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January 2019
Ant Bridges
Volume 40
Number 1
nts are some of the smallest creatures on Earth. But don’t let their size deceive
you. They also happen to be among the strongest creatures on Earth. They can
pick up something that is equal to 20 times their own body weight. And when
they work together, they accomplish amazing tasks. Among them is the ability to use
their bodies to create bridges. It happens slowly. Ants on the march will sometimes
come to openings along their route. The first ant to reach the opening will stop. The
ant following will climb over the halted ant. Then it will stop, too. Slowly, ants use
this process of climbing over the stopped ants and extending themselves across the
opening to form a bridge to the other side.
Discover American History
What a
January 2019
Disco er America
n Histor
i tory
Meg Chorlian, Editor
John Hansen, Art Director
Pat Murray, Designer
Ellen Bingham, Copy Editor and Proofreader
Naomi Pasachoff, Editorial Consultant,
Research Associate, Williams College
James M. O’Connor, Director of Editorial
Christine Voboril, Permissions Specialist
Frances Nankin and Hope H. Pettegrew,
Advisory Board
Eric Arnesen,
Professor of History
The George Washington University
Diane L. Brooks, Ed.D.,
Director (retired)
Curriculum Frameworks and
Instructional Resources Office
California Department of Education
Ken Burns
Florentine Films
Beth Haverkamp Powers, Teacher
Milford, New Hampshire
Maryann Manning, Professor
School of Education
University of Alabama at Birmingham
Alexis O’Neill, Author and
Museum Education Consultant
Lee Stayer, Teacher
Advent Episcopal Day School
Birmingham, Alabama
Sandra Stotsky,
Professor of Education Reform
21st Century Chair in Teacher Quality
University of Arkansas
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Indexed and/or Abstracted in:
Children’s Magazine Guide
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Readers’ Guide for Young People
Readers’ Guide to Periodical Literature
Many of the largest cities in the United States
have been built near water, and bridges make it
possible for people to get into and out of those
busy urban areas. Several major bridges connect the San Francisco area. Shown here is the
western section of the San Francisco–Oakland
Bay Bridge. It links San Francisco and Yerba
Buena Island in San Francisco Bay. The eastern
section connects the island to Oakland. Built
in 1936, the bridge opened six months before
another famous California bridge—the Golden
Gate Bridge.
4 Bridge Basics
16 Just Build It
by Andrew Matthews
by Will Bremen
10 Under Construction
by Cynthia Overbeck Bix
Chicago On the Move
by Patrick McBriarty
San Francisco’s Golden G
by Cynthia Overbeck Bix
31 Lessons Learned
by Kathiann M. Kowalski
35 The BEAST: Meet Bridge Engineering
Expert Dr. Franklin Moon
by Kathiann M. Kowalski
Maybe you’ll change
your mind, Colonel,
after you see what
engineering marvels
g are!
Use a bridge? No,
thank you! I like to
keep my feet on
solid ground.
That’s a strangge
thing for a bird
to say.
Getting Started
Mapping It!
Did You Know?
Going Global
by Bryan Langdo
Freeze Frame
Your Letters
Just for Fun
Dr. D’s Mystery Hero
by Dennis Denenberg
Spotlight On . . .
by Ebenezer
Say What?
Cartoon Connection
by K.E. Lewis
Getting Started
magine that you lived centuries ago. We’re talking before
the invention of trains, cars, or
airplanes. You get where you need
to go by walking. And the shortest
way to get from where you live to
a nearby village involves crossing a river. Usually, you ford the
river. But heavy rains can make
that route dangerous. When that
happens, you have to walk around
the river. That is a much longer
trip. One day, you discover that
a massive tree has fallen over the
river where you normally cross.
You recognize right away what the
tree offers. It is a way to reach the
other side of the river quickly and
with dry feet!
The first bridge most likely was
“discovered” in this way. And
what a discovery it was! People
Ford means to cross
a river or a stream at
a shallow point by
wading through the
Cast iron is a group
of iron-carbon
alloys that have
been melted and
then poured into a
mold and allowed to
become solid.
Wrought iron is a
metal material that
can be shaped by
applying heat and
using tools.
realized that bridges provided permanent solutions for crossing over
rivers or other difficult spaces.
They began to construct bridges
by using materials that they could
find nearby. Wood and stone were
naturally available and plentiful. Wood was cheap and easy to
work with. Stone was strong and
solid. But weather took a toll on
wood and weakened it. Gathering
together heavy stones and lifting
them into place was a lot of work.
In the United States, bridge
building really took off during the
Industrial Revolution in the 1800s.
Big changes took place in the fields
of transportation and manufacturing. People went from relying on
horses to get from one place to
another to traveling by steamboats
and trains. By 1869, the first transcontinental railroad stretched from
coast to coast. Almost overnight,
a journey across the country that
used to take several months by
horse- or oxen-drawn wagon could
be accomplished in less than two
weeks by train. But railways relied
on bridges to cross rivers and
gorges. As locomotives grew more
powerful, wooden bridges couldn’t
withstand their weight.
The manufacture of cast iron
and wrought iron in the mid-1800s
gave bridge engineers the materials
they needed to build stronger and
longer structures. By the late 1800s,
steel was being mass-produced.
Stronger but lighter than iron,
steel was perfect for bridge building. Concrete became another
valuable construction material
in the early 1900s. Most bridges
today are made of a combination
of steel and concrete.
With each modernization,
bridge designs became bolder. They
stretched across wider spaces and
reached greater heights. The first
man-made bridges spanned short
distances. Bridges today can cross
seas. The first bridges were low
and built over shallow rivers and
streams. Modern bridge spans can
hang at heights of more than 1,000
feet. The first bridges were footbridges for people to walk across.
Today, bridges can carry trains,
cars and trucks, bicyclists, and
pedestrians. Many have multiple
lanes and decks for traffic.
Most large cities develop around
water. Bridges of all sizes and
types serve an important function
in heavily populated areas. They
provide ways for people to keep
moving and stay connected.
Many of the bridges being
designed in the 21st century also
beautify their surroundings. They
use architectural designs, colors,
and lighting to inspire and impress.
