Engineers vs. Nature: An Impossible Bridge

Most engineers have the dream of completing an almost unattainable work of genius. Sometimes, the engineer dreams of creating a new way to propel a car, while others dream of fantastic buildings reaching new heights into the sky never before achieved by anyone else. 

With the completion of the Rio-Antirrio Bridge, the engineers involved in the project achieved greatness. In the early 19th-century, prime minister of Greece Charilaos Trikoupis wanted a bridge built over the Gulf of Corinth. The structure needed to be able to overcome serious natural and geological complications in order to have a sound, usable, and lasting construct. 

A plethora of problems needed to be solved to make a permanent structure. Read the article below to find out just how many issues engineers solved to create the modern marvel Rio-Antirio Bridge.

Challenge #1: No Solid Foundations Underwater

The Gulf of Corinth is 65 meters deep, and at the bottom is sand and silt for hundreds of meters down. Bridges usually need a solid base to be placed on, optimally bedrock. 

A solid foundation is vital for a bridge, especially in areas with frequent earthquakes. The silt on the bottom of the gulf combined with an earthquake causes liquefaction, making stable soil behave like a liquid.

Engineers needed to find a solution to anchor the bridge when an earthquake turns the silty soil into what effectively is water. They found it in the roots of a plant growing in India customarily used to make incense.

Vetiver grass grows in India's swamps, and the oil in their roots are used to make a sweet-smelling incense. The plant's roots went down seven meters, and the bridge designers noticed how the roots stabilized the soil around them. 

The wet sand was stabilized by installing 200 metal piles in the sand underneath three out of four piers. Engineers now had a solid foundation to keep the bridge from sinking into the silt in an earthquake. 

Challenge #2: Earthquakes Make the Ground Slide Laterally

While each pier weighs about 171 thousand tons and is 90 meters across, the shaking of an earthquake could cause it to fall. The piers sliding around on the ground means an edge could catch and dig into the soil, causing them to tip over and take the bridge down. 

Unless the piers had some give when the ground began to shake, the bridge would come down in the first significant earthquake after its completion. Engineers found if they replaced the sand underneath the base of the piers with gravel on top of the metal piles, it would give the base room to move. 

The piers would have room to slide on top of the ground and not catch while still having a good strong foundation underneath


Challenge #3: The Bridge Deck Could Buckle When the Piers Move

The good, solid foundation was an excellent start for the bridge, but how do you connect the deck? An already unconventional design for a foundation makes the bridge deck a problematic task. Connecting the deck to piers that are able to move across the gulf's bottom means that in an earthquake, the deck could buckle or break. 

To protect the bridge deck from collapse, the engineers came up with a way to separate the deck from the piers' movement and still be functional. They studied how a hammock stayed almost steady and smooth in rough seas, this gave designers the idea to suspend the bridge above the piers.

A hammock demonstrates the pendulum principle. Large cables hold the bridge deck on a pivot supported by the piers. If an earthquake shakes the piers, the bridge decking will swing back and forth over its original position.

However, by solving the issue of the shaking piers taking the decking in opposite directions and breaking it apart, the engineers created another problem.

Challenge #4: The Bridge Deck Could Hit the Piers as it Swings Freely

The freely moving deck meant a strong enough earthquake could send the 75,000 tons of road swinging into one of the piers' four arms. Something would have to be created to keep the bridge decking from swinging too far in an earthquake because once something that large starts moving, it is difficult to stop.

Engineers ended up creating something called a Viscous Damper, which is a brake system using a liquid to help slow down the motion of an object. The braking system designed for the bridge using oil and pistons is the largest in the world. It turns the kinetic energy of the bridge deck movement into heat as it slows movement to prevent the bridge from destroying itself. 

Challenge #5: The Gulf of Corinth is a Natural Wind Tunnel

Even with the dampers in place to help keep the bridge movements in check, another problem engineers had to solve was the wind on the Gulf of Corinth, causing the bridge to swing continuously. This made the bridge uncrossable unless they could find a way to keep the bridge deck from moving except during an earthquake. 

Engineers created a predictable failure: a fuse attached to the pistons under the bridge to keep the bridge steady, up to a point. High winds would cause the bridge decking to move without them keeping the pistons in place.

However, the fuse has a “fail” feature. When enough movement from an earthquake happens, the fuse holding the piston in place snaps off to allow the viscous dampers to move and protect the bridge decking. 

Challenge #6: Vortex Shedding

The gale-force winds coming off the gulf create another problem. Vortex shedding, or swirls of air around an object, cause the cables to vibrate and shake, which leads to metal fatigue that could cause the bridge decking to collapse. 

Installing a helical strake was the solution, a large metal pole with a coil like a metal spring winding its way to the top. Winds hit the spirals and break apart to even out over the air, preventing vortex shedding and keeping the cables safe.

Problem Solving to Greatness

The extraordinary Rio-Antirio bridge is a modern marvel created out of a chain of ingenuity to solve the many problems that arose when trying to create it. Even with the high seismic activity and high winds, the engineers figured out how to make the bridge sustainable and even got it finished four months ahead of schedule.

It makes all the difference when those working on a project have up-to-date technology in their repertoire. Being knowledgeable about your field is a skill that will save time, money, and other resources.  

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