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The World Trade Center in New York City, United States, (sometimes informally referred to as the WTC or the Twin Towers) was a complex of seven buildings in Lower Manhattan, mostly designed by architect Minoru Yamasaki and engineer Leslie Robertson and developed by the Port Authority of New York and New Jersey. It was initiated in 1960 by a Lower Manhattan Association created and chaired by David Rockefeller, who had the original idea of building the center, with strong backing from the then-New York governor, his brother Nelson Rockefeller.The World Trade Center, New York, like most World Trade Centers located around the globe, belonged to the family of World Trade Centers Association. Prior to its destruction, Larry Silverstein held the most recent lease to the complex, the Port Authority having leased it to him in July 2001.The complex, located in the heart of New York City's downtown financial district, contained 13.4 million square feet (1.24 million m?) of office space, almost four percent of Manhattan's entire office inventory at that time.

Best known for its iconic 110-story twin towers (101 usable floors, eight engineering-only "service" floors on top of a lobby which was three stories high) the World Trade Center was beset by a fire on February 13, 1975 and a bombing on February 26, 1993.

All seven original buildings in the complex were destroyed by terrorists linked to Al-Qaeda on September 11, 2001. Three of the buildings collapsed: One World Trade Center (1 WTC, the North Tower), Two World Trade Center (2 WTC, the South Tower), and 7 World Trade Center (7 WTC). The Marriott World Trade Center (3 WTC) was crushed by the collapses of 1 WTC and 2 WTC. 4 World Trade Center (4 WTC), 5 World Trade Center (5 WTC), and 6 World Trade Center (6 WTC) were damaged beyond repair and later demolished. Three buildings not part of the complex were also destroyed: St. Nicholas Greek Orthodox Church was destroyed by the collapse of 2 WTC. The Deutsche Bank Building and Borough of Manhattan Community College's Fiterman Hall were damaged beyond repair by the collapse of 7 WTC; these are currently being deconstructed.

1.1 Planning and construction

During the post-World War II period, the United States thrived economically, with increasing international trade. At the time, economic growth in New York City was concentrated in Midtown Manhattan, with Lower Manhattan left out. To help stimulate urban renewal, David Rockefeller, with support from his brother, New York Governor Nelson Rockefeller, suggested that the Port Authority should build a "world trade center" in Lower Manhattan.[6] Initial plans, made public in 1961, identified a site along the East River for the World Trade Center. Objections to the plan came from New Jersey Governor Robert B. Meyner, who resented that New York would be getting this $335 million project. Meanwhile, New Jersey's Hudson and Manhattan Railroad (H&M) was facing bankruptcy. Port Authority executive director, Austin J. Tobin agreed to take over control of the H&M Railroad, in exchange for support from New Jersey for the World Trade Center project.

With this acquisition, the Port Authority would obtain the Hudson Terminal, and decrepit buildings located above the terminal in Lower Manhattan. The Port Authority decided to demolish these buildings, and use this site along the Hudson River for the World Trade Center.The towers in the complex were designed by architect Minoru Yamasaki with Antonio Brittiochi and Emery Roth & Sons as associate architects. The World Trade Center was one of the most striking American implementations of the architectural ethic of Le Corbusier, as well as the seminal expression of Yamasaki's gothic modernist tendencies.

a. Structural design (see annexe 1.1. a)

The World Trade Center included many structural engineering innovations in skyscraper design and construction. The towers were designed as framed tube structures. There was a frame of closely-spaced columns tied together by deep spandrel beams along the exterior perimeter. The interior had 47 columns, all concentrated in the core. Engineer Felix Samuely used a similar concept in his "Mullion wall" buildings in the early 1950s as did Eero Saarinen in his US Embassy, London (1955-60); but these projects were low to medium rise and may not have been influences.

The perimeter columns supported virtually all lateral loads, such as wind loads, and shared the gravity loads with the core columns. All columns were founded on bedrock, which unlike Midtown Manhattan, where the bedrock is shallow, is at 65 feet (20m) below the surface. Above the seventh floor there were 59 perimeter columns along each face of the building. The perimeter columns had a square cross section, 14 inches on a side (36 cm), and were constructed of welded steel plate. The thickness of the plates and grade of steel were varied over the height of the tower, ranging from 36 ksi to 100 ksi, with the steel strength and plate thickness decreasing with height. The perimeter structure was constructed with extensive use of prefabricated modular pieces, which consisted of three columns, three stories tall, connected together by spandrel plates. The spandrel plates were welded to the columns at the fabrication shop. The modular pieces were typically 52 inches (1.3 m) deep, and extended for two full floors and half of two more floors.

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, and served to transmit shear flow between columns, thus 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 building's core housed the elevator and utility shafts, restrooms, three stairwells, and other support spaces. The core in 1 WTC was oriented with the long axis east to west, while that of 2 WTC was oriented north to south. The core of each tower was a rectangular area 87 by 135 feet (27 by 41 m) and contained 47 steel columns running from the bedrock to the top of the tower. The columns tapered with height, and consisted of welded box sections at lower floors and rolled wide-flange sections at upper floors. All of the elevators and stairwells were located in the core.

The large, column-free space between the perimeter and core was bridged by prefabricated floor trusses. The floors supported their