`Structural details=== Construction of the towers began in 1968 and was completed in 1970 (1 WTC) and 1972 (2 WTC). 1 and 2 WTC were nearly identical structures. Both had 110 stories, although 1 WTC was 1,368 feet (417.0 meters) tall and 2 WTC was 1,362 feet (415.1 m) tall. 1 WTC also supported a 360 foot tall (110 m) antenna.  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 towers were designed as framed tube structures, with columns grouped around the perimeter and within the core.  The perimeter columns supported virtually all lateral loads, such as wind loads, and shared the gravity loads with the core columns. It was said that Live load on those columns could have been increased more than 2,000% before failure occurred. “The World Trade Center towers would have an inherent capacity to resist unforeseen calamities. [..] For the perimeter columns (83% of total columns)”, “live loads on these columns can be increased more than 2,000% before failure occurs.” One “could cut away all the first story columns on one side of the building, and partway from the corners of the perpendicular sides, and the building could still withstand design live loads and a 100 mph wind from any direction.” All columns were founded on bedrock, which unlike Midtown Manhattan, where the bedrock is shallow, is at 65 feet 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 columns were connected with deep spandrel plates, which were typically 52 inches (1.3 m) deep. 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 perimeter was assembled from prefabricated modules consisting of three columns and three spandrel plates.  The spandrel plates were welded to the columns at the fabrication shop.  These modules 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 joints between modules were staggered vertically, so the column splices between adjacent modules were not at the same floor.  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 pre-fabricated floor trusses. The trusses had a span of 60 feet (18.2 m) in the long-span areas and 35 feet (11.0 m) in the short span area.  The trusses connected to the perimeter at alternate columns, and were therefore 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.  Approximately 10,000 of the trusses were also connected to the perimeter columns at their bottom chord through viscoelastic dampers to reduce wind-induced sway for occupant comfort.  The trusses supported a 4-inch-thick (10 cm) lightweight concrete floor slab, with shear connections for composite action.    The towers also incorporated a ``hat truss`` or ``outrigger truss`` located between the 107th and 110th floors.  The truss system consisted of six trusses along the long axis of 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. N