Civil What?

Engineering Challenges

next

• 1
• 2
• 3
• 4
• 5

Bridging space

Ever since our prehistoric ancestors used a fallen log to cross a stream, we've relied on bridges for transportation over rivers and lakes, valleys and canyons, roads and buildings. Although we often take them for granted, bridges make our lives much easier-they cut down on travel time, reduce traffic congestion, allow us to reach hard-to-get-to places, and help bring people closer together.

The art and science of bridges

In addition to their practical uses, bridges are often celebrated for their beauty and engineering ingenuity. They're among our most beloved landmarks-it would be hard to imagine San Francisco without the Golden Gate Bridge or New York without the Brooklyn Bridge! From the elegant aqueducts of ancient Rome to the soaring steel structures of today, bridges are one of our crowning achievements.

Bridge engineering 101

Bridges must be strong enough to support their own weight (called the dead load) as well as the weight of the people, goods, and vehicles travelling across them (called the live load). They can withstand these heavy loads because of continual pulling and squeezing on each part of the bridge. These two opposing forces are called compression (squeezing) and tension (pulling).

A balancing act between forces

Most bridges work by balancing compression and tension-if there's too much of either, a bridge becomes weak and may even collapse. Too much compression (squeezing) can make a bridge buckle or crumble. Too much tension (pulling) can make a bridge snap apart.

One way to keep tension and compression forces in balance is to spread them out over the bridge so they're not concentrated on one spot. In a suspension bridge, for example, the roadway hangs from massive steel cables that are draped over two towers. The cables are in tension (they pull up the road), while the towers are in compression (the weight of the cables squeezes and pushes down on them).

Forces of Nature

In addition to dead or live loads, other forces that act on a bridge include Mother Nature-winds, rain, hurricanes, and earthquakes. Engineers are problem solvers skilled in coming up with solutions to keep bridges strong, stable, and safe, even under the most difficult conditions.

Building materials

What you build a bridge out of is as important as how you build it. Bridges can be made from stone, wood, steel, concrete, or reinforced concrete (concrete with steel bars embedded in it for increased strength). Some materials, such as stone, can withstand lots of compression (squeezing) but not tension (pulling). Others, such as steel, are strong under both compression and tension.

Bridge engineering

Here are a few of the reasons civil engineers choose to design and build bridges:

1. It's creative: Imagine having a job that allows you to design these awe-inspiring structures!
2. It builds a better world: Our society needs and relies on bridges to function. Not only are they essential, but they add beauty to our surroundings.
3. It's about the power of teamwork: Such a large undertaking calls on lots of people with many different talents working together toward a common goal.

A variety of engineers contribute to designing and building a bridge:

Structural engineers plan and design a bridge's size, shape, span, and materials. They test and analyze the bridge's ability to withstand loads, forces, and stresses, ensuring it's strong and safe.

Geotechnical engineers test and evaluate the soil, rocks, and surrounding environment, making sure that the foundation is solid and stable.

Other engineers may also be part of the team, including construction engineers, who oversee the complex construction work and manage the many workers. Transportation engineers may weigh in about the traffic flow on the bridge, and environmental engineers may address any environmental concerns and offer guidance on ensuring that the bridge is in harmony with its natural surroundings.

Serving the public

Bridges are ambitious and large-scale projects, almost all of which are financed with public money. "Civil" refers to civilization, and it's the privilege and responsibility of civil engineers to serve the public good. We rely on them to create structures that are economical yet safe, efficient yet attractive.

Questions to ponder

When planning a bridge, engineers weigh many competing interests, including cost, efficiency, safety, design, and appearance. A suspension bridge, for example, is undeniably elegant and breathtaking. But it's also expensive and takes years to build. On the other hand, a beam bridge is much cheaper and quicker to build. But beam bridges can't always span long distances and are often less striking and attractive. What's the best choice? And when you factor in building materials, bridge and river traffic, and the surrounding environment, the puzzle becomes even more complex!

Fun Facts

next

• 1
• 2
• 3
• 4
• 5
• 6
• 7

The Golden Gate Bridge

The Golden Gate Bridge (1937), one of San Francisco's most striking landmarks, is painted a color called "international orange." The unconventional color was chosen to ensure that the bridge would be easily visible in San Francisco's frequently foggy weather.

