Fazlur Khan - Pushing the Limits: New Adventures in Engineering - page 105

shear racking,” the kind of sideways distortion that a deck of cards displays when pushed from the
side.
The tubular principle is common in nature, of course, being present in hollow-stemmed reeds
and grasses and in the feather shafts and bones of birds. Khan seems to have been inspired not by
nature, however, but by his thinking about how skyscrapers were traditionally framed, with steel or
concrete beams and columns arranged in a three-dimensional rectangular grid. He saw that when
the wind blew on a very tall building so constructed, each floor was sheared from a true rectan-
gular shape into somewhat of a parallelogram. By locating most of the structural material on the
periphery and tying it all together in such a way that the whole structure acted more like a single
continuous beam rooted in the ground like a stalk of bamboo, he was able to achieve tremendous
stiffness.
Gaining stiffness became essential if taller buildings were to be constructed without paying a
premium in structural material. A hundred-story skyscraper, for example, might require twenty-five
pounds of steel for every square foot of office space contained in the building, just to hold it up.
Such a building would be too flexible, however, if constructed in the way conventional skyscrapers
like the Empire State Building were, and the amount of steel would have to be more than doubled to
keep the skyscraper from moving too much in the wind. Khan termed the excess weight of steel re-
quired for stiffness over strength a “premium for height.” Using the tubular principle, however, the
structural engineer did not have to pay the premium for a supertall building, and thus Khan “con-
tributed greatly to the renaissance in skyscraper design in the late 60's.”
Although the ideal tubular building would have solid structural walls, buildings that are lived and
worked in need windows. Khan recognized this, and so he told Graham that “we punch small holes
in the tube for windows.” Just as the holes we make in floor beams and wall studs to allow wires to
pass do not appreciably weaken the structure of a house, so window holes in a tubular frame do not
significantly reduce its stiffness. The first application of the tubular principle was in 1963 in Chica-
go, with the construction of the forty-three-story reinforced-concrete apartment building located at
211 East Chestnut Street.
The first tubular building framed in steel was Chicago's John Hancock Center, completed in
1969. For years before that, the hundred-story skyscraper was news in the engineering and architec-
tural press for the way structural engineers and architects could work together to solve a problem.
According to Khan, “the engineer has to be an architect to the extent that the architect has to be an
engineer so that in combination they produce the creative building.”
The architectural problem posed by the Hancock Center was to provide one million square feet
of office space and an equal amount of apartment space, plus about eight hundred thousand square
feet for commercial use and parking. Since the center was to be an investment property, the cost
of the building was of prime concern. A typical architectural solution would have been to design
two buildings, so there would be no premium for height, which the investors did not wish to pay.
But two buildings on the single site would have crowded the complex at street level, which the
architects did not like. Hence, a structurally efficient single tall building was looked to, and Khan
proposed an “optimum column-diagonal truss tube,” as he then called the structure that has become
so familiar a part of the Chicago skyline. Because the office floors required more expanse than the