Bridges are structures that allow people to cross over obstacles
Bridges allow us to cross over many different obstacles. Clockwise from top left: Pedestrian Bridge, Spain, © JCVStock. Millau Viaduct, France, © PHB.cz (Richard Semik)/Shutterstock.com. Peace Bridge, Canada, © Jeff Whyte/Shutterstock.com. Tower Bridge, England, © pcruciatti.
Bridges are structures that allow people to cross over obstacles. An obstacle might be something found in nature, like a river or another large body of water like a bay. Other obstacles are man-made, like a freeway that you need to safely cross over to get to school. Bridges solve transportation problems by shortening the distance people have to travel to get from one place to another.
We live in a world full of bridges. There are over 600,000 bridges in the United States alone. Bridges are some of the most beautiful and well know structures in the world. Like all structures, bridges are load bearing. Unlike many structures, the structural components of a bridge are usually on display, making it easier to understand how they support their loads.
When helping to design a bridge, engineers must consider the obstacle to be crossed and the forces that will act on the bridge.
There are lots of facts engineers need to know before they can help design a bridge, including environmental factors like what obstacle needs to be crossed and how far down the ground is in case additional supports are needed. Engineers will also need to know if the area experiences any natural events like earthquakes. All of these environmental factors will help the engineer develop a set of design constraints, or requirements, that the bridge must meet to be safe. These requirements are called design constraints, because they will represent limitations on the design, or plan, of the bridge.
Another key design constraint is ensuring the bridge is strong enough to stand up to the forces (pushes and pulls) that will be acting it. If you’ve watched the structures video, you know that if a structure is load bearing, it is designed to stand up to forces acting on it. In addition to planning for all of the environmental factors above, engineers must also plan for the bridge to stand up to, or support, its own weight – its dead load – along with the weight of the users of the bridge – its live load. The environment also holds potential forces engineers must plan for, such as the possibility of high winds blowing against the bridge.
Along with the different forces and environmental factors, an important design constraint with every engineering project is the budget, or amount of money, available to complete the project. Materials, equipment, and labor – the work done by people – are expensive and will place additional restrictions on the design. Once an engineer understands all of the design constraints on building a particular bridge, she can figure out what kind of bridge design will work best.
Different types of bridges, including the beam bridge, the suspension bridge, and the arch bridge, use different structural elements to support their loads
A bridge has two components: the deck, which is the horizontal part of the bridge on which users cross, and the supports for that deck. The deck can be made up of one or more spans. A span is the distance between two supports. The key distinction between the different types of bridges is how the deck of the bridge – and therefore all of the users of the bridge – is supported. Let’s look more closely at three different types of bridges, the beam bridge, the suspension bridge and the arch bridge to see how they support their loads.
A beam is a horizontal structural element supported on both ends. With bridges, this beam is called a span. A beam bridge can have a single span or it can be continuous and have many spans and therefore many supports. As with all beams, each span of a beam bridge is under both tension and compression. Compression on the top of the deck from the weight of the deck and the people and vehicles using the bridge, and tension on the bottom of the deck, as the deck is pushed down from the load it is supporting.
A suspension bridge supports its deck with a pair of cables. These two cables run along the top of two towers and are then anchored to the earth at each end. Suspender cables attach the deck to these top two cables. All of the cables are under tension (being pulled) from the weight of holding the deck up. The towers are under compression (being pushed) from the weight of cables (and the deck they are supporting) pushing down on them.
An arch bridge works by diverting the force of its load to abutments on either side of the arch. Because of its semicircular shape, every part of an arch is under compression. If you were to stand on top of arch, it would not bend under your weight like a beam would. An arch bridge can be designed as a single arch with a deck on top, or as a series of arches that are connected with a deck on top. Arch bridges can also be built with the span of the deck passing through the arch, called a through arch bridge. A through arch bridge uses cables or beams to support the deck, which work like the suspender cables in a suspension bridge. The oldest bridges in the world that are still standing (some still in use) are arch bridges. Modern arch bridge designs look very different than ancient arch bridges, but they still rely on the strength of the arch to support their users.
One main distinction between the different bridge designs is the maximum length a single span of the bridge can be
Top: Lake Pontchartrain Causeway, © pisaphotograpy/Shutterstock.com, Bottom: Akashi Kaikyo Bridge, © Sean Pavone.
Just as skyscrapers challenge engineers to build vertically, or up, bridges challenge engineers to build horizontally, or across. Bridges are noted both for their overall length, and for the length of their longest span. Beam bridges are the longest bridges in the world, but a single span on a beam bridge is shorter than a single span on many of the other bridge designs. For example, the Lake Pontchartrain Causeway, part of the highway system in Louisiana, is 23.8 miles long, but the individual spans of its deck are only 56 feet.
On the other end of the spectrum, the longest suspension bridge in the world is the Akashi Kaikyo Bridge in Japan. Like the Lake Pontchartrain Causeway, the Akashi Kaikyo Bridge is part of a highway system. This bridge is only 2.4 miles long, but its longest span is 6,532 feet long. The Akashi Kaikyo Bridge is about a tenth the length of the Lake Pontchartrain Causeway, but its longest span is 116 times longer than any span on the Causeway.
These distances and lengths point out not only the differences in the bridge designs, but also the different design constraints that each bridge must meet. Bridges are a critical part of our transporation infrastructure, or system. They are each are designed to meet the unique requirements of the many obstacles that people cross every day on their way to work, school, and play.