As an interstate agency, the Port Authority was not subject to local laws and regulations of the City of New York, including building codes. Nonetheless, the Port Authority required architects and structural engineers to follow the New York City building codes. At the time when the World Trade Center was planned, new building codes were being devised to replace the 1938 version that was still in place. The structural engineers ended up following draft versions of the new 1968 building codes, which incorporated "advanced techniques" in building design.
The World Trade Center towers included many structural engineering innovations in skyscraper design and construction, which allowed the buildings to reach new heights and become the tallest in the world. Traditionally, skyscrapers used a skeleton of columns distributed throughout the interior to support building loads, with interior columns disrupting the floor space. The tube-frame concept, earlier introduced by Fazlur Khan, was a major innovation, allowing open floor plans and more space to rent. The buildings used high-strength, load-bearing perimeter steel columns called Vierendeel trusses that were spaced closely together to form a strong, rigid wall structure. There were 60 perimeter columns, narrowly spaced, on each side of the buildings. In all, the perimeter walls of the towers were 210 feet (64 m) on each side, and the corners were beveled. The perimeter columns were designed to provide support for virtually all lateral loads (such as wind loads) and to share the gravity loads with the core columns. Structural analysis of major portions of the World Trade Center were computed on an IBM 1620.
The perimeter structure was constructed with extensive use of prefabricated modular pieces, which consisted of three columns, three stories tall, connected by spandrel plates. The perimeter columns had a square cross section, 14 inches (36 cm) on a side, and were constructed of welded steel plate. The thickness of the plates and grade of structural steel varied over the height of the tower, ranging from 36,000 to 100,000 pounds per square inch (260 to 670 MPa). The strength of the steel and thickness of the steel plates decreased with height because they were required to support lesser amounts of building mass on higher floors. The tube-frame design required 40 percent less structural steel than conventional building designs. From the 7th floor to the ground level, and down to the foundation, the columns were spaced 10 feet (3 m) apart. All columns were placed on bedrock, which, unlike that in Midtown Manhattan, where the bedrock is shallow, is at 65–85 feet (20–26 m) below the surface.
The spandrel plates were welded to the columns to create the modular pieces off-site 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, transmitting shear stress between columns, 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 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 after the 66th floor, and consisted of welded box-sections at lower floors and rolled wide-flange sections at upper floors. The structural core in 1 WTC was oriented with the long axis east to west, while that of 2 WTC was oriented north to south. All elevators were located in the core. Each building had three stairwells, also in the core, except on the mechanical floors where they were located outside the core.
The large, column-free space between the perimeter and core was bridged by prefabricated floor trusses. The floors supported their own weight, as well as live loads, provided lateral stability to the exterior walls, and distributed wind loads among the exterior walls. The floors consisted of 4-inch (10 cm) thick lightweight concrete slabs laid on a fluted steel deck with shear connections for composite action. A grid of lightweight bridging trusses and main trusses supported the floors. The trusses had a span of 60 feet (18 m) in the long-span areas and 35 feet (11 m) in the short span area. The trusses connected to the perimeter at alternate columns, and were on 6-foot-8-inch (2.03 m) centers. The top chords of the trusses were bolted to seats welded to the spandrels on the exterior side and a channel welded to the core columns on the interior side. The floors were connected to the perimeter spandrel plates with viscoelastic dampers, which helped reduce the amount of sway felt by building occupants.
Hat trusses (or "outrigger truss") located from the 107th floor to the top of the buildings were designed to support a tall communication antenna on top of each building. Only 1 WTC (north tower) actually had an antenna fitted, which was added in 1978. The truss system consisted of six trusses along the long axis of the core and four along the short axis. This truss system allowed some load redistribution between the perimeter and core columns and supported the transmission tower.
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