WTC (Twin Towers I)

The WTC towers held the height record only briefly. As the building neared completion in 1973, work had already begun on Chicago's Sears Tower, which ultimately reached 1,450 ft (442 m). With the World Trade Center's destruction, the Empire State Building again became the tallest building in New York, after spending almost 30 years as the third-tallest.

To solve the problem of wind sway or vibration in the construction of the towers, chief engineer Leslie Robertson took a then unusual approach - instead of bracing the buildings corner-to-corner or using internal walls, the towers were essentially hollow steel tubes. Each tower thus contained 240 vertical steel columns called Vierendeel trusses around the outside of the building, which were bound to each other using ordinary steel trusses. In addition, 10,000 dampers were included in the structure. With a strong shell such as this, the internal floors could be simply light steel and concrete with internal walls not needed for structural integrity, creating a tower that was extremely light for its size. This method of construction also meant that the twin towers had the world's highest load-bearing walls. The exterior steel supports were spaced 22 inches (559 mm) apart, and narrow windows filled the gaps in between.

Of the 110 stories, eight were set aside for technical services (mechanical floors), in four two-floor areas evenly spread up the building. All the remaining floors were free for open-plan offices. Each tower had 350,000 m sq (3.8 million ft sq) of office space, ample room for companies to set up shop. Altogether the entire complex of seven buildings had 1.04 km sq (11.2 million ft sq) of space. During the 1990s some 500 companies, especially financial firms, had offices in the complex, including Morgan Stanley, Aon Corporation, Salomon Brothers, and the Port Authority itself.