Fazlur Khan - Pushing the Limits: New Adventures in Engineering - page 106

apartment floors, the building was given its characteristic taper, with the office space located in the
wider lower floors. The diagonal bracing, which some feared would make the rental space with ob-
structed windows less desirable, came to be coveted by tenants as having a cachet that marked it as
belonging to the distinctive 1,127-foot-tall building.
To explain why this new building was such an advancement over the skyscrapers of similar
height that were built decades earlier, Khan compared the designs of the two eras in a 1967 article
in Civil Engineering. His own words are worth quoting extensively to demonstrate how clear think-
ing, expressed in clear writing, can reduce what might appear to be a terribly complex problem in
structural engineering to one readily grasped by engineers and nonengineers alike:
The answer lies in three basic characteristics of the high-rise buildings built in the thirties. First, a
20-ft column spacing was considered adequate for office spaces. Today a minimum of about 40 ft
is considered adequate. In fact, the longer the spacing, the better the office space. Second, the par-
titions used were generally made of solid masonry from floor to floor, adding considerably to the
rigidity of the entire building. Today most partitions are removable, therefore very low in weight
and stiffness. Third, the exterior wall detail was generally made of solid masonry or stone, and the
window opening consisted of a small percentage of the total wall surface. Today the glass curtain
wall is generally attached to the frame as a non-rigid skin.
These three characteristics of the earlier framed structures all added to the lateral rigidity and
stiffness of the structural frame. Because of them, a building designed for a sway factor of as
much as 1 ⁄250 of its height would probably never sway over under the worst wind loads. It was
therefore possible, with many tall buildings built in the thirties, to design the structural frame for
strength only and not make extensive checks for the lateral drift under wind loads.
A similar problem presented itself when Sears, Roebuck and Company wanted to build 3.7 mil-
lion square feet of office space, and the architects “wanted to maintain a decent environment at
ground level,” which meant providing a plaza that could be “dotted with art pieces.” In order to
provide the desired floor space, which was to consist of large floor areas in the lower stories, which
Sears was to occupy, and decreasing floor areas as the building and the rents rose, Khan proposed
a connected cluster of nine tubes, which he often described as “a group of pencils bundled together
with a rubber band.” Just as such a bundle can stand up by itself much more easily than can a single
pencil, so the Sears Tower would be better able to resist the heavy winds to which its record-setting
height would be subjected.
Nine tubes, each seventy-five feet square, compose the 225-foot-square Sears Tower, which un-
like the John Hancock has parallel sides. To make the building more than a tall and slender rect-
angular box, the nine tubes are cut off at various heights, giving the building a different appear-
ance when viewed from different angles. This structural/architectural treatment of the tubes also
reduces the floor area in stages as the building rises, thus providing a variety of office configura-
tions and sizes. Each of the tubes has a columnless interior, which provides the unobstructed floors
that are so desirable in modern office space. At the lower levels, the nine tubes share interior walls
of columns, thus economizing on steel and adding additional stiffness to the structure. Khan estim-
ated that without the interior tic-tac-toe grid of columns, the skyscraper would have required twice