New and Future Bridges - Pushing the Limits: New Adventures in Engineering - page 79

ments could be made for additional spans across, say, the Hudson River or San Francisco Bay, the
best locations technically have already been developed. Thus, new spans would face great design
challenges, which often translate into prohibitively expensive solutions. Furthermore, the changed
regulatory climate might not allow new spans to be built at all. Conditions may change in the future,
but the present climate suggests that it is likely to be a distant future.
The future has already arrived elsewhere, however, and the closing years of the twentieth century
saw the opening of what were the two largest suspension bridges in the world. In June 1998, Den-
mark officially inaugurated its Great Belt Bridge, which crosses the 4.2-mile-wide strait between
the islands of Fyn and Sjælland, on which Copenhagen is located. Since Fyn was already connec-
ted across the narrower Little Belt to the Jutland peninsula, Copenhagen was finally linked to the
mainland. (It was only a few years later that the cosmopolitan city was also connected to Sweden,
when in 2000 the Oresund Crossing—a ten-mile-long fixed link incorporating a tunnel, an artifi-
cial island, and a 3,325-foot-long cable-stayed bridge with a 1,495-foot-long main span—was com-
pleted.) The suspended part of the Great Belt Bridge, with a 5,328-foot-long main span and its
8,838-foot total length between anchorages, could have made the bridge the longest in the world on
both counts, but those distinctions belong to a Japanese bridge.
The Akashi Kaikyo Bridge, which opened in 1998 near Kobe, is likely to be the longest suspen-
sion bridge for some time. Its towers were in place when a 1995 earthquake hit, and it left them
a couple of feet farther apart but otherwise undamaged. Since the roadway had not yet been hung
from the cables, its components were redesigned to fit properly into the main span, which is 6,529
feet between towers. The 12,828-foot length of the total suspended span means that drivers travel
almost 2.5 miles between the 350,000-ton anchorages.
The span across the Akashi Strait is only one part of the massive Honshu-Shikoku Bridge Project,
which links the islands of Honshu, on which Kobe is located, and Shikoku by way of the small is-
land of Awaji. The bridge has been designed to withstand an earthquake of magnitude 8.5, and its
survival of the 7.5 Kobe earthquake, albeit while still under construction, provides some confirm-
ation of the design's integrity but raises serious questions about the wisdom of building colossal
structures in earthquake zones.
Such experience may have severe implications for the design of a fixed link in another seismic
zone—across the Messina Strait between the Italian peninsula and Sicily, a project that was dis-
cussed at least as early as 1870, when a rail tunnel was proposed. An international competition was
held in 1969 to encourage conceptual designs, and more than 150 entries were submitted, about 90
percent of them from Italian engineers and firms. Among the schemes to cross the strait between the
presumed sites of the mythological Scylla and Charybdis, those taken most seriously were tunnels
and suspension bridges. Tunnels, however, are costly and would take a long time to construct at this
location, and so the bridge proposals have continued to attract the most attention. One design calls
for a bridge with a 9,500-foot main span, necessitated by the tower foundations being sited on the
shores. The Messina Strait bridge would also be three times as long as the longest suspension bridge
previously designed to carry railroad traffic, the Tagus River Bridge in Lisbon, Portugal, showing
further the uncharted waters into which daring projects enter.