Although it has been nine years since the terrorist attack on September 11, 2001, still this question still lingers in the minds of Conspiracy Theorists; and not without some justification, it seems. That the mighty towers came crashing down after being struck by Boeing commercial aircraft is, on second thought, quite implausible if the structural architecture is taken into consideration. According to WIKIPEDIA, The Twin Towers had a tube-frame structural design system with high-strength, load-bearing perimeter steel columns called Vierendeel trusses that were spaced closely together to form a strong, rigid wall structure, supporting virtually all lateral loads such as wind loads, and sharing the gravity load with the core columns.
The perimeter structure containing 59 columns per side was constructed with extensive use of prefabricated modular pieces each consisting of three columns, three stories tall, connected by spandrel plates. The spandrel plates were welded to the columns to create the modular pieces off-site at the fabrication shop. 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.
The core of the towers combined steel and concrete structures of each tower with a rectangular area 87 by 135 feet (27 by 41 m) and contains 47 steel columns running from the bedrock to the top of the tower. The large, column-free space between the perimeter and core was bridged by prefabricated floor trusses. The floors supported their own weight as well as live loads, providing lateral stability to the exterior walls and distributing wind loads among the exterior walls. The floors consisted of 4 inches (10 cm) thick lightweight concrete slabs laid on a fluted steel deck. A grid of lightweight bridging trusses and main trusses supported the floors. The trusses connected to the perimeter at alternate columns and were on 6 foot 8 inch (2.03 m) centers. The top chords of the trusses were bolted to seats welded to the spandrels on the exterior side and a channel welded to the core columns on the interior side. The floors were connected to the perimeter spandrel plates with viscoelastic dampers which helped reduce the amount of sway felt by building occupants. The trusses supported a 4-inch (100 mm) thick lightweight concrete floor slab with shear connections for composite action.
The impact from the aircraft smashing into the upper floors and the resulting fire from burning jet fuel would not be enough to melt steel girders and break down the reinforced concrete structures which were designed to withstand significant shear stresses, damaging a 107 story building to such an extent that it would come crashing down to the ground in a heap of twisted metal girders, dust and ash. Instead of the planes weakening the integrity of the buildings beyond a critical point, it is more likely that a simultaneous demolition-like detonatation of explosives at vulnerable areas, including possibly the elevator shafts, could be responsible for this tragedy. Even scientific tests, and forensic examination on any available evidence scattered among the remaining debris were not performed by independent, non-government laboratories and the results have not been made available to the general public.
So, yes, the Twin Towers were destroyed and over 3,000 American citizens of various nationalities, ethnicities and religions were indeed murdered by terrorists, but the real question remains largely unanswered, namely: Did all these former terrorists from Saudi Arabia orchestrate such a masterfully deadly plan on the American homeland by themselves or did they have help from inside the United States?
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September 28, 2010
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