Saturday, December 18, 2010

AKASHI KAIKYO BRIDGE

1. INTRODUCTION

The Akashi-Kaikyo Bridge, also known as Pearl Bridge, is a suspension bridge in Japan that crosses the Akashi Strait. It links Kobe in mainland Japan and the rural fishing island of Awaji as part of the Honshu-Shikoku Highway. The Akashi strait is one of the world’s busiest shipping lanes with 1000 ships plying through it daily.

At the time of completion, this bridge held 3 world records:-

1. At 280m, it was the highest suspension bridge in the world.

2. With the central span measuring almost 2km, it was the longest suspension bridge.

3. At a cost of $4.3 billion, it was the most expensive bridge project.

Fig. 1 Akashi Kaikyo Bridge

2. HISTORY

Before the Akashi Kaikyo Bridge was built, ferries carried passengers across the Akashi Strait in Japan. This dangerous waterway often experiences severe storms, and in 1955, two ferries sank in the strait during a storm, killing 168 children. The ensuing shock and public outrage convinced the Japanese government to develop plans for a suspension bridge to cross the strait. The original plan called for a mixed railway-road bridge, but when construction on the bridge began in April 1986, the construction was restricted to road only, with six lanes. Actual construction did not begin until May 1986, and the bridge was opened for traffic on April 5, 1998. The Akashi Strait is an international waterway that necessitated the provision of a 1,500m wide shipping lane.

Fig. 2 Location of the bridge

3. TIMELINE

1959 - Ministry of Construction commenced highway study.

1970 - Honshu-Shikoku Bridge Authority founded.

1973 - Ministry of Construction approved construction plans.

1985 - Government decided to construct Akashi Kaikyo Bridge.

1986 - Geological study of construction site commenced.

1987 - Construction survey for tower foundation commenced.

1988 - On-site construction commenced.

1998 - Opened for traffic.

4. REASONS TO BUILD THIS BRIDGE

1. As part of plans to modernize Japan, the various islands that constitute Japan had to be connected. So this bridge had to be built for that.

2. The only connection between Kobe and Awaji was by ferry. But the 1955 incident compelled the Japanese to think of this bridge soon enough.

3. An exercise of this magnitude had not yet been attempted. This was a symbol for Japan’s prosperity.

5. ARCHITECTURE

The bridge has three spans. The central span is 1,991 m (6,532 ft), and the two other sections are each 960 m (3,150 ft). The bridge is 3,911 m (12,831 ft) long overall. The central span was originally only 1,990 m (6,529 ft), but the Kobe earthquake on January 17, 1995, moved the two towers sufficiently (only the towers had been erected at the time) so that it had to be increased by 1 m (3.3 ft). The bridge was designed with a two-hinged stiffening girder system, allowing the structure to withstand winds of 286 kilometres per hour (178 mph), earthquakes measuring to 8.5 on the Richter scale, and harsh sea currents. The bridge also contains pendulums that are designed to operate at the resonance frequency of the bridge to damp forces. The two main supporting towers rise 298 m (978 ft) above sea level, and the bridge can expand because of heating up to 2 metres (7 ft) over the course of a day.

6. DIFFICULTIES FACED BEFORE CONSTRUCTION

· This proposed location of the Akashi Kaikyo Bridge was situated in a major earthquake zone.

· The Akashi Strait consists of currents of speed 40 km/hr. It is 100 m deep and a cradle for typhoons that generate winds of speed 280 km/hr.

· The proposed bridge had to have a length of almost 4 km, a distance that had not yet been attempted.

7. CONCEPT

The function of this bridge was to support the traffic load coming on the 6-lane freeway. But before that the bridge had to carry its own self weight. The load coming on the bridge was distributed as 91% to support its own weight and only the remaining 9% was for traffic load. The basic concept consisted of erecting two towers and passing steel cables through it. The girder was connected to the cable by means of hangar cables.

Fig. 3 A typical suspension bridge

8. CONSTRUCTION

The construction phase was divided into 4 stages:-

Stage 1:- Construction of tower foundation

Stage 2:- Construction of towers.

Stage 3:- Fixing of steel cables to towers.

Stage 4:- Placing the roadway.

