The Structural Systems oF Tall Buildings - Tall Buildings: Structural Systems and Aerodynamic Form - page 67

In tubular design, the rigidity of the structural system against lateral loads can be
increased with solutions such as:

• closer spacing of the perimeter columns
• increasing the depth of the spandrel beams connected to the perimeter columns
• adding shear trusses/braces or shear walls to the core
• adding an inner tube in place of the core (tube-in-tube)
• adding a truss (multi-storey braces) to the building exterior (trussed-tube)
• combining more than one tube (bundled-tube).

In tube systems, the tube formed around the building exterior is designed to resist all
lateral and vertical loads. If there is a structural core in the interior of the building, it
is assumed to support some part of the vertical loads. Adding a second tube instead
of a core can increase the stiffness of the structural system to support some part of the
vertical and lateral loads.
As well as its structural efficiency, in a tube system it increases the net usable area
of the building while reducing the dimensions of the structural elements in the core,
thanks to the tubular exterior frame supporting the entire lateral load. Tube systems
can be used in several geometrical forms like rectangular, square, triangular, circular
and even free-forms in the plan ( Figure 3.53 ).
Tube systems efficiently and economically provide sufficient stiffness to resist wind
and earthquake induced lateral loads in buildings of more than 40 storeys.
Tube systems can be divided into three types:

• framed-tube systems
• trussed-tube systems
• bundled-tube systems.

Figure 3.53

Some forms of tube systems