In the 1990s, there was no New York City skyline without the Twin Towers, hobnobbing with Lady Liberty and the Empire State Building. But on 11 Stemebr 2001, it changed as terrorists plunged two commercial aircrafts into of World Trade Center’s Twin Towers resulting in uncontrollable fires leading to collapse of 110 floors. On the 20 anniversary of collapse, often referred to as 9/11, media is agog with reports of tributes to thousands who perished, the unanswered questions related to the cause of collapse and the role of steel failure is back in focus.The original World Trade Center was a large complex of seven buildings in the Financial District of Lower Manhattan in New York City in United States. The core complex was built between 1966 and 1975, at a cost of USD 400 million It opened on April 4, 1973. At the time of their completion, the Twin Towers, the original 1 World Trade Center (the North Tower) at 1,368 feet and 2 World Trade Center (the South Tower) at 1,362 feet, were the tallest buildings in the world. Other buildings in the complex included the Marriott World Trade Center 3 WTC, 4 WTC, 5 WTC, 6 WTC, and 7 WTC. The complex contained 13,400,000 square feet of office space.Port Authority announced the selection of Japanese origin Mr Minoru Yamasaki as lead architect and Emery Roth & Sons as associate architects in 1962. Mr Yamasaki's design for the World Trade Center was unveiled to the public on January 18, 1964. The buildings were designed with narrow office windows 18 inches wide. The structural engineering firm Worthington, Skilling, Helle & Jackson worked to implement Mr Yamasaki's design, developing the framed-tube structural system used in the twin towers. The framed-tube design, introduced in the 1960s by Bangladeshi-American structural engineer Fazlur Rahman Khan, was a new approach that allowed more open floor plans than the traditional design that distributed columns throughout the interior to support building loads. Each of the World Trade Center towers had 236 high-strength, load-bearing perimeter steel columns which acted as Vierendeel trusses. The perimeter columns were spaced closely together to form a strong, rigid wall structure, supporting virtually all lateral loads such as wind loads, and sharing the gravity load with the core columns. The perimeter structure containing 59 columns per side was constructed with extensive use of prefabricated modular pieces, each consisting of three columns, three stories tall, connected by spandrel plates. The spandrel plates were welded to the columns to create the modular pieces off-site at the fabrication shop. 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 that the column splices between adjacent modules were not on the same floor. Below the 7th floor to the foundation, there were fewer, wider-spaced perimeter columns to accommodate doorwaysThe core of the towers 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 and contained 47 steel columns running from the bedrock to the top of the tower. 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, providing lateral stability to the exterior walls and distributing wind loads among the exterior walls. The floors consisted of 4-inch thick lightweight concrete slabs laid on a fluted steel deck. A grid of lightweight bridging trusses and main trusses supported the floors. The trusses connected to the perimeter at alternate columns and were on 6 foot 8 inch 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 that helped reduce the amount of sway felt by building occupants. The framed-tube design, using steel core and perimeter columns protected with sprayed on fire resistant material, created a relatively lightweight structure that would sway more in response to the wind compared to traditional structures. Although each tower was struck by fuel laden Boeing 767, both towers withstood the initial impact of the jet without immediate collapse, with WTC 1 and WTC 2 ultimately collapsing 102 and 56 minutes after impact, respectively showing survival from initial impact loads and the resulting structural damage. Both towers suffered from a progressive collapse, where steel columns lost strength due to exposure to high temperatures caused by burning jet fuel, thus surpassing their structural load capabilities and leading to a pancaked collapse, floor by floor. In the aftermath of the disaster, a series of investigations were launched with the goal of improving building performance and safety. Among the changes to building codes adopted are considerations which many architects today would consider second nature.