The World Trade Center towers, hereafter referred to as WTC 1 and WTC 2, were regarded as revolutionary when they were constructed in the late 1960s. Within the next two decades, five additional buildings were constructed in the World Trade Center Plaza. Among these was World Trade Center 7 (WTC 7) built in 1985 located north of WTC1 and WTC 2 across Vesey Street. The structural systems incorporated in the 110-story towers designed by Minoru Yamaski & Associates were unique, the process of construction was the first of its type, and the vertical fenestrations that adorned the skyscrapers aided in the towers' structural system and were very aesthetic. WTC 1 even garnered the title of world's tallest building upon its completion. Tragically, the Towers have also come to be known as the site of the largest loss of life from any single building event in United States history. This report will focus upon the structural design, the transference of vertical and lateral loads, and the failure mechanisms of WTC 1, WTC 2, and WTC 7.
[...] Once this process had been started, the collapse of the World Trade Center towers was inevitable. Fifty-six minutes after the attack, WTC 2 began to collapse. Twenty-nine minutes later, WTC 1 followed. At 5:20 PM EDT, WTC 7 began collapsing after burning for seven hours. WTC 7 did not collapse due to any direct structural damage from the attacks, however. The collapse of WTC 1 sent debris in all directions, some of which hit the south side of WTC 7. [...]
[...] As efficiently as the World Trade Center Towers had been designed, they eventually succumbed to the stresses resulting from the attacks of September and collapsed in a cloud of ash and debris. The effect caused by the impact of the Boeing 767-200ER aircrafts into the Twin Towers could not have been expected by the towers' designers. They had, however, considered the effect of an impact of a much smaller airplane in a Boeing 707. The greater speed, weight, and volume of fuel onboard the Boeing 767 resulted in a much more devastating impact and grave effect than the collision that the engineers had designed for. [...]
[...] Since there were no diagonal members in the towers' external walls, Vierendeel action was responsible for transferring the shear forces produced by the lateral loading between the walls. The towers' stiffness, therefore, was a result of the bending strength of the walls' structural members. Since the walls, themselves, transferred the shear stress, the WTC towers' structural frames can be modeled as tubular systems. (Figure The floors of WTC 1 and 2 outside of the central core were supported by main trusses, transverse trusses, and a metal decking. [...]
[...] As the FEMA report points out, “buildings are designed to withstand loading events that are deemed credible hazards.” When the WTC towers were constructed; such an attack was not considered a “credible hazard.” Hopefully, the next buildings on the scale of the World Trade Center towers will. The collapse of the World Trade Center towers is much more than a mere case- study in structural mechanics and design. September 11th is one of the darkest days in the history of the United States and the world as a whole. [...]
[...] In fact, the World Trade Center towers were some of the first structures to be designed with the aid of wind tunnels. One difference in their designs arose due to the fact that each building affected the wind loads on the other. WTC 1 and WTC 2's use of spandrel plates with short spans but considerable depth allowed the towers great stiffness laterally as well as vertically. Under lateral loading, such as that applied by wind, the windward side acted in tension with the leeward side in compression. [...]
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