Verktækni - 01.08.2002, Blaðsíða 18
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Side View
3911
m, 297 m measured from water level.
These are the tallest bridge towers in the
world. Due to the curvature of the earth
surface, the distance between the towers is
93 mm longer at top than at base. Among
long-span suspension bridges (>1000 m),
the majority of bridges have steel towers.
Some bridges have concrete towers, like
Great Belt, Runyang (1490 m; dt. 2005),
Humber, Jiangyin, Tsing Ma, Höga Kusten:
mainly bridges in Europe and China.
Main cables and hangers
The Akashi Bridge has 2 main cables,
each 1122 mm in diameter, a world
record. Each cable consists of 290 parall-
el-wire ropes, each containing 127 high-
strength wires 5,23 mm in diameter. Thus
each cable contains 36'830 parallel wires,
the total length of those being about
300'000 km. The wires are of special
high-strength grade, ultimate tensile
stength 1800 MPa (180 kgf/mm2). Thus
only 2 main cables are needed, while the
previous grade 1600 MPa would need 4
main cables. The 10 mm-diameter poly-
aramid fiber pilot rope was carried across
the strait by helicopter, a method first
time used for a major suspension bridge.
The wires of the cables are protected ca-
refully agaist corrosion. Also the at-
mosphere environment inside the cables
is improved by injecting dried air into the
cables.
Deck and underclearance
The deck of the Akashi Bridge consists of
strong steel truss girders, width 35,5 m,
depth 14,0 m.
The effective width of the Akashi Bridge
is 30,0 m and the underclearance is 65 m,
while the highest underclearance among
major suspension bridges (>1000 m) is 70
m of the Ponte 25 de Abril in Lisbon. The
deck of the Akashi Bridge accommodates
3 + 3 lanes for motorway traffic, design
speed for vehicles 100 km/h, no lanes for
light traffic. Depending on loads, the max-
imum displacements of the deck may vary
within the following ranges: vertically 8 m
upwards and 5 m downwards, horizontally
27 m each direction.There are expansion
joints at anchorages and hinges at towers.
Severe earthquake at Kobe
The Akashi Bridge is designed for winds
up to 80 m/s and also for severe earthqu-
akes of 8,5 on the Richter scale. On 17th
January 1995, an earthquake of 7,2 richters
occurred in Kobe area and about 6000
people were killed. The epicentre of the
earthquake was located close to the south
end of the Akashi Bridge, several kilo-
metres away from Kobe City itself. The
bridge experienced a severe earthquake
resistance test, as the magnitude at the
bridge site was nearly 8 richters. When the
earthquake occurred, the towers and the
cables of the bridge were newly complet-
ed, and the construction of the deck was
to be commenced. Buildings and bridges
collapsed even at a distance of 50 km from
the Akashi Bridge.
It appeared that the Akashi Bridge sur-
vived with only minor damages. Because
of the movement of the earth, the south
tower moved 80 cm southwards, and the
tops of the towers tilted 10 cm south-
wards. The main span increased 80 cm
and became nearly 1991 m. AIso, the
south side span increased 30 cm. Accor-
dingly, the total length of the suspension
bridge increased 110 cm. As a result the
sag of the main cables decreased by 130
cm. It is estimated that the damages
would not have been this slight had the
cables not been already installed. Despite
the earthquake, the Akashi Bridge was
completed according to the original
schedule in Spring 1998.