About this Article
Written by: Molly Kistler
Written on: November 1st, 2002
Tags: building & architecture, civil engineering, security & defense
Thumbnail by: Sander Lamme/Wikipedia Commons
About the Author
In Fall 2002, the author was a student at USC.
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Volume II Issue IV > Why The World Trade Center Towers Collapsed
On September 11, 2001, two Boeing 757's crashed into the World Trade Center's two high-rise towers. In addition to the structural damage the impacts caused, the leaking jet fuel led to extensive fires and rapid explosions in the buildings. The towers collapsed within two hours of the attack. While it may take investigators years to determine why the structures failed, we can examine the towers' structural design and some of the theories that have already been proposed to form our own hypothesis about the cause of the collapse.


The World Trade Center (WTC) towers 1 and 2 in New York City used to be the tallest buildings in the world. On September 11, 2001, a heinous act of terrorism directed at the United States brought the buildings plummeting to the ground. The terrorists hijacked two planes and flew them into the WTC North (1) and South (2) Tower. Fires and explosions ensued and eventually the buildings collapsed on themselves in a sooty, smoky mess. While the actual cause for the buildings' collapse is unknown, there are many theories about what ultimately led the two structures to fall. Before we can read theories concerning the cause of the collapse, we must examine the structure of the buildings and what they were designed to withstand.


The World Trade Center complex, which was officially opened on April 4, 1973, and consisted of seven individual buildings surrounding a cement plaza. The architect, Minoru Yamasaki, had been commissioned by the New York and New Jersey Port Authorities to create a complex providing 12 million square feet of office space on a 16-acre site [1]. Sixteen acres is approximately 700,000 square feet, or about the size of 12 football fields, so it was obvious to Yamasaki that this area would have to house a high-rise of some sort. He toyed with a few design ideas, and finally decided on the 7-building complex, in which two of the buildings would be 110 floor towers, containing a combined 9 million square feet of the required office space [1]. John Skilling and Leslie E. Robertson of the Seattle-based engineering firm Worthington, Skilling, Helle, and Jackson were hired to complete the actual structural design.


Anonymous. Courtesy of the Prints and Photographs Division. Library of Congress./Library of Congress
Figure 1: World Trade Center towers.
The whole complex was built over a six-story basement foundation [2], which ran 23 meters deep and housed shops, subway tunnels, and a parking garage . To accommodate such a foundational structure, 1.2 million cubic yards of earth and rock were removed from the site and relocated to a fill spot in the Hudson River; this spot has since become Battery City Park [2].
The structures were designed as a combination of three main components: the perimeter columns, the interior core column, and the floors. Each of the WTC Towers (Fig. 1) was constructed in a similar tubular formation, reaching heights of 1,362 feet and 1,368 feet for WTC 1 and 2 respectively [2]. Four 208-foot facades consisting of prefabricated steel lattices formed each building [1], and collectively they were the world's highest load-bearing walls [2]. The lattices were made up of more than 130 closely spaced vertical cantilevered steel columns [3], creating a "hollow tube" structure. Each column was itself a hollow 14-inch square box-shaped length of steel and its center was placed 39 inches from the center of the next column [2]. These exterior columns served as wind bracers for the building to resist overturning forces [1] (e.g. high speed winds). In the spaces between the columns were 43,600 narrow, slot-like windows, which made up to 30% of the facade. Each tower's inside core column, measuring 89 feet x 139 feet [4], was created to support only the gravity load [1] and the elevator system, and not the main structure. This design left the office space free of constricting interior columns.
Prefabricated steel trusses, 33 inches in depth, were welded to the exterior columns and supported the 3 inch deep concrete floor plates of the two towers. These floors spanned the entire 60 feet to the core column and helped to stiffen the exterior columns from buckling during extreme wind conditions. In all, the two towers required over 180,000 metric tons of structural steel.