Tension and Compression: Two Forces Every Bridge
Knows Well
What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to stretch over a distance seven times
that of an arch bridge? The answer lies in how each bridge type deals with the important forces of compression and tension.
Tension: What happens to a rope during a game of tug-of-war? Correct, it undergoes tension from the two sweaty opposing teams
pulling on it. This force also acts on bridge structures, resulting in tensional stress.
Compression: What happens when you push down on a spring and collapse it? That's right, you compress it, and by squishing it, you
shorten its length. Compressional stress, therefore, is the opposite of tensional stress.
Compression and tension are present in all bridges, and as illustrated, they are both capable of damaging part of the bridge as varying
load weights and other forces act on the structure. It's the job of the bridge design to handle these forces without buckling or snapping.
Buckling occurs when compression overcomes an object's ability to endure that force.
Snapping is what happens when tension surpasses an object's ability to handle the lengthening force.
The best way to deal with these powerful forces is to either dissipate them or transfer them. With dissipation, the design allows the
force to be spread out evenly over a greater area, so that no one spot bears the concentrated brunt of it. It's the difference in, say, eating
one chocolate cupcake every day for a week and eating seven cupcakes in a single afternoon.
In transferring force, a design moves stress from an area of weakness to an area of strength. As we'll dig into on the upcoming pages,
different bridges prefer to handle these stressors in different ways.
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Build a Bridge
by Kate Hudec
Arch Bridge
Arch bridge—Bixby Creek Bridge, Monterey, CA Enlarge Photo credit: © Jay Spooner/iStockphoto
One of the oldest types of bridges, arch bridges have great natural strength. Instead of pushing straight down, the weight of an
arch bridge is carried outward along the curve of the arch to the supports at each end. These supports, called the abutments, carry the
load and keep the ends of the bridge from spreading outward.
When supporting its own weight and the weight of crossing traffic, every part of the arch is under compression. For this
reason, arch bridges must be made of materials that are strong under compression.
The Pont du Gard aqueduct Enlarge Photo credit: © Agnieszka Gaul/iStockphoto
The Romans used stones. One of the most famous examples of their handiwork is the Pont du Gard aqueduct near Nîmes,
France. Built before the birth of Christ, the bridge is held together by mortar only in its top tier; the stones in the rest of the structure
stay together by the sheer force of their own weight.
The New River Gorge Bridge Enlarge Photo credit: © John Brueske/iStockphoto
Today, materials like steel and prestressed concrete (see sidebar at right) have made it possible to build longer and more
elegant arches, including a spectacular 1,700-foot span in New River Gorge, West Virginia. (More typically, modern arch bridges
span between 200 and 800 feet.)
The Natchez Trace Parkway Bridge Enlarge Photo credit: © J. Paul Moore/iStockphoto
One of the most revolutionary arch bridges in recent years is the Natchez Trace Parkway Bridge in Franklin, Tennessee,
which was opened to traffic in 1994. It's the first American arch bridge to be constructed from segments of precast concrete, a highly
economical material. Two graceful arches support the roadway above. Usually arch bridges employ vertical supports called spandrels
to distribute the weight of the roadway to the arch below, but the Natchez Trace Parkway Bridge was designed without spandrels to
create a more open and aesthetically pleasing appearance. As a result, most of the live load is resting on the crowns of the two arches,
which have been slightly flattened to better carry it. Already the winner of many awards, the bridge is expected to influence bridge
design for years to come.
Beam Bridge
Beam bridge Enlarge Photo credit: © Andrea Pelletier/iStockphoto
A beam or "girder" bridge is the simplest and most inexpensive kind of bridge. According to Craig Finley of Finley/McNary
Engineering, "they're basically the vanillas of the bridge world."
In its most basic form, a beam bridge consists of a horizontal beam that is supported at each end by piers. The weight of the beam
pushes straight down on the piers.
Prestressed concrete (see sidebar at right) is an ideal material for beam bridge construction. The concrete withstands the
forces of compression well, and the steel rods embedded within resist the forces of tension. Prestressed concrete also tends to be one
of the least expensive materials in construction.
But even the best materials can't compensate for the beam bridge's biggest limitation: its length. The farther apart its supports,
the weaker a beam bridge gets. As a result, individual beam-bridge girders rarely stretch more than 250 feet. This doesn't mean beam
bridges aren't used to cross great distances; it only means that they must be daisy-chained together, creating what's known in the
bridge world as a continuous span.