Tower Bridge

One of London's most celebrated attractions, the Tower Bridge (1894) is a combination suspension and bascule bridge (a drawbridge), which allows for river traffic to pass. In 1952, a double decker bus was crossing when the bus driver realized the bridge was in the process of opening-even though no warning had been issued! The driver stepped on the gas and the bus leapt across the 3-foot gap, landing safely on the other side.

The Millau Viaduct

The Millau Viaduct (2004) in France is the tallest bridge in the world. One of its piers rises 1,125 feet (343 m)-and is taller than the Eiffel Tower. This elegant cable-stayed bridge stretches 1.6 miles across a valley, and has greatly reduced the area's heavy traffic congestion.

Mostar Bridge

Built by Ottoman Turks in 1566 in the city of Mostar, Bosnia-Herzegovina, this graceful pedestrian arch bridge connected two parts of the city, whose people were divided by religion and ethnicity. But in 1993, during the country's bitter civil war, the 427-year-old bridge was blown up. Painstakingly reconstructed in 2004, Stari Most ("Old Bridge") once again unites the city and serves as a symbol of hope and reconciliation.

Lake Pontchartrain Causeway

Made of two concrete spans nearly 24 miles long, this beam bridge (1956) in Louisiana is the world's longest highway bridge over water. When a car reaches its midpoint, neither shore is visible-there's just water in every direction!

Cape Cod Canal Railroad Bridge

This type of movable bridge is called a vertical lift bridge-the span moves up and down like an elevator. Connecting Cape Cod with the rest of Massachusetts, this majestic truss bridge is normally kept in a raised position to let ships pass, and lowered to allow trains to cross the canal. When it was built in 1935, it was the longest vertical lift span in the world.

Banpo Bridge

This double deck bridge in Seoul, South Korea (1982) features a spectacular water fountain that shoots 190 tons of water per minute from its sides into the river. During the day, the fountain resembles a willow tree swaying in the wind. At night, the fountain lights up in a rainbow of colors, synchronized to music.

Types of Bridges

next

• 1
• 2
• 3
• 4
• 5
• 6

Which bridge is best?

Before designing and building a bridge, engineers must consider many things-including the purpose of the bridge, the distance to be spanned, and the kinds of materials available. There are five basic types of bridges, and many creative variations on each.

Beam Bridge

A beam bridge is the simplest and oldest type of bridge. It's made of a horizontal beam supported by vertical piers. The weight of the beam pushes straight down on the piers. The beam itself must be strong so that it doesn't fail under its own weight and the added weight of crossing traffic. The farther apart its piers, the weaker the beam becomes. This is why a single beam bridge rarely spans more that 250 feet. To create a longer bridge, several beam bridges can be linked together, creating what is called a continuous span bridge.

Truss Bridge

A truss bridge is stronger than a simple beam bridge. It's made of a series of straight steel bars that form triangles or other stable, rigid shapes. The trusses create a strong framework that distributes the weight from a single point over a wider area. Trusses can be used above or below the deck and come in a variety of patterns-many are geometric masterpieces.

Arch Bridge

The ancient Greeks and Romans invented arch bridges more than 2000 years ago, and some examples of their sturdy designs still survive today. Instead of pushing straight down, the weight of an arch bridge is carried outward along the curve of the arch. As the arch tries to spread outward, vertical supports-called abutments-push back on the arch at each end. An arch bridge can span up to 800 feet.

Suspension Bridge

The roadway of a suspension bridge is suspended from vertical hangers that are attached to two massive steel cables. These cables are draped over two towers and secured into solid concrete blocks, called anchorages, on both ends of the bridge. When weight like cars (called the live load) pushes down on the road, the cables and hangers holding it up transfer the weight to the towers, which support most of the bridge's weight. Suspension bridges can span 2,000 to 7,000 feet-much farther than any other type of bridge! Most have a truss system beneath the roadway to keep them from bending and twisting.

Cable-stayed Bridge

Cable-stayed bridges are the newest type of bridge. Like suspension bridges, they support the roadway with massive steel cables, but in a different way. The cables are directly attached to the deck, and run from the roadway up to a tower, forming a unique "A" shape. Unlike a suspension bridge, a cable-stayed bridge has no anchorages-the towers alone bear the weight.