Tall Building case studies - Tall Buildings: Structural Systems and Aerodynamic Form - page 135

to carry the lateral loads was first used by the structural engineer Fazlur Rahman
Khan in the structural design of the John Hancock Center. The expressive trussed-tube
structure of the building was integrated with the architectural design and became
prominent. The John Hancock Center represents an icon of Chicago and symbol
of structural expression in architecture. Fazlur Rahman Khan commented that “The
social and visual impact of buildings is really my motivation for searching out new
structural systems and to get the right visual impact, a building's natural strength
should be expressed” (Mufti and Bakht, 2002) . The John Hancock Center was granted
the “Distinguished Building Award” in 1970 by AIA (American Institute of Architects).
The John Hancock Center in Chicago has an important place among tall buildings.
The exterior mega cross braces of the trussed-tube system, which can be seen on the
tapering facade, emphasise the architectural aesthetic of the structural system and are
the most striking design feature. The building is an architectural icon, with a sloping form
(truncated pyramid), narrowing as it rises and symbolises the integration of structural
expression with the architecture. Thanks to its original design, in 1970 it won the “Honor
Award of the AIA Chicago Chapter” in the architectural and structural engineering
section and in 1999 it won the “25-Year Architectural Excellence Award of the AIA” for
its preservation of architectural excellence and authenticity for more than 25 years.
Bruce Graham (the architect) and John Hancock Insurance (the owner) are said
to have wanted to remove the exterior X-braces on the upper 10 storeys of the
building because they blocked the view; however, Fazlur Rahman Khan (the structural
engineer), aiming to integrate the architecture with the structural system, asserted
that the structure would give great aesthetic value to the building and convinced the
architect and owner to repeat the exterior braces, consisting of trusses, continuously
throughout the height of the building. Thanks to Fazlur Rahman Khan's design, the
continuous X-braces expressed on the facade with their reduced dimensions and
cross-sectional areas towards the top of the building contribute to the design approach
of the tapered form of the building, which narrows as it rises, creating the impression
that the building is taller than it is in reality.
Mega X-braces on the facade, designed as truss elements with 45º angles between
them, support a large part of the wind induced lateral loads that are converted to
an axial load and also support a part of the vertical load, ensuring the system's pure
tubular behaviour.
The John Hancock Center's truncated pyramid form and thus reduction of the floor
plan area, has several advantages. The placing of office areas, which need large spaces
and long lease spans, on the lower floors and residential areas, which need smaller
spaces and shorter lease spans, on the upper floors, provides a continuous relationship
between form and function. With the tapering form by the inward slope of the facade,
the surface area affected by the wind on the higher levels of the building is reduced,
as is the wind intensity and thus the excess wind pressure/load ( Section 6.1 ). In calcu-
lating the aspect ratio (the ratio of the structural height of a building to the narrowest
structural width on the floor plan), because the widest dimensions of the floor plan of
the building were taken as a basis, the wide ground floor layout improves this ratio
and thus the slenderness and flexibility of the building. Due to the widening of the
building facade as it descends towards the ground, the potential for the view from the
higher floors to cause dizziness and vertigo is reduced.