8.1 Construction of Tower Foundation

The first problem faced was the erection of the towers in the bed of the Akashi strait. Due to the enormous depth of 110m and presence of fast currents, the usual method of building foundation by placing pre-cast concrete cylinders over each other was abandoned. So a new solution was brought up. Two enormous steel moulds were built in the dry docks and were then towed to sea and sunk at the precise location. The steel moulds were 70m tall, 80m wide and weighed about15000 tonnes. The moulds were sunk by filling with seawater. But, the next problem encountered was filling the mould with concrete. Since the mould contained seawater, concrete could not set. So, the Japanese engineers invented a new type of super-concrete which hardened in seawater.

8.2 Construction of Towers

The towers had to withstand not only the self-weight but also the load due to earthquakes. So it was decided to construct the towers out of steel. They were built block by block; 90 blocks constituted a tower. By this the second stage was completed. The towers were tested by placing a dozen workers on the top and they were asked to sway. This was done to test the earthquake load.

8.3 Fixing of Steel Cables to Tower

The next stage consisted of fixing the cables onto the tower. 37,000 strands of wire were entwined to constitute one cable. Super-strength steel wire was developed for this purpose. The cable was threaded over the tower using a helicopter. The cable was lifted from Kobe threaded around the first and second towers and the cable was tied at the Awaji end. The next stage involved the construction and placing of the six lane roadway. The deck had to be strong to support traffic and self-weight. It also had to be slender to allow wind to pass through it. The deck was made of steel girder which was arranged in a triangular shape. For extra strength they gave a vertical stabilizer throughout the length of the bridge. When wind blows, the vertical stabilizer balances pressure below the roadway and reduces vibrations, which destroyed Tacoma. They also installed steel mesh grating down the centre and along sides. This allows wind to pass right through the roadway and stops the pressure building up.

8.4 Placing the Roadway

The final phase consisted of placing the roadway but the earthquake at Kobe brought about a change in plan. As a result of the earthquake the towers had moved sideways over a metre. This stretched the bridge length a full metre. In order to increase the bridge length the engineers decided to space out the anchor cables. The massive 100 ton steel sections of the roadway was carried out by floating cranes and assembled.

9. A NEAR ESCAPE?

When the earthquake struck Kobe, the epicenter was just 4 km from the bridge, it was partly due to luck and partly due to the fore-sight of the engineers that the bridge did not sustain serious damage.

· Since the roadway had not been constructed by then, the structure did not have to suffer extensive losses that would have resulted from the possible collapse of the bridge.

· Since the tower had been constructed of steel, they were flexible to the effect of earthquake. Also, there were 20 shock absorbers within each tower. This helped to keep the tower in place after the earthquake.

· After the earthquake, detailed surveying showed that the tower on the Awaji side had shifted a meter apart due to the quake. It could have been more dangerous than this.

10. NEW TECHNIQUES DEVELOPED FOR AKASHI KAIKYO BRIDGE

The Japanese engineers delivered their true potential when met with stumbling blocks. Few of the innovations are:-

1. When the usual method of constructing bridge foundation by laying concrete cylinders on top of each other was not possible here due to the violent nature of the sea, engineers introduced a new concept of casting the foundation in steel moulds.

2. Super-concrete was developed to aid the setting of cement in seawater. Since the foundation moulds had been filled with seawater, this new type of material was required.

3. Super-strength steel wire was developed for the cable by changing the alloy proportions. This super-strength cable was so strong that a 5mm thick wire could carry 3 family cars.

11. CONCLUSION

We know that the Akashi Kaikyo Bridge is located at a place where weather and sea is very rough and it is in an earthquake zone. Still the determination and dedication of the Japanese engineers was instrumental in gifting this marvel. The Akashi Kaikyo Bridge has been set as a benchmark for future bridge constructions. The Mazena Bridge in Italy will soon overtake Akashi Kaikyo as the world’s longest bridge. With the advent of carbon fibre, it may be used in cables and if this being the case, longer bridges can be constructed. Normally as the span increases the length of the cable increases and this leads to an increase in the load to be carried by the bridge. But if carbon fibre is used, it will be lighter than steel and stronger leading to huger spans.

12. REFERENCES

1. Wai Tak Yim (2007) Akashi Bridge, Bridge Engineering Conference

2. Masaru Takeno, Yasuhiro Kishi (1997) Cable Erection Technology for Akashi Kaikyo Bridge, Nippon Steel Technical Report No. 73, pp 59-70

3. James Hill, Rola Idriss (2006), Bridge Construction, Committee on construction of bridges and structures

4. <www.wikipedia.com>

5. ‘Megastructures’, National Geographic Channel

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