Bridges still provide the fastest way
to get from point A to point B, but
they have become beautiful engineering marvels along the
Th ancient
i t Romans
were the first to mix
volcanic ash, called
pozzuolana, with water
to make cement. The
cement was used to seal
the stonework of arch
bridges. Many examples
of Roman arch bridges
still can be seen in the
world today.
Steel is a strong, hard,
shapeable alloy of iron
and carbon.
Concrete is a
construction material
consisting of sand,
gravel, pebbles, broken
stone, or slag. When
combined with water,
it can be poured and
molded before it
hardens into shape.
by Andrew Matthews
ere’s a look at
six basic bridge
types. For help
with bridge terms, check
out pages 8–9.
Bridge Key
■ Tension
■ Compression
A beam bridge was one of the earliest bridges. It is
also one of the simplest and least expensive to build.
Its main elements are a straight horizontal beam
supported by a pier at each end. The load pushes
straight down on the beam. It is great for short distances—usually not more than 250 feet. Modern
beam bridges made of multiple girders supported by
piers can be much longer. Beam
Lake Pontchartrain Causeway
bridges frequently are used to
carry one road over another
road. The longest modern beam
bridge over water in the United
States is Lake Pontchartrain
Causeway in Louisiana. It is two
parallel bridges, the longer of
which is 23.83 miles. The longest
bridge in the world is China’s
Danyang-Kunshan Grand Bridge
at 102.4 miles. It spans a variety
of surfaces that include land
and water.
Girders are horizontal
beams, mostly made
of concrete or steel,
that are the main
supports for a bridge.
Arch bridge date ack to the ancient Greeks and
Romans. Many examples of ancient Roman arch
bridges remain standing today. An arch bridge
spreads out the load so that it transfers down and
outward at the same time. The arch is anchored
on each end by piers. The earliest versions of these
durable bridges were made of stone, a material that
was naturally available. But they covered short spans
and were labor intensive to build. As large quantities
of steel became available in the late 1800s, it made
longer single arch bridges possible. The longest arch
bridge in the United States is the New River Gorge
Bridge in West Virginia at 1,699
Durable means able
feet. The longest arch bridge in
to withstand damage
or wear.
the world is China’s Chaotianmen
Bridge at 1,811 feet.
New River Gorge Bridge
Astoria-Megler Bridge
A truss bridge uses a series of frames to evenly distribute the load along its length. The basic truss form
is a triangle, but many varieties of designs exist. The
Pratt truss and the Warren truss are the most com-
mon in the United States. These bridges originally
were made of wood and generally covered a short
span. By the mid-1800s, railroads were crisscrossing the country. To create more durable bridges for
newer and more powerful trains, iron and then steel
replaced wood trusses. The longest truss bridges
today are continuous truss bridges. They have three
or more supports that are rigidly connected. The
longest continuous truss bridge in the United States
is the Astoria-Megler Bridge in Oregon at 1,232 feet.
The longest continuous truss bridge in the world is
Japan’s Ikitsuki Bridge at 1,312 feet.
A cantilever is a structure that is anchored at on
end while the other end extends out horizontally
into space. A cantilever bridge projects two arms
out toward each other while the other ends are
connected to land. The arms meet in the middle.
Cantilever bridges are rigid and strong. They can
be longer than beam and arch bridges, and their
design leaves the main span open for water trafCommodore John Barry Bridge
fic below. The first cantilever bridges were built
of iron in the mid-1800s. They mostly are made
of steel or concrete
today. The longest
cantilever bridge in
the United States is
the Commodore John
Barry Bridge between
Pennsylvania and New
Jersey at 1,644 feet.
The longest cantilever
bridge in the world is
Canada’s Quebec Bridge
at 1,801 feet.
A suspension bridge suspends its deck or roadway
from above rather than supporting it from below.
The deck hangs from two enormous main cables.
The cables drape over tall towers and run the length
of the bridge. They are secured in massive anchorages on land. The main cables hold the bridge
in tension while the towers are compressed and
transfer the bridge’s load to the ground. Suspension
bridges have the longest main spans. They require
less material so they can be less expensive to build
and to maintain. They are not as rigid as other
bridges. That helps them to withstand earthquakes.
But extreme, severe winds can cause their flexible
decks to move dangerously. Suspension bridges also
Verrazzano-Narrows Bridge
cannot handle the heavy load of railway traffic. The
first suspension bridges were built in the United
States in the mid-1800s. The longest suspension
bridge in the United States is the VerrazzanoNarrows Bridge in New York at 4,260 feet. The
longest suspension bridge in the world is Japan’s
Akashi Kaikyo Bridge at 6,532 feet.
A cable-stayed bridg uses a suspended deck or roadway, too. But the deck cables are attached directly
to the tower or towers rather than hung from main
cables. The cables often are attached in a fan- or
harp-like manner to the towers, which carry the load.
Cable-stayed bridges do not require anchorages to
secure the cables. Thus, they require less material
than suspension bridges. They can span a greater
distance than a cantilever bridge, up to 3,000 feet.
Some cable-stayed bridges were built in the 19th
century. Their low-cost, fast-construction time have
made them popular in the 21st centur The lo gest
cable-stayed bridge in the United States is the Arthur
Ravenel Jr. Bridge in South Carolina at 1,546 feet. The
longest cable-stayed bridge in the world is Russia’s
Russky Bridge at 3,622 feet.
Arthur Ravenel Jr. Bridge
A tall steel
structure that
supports hightension wires
A h
The place at
which the main
cables and other
elements are
secured to the
The structure that
supports the point
at which the bridge
connects with land
The lower support
The structure that becomes
an underwater foundation
The load-bearing horizontal
structures that extend a bridge’s
length and support the deck
hen designing a bridge, engineers and architects factor in different loads. Over time,
loads can impact a bridge’s stability. Dead
load is the weight of the bridge itself. Live load
is the changing, moving weight of people and
vehicles on the bridge. Dynamic load is wind or
extreme weather. This load is difficult to measure,
but it must be factored in as a potential impact
on a bridge. Bridges need to be able to withstand
other forces, too:
Tension is the condition of being pulled or
stretched outward. Think of pulling on a tug-of-war
Main Span
The distance between
the two main towers
or piers
Compression is the state of being pressed inward into
less volume or space. Think of sitting down on a chair.
Torsion is the act of twisting or turning. Think of
wind twisting a hammock.
