During the previously mentioned studies of high piers undertaken by the
Eiffel firm, it was established that as the base width of these piers
increased in proportion to their height, the diagonal bracing connecting
the vertical members, necessary for rigidity, became so long as to be
subject to high flexural stresses from wind and columnar loading. To
resist these stresses, the bracing required extremely large sections which
greatly increased the surface of the structure exposed to the wind, and
was, moreover, decidedly uneconomical. To overcome this difficulty, the
principle which became the basic design concept of the Tower was
developed.
The material which would otherwise have been used for the continuous
lattice of diagonal bracing was concentrated in the four corner columns of
the Tower, and these verticals were connected only at two widely
separated points by the deep bands of trussing which formed the first and
second platforms. A slight curvature inward was given to the main piers to
further widen the base and increase the stability of the structure. At a
point slightly above the second platform, the four members converged to
the extent that conventional bracing became more economical, and they were
joined.
[Illustration: Figure 4.--The proposed 1,000-foot iron tower designed by
Clarke, Reeves & Co. for the Centennial Exhibition of 1876 at
Philadelphia. (From _Scientific American_, Jan. 24, 1874, vol. 30, p.
47.)]
That this theory was successful not only practically, but visually, is
evident from the resulting work. The curve of the legs and the openings
beneath the two lower platforms are primarily responsible for the Tower's
graceful beauty as well as for its structural soundness.
The design of the Tower was not actually the work of Eiffel himself but of
two of his chief engineers, Emile Nouguier (1840-?) and Maurice Koechlin
(1856-1946)--the men who had conducted the high pier studies--and the
architect Stephen Sauvestre (1847-?).
In the planning of the foundations, extreme care was used to ensure
adequate footing, but in spite of the Tower's light weight in proportion
to its bulk, and the low earth pressure it exerted, uneven pier settlement
with resultant leaning of the Tower was considered a dangerous
possibility.[2] To compensate for this eventuality, a device was used
whose ingenious directness justifies a brief description. In the base of
each of the 16 columns forming the four main legs was incorporated an
opening into which an 800-ton hydraulic press could be placed, capable of
raising the member slightly. A thin steel shim could then be inserted to
make the necessary correction (fig. 5). The system was used only during
construction to overcome minor erection discrepancies.
In order to appreciate fully the problem which confronted the Tower's
designers and sponsors when they turned to the problem of making its
observation areas accessible to the fair's visitors, it is first necessary
to investigate briefly the contemporary state of elevator art.