Bridge Basics
There are five major types of bridges:
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The beam bridge
The truss bridge
The arch bridge
The suspension bridge
The cable-stayed
The biggest difference between the five is the distances they can cross in a single span. A span is the
distance between two bridge supports, whether they are columns, towers or the wall of a canyon. A
modern beam bridge, for instance, is likely to span a distance of up to 200 feet (60 meters), while a
modern arch can safely span up to 800 or 1,000 feet (240 to 300 m). A suspension bridge, the pinnacle
of bridge technology, is capable of spanning up to 7,000 feet (2,100 m).
What allows a suspension bridge to span greater distances than a beam bridge, truss bridge or an arch
bridge? The answer lies in how each bridge type deals with two important forces called compression
and tension:
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Compression is a force that acts to compress or shorten the thing it is acting on.
Tension is a force that acts to expand or lengthen the thing it is acting on.
A simple, everyday example of compression and tension is a spring. When we press down, or push the
two ends of the spring together, we compress it. The force of compression shortens the spring. When
we pull up, or pull apart the two ends, we create tension in the spring. The force of tension lengthens
the spring.
Compression and tension are present in all bridges, and it's the job of the bridge design to handle these
forces without buckling or snapping. Buckling is what happens when the force of compression
overcomes an object's ability to handle compression, and snapping is what happens when the force of
tension overcomes an object's ability to handle tension. The best way to deal with these forces is to
either dissipate them or transfer them. To dissipate force is to spread it out over a greater area, so that
no one spot has to bear the brunt of the concentrated force. To transfer force is to move it from an area
of weakness to an area of strength, an area designed to handle the force. An arch bridge is a good
example of dissipation, while a suspension bridge is a good example of transference.
Beam Bridge
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.
The beam itself must be strong so that it doesn't bend under its own weight and the added weight of
crossing traffic. When a load pushes down on the beam, the beam's top edge is pushed together
(compression) while the bottom edge is stretched (tension).
Pre-stressed concrete is an ideal material for beam bridge construction; the concrete withstands the
forces of compression well and the steel rods imbedded within resist the forces of tension. Pre-stressed
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, beam bridges rarely span
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."
In fact, the world's longest bridge is a continuous span beam bridge. Almost 24 miles long, the Lake
Ponchartrain Causeway consists of two, two-lane sections that run parallel to one another. The
Southbound Lane, completed in 1956, is made up of 2243 separate spans, while the Northbound Lane,
completed in 1969, is pieced together from 1500 longer spans. Seven cross-over lanes connect the two
main sections and function as pull-over bays in emergencies. Although impressive, the Lake
Ponchartrain Causeway Bridge underscores the drawback of continuous spans: they are not well suited
for locations that require unobstructed clearance below.
Truss Bridge
Wooden truss bridges were used as early as the 1700’s, but the first metal one completed in 1841.
They are very strong and have been used for railroads bridges mainly because of the heavy loads they
can support. A truss, a rigid support structure that is made up of interlocking triangles, holds up the
roadbed and is set between two piers. The triangle is used because it is the only shape that is
inherently rigid.
If the beam were designed so that there was more material on the top and bottom, and less in the
middle, it would be better able to handle the forces of compression and tension. As traffic pushes down
on the roadway, compression acts on the upper horizontal members of the truss structure. Tension
acts on the bottom of the horizontal members of the truss structure. The forces of tension and
compression are shared among the angled members.
A truss system takes the concepts of beam bridges one step further. Think of one side of a truss bridge
as a single beam. The center of the beam is made up of the diagonal members of the truss, while the
top and bottom of the truss represent the top and bottom of the beam. Looking at a truss in this way, we
can see that the top and bottom of the beam contain more material than its center (corrugated
cardboard is very stiff for this reason).
In addition to the above-mentioned effect of a truss system, there is another reason why a truss is more
rigid than a single beam: A truss has the ability to dissipate a load through the truss work. The design of
a truss, which is usually a variant of a triangle, creates both a very rigid structure and one that transfers
the load from a single point to a considerably wider area.
Truss bridges are considered to be strong, but can be difficult to construct. Truss bridges difficult
construction can also lead to high maintenance and difficult to widen if necessary.
Arch Bridge
Arch bridges are one of the oldest types of bridges and 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 out.
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 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.
Today materials like steel and pre-stressed concrete have made it possible to build longer and more
elegant arches, including a spectacular 1700 foot span in New River Gorge, West Virginia. (More
typically, modern arch bridges span between 200-800 feet.)
Constructing an arch bridge can be tricky, since the structure is completely unstable until the two spans
meet in the middle. One technique is to build elaborate scaffolding, or "centering," below the spans to
support them until they meet. A newer method supports the spans using cables anchored to the ground
on either side of the bridge. In situations where there is an active water or road way below, this method
allows contractors to build without disrupting traffic.
One of the most revolutionary arch bridges in recent years is the Natchez Trace Bridge in Franklin,
Tennessee, which was opened to traffic in 1994. It's the first American arch bridge to be constructed
from segments of pre-cast 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 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.
Suspension Bridge
Aesthetic, 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 on top of high towers and are
secured at each end by anchorages.
The towers enable the main cables to be draped over long distances. Most of the weight of the bridge is
carried by the cables to the anchorages, which are imbedded 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 nineteenth
century, suspension bridges used iron chains for 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 Akashi Kaikyo Bridge in Japan has world's longest center span—measuring a staggering
6,527 feet, linking the islands of Honshu and Shikoku. The bridge's center section stretches. 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 of
cars reported that vehicles 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 the structure was designed 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 become transformed into a
catastrophic force? To answer these questions engineers began applying the science of aerodynamics
to bridge designs. 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.
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 Bridge
Cable-stayed bridges may look similar to suspensions 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 are connected 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.
The cables can be attached to the roadway in a variety of 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 other.
Even though cable-stayed bridges look futuristic, the idea for them goes back a long way. The first
known sketch of a cable-stayed 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 stay bridges have begun to be erected 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. Compared to suspension bridges, cable-stayeds require less cable, can be
constructed out of identical pre-cast concrete sections, and are faster to build. The result is a costeffective bridge that is undeniably beautiful.
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.
In your notes: (There may be a pop quiz on this, hint, hint.)
Read through ‘Bridge Basics’
 Identify the 5 major types of bridges
 List the strengths and weaknesses of each type of bridge
 Draw a sketch of each of the bridges
 Label where tension and compression are involved in each bridge