Shear forces push one part of a body in one direction and the other part of the body in an opposite
direction. Think of the pressure of air on the front
of an airplane wing.
Resonance is a strong, prolonged effect after an initial impact. Think of the growing ripples that result
from a stone dropped in a pond.
The elements that
absorb the live load
The vertical structure
that rests on the
ground and that
supports connecting
beams or girders
The area or path over which
people and vehicles travel
Main Cable
The cable that drapes
over the towers
and from which the
deck or roadway is
Vertical/Suspender Cable
The cables that hang
vertically from the main
cable to hold up the
deck or roadway on a
suspension bridge
The base through
which the load is
transferred to the
Side Span
The distance between
the initial access to
the bridge and the first
tower or pylon
by Cynthia Overbeck Bix
Where are
our hard hats?
ow does a bridge get built?
It’s a massive job that takes
a lot of planning. Many
questions are considered before any
building starts. What kind of bridge
is needed? How long does the bridge
have to be? Will the bridge cross
over water or land? What will use
it? Cars? Trains? People? How much
traffic will it carry? Will that change
in the future? What weather issues
will impact it? How much money
will it cost?
Who Pays?
W hether building a new bridge or
repairing an old bridge, it costs a
lot of money. State and local governments are responsible for the
roadways and bridges within their
boundaries. To help pay for major
construction projects, most local
governments collect gas and vehicle
taxes or fees from their residents.
Highway tolls are a source of revenue, too. Local governments also
borrow money or sell bonds to raise
money. The federal government
helps by contributing some funds,
It Takes a Team
Bridge construction requires a
team of skilled professionals. They
include civil engineers, designers,
architects, structural engineers,
surveyors, soil engineers, environmentalists, and different crews of
construction workers.
Civil engineers are the brains
behind any bridge project. They
understand how different forces,
such as compression and tension,
work on bridges. They understand
advanced mathematics. They are
familiar with modern building
materials. They know how conditions such as temperature and stress
might affect those materials.
A chief engineer is in charge of
the overall bridge construction. The
designer and architect work with
the chief engineer to decide how the
bridge will look. They make precise
calculations and create a blueprint.
The structural engineer makes
sure that the bridge’s basic design is
strong enough. Surveyors and soil
engineers study the proposed bridge
site. They decide where the bridge
should be built and what needs to be
done to ensure a stable foundation.
Environmental engineers study the
impact a bridge and its construction might have on the area. On
the actual building site, a project
supervisor oversees the work. Many
construction workers provide the
hands-on labor. Workers often are
highly skilled in particular tasks,
such as welding or masonry.
Bonds are certificates
of debt issued by
a government that
promise to repay an
initial investment,
plus some interest.
A blueprint is a
reproduction of
architectural and
technical drawings.
Welding means joining
metals by applying heat.
Masonry is work done
with stone or brick.
A blueprint is like a map. It
provides all the details about
materials and measurements
for a construction project.
All bridges begin with a strong
Beam bridges often are
used today to raise roads
above other roads.
Bedrock is the solid rock
beneath loose sand, soil,
clay, or gravel.
Cofferdams are
temporary watertight
enclosures that are
pumped dry so that
construction can be
completed underwater.
Caissons are
watertight structures
in which underwater
construction work can
be done and which
remain permanently
Falsework is a temporary
frame designed to hold
a structure until it
can bear its load.
A Strong Foundation
The construction processes vary for
different bridges, but every bridge
needs a strong foundation to hold
its enormous load. For centuries,
concrete bases have served as foundations. First, large holes are needed
to hold the concrete. Soil and loose
sediment are removed down to
bedrock. It’s hard work to dig a foundation on land, even with dynamite
and modern excavating machines.
When the proposed bridge needs to
cross water, it’s more complicated.
To build a foundation in water,
workers use cofferdams and caissons.
Those structures basically function
as underwater rooms. A barge tows
a cofferdam or caisson out to the
foundation site. A crane lowers the
cofferdam or a caisson into the
water. A pump sucks out the water,
and a continuous supply of fresh air
is pumped in. Inside the structure,
workers remove loose sediment.
Mud and debris are moved to the
surface through a separate shaft.
Large cranes with clamshell buckets
and other machines help with the
digging and the removal.
As a caisson sinks toward bedrock, weight is applied on top. It
helps to push the caisson down and
keep it stable. Once a cofferdam
reaches its resting point, workers
pour in concrete to create a solid
foundation. A cofferdam is removed
once the concrete is poured. A
caisson remains underwater and
becomes part of the foundation.
Supporting Roles
Once foundations are set, supports
can be added. Different kinds of
bridges use different kinds of supports. A beam bridge includes piers
at each end of a beam for support.
A beam bridge with two piers can
safely span only about 250 feet.
Longer beam bridges can be made
by connecting multiple beams and
supporting piers for each additional
Arch bridges also rely on piers
at each end for support. Early arch
bridges were constructed around a
temporary frame called falsework.
Workers fitted wedge-shaped stones
around the falsework from each side
toward the middle. The final stone,
at the top center, was the keystone.
It locked all the other stones into
place. The ancient Romans were
the first to use a form of cement to
further strengthen the arch bridge.
Once the arch was set, the falsework
was removed.
Eventually, engineers figured out
how to construct arches of iron and
steel. Today, reinforced steel sections
can be made at a factory and then
incorporated into the bridge at the
site. Instead of relying on falsework,
engineers use cables and cranes to
hold the pieces of the arch in place
as the bridge takes shape.
BELOW: Falsework provides
a temporary frame to hold
an 1800s arch bridge in
place until it was complete.
BOTTOM: Today, cables can
be used to hold the two
sides of an arch bridge until
the anchoring center piece
locks it all into place.
The deck for New York City’s
Manhattan Bridge, built to
span the East River in 1909, is
built out from the completed
This is amazing!
In beam and arch bridges, the
deck or roadway is supported from
below. But in a suspension bridge,
the deck is suspended from above.
Main cables drape over towers.
Vertical cables hang from the main
cables to hold the deck.
Workers rely on temporary hanging platforms to build a suspension
bridge’s tall towers. Large preformed pieces are brought in and
attached to slowly build the tower’s
height. Workers install cable saddles
on top of the towers. They will hold
the thick main cables in place.
