Can you imagine being an early explorer? Setting out to discover new lands, you couldn’t predict what you would find.

Sooner or later, you would surely run into rivers or other bodies of water that you would have to cross to continue your journey.

Early settlers who chose to make a home in a new area often had to decide where to live based on the location of rivers and streams. Not only were these waterways sources of water, but they also enabled settlers to travel to other areas.

To make travel between settlements separated by bodies of water easier, bridges were eventually built. Today, we take them for granted. But think about how much more difficult life might be if they didn’t exist!

The first bridges were most likely simple logs placed over small streams. Now, bridges of many types are used to make it easier to cross over a variety of obstacles, including waterways, valleys, roads and railroad tracks.

Believe it or not, there are more than 500,000 bridges in the United States today. They come in a variety of lengths and styles, but they almost all fall into one of three basic types of bridges: beam, arch or suspension bridges.

The main difference between the three types of bridges is what distance they can cross in a single span. A span is how far it is between bridge supports, which can be columns, towers or even the walls of a canyon.

Beam bridges consist of a horizontal beam (often concrete or steel) supported at each end by columns or piers. The weight of the beam and traffic on the bridge is directly supported by the piers.

The farther apart the piers are, the weaker the beam will be. This is why beam bridges usually span no more than 250 feet.

They’re common on highway overpasses. Some beam bridges use supporting latticework — called a “truss” — to add strength.

Arch bridges have great natural strength due to their shape. Semicircular in shape with abutments on each end, arch bridges are usually made of steel or concrete and can span up to 800 feet.

The shape of arch bridges naturally distributes weight away from the center toward the abutments.

Suspension bridges use cables strung across tall towers to support the bridge surface, called the “deck.” Because the cables and the towers can distribute a tremendous amount of weight, suspension bridges can span from 2,000 to 7,000 feet!

Almost all suspension bridges also have supporting truss systems underneath the bridge deck to add strength and support.

So how long are some of the longest bridges in the world? Because there are different ways to measure the length of bridges, a bridge can be the longest bridge in one of many categories.

Check out these amazing bridges:

• Danyang–Kunshan Grand Bridge: Located in China, the Danyang-Kunshan Grand Bridge does not span water. It is used for an elevated high-speed rail line and totals more than 102 miles in length.
• Qingdao Haiwan Bridge: Also in China, the Qingdao Haiwan Bridge opened in 2011 and is now the world’s longest sea bridge at 26.4 miles in length.
• Lake Pontchartrain Causeway: The longest bridge in the United States consists of two parallel bridges across Lake Pontchartrain in southern Louisiana. The longer of the two bridges measures 23.87 miles long and is the seventh-longest in the world and the second-longest over water.