Benjamin Franklin Bridge - Pushing the Limits: New Adventures in Engineering - page 14

Benjamin Franklin Bridge
Cities and water go together. It was natural to establish early camps, settlements, and forts beside
streams, rivers, lakes, and bays. Such locations provided ready access not only to the substance es-
sential to life but also to fishing, transportation, and security. It is no accident that the nucleus of
New York City was at the tip of lower Manhattan, which commands a strategic view of the harbor,
or that the U.S. Military Academy sits on West Point, which is located at a double turn in the Hud-
son River, where enemy ships were forced to navigate slowly, thus making them easy targets.
As forts grew into towns and cities, waterways were both blessing and curse. Chicago's location
on Lake Michigan gave it what at first must have seemed a near inexhaustible source of fresh water,
and the Chicago River that flowed into the lake provided a natural sewer. However, the refuse even-
tually polluted the drinking water, producing a serious health hazard. The situation drove the city to
undertake the enormous task of reversing the flow of the river, sending Chicago's sewage westward
into the Des Plaines River and, eventually, into the Mississippi, which supplied water to other cities,
including St. Louis. Fortunately, as predicted, the water was purified in the course of the long and
turbulent journey.
Swift-running rivers and streams not only cleaned themselves but also provided power for mills.
Yet the same flow that drove waterwheels complicated up-, down-, and cross-stream traffic, which
was essential for distributing the products of the mills. The continued development of a mill town
into a city necessitated the construction of canals, locks, and especially bridges, the last of which
was relatively easy for narrow crossings but a technological challenge for wider waterways. The
growth of many a city was hampered as much by its lack of adequate bridges as by anything else.
The development of the railroad in the nineteenth century threatened the economic future of cities
that could not provide bridges across wide rivers. Had the Eads Bridge not been built at St. Louis,
virtually all long-haul rail traffic crossing the Mississippi River might have been routed through
Chicago and northern Illinois, where the river was bridged, leaving the more southern city without
significant commerce. The development of the automobile in the early twentieth century presented
new challenges for bridge builders, requiring new spans to move traffic across previously unbridged
waters.
Every river city has at least one bridge story, and Philadelphia is no exception. Philadelphia sits
at the confluence of the Schuylkill and Delaware rivers, with the former being a tributary of the
latter. The smaller Schuylkill was naturally the first to be bridged. The most famous of its early-
nineteenth-century crossings was located at Fair-mount, now a part of the city but then a bit out-
side Philadelphia proper. In 1813, Louis Wernwag completed a combination arch-and-truss covered
bridge there, which had a clear span of 340 feet. At the time, it was the longest wooden bridge
in America. According to David B. Steinman and Sara Ruth Watson in their classic Bridges and
Their Builders, “it was a beautifully and originally designed bridge, worthy in all respects of its
nickname,” the Colossus. The great bridge has also been described by Donald C. Jackson in his