The Structural Systems oF Tall Buildings - Tall Buildings: Structural Systems and Aerodynamic Form - page 37

Mega columns can also be used solely to provide large spaces at the building
entrance, as an aid to the main structural system for the levels above the entrance,
without running continuously throughout the height of the building. As the number
of mega columns at the entrance is much lower than the number of columns on the
upper storeys, the structural transition between them is achieved using deep transfer
beams. In such cases, the cross-sectional dimensions of the column at the entrance
are large enough for it to be classified as a “mega column”, but the structural system
cannot be classified as a “mega column system”.
Tall buildings where this approach has been used include the 63-storey, 283 m
high Cheung Kong Centre (Hong Kong, 1999) ( Figure 3.24 ), which has an outriggered
frame system and 8 composite mega columns at the ground floor with 2.5 m diameter
circular cross-sections, and the 59-storey, 279 m high Citigroup Center (New York,
1977) ( Figure 3.66 ), which has a trussed-tube system and 4 steel mega columns at the
ground floor with rectangular cross-sections of approximately 6.5×7 m.
Mega columns, in cases where they run continuously throughout the height of the
building, can be used with an outriggered frame system or a tube system. In such
cases, when they are used for a purpose such as reducing the number of columns,
the structural system cannot be classified as “mega column system”, since the mega
columns are not the only structural elements that resist the external loads. Tall
buildings with outriggered frame systems include:
• the 101-storey, 508 m high Taipei 101 (Taipei, 2004) ( Figure 3.36 ), which has 8
composite mega columns at the ground floor with rectangular cross-sections of
2.4×3 m
• the 88-storey, 421 m high Jin Mao Building (Shanghai, 1999) ( Figure 3.40 ), which
has 8 composite mega columns at the ground floor with rectangular cross-
sections of 1.5×4.9 m
• the 88-storey, 412 m high Two International Finance Centre (Hong Kong, 2003)
( Figure 3.41 ), which has 8 composite mega columns at the ground floor with
rectangular cross-sections of 2.5×3.5 m.
3.8 mega core systems
Mega core systems consist of reinforced concrete or composite core shear walls with
much larger cross-sections than normal, running continuously throughout the height
of the building ( Figure 3.25 ). Since the mega core can resist all vertical and lateral
loads in this system, there is no need for columns or shear walls on the perimeter of
the building. In mega core systems, floor slabs are cantilevered from the core shear
wall ( Figure 3.25a ). Mega core systems can also be used with strengthened cantilever
slabs ( Figure 3.25b ). In this case, floor slabs are supported by the core shear walls and
discontinuous perimeter columns. Perimeter columns are supported by strengthened
cantilever slabs repeated on some storeys. Strengthened cantilever slabs protrude
from the core, and are strengthened in order to support the load coming from the
storeys above.