Tall Building case studies - Tall Buildings: Structural Systems and Aerodynamic Form - page 121

transparency”. The resulting design is a building with an inner glazed facade and
an outer transparent screen layer composed of closely spaced horizontal ceramic
rods mounted outboard of the facade. Ceramic rods are not only features of the
architectural design, but are also sun shading without disturbing the transparency of
the building. In this way, the transparent double-skin facade system with the shade
screen provides the connection between the journalists working in the building and
the people outside. Moreover, the transparency of the building is emphasised also by
expressing the structure in large corner notches (recessions) by X-braces and columns
where the exterior screen layer is omitted.
Above the lobby at the ground floor, floors 2-4 are the newsrooms, 5-27 and
29-50 are for the building occupants. Outriggers and mechanical rooms are located
at floors 28 and 51.
The structural core is a centrally located braced core with 27.4×19.8 m in dimen-
sions in the north-south and east-west directions respectively. The least span between
the core and the perimeter columns on the long east and west faces is about 12 m.
North and south faces have cantilevered floor areas with 19.8×6.1 m in dimensions.
Columns on the east and west faces are spaced at 9.14 m centres ( Figure 4.22 ).
Although the structural core area remains constant throughout the building height,
above the 27th floor, since the elevators serving the lower half of the building are no
longer required, the area left for the service core decreases. This situation changes the
bracing configuration of the central core and so increases the space efficiency ratio of
the floors in the lower half (floors 5-27) and upper half of the building (floors 29-top)
from 0.746 to 0.833 respectively ( Figure 4.23 ) .
The Tower has 30 columns extending to the ground level. Exposed columns are
made of built-up box sections with dimensions of 76×76 cm. In order to improve the
lateral stiffness of the structural system, some of the columns at the lowest floors have
solid sections. Inner columns are wide-flange section columns. However, perimeter
columns of the cantilevered floor areas do not extend to the ground level.
Two-storey-deep outriggers are located at the mid-height and top mechanical floors
of the building. The lateral stiffness of the outriggered frame system is improved by
2-storey-high pretensioned X-braces on the north and south faces of the four notched
corners of the building. These braces reduce the maximum lateral drift/sway (drift
index) from 1/350 to 1/450 of the building height (90 to 70 cm).
The X-bracing system of the building is very efficient since both the diagonal steel
rods are pretensioned so that compression force cannot develop and both the braces
work simultaneously.
In general, conventional X-braces are composed of two single diagonal members
placed with a certain eccentricity while crossing each other at the middle of the
bay (which creates column torsion) ( Figure 4.24a ), or a crossing joint is placed
at the middle of the bay where the braces intersect (to overcome this problem)
( Figure 4.24b ). However, in the New York Times Building, X-braces are composed
of pairs of diagonal members by placing one pair side-by-side and the other pair
over-and-under ( Figure 4.24c ). In this way, the column torsion problem that gains
importance because of pretensioning is overcome.