The main cables are made of
many individual wires. The process
of creating those large cables is
called “cable spinning.” A single
wire is attached to a steel fastener
embedded in one of the bridge’s
anchorages. Then the wire is looped
over a large wheeled traveler. The
traveler carries the wire over the
tower cable saddles to the opposite
anchorage. There the wire is looped
around fasteners in that anchorage.
Then the traveler returns to its starting point.
The process is repeated thousands
of times. At the top of the tower,
workers bundle the individual wires
into strands. When all the wires
are bundled into a series of thick
strands, the strands gets squeezed
by machine into one thick, superstrong main cable. The cables are
secured in deep concrete anchorages
on land at either end of the bridge.
On Deck
Once a bridge’s pier or towers are
in place, and—in a suspension
bridge—the main cables are strung,
it’s time to build the bridge’s span.
Concrete or steel beams called girders are attached along the length of
the bridge, piece by piece. Gigantic
cranes lift and lower the girders
into place. Until about 50 years ago,
rivets held most bridges together.
Today, workers bolt or weld together
parts. Workers start at the towers or
the ends of the bridge and meet in
the middle.
Finally, steel panels or concrete
slabs are laid across the girders for
the deck or roadway. The concrete
can either be pre-formed at a factory
or poured in place into forms right
on the bridge. The workers may add
a final covering of asphalt. After
adding railings and signs, the bridge
is open for traffic to cross it.
Cynthia Overbeck Bix loves learning about the people
who shaped American history. She has written books
and articles about history, natural science, and the arts.
ABOVE: Cable spinning is
underway on a new suspension
bridge. LEFT: Anchorages for a
suspension bridge are massive
structures designed to hold
fast the bridge’s loadbearing
Rivets are metal bolts
or pins having a head
on one end that is
inserted through
aligned holes in the
pieces to be joined
and then hammered
on the plain end
to create a second,
locking head.
by Will Bremen
ere are some hands-on activities to understand the ways a few basic bridges work.
You Need
4 craft sticks
2 hardcover books that are the same thickness
glue stick or bottle of glue
1. Put the 2 books cover side down about 4
inches apart on a flat surface. Rest the craft
stick across the gap between the books so
that about a half inch of each tip rests on
the books.
2. Gently press down on the center of the
stick with your index finger. Your finger is
applying force that compresses the top of
the stick and puts the bottom of the stick in
tension, like live load on a bridge’s deck.
3. Glue 3 craft sticks together so that 1 vertical
stick is glued between 2 horizontal sticks.
Once it is dry, rest it across the gap between
the books.
4. Press down on it. Does it feel stronger?
Some modern bridges use girders called
“I-beams.” They are called I-beams because
they are shaped like an “I.” I-beams make
strong bridge components. They more easily
withstand forces that push and pull.
You Need
2 hardcover books, of equal size in height
and thickness
6 sheets of 8½-x-11-inch paper
about 40 pennies
Beam Bridge
1. On a flat surface, stand the 2 closed books
about 10 inches apart. Place a single sheet
of paper over the top so that the ends of
the paper line up with the outer side of
each book. You may have to adjust the
books to make sure that the paper rests flat
and straight. Lightly tape the ends of the
paper to each book.
2. Place 1 penny at a time on the center of the
paper. How much “load” (pennies) can the
“deck” (paper) hold before it starts to sag
and pull on the “piers” (books)?
Arch Bridge
1. Begin with the beam bridge.
2. Arrange a second sheet of paper between
the books, below the straight sheet of
paper. Make it curve so that the top of the
curve is flush with the flat sheet. Lightly
tape the edges of the curved paper to the
books on each side.
3. Place 1 penny at a time on the center of
the paper. How much “load” can the “deck”
hold before it starts to sag and pull on the
“piers”? Is it greater than or less than the
first bridge?
Truss Bridge
1. Remove the arched paper from the bridge,
but keep the single flat sheet of paper in
2. Fold, fan-like and lengthwise, 3 sheets of
paper in 1-inch increments. Place the folded
papers lengthwise, side by side, on top of
the paper on the books.
3. Place a second sheet of paper on top of the
folded papers. Lightly tape the top paper to
stay in place.
4. Place 1 penny at a time on the center of
the paper. How much “load” can the “deck”
hold before it starts to sag and pull on the
“piers”? How does it compare to the other
2 bridges?
Can you think of ways to make
the bridges stronger? What other
materials could you use? What if
the bridge spans were longer? What
if the load was heavier? Lighter?
Try experimenting!
This looks
like fun.
Let’s try it.
I’ll get the
by Patrick McBriarty
umans have engineered
all kinds of bridges.
Some of the most interesting are moveable bridges, or
drawbridges. Moveable bridges are
designed to allow their main spans
or decks to be temporarily moved
out of the way to allow water traffic
to pass below.
For a city such as Chicago, moveable bridges solved a lot of problems.
The city is divided into three by the
Y-shaped Chicago River. It also is
located at the southwest tip of Lake
Michigan. Moveable bridges allowed
the city to grow while providing ways
for people to easily get around.
Chicago began as a swampy
French and Native American trading outpost on the flat Midwest
prairie. By 1837, it was incorporated
as a city. It grew rapidly, thanks to
its location on Lake Michigan and
the Chicago River. It was the age of
sailing ships. The city became a bustling center for commerce, industry,
trade, and transportation.
Eventually, a system of canals
(the Illinois and Michigan Canal in
1848, replaced by the Sanitary and
Ship Canal in 1900) connected the
Great Lakes and the Mississippi River
watersheds. That waterway carried
people and goods from the Atlantic
Ocean to the Gulf of Mexico. It
anchored Chicago’s position as the
largest city in the Midwest.
By 1900, Chicago had more than
1.6 million residents. It was a major
shipping port. It also had become
a railway hub. The first decades of
the 1900s added automobiles to the
city—and roadways on which to
drive them. The city needed ways
to keep all its people, vessels, trains,
and vehicles moving.
Watersheds are the
regions that drain
into a river or a river
A hub is a junction
where multiple
lines or forms of
N th l d ’
The Netherlands’
Amsterdam is the only
city in the world that
has more drawbridges
than Chicago.
A historic Chicago railroad
swing bridge
Chicago’s pontoons
were floating wooden
boxes sealed with tar.
Timber is wood used as
a building material.
