Suspension Bridge

Station

Overview

Suspension Bridge Station Simulation

What are the anchorages for?

Resources needed - 6 strands of string, each at least 3’ long, and perhaps longer

Golden Gate Bridge

Instructions

Tie two loops of string around the tops of two hardcover books of similar size. Tie a third piece of string to each loop so that it hangs loosely between the books. Press down on the center string. What happens ?

Next, stand two books upright about 10 inches apart. Put a stack of heavy books on one end of a string to secure it to the table. Then pass the string over each standing book (letting some string hang loosely between the books). Place a second stack of books on the other end of the string. Press again on the center of the string. What happens? Notice how the anchorages (stacks of books) help to stabilize the bridge.

Suspension

More Background Information

This is more for the teachers and facilitators to acquire more background knowledge of each type of bridges in order to facilitate this session.

Suspension bridges are pleasing to look at, light, and strong, and can span distances from 2,000 to 7,000 feet—far longer than any other kind of bridge. They also tend to be the most expensive to build. True to its name, a suspension bridge suspends the roadway from huge main cables, which extend from one end of the bridge to the other. These cables rest atop high towers and are secured at each end by anchorages.

The towers enable engineers to stretch the main cables over long distances. The cables carry most of the bridge’s weight to the anchorages, which are embedded in either solid rock or massive concrete blocks. Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free.

Some of the earliest suspension-bridge cables were made from twisted grass. In the early 19th century, engineers began using iron chains for such cables. Today, the cables are made of thousands of individual steel wires bound tightly together. Steel, which is very strong under tension, is an ideal material for cables; a single steel wire only 0.1-inch thick can support over half a ton without breaking. Currently, the Humber Bridge in England has the world's longest center span—measuring 4,624 feet.

But this record won't stand for long. In 1998, the Japanese will unveil the $7.6 billion Akashi Kaikyo Bridge, linking the islands of Honshu and Shikoku via Awaji Island. The bridge's center section stretches a staggering 6,527 feet. To keep the structure stable, engineers have added pendulum-like devices on the towers to keep them from swaying and a stabilizing fin beneath the center deck to resist typhoon-strength winds.

Because suspension bridges are light and flexible, wind is always a serious concern—as the residents of Tacoma, Washington can surely attest. At the time it opened for traffic in 1940, the Tacoma Narrows Bridge was the third-longest suspension bridge in the world. It was promptly nicknamed "Galloping Gertie," due to its behavior in wind. Not only did the deck sway sideways, but vertical undulations also appeared in quite moderate winds. Drivers reported that cars ahead of them would completely disappear and reappear from view several times as they crossed the bridge.

Attempts were made to stabilize the structure with cables and hydraulic buffers, but they were unsuccessful. On November 7, 1940, only four months after it opened, the Tacoma Narrows Bridge collapsed in a wind of 42 mph—even though engineers had ostensibly designed the structure to withstand winds of up to 120 mph.

The failure came as a severe shock to the engineering community. Why did a great span, more than half a mile in length and weighing tens of thousands of tons, spring to life in a relatively light wind? And how did slow, steady, and comparatively harmless motions suddenly transmogrify into a catastrophic force?

The Humber Bridge Photo credit: © Paul Hutchings/iStockphoto

Akashi Kaikyo Bridge Photo credit: © GYRO PHOTOGRAPHY / amanaimages / Corbis

Photo credit: © Lawrence Freytag/iStockphoto

To answer these questions, engineers began applying the science of aerodynamics to bridge design. Technical experts still disagree on the exact cause of the bridge's destruction, but most agree the collapse had something to do with a complex phenomenon called resonance, the same force that can cause a soprano's voice to shatter a glass.

This Tacoma Narrows Bridge opened in 1950, replacing the collapsed "Galloping Gertie." An even newer bridge now stands beside this one.

Today, wind-tunnel testing of bridge designs is mandatory. As for the Tacoma Narrows Bridge, reconstruction began in 1949. The new bridge is wider, has deep, stiffening trusses under the roadway, and even sports a slender gap down the middle—all to dampen the effect of the wind.