ENGINEERING MARVELS 1

By: S.Niranjana, B.E.CIVIL 2008-2012

Akashi-kaikyo Bridge- The Pearl Bridge

If you take up engineering project what would be your main goal?

Before all the answers like quality, deliverance, cost, etc., I’m sure the project completion time is of utmost importance. But that’s exactly one factor that didn’t quite workout in this case. This

is an engineering marvel that pushed the barriers of engineering further. The whole project including the project planning, designing, execution spanned over 30 years. But all the efforts put in, paid rich dividends later.

The need:

The Akashi-kaikyo Bridge connects the city of Kobe and Iwaya across the Akashi strait of Japan. The need for the bridge arouse when the ferries, which were the only mode of transport to fishermen and their family across the Akashi straits, started facing devastating mishaps. There were heated debates on whether Japan needed to invest so much in building just one bridge. The final straw was the accident in 1955 involving 168 students who died along with some adults. And it was decided, Japan did need that bridge.

This bridge holds the record for

• The longest bridge (it spans over a length of 2km)
• Tallest tower to hold the suspension cables
• Most expensive construction (4 billion dollars)

It was designed to serve a design life of 200 years. It took two million workers ten years to construct the bridge, 181,000 tonnes of steel and 1.4million cubic metres of concrete. The steel cable used would circle the world seven times.

Obstacles and the solutions provided by the Japanese engineers:

When one plans something so huge, there will be hindrances along the way. Here the hindrances were as huge as the project. The water body over which the bridge had to be built is one that was 100m deep, having high tides and attracted high magnitude typhoons, that even the sailors found it tough to sail through it. The other main problem was, Japan lay in the high seismic activity zone. The final problem was the heavy traffic which made it impossible to shut down traffic in even a small part of the water way for construction. That called for perfect planning.

The first step was the construction of pier. Since the water was so deep the concrete blocks of large size could not be dropped to form the pier of the bridge. Neither could any in-situ work be carried out using concrete. So the engineers developed a new innovative technique. They made use of an 80m high cylindrical (steel) mould built in the dry site. Then the used so many boats to tow the mould to the spot where the foundation was required. Then slowly water was filled and the cylinder settled on the bed of the water body. Further surveying revealed that the mould had settled exactly where it was intended to be.

The concrete is a heterogeneous material, which when exposed to water prior to curing would dissolve like aspirin. So the challenge was to develop a concrete that could hold itself in water and cure perfectly. The Japanese came up with an amazing type of concrete called the super concrete which served this purpose. The next step in the construction was building the tower that would hold the cables of this suspension bridge. The towers were built of steel and could withstand earthquakes up to 8.5 Richter.

The next process was suspending the cables over the two towers. The suspension cable was to withstand high loads and so a new technology involving super strong steel wire with added silica was employed. The cable was made of 37,000 strands packed together making the width of the cable as 1m.It was planned to be carried out by shipping the cables from one island to the other and releasing the cables slowly along the route. But this was declared impossible as the water way traffic was so high to be disturbed. In 1993, the cables were passed by air from one island to other after passing and fixing them over the two towers. After securely fixing the main cables in place the vertical suspension cables were fixed at regular intervals so that the lower end of these suspension cables reach only till the deck level.

The final step was the deck and roadway laying. The commencement of the deck slab construction and hence the road way laying began in January 1995. The deck was designed to be strong and hold against any sway. Beneath the deck the structural steel members were arranged in triangular shapes to allow the wind to pass through, so that the wind load on the bridge was reduced to some extent. This deck construction had just begun when Japan faced a major disaster.

In January 1995, a major earthquake of 7.5 Richter hit Japan. But the wonder was the construction withstood the seismic forces perfectly. The tower stood still due to the flexibility of the steel. But before the work could commence again, inspection of the piers underwater was essential to make sure no damage had occurred in the base. Only when this inspection was done with the aid of video cameras did the engineers know that the pier was resting on a fault line. And when a survey was conducted on land, the other shocking discovery was made. Due to the earthquake and a fault line presence they hadn’t detected earlier, the two towers moved a little away from its original position, as a result of which the bridge had to be 1m longer. The design was to be changed now for the roadway and the intervals between suspension cables had to be increased. Then roadways were laid. The 100 tonne steel road way pre-fabricated was taken using floating cranes and placed in position. In 5^th April 1998, their efforts bore fruits and the bridge was open to traffic. The most important thing that makes this construction even more wonderful is there were no fatalities throughout the 10 year construction period.

And that’s how 3 decades of planning, 1 decade of construction and 4 billon dollars have made the impossible, possible.