The boundary columns supported nearly all lateral loads, like the wind loads, and also shared the gravity loads with the core columns. In total, above the 7th floor there were 59 boundary columns along each face of the building and there were 47 heavier columns in the core. Besides, all of the elevators and stairwells were located in the core, leaving a large column-free space between the boundary that was bridged by pre-fabricated floor trusses. The tower also consisted of truss system that allowed some load redistribution between the perimeter and core columns and supported the transmission tower The towers also consisted of a "hat truss" or "outrigger truss" located between the 107th and 110th floors, which consisted of six trusses along the long axis of core and four along the short axis. It was found to play a key role in the collapse sequence (NIST 6-17, 2005). WTC towers like all modern day skyscrapers were designed in a perfet way to survive major fires. After the fire incidence in 1975 that spread to six floors before being extinguished, fireproofing was also added to the two towers (Hamburger 2:1-40).
It is said that while designing these towers, the possibility of any accedent by aircraft was also taken into consederation. John Skilling, who had been in charge of the structural design of the buildings said that an aircraft impact would cause a great deal of damage and loss of life, mainly because of the cunsequent fires, but he claiment that the structure would not collapse (Nalder n. pag).
Leslie Robertson, who had also participated in the structural design of the towers, after the 9/11 attack said that the towers had in fact been designed to withstand the impact of the largest airliner such as the Boeing 707-320. According to Robertson, the modeled aircraft weighed 263,000 lb (119 metric tons) with a flight speed of 180 mph (290 km/h), as in approach and landing. However, this would have been much slower than the actual impacts of 9/11. He also said that they lacked a good understanding and did not antiocipate the effects of such large fires on the structures (Robertson n. pag). Unfortunately, the designers Skilling and Robertson and many others had not anticipated sush an event while designing the buildings.
Impact of the Fire
Though both the towers were cinstructed in such a mannar with fireproofing, none of us anticipated such a attack. Once the aircrafts hit the towers the light construction and hollow nature of the structures allowed the jet fuel to penetrate deep inside the towers simultanously igniting many large fires over a wide area of the impacted floors. Once the fuel from the planes burned out the contents of the buildings cought fire and burned over the next hour or hour and a half (Field 2004). Many people has suggested that the fires may not have penetrated deep into the core of the building rathere the debris and fuel would likely have remained mostly outside the buildings or concentrated in more peripheral areas away from the building cores. But in this scenario, the towers might have stood far longer, perhaps indefinitely (Gross 2004). The fires were hot enough and were responsible for weakening the columns and cause floors to sag down resulting in pulling perimeter columns