The structural engineering firm Worthington, Skilling, Helle Jackson, worked to implement Yamasaki designing, developing structural tube frame used on the facade of the Twin Towers. The engineering department of the as project engineers recruited Joseph R. Loring Associates as an electrical engineer, and Jaros, Baum Bolles as mechanical engineers. Tishman Realty Construction was the general contractor on the project of the WTC. Guy F. Tozzoli, director of the World Trade Department and the Port Authority, and Rino M. Monti, the chief engineer of the Port Authority, oversaw the project. As a state agency, the Port Authority was not subject to local laws or regulations of the City of New York including building codes.The tube frame design was introduced by Fazlur Khan, and became a new commitment of engineering that allowed the construction of buildings more “open” instead of columns distributed throughout the interior to support the building loads as traditionally had done. Modeling of the outer panels of the Twin Towers. The Twin Towers used the height as the main attraction: for that the engineers made steel columns at the perimeter, called “lattice-Vierendeel” which allow, due to its design, distribute the total load of the building. They were designed rectangular to form a strong structure, reinforced later by the walls, all of which allows the lateral loads caused by wind resistance, are absorbed by the building’s facade, thus leaving the main columns as the heart of the structure.The entire perimeter that contains 59 columns per side, was built using “dry modules” which had the advantage of being anti fire but both were very weak. The plates were welded to the columns in order to create modular offices inside the towers. The so-called dry modules were thus enclosing the main columns, all that is conveyed in a rigid main structure. The modules were located on each floor, which helps, together with the structure of the facade, to convey the tension between columns, allowing the entire building to resist lateral loads. The bonds between the different modules were made vertically. The heart of the tower housed the elevators and their engines, services, ladders, and other support spaces. The main columns were made of steel combined with the concrete lining.
Each tower was a rectangular area of 27 by 41 m and contained 47 steel columns running from the base to the top of each tower.The more open central space, no columns between the perimeter and the heart of the building was shortened by prefabricated floor trusses. The floors supported all his weight on the structure of the facade, providing lateral stability to the outer walls and distributing wind resistance between the outer tube frames. The floors consisted of thick concrete slabs 10 cm thick, very light resting on a ledge in the steel structure. A lightweight grid gave strength to the floors. The trusses connected to the perimeter at alternate columns, were at 2.03 m from the center. The floors were supported in seats welded to the front of the building, while the main columns in the interior. The floors were so firmly anchored to the front and kept it off to the outside while also helping to reduce the amount of sway felt by the constant traffic of tenants.The trusses were located from the 107th floor to the top of the buildings, which were designed to support high communications antenna on top of each tower. In fact only the north tower antenna was anchored in the center of the roof, which was added in 1978. The truss system consisted of six trusses along the main axis and four along the shaft. This truss system allowed the redistribution of load between the perimeter and the main columns, while supporting the transmitting antenna. The main pillars sustaining the building and perimeter columns were protected by spraying foam on them resistant to fire, all that allowed a relatively lightweight structure that is more balancear a in response to wind compared to traditional rigid structures like the Empire building State, which is quite heavy for its masonry and make it a structure fire resistant, yet heavy.During the process of design, wind tunnel tests were done to establish wind pressures which would be exposed once constructed buildings. The experiments were also done to evaluate how much sway could comfortably tolerate tenants of the towers, but many tend to suffer from vertigo which works against the construction of skyscrapers in general.