Bridge Design

A pedestrian bridge.

There are more than half a million bridges in the United States, and you rely on them every day to cross obstacles like rivers, streams, valleys, and railroad tracks. But do you know how they work? 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 bridges, truss bridges and suspension bridges.

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. NOTE: The blue arrows indicate tension (pulling apart), the red arrows represent compression (pushing together).

Some beam bridges need extra support in the middle.
A railroad bridge.
The north end of the Golden Gate bridge is a section of a beam bridge.

The truss bridge…
consists of an assembly of triangles. Truss bridges are commonly made from a series of straight, steel bars. NOTE: The blue arrows indicate tension (pulling at each end), the red arrows represent compression (pushing down).

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.

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 with counterweights like a drawbridge; and the deck of a lift bridge is raised vertically like a massive elevator.
A truss bridge that also rotates to let boat traffic pass.

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.

My friend Charlie designed this arch bridge (his first bridge design).
Roman arches.
How are arch bridges built?

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.
The new bridge next to the Hoover dam. The Mike O’Callaghan–Pat Tillman Memorial Bridge is an arch bridge that spans the Colorado River between the states of Arizona and Nevada.


Next is the Suspension Bridge.

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

NOTE: The blue arrows indicate tension (pulling at each end), the red arrows represent compression (pushing down).
Golden Gate Bridge
A hypothetical suspension bridge.
The Tacoma Narrows bridge was designed to be 20 feet thick, but the city decided to make the deck 8 feet thick to cut costs. Winds set up undulations (it was called Galloping Gerdy) that tore it apart. It did not have a truss system under the deck sufficient to stop the twisting. All bridges after this one have had to have an analysis of wind effects performed before it is built.

Cable-Stayed Bridge

In 1978, SOM architect Myron Goldsmith and engineer T.Y. Lin designed a bridge to span the middle fork of California’s American River, literally turning the discipline of bridge-building on its side. Their bridge scheme for the Ruck-a-Chucky gorge was an elegant solution to a challenging topographic problem: a wide, deep river bordered by banks with 40-degree slopes. To build a straight bridge, deep cuts and tunnels were required to create the approaches, and 500 foot-tall partially submerged pylons, which would have been difficult and expensive to make seismically stable, were needed.

With these concerns in mind, Goldsmith proposed what he called “a hanging arc,” a curved bridge suspended by 80 stayed, high-strength cables attached to anchors embedded in the hillsides. The weight of the deck would have been balanced by the surrounding array of tensile forces.
Bridge to the town of Pasco in Washington State.

Completed in 1987, the Sunshine Skyway bridge is the world’s longest cable-stayed concrete bridge. It is probably the best known of the several dozen cable-stayed bridges that have been built in the United States since the late 1970s. Its popularity may be due to its unique color — its cables are painted a bright taxicab yellow.
I was the environmental engineer on the project.
The cable-stayed bridge supports the roadway with massive steel cables, 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.
Bridge complete.

I was the Environmental Engineer for the Sunshine Skyway project. As such I designed a groin and plantings at the south end of the project.

A groyne (in the U.S. groin), built perpendicular to the shore, is a rigid hydraulic structure built from an ocean shore (in coastal engineering) or from a bank (in rivers) that interrupts water flow and limits the movement of sediment. It is usually made out of wood, concrete, or stone. In the ocean, groynes create beaches, prevent beach erosion caused by longshore drift where this is the dominant process and facilitate beach nourishment.
My design was semicircular to encompass the area needed for mitigation of the loss of mangroves as the highway came to shore.