Report of a case study on Akashi Kaikyo Bridge, longest suspension bridge in the world, Japan

1. 1
Akashi Kaikyō Bridge – Longest Suspension Bridge in
the World
Adarsh
Abstract: The Akashi Kaikyō bridge is the longest suspension bridge in the world with a central span
of 1,991 m and a total length of 3,911 m. It is situated in Japan and is a bridge engineering masterclass
in terms of technological advances. This report deals with the materials required in the construction of
the bridge, construction of the foundation for the anchorage of the cables, use of stronger cables which
support huge weight of the roadway and a large no. of vehicles, construction of foundation for the
main towers of the bridge, dependence of the height of the towers on the length of the roadway and
their construction, design for resisting huge forces of the wind and the technology involved in making
the bridge resistant against high magnitude earthquakes. Discussion of each of the above topics helps
us realise that Japanese engineers pushed technology to its limits to make the dream of this
megastructure a reality. Comparisons are done with other significant examples of the suspension
bridges around the world and also the design concepts adopted from the previous bridges built. The
Akashi bridge proves the strength of its record breaking design in one of the most challenging
environments and is a combination of 2 centuries of engineering breakthroughs and innovations.
Keywords: Suspension bridge, Caisson, Foundations, Open Box Girders, Tuned Mass Dampers
Introduction
The Akashi Kaikyō bridge is a suspension
bridge built over the Akashi Strait which links
the Kobe city on Japanese mainland to Iwaya
on Awaji island. The design engineer and
architect of this bridge was Dr. Satoshi
Kashima and the major construction company
involved it the construction of this bridge was
The Matsuo Bridge Co.
Originally the plan proposed to
construct a bridge with both railway and
roadway but the plan was about to be cancelled
by the government. In 1985, after much
consideration the plan was kept down to only a
road bridge. The construction of the bridge
started in 1988 and was opened for the public
in 1998. This bridge is also popularly known as
the ‘Pearl Bridge’. It is one of the key links of
the Honshu-Shikoku Bridge Project which also
includes the Great Seto Bridge and Kurushima
Kaikyō Bridge.
Features
The Akashi bridge has the longest central span
of all the suspension bridges in the world i.e.
1,991 metres and has two side spans of 960
metres which constitutes to a total length of
3,911 metres. It also holds the record of having
the tallest main towers in a suspension bridge
at a height of 297 metres. This bridge is
considered to be the most expensive bridge
ever built with an estimated cost of 500 billion
yens or $ 4.5 billion.
Around 2 million workers were
involved in the construction of the bridge. As
far as safety of the workers was concerned,
only 11 were injured and there were no deaths
which can be considered as a remarkable
achievement taking into account this majestic
project. The reason for this was that some of the
toughest works were done mechanically rather
than manually such as erection of the towers,
underwater construction of the foundation for
those towers, etc.

