Structural details Construction of the towers began in  and was completed in   WTC and   WTC  and  WTC were nearly identical structures Both had  stories although  WTC was  feet  meters tall and  WTC was  feet  m tall  WTC also supported a  foot tall  m antenna  The core in  WTC was oriented with the long axis east to west while that of  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  before failure occurred The World Trade Center towers would have an inherent capacity to resist unforeseen calamities  For the perimeter columns  of total columns live loads on these columns can be increased more than  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  mph wind from any direction All columns were founded on bedrock which unlike Midtown Manhattan where the bedrock is shallow is at  feet below the surface  Above the seventh floor there were  perimeter columns along each face of the building  The perimeter columns had a square cross section  inches on a side  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  ksi to  ksi with the steel strength and plate thickness decreasing with height  The columns were connected with deep spandrel plates which were typically  inches  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 midspan 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  by  feet  by  m and contained  steel columns running from the bedrock to the top of the tower The columns tapered with height and consisted of welded boxsections at lower floors and rolled wideflange sections at upper floors  All of the elevators and stairwells were located in the core  The large columnfree space between the perimeter and core was bridged by prefabricated floor trusses The trusses had a span of  feet  m in the longspan areas and  feet  m in the short span area  The trusses connected to the perimeter at alternate columns and were therefore on  foot  inch  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  of the trusses were also connected to the perimeter columns at their bottom chord through viscoelastic dampers to reduce windinduced sway for occupant comfort  The trusses supported a inchthick  cm lightweight concrete floor slab with shear connections for composite action    The towers also incorporated a hat truss or outrigger truss located between the th and th 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