I think moveable
bridges are pure
Chicago had been experimenting
with moveable bridges for decades.
At first, the city used pontoons. A
flood in 1849 destroyed most of
them. Swing bridges replaced pontoon bridges, but they weren’t always
stable. In 1863, the Rush Street
Bridge collapsed under the weight
of a herd of cattle. Then, the Great
Chicago Fire of 1871 destroyed eight
timber-framed swing bridges.
By the late 1800s, larger and
wider commercial ships were coming through the Great Lakes. Swing
bridges created an obstruction for the
new ships. A swing bridge rests on a
center pier from which it swings open
and close. Built in the middle of the
river, the center piers narrowed down
the waterway and made it difficult
for large ships to get around. Newer,
different drawbridges were needed.
Chicago tried a variety of lift
bridges before it solved its problem
by inventing the trunnion bascule
bridge. A trunnion is a pin or an axis
upon which something can pivot.
Bascule is French for “seesaw.” It
describes the way the bridge operates—like a giant seesaw. When
down, the road can carry train,
vehicle, and pedestrian traffic. But
the bridge can pivot up to allow tall
boats to travel the waterway below.
The bridge’s design relies on a
counterweight usually attached
under the roadway near the approach
of the bridge. The counterweight
balances against the leaf that projects
over the waterway. It also puts the
center of gravity close to the point of
rotation. That means that relatively
small electric motors (about the size
of a Volkswagen Beetle engine) can
operate the many-thousand-pound
bridges. Today, the trunnion bascule
bridge is known as the Chicago-style
Chicago’s first trunnion bascule
bridge was built in 1902. Although
no longer moveable, the Cortland
Street Drawbridge remains standing.
It is used by vehicles and pedestrians. Since the 1980s, the bridge
has been restored, and repainted.
In 1991, it was designated a historic
Chicago landmark.
Chicago’s most-iconic trunnion
bascule bridge is the double-deck,
double-leaf Michigan Avenue
Bridge. It also is known as the
DuSable Bridge, after Chicago’s first
permanent resident, Jean Baptiste
Point DuSable. Built in 1920, it is
one of four double-deck trunnion
bascule bridges in the city. It is the
only one designed to carry automobile traffic on both decks. The other
double-deck bridges were designed
to carry trains on one level.
One of Chicago’s best-kept secrets
is the McCormick Bridgehouse
& Chicago River Museum. It is
located in the southwest tower of the
Michigan Avenue Bridge. Visitors
can explore the museum’s five levels.
They can stand two feet from the
gears when the bridge is in operation. And they can enjoy spectacular
views of the city and the river from
the top floor.
Today, 60 drawbridges cross
the Chicago River, the Calumet
River to the south, and the city’s
canals. Thirty-seven are still moveable. But large commercial ships
stopped traveling on the Chicago
River in the 1980s. Today, sand,
gravel, and concrete barges are the
waterway’s primary commercial
traffic. Tugboats with specialized
wheelhouses push the barges. Set on
a hydraulic lift, the wheelhouses can
be raised for better visibility over
the low barges and then lowered to
pass under the bridges.
The city’s South Side has become
the main commercial port. Industrial
and manufacturing goods are carried into and out of this area. More
than a dozen railroad drawbridges
cross the South Side’s Calumet River.
About half of them are still in use
today. The vertical-lift bridges are the
most dramatic. They raise and lower
like an elevator and offer 150 feet of
clearance for ships.
Chicago’s bridges used to open
on demand for any ship because
water traffic had the right of way.
Since 1995, the bridges on the
Chicago River are raised on a
schedule each spring and fall to
allow recreational boats to get to
Lake Michigan for the summer
and then back to boatyards before
winter. During these “river runs,”
several crews of bridge tenders
leapfrog from bridge to bridge.
They open and close the bridges in
a sequence to allow the many boats
to pass through Chicago’s busy
downtown. Today, Chicago takes
pride in its nickname of “Drawbridge
Capital of the World.”
Patrick McBriarty is the author of Chicago River
Bridges, which was published by the University of Illinois
Press in 2013.
Hydraulic refers to a
fluid (oil) moving in a
confined space under
The Michigan Avenue Bridge
is a double-deck, doubleleaf trunnion bascule bridge.
he Chicago Department of Transportation
(CDOT) maintains the city’s 277 bridges.
Included in that number are viaducts, overpasses,
fixed bridges, and moveable bridges. All the
bridges receive a complete inspection every two
years to make sure that they are safe.
Construction materials for Chicago’s bridges
have changed over time. The first bridges were
made of timber. Then iron. Then steel. Then steel
and concrete. Over time, nearly all of Chicago’s
bridges have undergone some form of repair or
The CDOT repaints about four drawbridges
each year. Just painting a bridge can add several
thousand pounds to its load. That may require
adjusting the counterweight to keep it in balance.
After any repairs, a bridge is checked to ensure
its smooth operation. Each spring, the CDOT
also performs test lifts on each of the still-active
drawbridges to identify any necessary repairs or
The city has not constructed any new drawbridges since the 1980s. And it has replaced several
drawbridges on the North Branch of the Chicago
River with fixed bridges. But it is committed to
maintaining its moveable bridges on the other
branches of the Chicago River and on the Calumet
River. A recent example was the rehabilitation of
the 91-year-old Wells Street Bridge. It was completed in 2013. Workers focused on half of the
bridge at a time. They cut off one of the doubledeck trusses. It was supported by a barge on the
river and towed away for scrap. A new truss was
floated into place and connected to the bridge—in
less than nine days. A month later, the other truss
was removed and a new one put in its place.
Except for the two 9-day shutdowns, elevated
trains continued to run on the upper deck of the
bridge throughout the yearlong project. That was
key for the many commuters who use the city’s
rapid transit system. Such a feat had never been
done before. —P.M.
Viaducts are a series of
spans or arches used in
a roadway or a railroad
to pass over a wide
valley or over other
Overpasses are
passages, roadways, or
bridges that pass over
another roadway.
Rehabilitation means
the act of restoring
something to good
Crews attach the new truss to the south
approach of the Wells Street Bridge.
Many bridges are the key to
keeping people, bicyclists,
vehicles, trains, and boats
moving in Chicago.
people live in the greater
Chicago area today. It is
the third-largest city in
the United States.
