Portions of this page have been extracted verbatim from the webpage "BUILDING BIG: Bridge Basics." http://www.pbs.org/wgbh/buildingbig/bridge/basics.html
BEAM BRIDGES

There are more than half a million bridges in the United States, and you rely on them every day to cross obstacles like streams, valleys, and railroad tracks. But do you know how they work? Or why some bridges are curved while others are straight? Engineers must consider many things -- like the distance to be spanned and the types of materials available -- before determining the size, shape, and overall look of a bridge.

Since ancient times, engineers have designed three major types of bridges to withstand all forces of nature.

Beam bridge

The beam bridge consists of a horizontal beam supported at each end by piers. The weight of the beam pushes straight down on the piers. The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely span more than 250 feet.

Forces that act on beam bridges...

When something pushes down on the beam, the beam bends. The top end pushes together, and the bottom ens pulls apart.

Other types of beam bridges...

Continuous span
beam bridge

The continuous span...
gives beam bridges the ability to span great distances. A single beam bridge rarely spans more than 250 feet. But, as in the Chesapeake Bay Bridge-Tunnel, several beam bridges can be linked together, creating what is called a continuous span.


Swing bridge:
George P. Coleman Bridge
The movable bridge...
like the George P. Coleman Bridge, has a deck that moves. A swing bridge has a deck that rotates around a center point; a drawbridge has a deck that can be raised and lowered; a bascule bridge deck is raised withcounterweights like a drawbridge; and the deck of a lift bridge is raised vertically like a massive elevator.

TRUSS BRIDGE

Cantilever bridge:
Firth of Forth

The truss bridge...
consists of an assembly of triangles. Truss bridges are commonly made from a series of straight, steel bars. The Firth of Forth Bridge in Scotland is a cantilever bridge, a complex version of the truss bridge. Rigid arms extend from both sides of two piers. Diagonal steel tubes, projecting from the top and bottom of each pier, hold the arms in place. The arms that project toward the middle are only supported on one side, like really strong diving boards. These "diving boards," called cantilever arms, support a third, central span.

Forces that act on truss bridges...



Every bar in this cantilever bridge experiences either a pushing or pulling force. The bars rarely bend. This is why cantilever bridges can span farther than beam bridges.

ARCH BRIDGE
Ancient Roman aqueduct

The arch bridge...
has great natural strength. Thousands of years ago, Romans built arches out of stone. Today, most arch bridges are made of steel or concrete,and they can span up to 800 feet.

Forces that act on arch bridges...



The arch is squeezed together, and this squeezing force is carried outward along the curve to the supports at each end. The supports, called abutments, push back on the arch and prevent the ends of the arch from spreading apart.


How arch bridges are built...

Arch bridge
being constructed with cables

Building an arch bridge isn't easy, since the structure is completely unstable until the two spans meet in the middle. For years, engineers used a technique called centering, in which a wooden form supported both spans until they locked together at the top. A newer method supports the spans using cables anchored to the ground on either side of the bridge. This is how the New River Gorge Bridge in West Virginia was built.

SUSPENSION BRIDGES

Suspension bridge:
Golden Gate Bridge

The suspension bridge...
can span 2,000 to 7,000 feet -- way farther than any other type of bridge! Most suspension bridges have a truss system beneath the roadway to resist bending and twisting.

 


Forces that act on suspension bridges...


In all suspension bridges, the roadway hangs from massive steel cables, which are draped over two towers and secured into solid concrete blocks, called anchorages, on both ends of the bridge. The cars push down on the roadway, but because the roadway is suspended, the cables transfer the load into compression in the two towers. The two towers support most of the bridge's weight.


Another type of suspension bridge...

Cable-stayed bridge:
Sunshine Skyway Bridge
Cable-Stayed Bridge
The cable-stayed bridge, like the suspension bridge, supports the roadway with massive steel cables, but in a different way. The cables run directly from the roadway up to a tower, forming a unique "A" shape.

Cable-stayed bridges, like the Sunshine Skyway in Florida, require less cable and can be built much faster than suspension bridges. Cable-stayed bridges are becoming the most popular bridges for medium-length spans (between 500 and 3,000 feet).

Now that you've mastered the bridge basics, test your bridge-building skills in the Bridge Challenge!