BRIDGE TYPES

1- Arch Bridge

2- Beam Bridge

3- Suspension Bridge

4- Cable Stayed Bridge

1- 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.
Arch Bridge
Bixby Creek Bridge, Monterey, CA
Try It!

How do the abutments support an arch bridge?

Cut a strip of cardboard that's about one inch by 11 inches. Gently bend the strip so that it
has a curve. Position the cardboard on a table so that it resembles
an arch. Press down on the center of the arch. What happens to the
ends of the cardboard?

Next, place a stack of books at each end of the arch.

Press again. Now what happens? Notice how the stacks
of books act as abutments, keeping the ends of the arch from spreading apart.

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.

The Pont du Gard

aqueduct

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.)

The New River bridge

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 The Natchez Trace
bridge
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 Centering under
for years to come. construction

2- 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."
Beam bridge
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).

Try It!

What happens when a load pushes down on a beam bridge?

Take a flat eraser or a small sponge and slice a shallow notch across the top and bottom.
Create a beam bridge by supporting each end of the eraser (or sponge) with
a stack of books. Press down on the center of the bridge. What happens to
the top and bottom notches? Notice how the top notch squeezes together in
compression, while the bottom notch spreads apart under 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." Lake Ponchartrain Causeway,
Louisiana
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.

3- 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
Suspension bridge
weight of the bridge is carried by the cables Golden Gate Bridge,
to the anchorages, which are imbedded in San Francisco, CA
either solid rock or massive concrete
Suspension bridge blocks. Inside the anchorages, the cables
anchorage are spread over a large area to evenly distribute the load and to
prevent the cables from breaking free.

Try It!

What are the anchorages for?

Tie two loops of string around the tops of two hard cover 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 about 10 inches apart. Put a stack of heavy
books on one end of string to secure it to the table. Then pass the
string over each 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.

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 Humber bridge in England has world's

The Humber
bridge 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 Akashi bridge
islands of Honshu and Shikoku. The bridge's center section stretches a cable assembly
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 Tacoma Narrows Tacoma Narrows
appeared in quite moderate winds. Drivers of Bridge Oscillation Bridge Collapse
cars reported that vehicles ahead of them
would completely disappear and reappear RealVideo: isdn | 28.8 RealVideo: isdn | 28.8
from view several times as they crossed the QuickTime: (0.6MB) QuickTime: (0.4MB)
AVI: (0.6MB) AVI: (0.4MB)
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.
New Tacoma Narrows Bridge

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.

4- 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

Cable-stayed bridge
Clark Bridge, Alton, IL
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-stayeded
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.

Parallel attachment pattern

Radial attachment pattern

Try It!

How do cable-stayeds work?

Stand up and hold your arms out horizontally at each side. Imagine that your arms are a
bridge, and your head is a tower in the middle. In this position, your muscles are holding
up your arms.

Try making cable-stayeds to support your arms. Take a piece of

rope (about five feet long), and have a partner tie each end of the
rope to each of your elbows. Then lay the middle of the rope on
top of your head. The rope acts as a cable-stayed and holds your
elbows up.

Have your partner tie a second piece of rope (about 6 feet long) to each wrist. Lay the
second rope over your head. You now have two cable-stayeds. Where do you feel a
pushing force, or compression? Notice how the cable-stayeds transfer the load of the
bridge (your arms) to the tower (your head).

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 cost-effective 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.

Sunshine Skyway bridge