Meet some famous U.S. bridge engineers. They played
key roles during the country’s great bridge-building era
in the late 1800s and early 1900s.
Polish-born Ralph Modjeski (1861–1940)
immigrated to the United States in 1876. By
the early 1890s, he had opened his own
bridge-building firm in Chicago. He built
suspension and railroad bridges all over
the country. His most famous bridge
is the San Francisco–Oakland Bay
Bridge. He also hired Joseph B.
Strauss. Strauss later opened
his own firm and revolutionized
the concept of bascule bridges.
Strauss is most famous for
building the Golden Gate Bridge
in San Francisco.
John A. Roebling (1806–
1869) immigrated to the United
States from Germany in 1830.
He invented a way to make wire
rope to hold suspension bridges.
He built his first bridges in the
Pittsburgh area. Then, he built
the first railroad suspension
bridge from Niagara, New York, to
Canada. His most famous bridge—
New York’s Brooklyn Bridge—is a
cable-stayed suspension bridge.
He died after an accident on
that site. His son Washington
completed the bridge by following
his careful plans.
Othmar Ammann (1879–1965) was born in
Switzerland. He settled in New York City in 1904. He
worked on some of the most famous spans that keep
that city connected. They include the George Washington
Bridge, the Triborough Bridge, the Goethals Bridge, and
the Verrazzano-Narrows Bridge. Ammann was known for
his ability to create structures that combined beauty with
Conde McCullough
(1887–1946) grew up in Iowa.
His reputation took off
when he moved to Oregon in
1916. He oversaw the state’s
Department of Transportation
for 25 years. During that
time, he helped build many
bridges along Oregon’s newly
constructed coastal Highway
101. He combined artistic styles
with practical function. He
became known for his use of
simple but elegant arches.
by Cynthia Overbeck Bix
Iconic means having
the characteristic of
being an important
hrouded in swirling fog or shining in the sun, the Golden Gate
Bridge has become an iconic
part of California’s landscape. But
it wasn’t always there. The story of
how it came to be started with a city’s
need to expand. But it also involved
some hard-working, determined
people who overcame significant
engineering challenges. It proved to
be an idea that wouldn’t die.
Impossible Dream?
In 1848, San Francisco was a small
pre-gold rush town of fewer than
1,000 people. By 1900, it was the
largest city in the American West.
With a population of more than
1 million people, the city needed
to find ways to expand beyond
its tight geographic boundaries.
Highway 101, a major road traveling
south–north through California,
terminated in San Francisco.
San Francisco was built on a
peninsula. It is surrounded by water
on three sides. The Pacific Ocean is
to the west. San Francisco Bay is to
the east. And between the two is a
strait known as the Golden Gate. It
separates San Francisco from Marin
County to the north.
San Francisco’s leaders looked to
Marin County as an important gateway to northern California. But the
only way to travel between the two
places in 1900 was by boat. Some
people wondered if a bridge could be
built across the strait.
Most experts said it couldn’t be
done. At its narrowest point, the
strait is more than a mile wide.
Heavy winds blow through it. The
water churns with powerful waves
and strong currents. San Francisco
is near a fault line, which means it
experiences earthquakes. A bridge
Yup, that’s fog rolling
across the Golden Gate Bridge!
O’Shaughnessy had his eyes on
San Francisco’s future. He believed
that constructing a bridge over the
strait was possible. In 1919, he asked
civil engineers to submit plans.
Joseph B. Strauss answered that call.
A Grand Vision
Strauss was a grand dreamer
and a tireless promoter. He
had already designed more
than 400 bridges. His strong
personality helped push the
project forward. He and
O’Shaughnessy created the
Bridging the Golden Gate
Association to promote their
plans. They ordered studies
on the surrounding land and
water. They gathered public
support. They raised money.
By 1923, San Francisco
needed a bridge more than
ever. It was the largest major city in
the United States that relied completely on ferry boats to transport
people. And by then, more and more
people were driving new automobiles. Roadways were being built
to make driving easier. But to get
between San Francisco and Marin
County, cars had to be carried across
the strait by ferry—a 30-minute ride
today. During busy times, the wait
for a ferry could be as long as three
would have to be strong enough to
withstand rough waters, gale-force
winds, and potential earthquakes.
It would have to be tall enough for
ocean-going ships to pass under
it as they go into and out of San
Francisco Bay. And, it would have
to be visible to ships through thick
ocean fog.
Then, in 1906, a huge earthquake
rocked San Francisco. About 80
percent of the city was destroyed.
More than 100,000 citizens were left
homeless. The city’s leaders quickly
decided to rebuild the city. To
supervise the massive project, they
hired an energetic new city engineer
named Michael O’Shaughnessy.
Golden Gate
By the early 1900s, San
Francisco’s location on a
peninsula was limiting its
hours. Loading and unloading at
each end took time, too.
Still, powerful forces continued
to oppose the bridge. Ferry companies didn’t want competition.
Conservationists argued against disturbing the beautiful scenery of sea,
sky, and land. After years of challenging their opponents, however,
the bridge supporters won out.
A Team Effort
Chief Engineer Joseph B.
Strauss devoted nearly 20
years to the Golden Gate
Bridge. A year after the
bridge opened, he died.
BELOW: The south side’s pylon
captures the enormity of the
project. RIGHT: The bridge’s
two towers were underway by
1934. The far tower is the north
tower in Marin County.
Strauss got busy assembling a team
of experts. In 1922, he hired Charles
A. Ellis. Ellis was a quiet, studious
university professor of structural
and bridge engineering. He spent
nearly two years making the calculations needed to ensure the strength
and flexibility of the bridge design.
In those pre-computer years, he did
the figuring by hand. He worked on
the bridge for nearly a decade before
Strauss abruptly fired him. Ellis
was replaced by
Clifford E. Paine,
who oversaw the
bridge’s actual
Strauss never acknowledged Ellis’s
pivotal role in the bridge’s design
and planning phase.
Strauss had shared his original
proposal for the bridge—an unusual
hybrid structure—in 1921. It had
cantilevers at both ends and a suspension bridge in the middle. City
officials decided that Strauss’s design
was ugly. One person described it
as looking like “an upside-down rat
Strauss asked bridge designer
Leon S. Moisseiff for help. Moisseiff
designed a new kind of suspension
bridge. It would be the longest one
ever built. But it would be lighter
and more flexible.
