AT 411M high, the New York World Trade Center's twin towers became the tallest buildings on the planet when completed in 1977 following a seven year construction programme.
The 110 storey towers were constructed from a steel lattice frame with a glass curtain wall and composite floors.
The conventional solution for handling wind loads - diagonal bracing or internal shear walls in the central core - were considered impractical, as the World Trade Centre is open plan at each floor.
Instead, each tower consists of 240 exterior wall columns spaced at 1,015mm centres with 1,340mm deep steel spandrels at each floor to form Vierendeel trusses. Around 180,000t of steel was used for the project.
Each 63.5m by 63.5m square tower was also thought to provide excellent torsional resistance.
Ten thousand innovative viscoelastic dampers were installed in each tower to control sway.
'These dampers or shock absorbers dissipate sway energy in the form of heat of friction, ' said a paper produced by Lester Feld, structural steel and concrete administrator for the project client New York Port Authority. The dampers are bolted between exterior wall columns and the end of the bottom chord of each floor truss.
The steel trusses are set in pairs at 1,850mm centres spanning up to 18m between the steel core and external wall.
Sitting on top of each truss was a permanent corrugated formwork deck used to support the 100mm thick lightweight concrete floor slabs. The top knuckle of each web acts as a shear connector by projecting 75mm into the concrete, to form a composite floor.
Moment connections at each corner of the building achieve a two-way truss action. Bridging trusses at 4,065mm centres provide further lateral bracing.
Designer Minoru Yamasaki Associates derived the towers' design from the IBM building in Seattle.
Yamasaki said of the building:
'The World Trade Center should, because of its importance, become a living representation of man's belief in humanity, his need for individual dignity, his belief in the co-operation of men, and through this co-operation his ability to find greatness.'