Verktækni - 01.08.2002, Blaðsíða 17

Two long-span suspension bridges
of the past and new millennia
Abstract
In this this article, 2 long-span suspension
bridges are described: the Akashi-Kaikyo
Bridge in Japan and the Runyang South
Bridge in China. The Akashi Bridge is
dated 1998. With its main span of 1991 m,
it is the longest-span bridge ever built and
the greatest bridge of the past Millennium.
The Runyang Bridge (1490 m) was
commenced in 2000 and is due for comp-
letion in 2005. Then it will the 3rd longest-
span bridge ever built, and for some time
the greatest bridge in the beginning of the
new Millennium.
History of the Akashi Bridge
The 4 km wide Akashi Strait is located
between Awaji and Honshu. The Akashi-
Kaikyo Bridge is named thereby: Japanese
word „Kaikyo" means „Strait" in English.
The bridge across the Akashi Strait is
located some tens of kilometres west
Kobe, which itself is west of Osaka. Heavy
sea traffic along the Akashi Strait (some
1400 ships a day) dictates a long main
span for the bridge.
The Akashi Bridge was first proposed
in the 1950's. The main span was all the
time increasing. In the 1960's, the main
span was considered to be 1300-1500 m,
a world record in those times . In the
1970's, the main span was increased to
1780 m and the bridge was proposed for
both road and rail traffic. In 1985, the
rail alternative was abandoned and a
decision was made to restrict the bridge
for highway use only. According to ca-
reful consideration (i.a. heavy sea traffic,
difficult foundation conditions), the
optimal main span range appeared to be
1950-2050 m.The final main span was
to be 1990 m, annoyingly less than the
magic 2 km mark. Even this exceeds by
41% the previous record: Humber
Bridge in Britain, main span 1410 m,
dated 1981.
Construction of the Akashi Bridge was
commenced in May 1988, and the
construction period lasted 10 years. Of its
type the bridge is a 3-span 2-hinged
suspension bridge with steel truss stiffen-
ing girders for deck structure, designed
spans 960 + 1990 + 960 m = 3910m, actual
spans somewhat longer. The bridge is
located between Maiko (in Shikoku) in the
north and Matsuho (in Awaji) in the
south. At each end there is a short viaduct
made of concrete.
It should be mentioned that actually
this suspension bridge length of nearly 4
km is roughly only 10% of the longest
bridge in the world. That record belongs
to the Second Lake Pontchartrain
Causeway, in Louisiana, U.S.A., dated
1969. It is a multi-span bridge, total lengt
38 km 422 m, while its spans are only
about 25 m on an average. So the Akashi
Bridge is the world's longest-span bridge.
But it is definitely not the longest bridge
in the world.The main span of the the
Akashi Bridge is the first ever exceeding
the English mile (1609 m), and currently
the only one exceeding even the nautical
mile (1852 m). With a full reason it may
be said that in many respects this is the
most remarkable bridge of the past
millennium. During the new millennium,
some longer-span bridges may be built.
To mention some of the proposed gigant-
ic bridges of the future:
• across the Messina Strait between
Sicily and mainland Italy: a suspension
bridge, main span 3,3 km.
• across the Tsugaru Strait in Japan: a
hybrid suspension/cable-stayed bridge
with 2 successive main spans of 4 km.
• across the Gibraltar Strait between
Spain and Morocco: a suspension
bridge with 2 successive main spans of
5 km, or a cable-stayed bridge with an
enormous main span of 8,4 km.
Foundations and towers
The huge cable anchorages (anchor
blocks), made of concrete, are located on
both shores. To prevent the corrosion of
the cable wires, the relative humidity of
the air inside anchorages is kept at max.
60%, practically about 40%.
The both towers, located in the sea, are
founded on large caissons 80 m in diamet-
er, foundation depth about 70 m below
water level, which is near a record. Among
long-span bridges (>1000 m), the south
tower of the the Ponte 25 de Abril in Lis-
bon has the world's deepest foundation
depth below water level (80 m).
The towers of the Akashi Bridge are
made of steel, height above the piers 283