To be worthy of its dramatic
setting, the new bridge had to be
beautiful. Strauss added highly
respected local architects to the
team. Irving and Gertrude Comfort
Morrow designed two main towers
in the Art Deco style. Each tower
tapered slightly toward its top and
had four giant “windows.” The
towers’ surfaces were covered with
indented vertical panels that caught
the light and cast a dramatic pattern
of shadows. The Morrows also specified the bridge’s color: a distinctive
burnt orange known as International
Orange. It remains one of the bridge’s
most eye-catching elements.
On the practical side, the structure
needed strong piers at each end.
Strauss asked for help from University
of California, Berkeley, geologist
Andrew C. Lawson. Lawson studied
and tested the land to make sure that
solid piers could be built. The northern pier would be anchored deep in
solid bedrock. The southern pier had
to be built in the water. Divers would
have to blast a giant hole on the ocean
floor that could be filled with concrete to create a stable base.
Making It Happen
After a decade of designing and
planning, Strauss was ready to begin
building in 1929. By that time, he had
become the chief engineer. Then, the
stock market crashed. Throughout the
country, banks failed and businesses
folded. People lost their jobs and their
homes. The nation headed into the
decade-long Great Depression.
The Great Depression resulted in
suffering for many Americans. By the
early 1930s, people were desperate to
find employment. Men heard about
the bridge project and headed to
San Francisco. They lined up by the
thousands hoping to be hired. Since
the work was dangerous, the pay was
high. Men with construction experience earned up to $11 a day. That was
a good wage for the time.
Length of bridge’s main suspension span
(between the two towers): 4,200 feet
Length of bridge’s total suspension span:
6,450 feet (1.2 miles)
Length of one cable: 7,650 feet
Weight of one cable: 12,000 tons
Diameter of cable: slightly greater than 3 feet
Number of wires in cable: 27,572 wires,
about 80,000 miles
Height of towers above the water: 746 feet
Height of towers above the road: 500 feet
Amount of steel in towers: 88,000 tons
Number of rivets in each tower: 600,000
By late 1930, San Francisco area
residents also made it clear that
they wanted the bridge. Despite the
terrible economic conditions in the
country, they voted to support a
bond to raise money to finance it.
Locals offered their homes, farms,
and businesses as security.
Construction began on January 5,
1933. Over the next four years and
five months, workers risked their
lives to do the difficult task of erecting the bridge. Eleven men died
while working on it. Ten of those
men fell to their deaths when the
piece of scaffolding they were on
broke off. It ripped through the
safety net that had been attached
below the bridge before plunging
into the water. But 19 other men
were saved by the safety net. All
along, Strauss had stressed worker
safety. In addition to the safety net,
Art Deco is an architectural
style from the 1920s and
1930s that originated
in France and that
incorporated geometric
designs and bold colors.
Th U
Navy wanted
t d
the bridge to be painted
with black and yellow
stripes to make it visible.
he also insisted that workers wear
hard hats, goggles, and headlamps.
An Icon Rises
Th G
ld Gate
G t B
has been closed three
times for high winds.
The Golden Gate Bridge was
the longest suspension bridge
in the world until 1964.
First the piers and anchorages were
built, north and south. Then the
towers, north and south. Next,
the two main cables were spun.
Then, working from land out to the
middle of the bridge, the suspended
steel deck was built from each end.
It was attached to the bridge by
vertical suspension cables. The deck
was paved, and the bridge was done.
On Opening Day—May 27,
1937—more than 200,000 people
walked (some even roller-skated)
across the new Golden Gate Bridge.
The next day, the first cars crossed
to the celebratory sound of fire
sirens, ship whistles, and foghorns.
Its final price tag was $27 million,
more than $1 billion today.
When it was completed, the
Golden Gate Bridge was the longest
and tallest bridge ever built. It no
longer holds those records, but the
American Society of Civil Engineers
ranks it as one of the Seven Wonders
of the Modern World. The impossible feat has become one of the
most famous and visited sites in
the world
Less Learned
by Kathiann M. Kowalski
Shoddy means of
poor quality.
The new Sunshin
e Skyway
Bridge is taller an
d has
protective island
s around
its piers.
couldn’t stop. I skidded off the
edge and fell 140 feet down into
the stormy seas,” Wesley McIntire
said. He was driving his pick-up
truck when a 1,400-foot section of
Florida’s Sunshine Skyway Bridge
collapsed in 1980. He was lucky.
He survived a plunge into Tampa
Bay. A freighter ship had struck
the bridge, and 35 people died.
The tragedy is one of more than
50 significant bridge failures in the
United States since 1930.
Crashes caused some catastrophes. Natural disasters triggered
others, as when a 1989 earthquake
collapsed part of the double-decker
Cypress Freeway Bridge in Oakland,
California. And some failures were
the fault of poor design, shoddy
inspections, or other flaws.
Although those failures are
now history, they matter. After a
collapse, experts investigate what
went wrong. “We always learn
something,” says Anil Agrawal, a
structural and bridge engineer at
the City University of New York
(CUNY). “Frequently we come out
with some new knowledge or some
An Ongoing Problem
A new southern
span of the Sunshine
Skyway Bridge
opened in 1987. It was
taller than the old one.
It also had a wider
area for ships to pass
underneath. Designers
added concrete islands
to guard against future
crashes, too.
Galloping Gertie
In 2007, Minn
-35 Bridge
busy Interstate
e repairs
collapsed whil
were being m
Deficient means
inadequate or lacking
in an essential quality.
That same year, though, a New
York State Thruway bridge collapsed
over the Schoharie Creek. Heavy rain
and melting snow had swollen the
creek waters. There wasn’t enough
protection around the piers. Moving
water eroded soil around one of them
until it gave way. A careless bridge
inspection system failed to spot the
problem beforehand. Afterward, the
Federal Highway Administration
adopted stricter requirements.
