New and Future Bridges - Pushing the Limits: New Adventures in Engineering - page 78

New and Future Bridges
An era of great bridge building in America reached a peak in the 1930s, when the George Washing-
ton, Golden Gate, and San Francisco-Oakland Bay bridges were completed. The first two of these
had record-setting main suspension spans of 3,500 and 4,200 feet, respectively, whereas the third,
costing seventy-five million Great Depression dollars, was the longest and most expensive bridge
project to date. Many other structurally significant but lesser-known spans were also built in those
hard times, but the era ended abruptly in 1940 with the collapse of the Tacoma Narrows Bridge. The
dramatic failure was recorded on film, which became incorporated into newsreels and embedded in
the minds of people everywhere as a symbol of the hubris of engineers.
In addition to the obvious fact that bridge designers could no longer claim complete knowledge
of their art, there were other factors that contributed to the subsequent hiatus in long-span bridge
building. That interruption, which would last for a decade, was also brought about by World War II
and the rationing of crucial materials that accompanied it. Furthermore, the most critically needed
fixed links for carrying the country's growing automobile traffic were then in place. However, there
were great bridges still to be built.
Although a crossing between the lower and upper peninsulas of Michigan would have alleviated
long lines of cars waiting for ferries, the traffic there comprised mainly hunters and vacationers and
was thus seasonal and not so pressing a problem to be solved. Another obvious location for a bridge
was at the entrance to New York Harbor, between Brooklyn and Staten Island, but the latter bor-
ough of New York City was not very highly populated at the time, so that bridge also could wait
until the war ended and the Tacoma Narrows collapse could be convincingly explained.
A redesigned Tacoma Narrows Bridge opened in 1950, and the Mackinac Straits Bridge eventu-
ally was built, opening in 1957, with special design attention paid to how the deck would behave
in the wind. Unlike the original Tacoma Narrows Bridge—and the George Washington Bridge,
whose state-of-the-art design it had followed—the Mackinac employed a very deep deck truss and
an open-gridded roadway portion to allow wind to pass through the bridge rather than wreak havoc
on it. (It was claimed that this feature made the bridge stable in any wind.) The Verrazano-Narrows
Bridge across New York Harbor was completed in 1964, with its lower deck in place from the be-
ginning— not because traffic demanded it but as a conservative design measure to stiffen the world-
record 4,260-foot central span. Even before the completion of the Verrazano-Narrows, America had
all the top-ranking suspension bridges in the world, but it would not retain that distinction for long.
When the 4,626-foot Humber Bridge was completed in England in 1981, America lost its claim
to having the world's greatest suspension bridge, at least as measured by length of main span, and
the United States is not likely to recover the title in the forseeable future, if ever. (The longest sus-
pension bridge built in America in the last forty years is the 2,390-foot Alfred Zampa Memorial
Bridge, completed in 2003 across Carquinez Strait at the northern end of San Francisco Bay.) The
country's great rivers, straits, and harbors have by and large all been bridged, and, although argu-