2. 2
The bridge carries six lanes of roadway
as the design was restricted to only a road
bridge. It is used by around 25,000 cars daily
and the toll charged is 2,300 yen/car which
results in an annual revenue of around 19
billion yen.
Need for the Bridge
The Akashi bridge is built over the Akashi
Strait which is very prone to typhoons,
tsunamis, earthquakes, etc. Hence it was very
difficult for the ships to carry passengers and
goods from one end to the other. Moreover,
around 168 children and adults died as a result
of the collision of two ferries in a heavy fog
over the strait. This created political and public
outrage which forced the government to
consider over the construction of the bridge.
The Akashi strait is used by around
1400 ships per day with an important port of
supply and release of goods. Thus the engineers
did not want to block the sea traffic and hence
came up with the design of a suspension bridge.
The bridge provides about 1500 metres of
waterway (Fig 1.) for the bridges which can
pass easily below the bridge owing to the
sufficient height of its deck (the bridge).
Fig 1. Waterway through the bridge
Structural Features
Materials Used
Since the bridge was going to be enormous in
every aspect the weight that the bridge had to
carry was very huge. Therefore, the engineers
had to consider about using a lighter material
for its construction. Stone towers were not an
option or using concrete deck. All the factors
indicated that steel was the most favourable
material regarding these requirements and also
with respect to the environmental conditions.
As a result of this, steel was the major
component used, most of the parts were pre-
fabricated and total amount of steel constitutes
to 250,000 tonnes.
Concrete is used in the anchorage
foundations for the cables and also for the
foundation of the main towers. Around 460,000
cubic metres of concrete was used in the
anchorage part and a similar amount in the
foundations.
The Suspension Bridge
One of the major part of a suspension bridge is
the use of strong cables and since they carry a
large amount of weight they need to be
anchored properly so that load can easily be
transferred to the ground. For this bridge,
anchorage foundations had to be built on both
sides of the strait as there were no natural
features such as rocks to which the cables could
be anchored.
Two big holes were dug into the ground
of diameter 80 metres and depth of around 60
metres. Each one of them was filled with
approximately 230,000 cubic metres of
concrete. Huge steel cages were placed on it
upto a height of 15 metres and concrete was set
in 5 different parts with spaces in between so as
to allow the heat to escape which could lead to
the cracking of the concrete. After the concrete
was fully set the steel cages were covered by

3. 3
casting the concrete over it. It consists of two
conical parts to which the cables were attached.
It can be considered very similar to an iceberg
with some height above the ground and a huge
depth below the ground.
Stronger Cables
The vertical weight of the bridge was going to
be very large and hence stronger cables were
required to lift the deck with the heavy traffic
that would use the bridge later. The main cables
weigh 25,000 tonnes each and have a tensile
strength of 62,500 tonnes. The diameter of
these cables are 112 cm and 36,830 wires
comprise of one cable. Super strength wires are
used, the strength of which can be imagined by
the fact that a wire of 5 mm thickness can
support the weight of 3 family cars. The strand
length of the cables is 4070 m approx. A total
of these wires constitute to a length of 3,00,000
kilometres which is enough to circle the earth 7
times. To prevent the corrosion in these wires,
dry air is pumped through them (Fig 2.).
Fig 2. DryAir Injection System
Building Underwater
For the suspension bridges, underwater
construction involves the construction of
foundation for the main towers. This is done
with the help of caisson which is made up of
steel, is further sunk into the water to make up
the foundation. This can be done manually and
mechanically.
Since the strait in which the work was to be
carried out was very prone to storms, typhoons
etc. so the lives of the workers were not risked.
Huge caissons were built of dia 80m and depth
70 m and they were taken into its required place
with the help of ferries. It was then flooded
with seawater to sink it upto the bed. Then the
seawater was replaced by pouring wet concrete
into them in order to strengthen the case-on and
finally a lid was placed over it to complete the
formation of the foundation. Also the engineers
found that normal concrete was hard to set
under seawater. Thus super concrete was used
for this purpose which had a speciality of
hardening under seawater.
Taller Towers
Experiments were conducted to prove that
longer was the central span of a suspension
bridge, taller would be the towers. Since taller
towers had to be built so stone or concrete
towers were not an option because they would
fail under the load of buckling. Hence steel
towers were built because of the flexibility of
steel as it would move under the action of load
without causing any damage. The bridge has a
record of having tallest towers in a suspension
bridge i.e. 297 m. the towers have a cruciform
cross-section which provides stability. About
1.5 million bolts are used to join separate steel
section to form the tower.
Fig 3. Tower cross-section

