feet (64 m) on each side, and the corners were beveled. The perimeter columns were designed to provide support for virtually all lateral loads (such as wind loads) and to share the
gravity loads with the core columns.[48] Structural analysis of major portions of the World Trade Center were computed on an IBM 1620.[49]
The perimeter structure was constructed with extensive use of prefabricated modular pieces, which consisted of three columns, three stories tall, connected by spandrel plates. The perimeter columns had a square cross section, 14 inches (36 cm) on a side, and were constructed of welded steel plate.[50] The thickness of the plates and grade of structural steel varied over the height of the tower, ranging from 36,000 to 100,000 pounds per square inch[51] (260 to 670 MPa). The strength of the steel and thickness of the steel plates decreased with height because they were required to support lesser amounts of building mass on higher floors.[50] The tube-frame design required 40 percent less structural steel than conventional building designs.[52] From the 7th floor to the ground level, and down to the foundation, the columns were spaced 10 feet (3 m) apart.[53] All columns were placed on bedrock, which, unlike that in Midtown Manhattan, where the bedrock is shallow, is at 65–85 feet (20–26 m) below the surface.[54]
The spandrel plates were welded to the columns to create the modular pieces off-site at the fabrication shop.[55] The modular pieces were typically 52 inches (1.3 m) deep, and extended for two full floors and half of two more floors.[50] Adjacent modules were bolted together, with the splices occurring at mid-span of the columns and spandrels. The spandrel plates were located at each floor, transmitting shear stress between columns, allowing them to work together in resisting lateral loads. The joints between modules were staggered vertically, so the column splices between adjacent modules were not at the same floor.[50]
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The tube frame design using steel core and perimeter columns protected with sprayed-on fire resistant material created a relatively lightweight structure that would sway more in response to the wind, compared to traditional structures such as the Empire State Building that have thick, heavy masonry for fireproofing of steel structural elements
