Benjamin Franklin Bridge - Pushing the Limits: New Adventures in Engineering - page 18

Before such calculations could be made, however, the overall proportions of the structure had
to be established. There are no rigid formulas for doing this; it involves a combination of working
within the constraints of the bridge location and local geology and of making aesthetic judgments
about what just looked right. In taking this all-important design step of setting the proportions of a
major structure, engineers act more like artists than scientists. And only after the overall defining
geometry is set down can the theories and formulas of engineering science be applied to the details.
Modjeski and Moisseiff were each masters of both the art and the science, but as chief engineer
Modjeski would have the final word and responsibility for the way the bridge looked.
Although a suspension bridge is the purest of structural forms, needing no decorative treatment
or facade to give it an aesthetic presence, architects are often involved to recommend finishing
touches. Modjeski engaged Paul Cret as architect to provide advice on details large and small. Cret
had considerable input on such aspects of the bridge as the exterior design of the enormous anchor-
ages that it required, and the stone pylons that frame the steel towers for everyone driving across
the bridge. Yet for all of the thought that went into the materials, proportions, and appearance of the
structure, the artist Joseph Pennell would still call the bridge under construction over the Delaware
“the ugliest bridge in the world.” That judgment was reinforced by the aging master bridge engineer
Gustav Lindenthal, who considered the towers to be designed “too much on the utilitarian principle
of braced telegraph poles or derricks, holding up ropes.” Such opinions were disputed by many oth-
er observers, who found the structure beautiful.
Whatever the aesthetic verdict on the bridge, from a purely engineering point of view it was an
innovative and record-breaking achievement. Modjeski's board submitted its defining recommend-
ation for the location and type of structure on June 9, 1921, and it was accepted by the joint com-
mission just two weeks later. The total cost of the bridge was estimated to be almost $29 milli-
on, with the State of New Jersey responsible for about $12.5 million and the Commonwealth of
Pennsylvania and the City of Philadelphia responsible for just over $8 million each. The board
declared that the bridge could be finished for the country's sesquicentennial celebrations being
planned for July 4, 1926. It would open three days early, almost fifty years to the day from when
Modjeski first came to America. The event took place as planned despite construction delays caused
by a debate over whether tolls would be charged. In the end, tolls were used to pay for the project,
which was believed to be the largest public-toll enterprise up to that time.
The span, width, and traffic capacity of the bridge required cables of unprecedented size. Each
cable was spun in place and was made up of twenty thousand individual steel wires, which meant
the spinning operation had to handle almost twice as many strands as on any previously built
suspension-bridge project. When compacted, the finished cables measured thirty inches in diamet-
er, which made them half again as large as any then built. The use of new and stronger materials in
the cables and lighter materials in the roadway reduced the overall weight of the structure, as did
the use of lightweight trusses. These things in turn kept the overall cost down.
For all of the attention to technical innovations, the bridge was also designed with full regard for
the people who would use it. Vehicle lanes were located in the center of the bridge, flanked and
guarded on either side by sets of train rails and trusses. One set of tracks ran inside and one out-
side each of the bridge's two stiffening trusses. Over the outside tracks, cantilevered out from the