4. 4
Wind Resistant
The environment in which the bridge was
going to be built was very challenging. It had
to be designed to resist typhoons, tsunamis, etc.
Initially an open box triangular girder was
provided in the design because steel boxes
would have been too heavy for the cables to lift.
The prototype of the bridge was made at a scale
of 1:100 and then tested in one of the largest
wind tunnels against a wind speed of 300
km/hr. Results suggested that it was required
for the bridge to stiffen it against the wind and
also deflect it.
As required, a mesh grid was provided
in the centre of the roadway and a massive steel
beam was provided below the deck in the
centre which runs along the length of the
bridge. It works in such a manner that
whenever any cross-wind hits the bridge, some
part of it is deflected through the grid provided
above and the rest downwards through the open
girders which neutralizes the effect of the wind.
Triangular girders are provided because it
provides stiffness to the structure. This design
is adopted from that of the Verrazano Narrows
Bond in USA.
The girder system is a two-hinged
stiffening girder system which makes it resist
upto 286 km/hr of wind (Fig 3.).
Fig 3. Two-hinged Stiffening Girder System
Earthquake Resistant
As it is well known, Japan is one of the most
earthquake affected regions in the world.
Engineers had to consider it as one of the most
important aspects and thus find a way to resist
it. They decided to use Tuned Mass Dampers
(TMD), which are the pendulums which
oscillate at the resonant frequency of the object
in which it is being placed. Around 20 TMDs
were used in one tower and each of them
weighs 10 tonnes. In order to test it, engineers
created a man-made quake in which around 100
men were made to move in perfect harmony,
standing at the top of the tower. It caused very
less deflection in the towers due to the use of
TMDs. Hence it was considered to be a safe
design.
Fig 4. Tuned Mass Damper used in towers of the bridge
In 1995, an earthquake of magnitude 7.5 on
Richter Scale hit the city of Kobe which
resulted in the death of around 6000 people.
Fortunately for the bridge the deck had not
been constructed because the effect on the
bridge was that the bed just below the
foundations moved and thus increased the span
of the bridge by 1.1 metres. One can imagine
the devastation that would have taken place if
the deck was destroyed. The bridge overcame
this disastrous situation which was enough to
prove its strength. Also it reduced the sag in the

5. 5
main cable in the central span of the bridge by
1.3 metres.
Overall the bridge has two layers of
protection against the earthquake. One is its
flexible steel towers and other is the TMD
which reduces the vibrations and thus the
capable of resisting 8.5 magnitude earthquake.
Fig 5. Change in the span of bridge
Other SuspensionBridges
The Akashi Kaikyō bridge draws inspirations
from some of the great suspension bridges built
in the past. Those are the Menai Strait Bridge,
Brooklyn Bridge, Golden Gate Bridge,
Verrazano Narrows Bridge. Each of them
provides for some of the technologies used in
the Akashi bridge in an advanced way. For
example, Verrazano Narrow provides for the
open deck girder design, Golden Gate for the
taller towers, Brooklyn bridge for the tower
foundations, etc.
As said, the Akashi Bridge has used
basic techniques to gain an advanced level of
technology and is a result of the major
engineering achievements in the field of bridge
engineering. It can be assumed that we can
achieve even bigger in this field if we are able
to follow the basic principles and using them
on a bigger scale.
References
En.wikiarquitectura.com (2014)."Akashi Kaikyō
Bridge-Architecture of The World-Wikiarquitectura".
<https://en.wikiarquitectura.com/index.php/Akashi_K
aikyō _Bridge> (August 20, 2016).
Jb honshi.co.jp -"Akashi-Kaikyo Bridge - Bridge World
- Enjoy The Top of the Bridge, The LONGEST
BRIDGE in The World!". <http://www.jb-
honshi.co.jp/english/bridgeworld/bridge.html>
(August 20, 2016).
Wikipedia (2016). "Akashi Kaikyō Bridge".
<https://en.wikipedia.org/wiki/Akashi_Kaikyō_Brid
ge> (August 20, 2016).
Seminarsincivil.blogspot.in. (2010) "Engineering
Seminars: AKASHI KAIKYO BRIDGE".
<http://seminarsincivil.blogspot.in/2010/12/akashi-
kaikyo-bridge.html> (August 20, 2016).
YouTube (2014) "The Longest Suspension Bridge In The
World".<https://www.youtube.com/watch?v=N9fbRc
RJY34> (August 19, 2016).