What is tube system ?
In structural engineering, the tube is the name given to the systems where in order to resist lateral loads (wind, seismic, etc.) a building is designed to act like a three-dimensional hollow tube, cantilevered perpendicular to the ground. The system was introduced by Fazlur Rahman Khan . Materials
The system can be constructed using steel, concrete, or composite construction (the discrete use of both steel and concrete). It can be used for office, apartment and mixed -use buildings. Most buildings in excess of 40 stories constructed since the 1960s are of this structural type.
Concept
The tube system concept is based on t he idea that a building can be designed to r esist lateral loads by designing it as a hollow cantilever perpendicular to t he ground. In the simplest incarnation of the tube, the perimeter of the exterior consists of closely spaced columns that are tied together with deep spandrel beams through moment connections. This assembly of columns and beams forms a rigid frame that amounts to a dense and stro ng structural wall along the exterior of t he building.
structural concept : BUNDLED TUBE Instead of one tube, a building consists of several tubes t ied together to resist the lateral forces. Such buildings have interior columns along the perimeters of the tubes when they fall within the building envelope. Example of this is the Willis Tower ( now called as Sears Tower ) The bundle tube design was not only highly efficient in ec onomic terms, but it was also "innovative in its potential for versatile formulation of architectural space. Efficient towers no longer had to be box-like; the tube-units could take on various shapes and could be bundled together in different sorts of groupings." The bundled tube structure meant that "buildings no longer nee d be boxlike in appearance: they could become sculpture."
SEARS TOWER
The Willis Tower draws its strength, both visual and physical, from its structural form, the bundled tube. The building plan consists of 9 squares, each 75 feet across, placed in a three-by-three grid arrangement Each square has 5 columns per side spaced 15 feet on centers, with adjacent squares sharing columns. As the columns rise up the building, each square in the plan forms a tube, which can be seen on the exterior of the building. These tubes ar e independently strong but are further strengthened by the interactions between each other through truss connections. While the tubes connectat each floor level with beams and floors trusses, several large tr ussed levels act as the main horizontal connectors in the buildings. These trussed levels, which also contain the mechanical systems for the building, appear as black horizontal bands on the façade (Figure 2). While the louvres covering the trussed levels mask the structural details, the purpose of these levels remains abundantly clear visually.
Purpose and advantages
One of the greatest concerns for the bundled tube system was achieving sufficient lateral stiffness with an economic use of steel. he choice of steel likely stemmed from the building practices of the time, which used steel for tall buildings rather than concrete in Chicago. Though the Sears Tower is significantly taller than the John Hancock Center, the structural system uses a comparable quantity of steel per unit area. The closely spaced interior and exterior columns are tied at each floor with deep spandrel beams. At the truss levels, these tubes are tied together.10 These ties resulted in a stiffer structure, as the building acts as a unified system of stiffened tubes. The interaction between the individual tubes and the belt trusses at mechanical levels allows the building to attain its extreme height. These trusses serve an additional purpose beyond stiffening the tube structure against winds. Due to the drop offs, the gravity loading on the system is not evenly distributed along the height of the building. These trusses take the gravity loads from above and redistribute them evenly onto the tubes below. This is particularly important for the uppermost section of the tower, due to its asymmetry about the central axis of the building. Because the section is o ffset, its weight causes columns on one side of the building
to experience a greater load than those on the other side. The presence of the belt trusses help to mitigate these effects of differential settlement, which cause the building to tilt.
