linear-elastic material was a conser-
Tower in 1973.Khan's central innova-
World Trade Center, Petronas Twin
vative approximation of its behaviour.
tion in skyscraper design and
Towers and Taipei 101.
Concrete design and analysis has
construction was the idea of the "tube"
In 1987 Jörg Schlaich and Kurt
been progressing ever since, with the
and "bundled tube" structural systems
Schafer published the culmination of
development of analysis methods
for tall buildings. He defined the
almost ten years of work on the strut
such as yield line theory, based on
framed tube structure as "a three
and tie method for concrete analysis -
plastic analysis of concrete (as
d i m e n s i o n a l
s p a c e
s t r u c t u r e
a tool to design structures with discon-
opposed to linear-elastic), and many
composed of three, four, or possibly
tinuities such as corners and joints,
different variations on the model for
more frames, braced frames, or shear
providing another powerful tool for the
stress distributions in concrete in
walls, joined at or near their edges to
analysis of complex concrete geome-
compression.
form a vertical tube-like structural
tries.
Prestressed concrete, pioneered by
system capable of resisting lateral
In the late 20th and early 21st centu-
Eugène Freyssinet with a patent in
forces in any direction by cantilevering
ries the development of powerful
1928, gave a novel approach in over-
from the foundation. Closely spaced
computers has allowed finite element
coming the weakness of concrete
interconnected exterior columns form
analysis to become a significant tool
structures in tension. Freyssinet
the tube. Horizontal loads, for exam-
for structural analysis and design. The
c o n s t r u c t e d
a n
e x p e r i m e n t a l
ple wind, are supported by the struc-
development of finite element
prestressed arch in 1908 and later
ture as a whole. About half the exterior
programs has led to the ability to accu-
used the technology in a limited form
surface is available for windows.
rately predict the stresses in complex
in the Plougastel Bridge in France in
Framed tubes allow fewer interior
structures, and allowed great
1930. He went on to build six
columns, and so create more usable
advances in structural engineering
prestressed concrete bridges across
floor space. Where larger openings
design and architecture. In the 1960s
the Marne River, firmly establishing
like garage doors are required, the
and 70s computational analysis was
the technology.
tube frame must be interrupted, with
used in a significant way for the first
Structural engineering theory was
transfer girders used to maintain
time on the design of the Sydney
again advanced in 1930 when
structural integrity. The first building to
Opera House roof. Many modern
Professor Hardy Cross developed his
apply the tube-frame construction was
structures could not be understood
Moment distribution method, allowing
in the DeWitt-Chestnut Apartment
and designed without the use of
the real stresses of many complex
Building which Khan designed in
computational analysis.
structures to be approximated quickly
Chicago. This laid the foundations for
Developments in the understanding of
and accurately.
the tube structures used in later
materials and structural behavior in
In the mid-20th century John
skyscraper constructions, including
the latter part of the 20th century have
Fleetwood Baker went on to develop
the construction of the World Trade
been significant, with detailed under-
the plasticity theory of structures,
Center.
standing being developed of topics
providing a powerful tool for the safe
Another innovation that Fazlur Khan
such as fracture mechanics, earth-
design of steel structures.
developed was the concept of X-
quake engineering, composite materi-
High-rise construction, though possi-
bracing, which reduced the lateral
als, temperature effects on materials,
ble from the late 19th century
load on the building by transferring the
dynamics and vibration control,
onwards, was greatly advanced
load into the exterior columns. This
fatigue, creep and others. The depth
during the second half of the 20th
allowed for a reduced need for interior
and breadth of knowledge now avail-
century. Fazlur Khan designed struc-
columns thus creating more floor
able in structural engineering, and the
tural systems that remain fundamental
space, and can be seen in the John
increasing range of different struc-
to many modern high rise construc-
Hancock Center. The first sky lobby
tures and the increasing complexity of
tions and which he employed in his
was also designed by Khan for the
those structures has led to increasing
structural designs for the John
John Hancock Center in 1969. Later
specialization of structural engineers
Hancock Center in 1969 and Sears
buildings with sky lobbies include the
23
Issue 71 December 2013
2013 71

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