The Lake Pontchartrain Causeway, Louisiana Enlarge Photo credit: © Gary Fowler/iStockphoto
In fact, the world's longest bridge is a continuous-span beam bridge. Almost 24 miles long, the Lake Pontchartrain Causeway
consists of a pair of two-lane sections that run parallel to each other. The Southbound Lane, completed in 1956, comprises 2,243
separate spans, while the Northbound Lane, completed in 1969, includes 1,500 longer spans. Seven crossover lanes connect the two
main sections and function as pull-over bays in emergencies. Although impressive, the Lake Pontchartrain Causeway bridge
underscores the drawback of continuous spans—they are not well suited for locations that require unobstructed clearance below.
Suspension Bridge
Suspension bridge—Golden Gate Bridge, San Francisco, CA Enlarge Photo credit: © ekash/iStockphoto
Pleasing to look at, light, and strong, suspension bridges 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.
The Humber Bridge Enlarge Photo credit: © Paul Hutchings/iStockphoto
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.
Akashi Kaikyo Bridge Enlarge Photo credit: © GYRO PHOTOGRAPHY / amanaimages / Corbis
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?
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. Enlarge Photo credit: © Lawrence Freytag/iStockphoto
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.
Cable-stayed bridges
Cable-stayed bridge—William H. Natcher Bridge, Rockport, IN Enlarge Photo credit: © David Sailors/CORBIS
Cable-stayed bridges may look similar to suspension bridges—both have roadways that hang from cables, and both have
towers. But the two bridges support the load of the roadway in very different ways. The difference lies in how the cables connect to
the towers. In suspension bridges, the cables ride freely across the towers, transmitting the load to the anchorages at either end. In
cable-stayed bridges, the cables are attached to the towers, which alone bear the load.
Parallel attachment (left) and radial attachment patterns in cable-stayed bridges Enlarge Photo credit: © WGBH Educational
Foundation
The cables can be attached to the roadway in either of two main ways. In a radial pattern, cables extend from several points
on the road to a single point at the top of the tower. In a parallel pattern, cables are attached at different heights along the tower,
running parallel to one another.
Even though cable-stayed bridges look futuristic, the idea for them goes back a long way. The first known sketch of a cablestayed bridge appears in a book called Machinae Novae published in 1595, but it wasn't until this century that engineers began to use
them. In post-World War II Europe, where steel was scarce, the design was perfect for rebuilding bombed-out bridges that still had
standing foundations. Cable-stayed bridges have gone up in the United States only recently, but the response has been passionate.
For medium-length spans—those between 500 and 2,800 feet—cable-stayeds are fast becoming the bridge of choice. And even longer
cable-stayeds are going up, though suspension bridges are still used for the very longest spans. Compared to suspension bridges,
cable-stayeds require less cable, can be constructed out of identical precast concrete sections, and are faster to build. The result is a
cost-effective bridge that is undeniably beautiful.
The Sunshine Skyway Bridge Enlarge Photo credit: © Tinik/iStockphoto
In 1988, the Sunshine Skyway bridge in Tampa, Florida won the prestigious Presidential Design Award from the National
Endowment for the Arts. Painted yellow to contrast with its marine surroundings, the Sunshine Skyway is one of the first cable-stayed
bridges to attach cables to the center of its roadway as opposed to the outer edges, allowing commuters an unobstructed view of the
magnificent bay. Recently, in Boston, Massachusetts, a cable-stayed design was selected for a new bridge across the Charles River—
even though cheaper options were proposed. City officials simply liked the way it looked.
PLEASE SURVEY THE FOLLOWING SITES AND FOLLOW THE DIRECTIONS AT THE BOTTOM OF THIS PAGE.
Here are the four sites that need bridges. Take a good look at each one and notice how each differs from all the others.
Notes:
A 4,200-foot span across an ocean bay where huge ships come and go Enlarge Photo credit: © pkujiahe/iStockphoto
Notes:
A 300-foot span across a narrow waterway Enlarge Photo credit: © Dariusz Paciorek/iStockphoto
Notes:
A 3,000-foot main span across a busy shipping channel in a major city Enlarge Photo credit: © Yang Liu/CORBIS
Notes:
A 1,200-foot span across a deep river gorge Enlarge Photo credit: © Catharina van den Dikkenberg/iStockphoto
DIRECTIONS:
Okay, now you're ready to consider what's needed where to best span the gap. Choose one location out of the four
shown above. Make some notes in the box about the location and possible bridge types for the location. On page
30 in your science notebook, explain which type of bridge you would choose for that location and why you would
choose that type of bridge. You should give specific reasons (use the article) and not just “because I think so”
answers.