The tube, as opposed to the core moves the lateral load resisting system to the perimeter of the steel structure rendering the internal structure secondary thus maximizing the lateral stiffness of the structure while maximizing the available floor space. The construction of tube structures will differ from that of core structures in that the steel floors are erected before the concrete floor is cast, where with a core, the steel floors follow behind the core construction. Wide experience gained with both methods shows that with proper planning the advantages and disadvantages are fairly well balanced.
The examples indicated in the following diagram represent, once again, a sample of many possible configurations which were selected to give the reader a cursory insight as to their applications, they include:
Here, the perimeter structure is constructed as a conventional rigid, moment resisting frame in both directions. Up until the development of the framed tube it limited building heights to no more than 30 storeys, beyond that height meant that the excessive material in the connections and splices was rendering the construction method uneconomic.
The framed tube was developed in the early to mid-sixties and enabled the structures to extend to 80 storeys and more. Structurally, the framed tube is superior to the rigid-frame in that it spaces the columns closer together and the horizontal members between the columns tend to be deeper than that for the more conventional framed structure. This results in a much stiffer structure while the lateral forces are spread out among the additional columns
Braced Frame or Exo-Skeleton
The braced frame is essentially a series of vertical trusses placed at the corners of the structure, which are tied at intervals by means of horizontal triangulated trusses. Such buildings, though structurally efficient, allowing buildings to be constructed at height of 90 storeys or more, may have an impact on the external surfaces of the building by limiting the vision of the architect. The bracing may interfere with window openings, thus dictating the interior layout.
The term is derived from ‘Diagonal Grid’ and was developed by the Russian engineer Vladimir Zhukov in the early 19th The diagrid is a very efficient matrix eliminating the need for large corner columns and is very stable under lateral load conditions. Its big advantage is that it reportedly uses less steel that any other form of high-rise construction and in the right environment can be aesthetically pleasing. The diagrid may be almost any shape provided its symmetrical, including square, round, oval, many examples of which can be seen around the world. The problem is that the connections may be quite complex, and if the shape is not square the connections may be even more so. However, the repetitive nature of this design can negate this disadvantage. Structures of this type may exceed 100 storeys.
The Bundled Tube is basically a number of separate tubes ‘bundled’ together, often these ‘bundled’ tubes are of differing heights creating a pleasing aesthetic. This type of construction results in interior columns which will, to some extent, dictate the interior layout of the building. Such structures may be erected to a height exceeding 110 storeys
The Braced Tube, also known as the ‘trussed tube’ allows the columns to be spaced further apart, effectively using fewer, but heavier, columns by tying them together with vertical bracing. The bracing system is generally designed for compression, meaning it will be heavy to resist bending. Such structures may be erected to a height exceeding 120 storeys