Inspections aren’t always enough,
however. The Interstate-35 Bridge
over the Mississippi River in
Minneapolis, Minnesota, was listed
as “structurally deficient” in a 2005
Department of Transportation
(DOT) inventory. “The fact that a
bridge is ‘deficient’ does not imply
that it is likely to collapse or that it
is unsafe,” said a Federal Highway
Administration fact sheet. “It means
it must be monitored, inspected,
and maintained.” Yet the bridge
collapsed in 2007, while repairs were
People also knew about problems
with the Tacoma Narrows Bridge in
Washington. The suspension bridge
crossing Puget Sound opened in
1940. It often bounced in the wind,
so locals nicknamed it Galloping
Gertie, after a popular saloon.
News editor Leonard Coatsworth
described his experience driving
across the bridge on November 7.
“Before I realized it, the tilt
became so violent that I lost control
of the car,” Coatsworth recalled.
Listening to concrete crack around
him, Coatsworth got out of the car
and crawled to safety. When the
bridge finally broke apart, the family dog went down with it.
The long, narrow bridge couldn’t
cope with twisting forces caused
by the wind. As the twisting action
increased, the bridge absorbed more
of the wind’s energy. Then the deck
itself generated even more motion
and twisted until it broke apart.
Back in the late 1930s, the designers for the Tacoma Narrows Bridge
hadn’t worried about the wind. They
knew about other bridge failures but
blamed poor workmanship or heavy
loads on those structures. “There
seemed to be almost no recognition
that wind created vertical movement
at all,” noted David Billington, who
was a Princeton University engineering professor. Afterward, bridge
engineers were more aware of the
need to design bridges to deal with
wind and other phenomena.
Now What?
The good news is that the government has standards. The DOT’s
Federal Highway Administration
and Federal Railroad Administration
each write rules to ensure the safety
of bridges. State and local authorities
have duties as well.
For example, the Federal-Aid
Highway Act of 1968 and later
laws led to the DOT’s
National Bridge Inventory.
That database tells when
thousands of U.S. bridges
and tunnels were last
inspected. It notes what
condition they were in.
Other rules were added.
New requirements
were made for inspecting underwater areas.
Funding was found for
improvements to better
withstand earthquakes.
The Federal Highway
Administration updated
its rules for roads and
bridges yet again in
2017. The first four-year
performance period
runs through 2021.
Advances in engineering bring more
good news. Accelerated Bridge
Construction (ABC) addresses
ways to speed up planning and
construction of bridge repairs and
replacements. “What ABC does
is really minimize the impact on
mobility,” says Agrawal at CUNY.
For example, a bridge can stay open
while replacement sections are made
off-site. Those sections then can be
slid into place to connect with an
existing bridge element. That means
a bridge’s downtime can be a matter
of days or weeks, instead of months
or years.
of the 1940 collapse
of Washington’s
Tacoma Narrows
Bridge, also known as
“Galloping Gertie,” at
The Tacoma
Narrows Bridge
may be the most
famous bridge
failure. It collapsed
in high winds just
four months after
Maintaining and
bridges have beco
important issue
s for
the nation’s heal
The bad news is that there’s
a lot of work to be done. The
United States’ bridge system got a
grade of only C+ on the American
Society of Civil Engineers’ 2017
Infrastructure Report Card. And
more than 54,000 of the country’s
600,000-plus bridges are rated
“structurally deficient.” That
report came from the American
Road & Transportation Builders
Association in 2018. The group
analyzed data from the DOT’s 2017
National Bridge Inventory database. As Galloping Gertie shows,
though, bridge problems aren’t
always fixed in time.
Surprises continue to happen,
too. In 2018, a footbridge over an
eight-lane highway collapsed at
Florida International University. An
initial report by federal investigators noted that workers had been
tweaking the tension for part of
the bridge. Work on a final report
about that incident continues as
COBBLESTONE goes to press.
Kathiann M. Kowalski writes often for COBBLESTONE.
She spent more than an hour each way on a recent road
trip crossing the George Washington Bridge across the
Hudson River in New York.
Infrastructure refers
to the basic structures
and systems that
society relies on.
as under
rnational University w
The footbridge near Sw
killing six people.
construction when it su
by Kathiann M. Kowalski
ridges need to stand up to
lots of stress. Heavy truck
loads. Swings in temperature.
Water. Salt and other corrosive
chemicals. Now many of America’s
600,000 or so bridges are aging.
Some must be replaced. Others can
be repaired. Still others need ongoing
Many new materials and ideas
can improve the country’s bridge
system. The tricky part is figur-
ing out which ones to use. That’s
where the BEAST comes in. It’s the
Bridge Evaluation and Accelerated
Structural Testing lab at
in New
Jersey. The
developed by
the Center
for Advanced
and Transportation (CAIT).
Engineer Dr. Franklin Moon talked
with COBBLESTONE about the
he Center for Advanced
BEAST and other trends in
Infrastructure and Transportation
bridge engineering.
(CAIT) works to maintain and improve the
country’s infrastructure. CAIT researchers
develop practical tools and methods to help solve
tough problems that transportation agencies strugI love things that
gle with. How? They do things such as assess and
are big. The largest
monitor the health of bridges, roads, and pipelines.
things that humans
They create technologies, materials, and tools that
build are bridges
increase the life span of infrastructure and promote
and buildings. I
safety. And they try to minimize costs and make the
love going out to site
nation’s transportation systems more environmenvisits.
tally friendly.
Heavy truckloads (TOP),
extreme hot-and-cold
weather (MIDDLE), and
corrosion (BOTTOM) all are
factors that can impact the
life of a bridge.
Corrosive describes
the process by which
a metal or alloy is
gradually destroyed as it
goes through chemical
reactions with things in
its environment.
Bridge expert Dr. Franklin
Moon is a civil engineering
professor at Rutgers
University and a researcher
at CAIT.
It’s a time machine. We use it to
accelerate how a bridge ages. After
about nine months, you can see
what a bridge would look like 15 or
20 years into the future.
If I build a bridge today, it may
take 15 to 20 years before I see
signs of deterioration. There’s
an explosion of new
materials now. How do
you know which ones
will be cost-effective to
use in bridges? It may
take 15 or 20 years to
figure that out. It really
slows down innovation.
Deterioration is a
decline in quality
over time.
There are basically three things that
cause bridges to deteriorate. The
first is trucks. We can take 15 to
20 years’ worth of trucks, and we
can apply it to the bridge in nine
months. Another thing that causes
a lot of problems is temperature
changes. That causes cracking. We
create almost 15 to 20 years’ worth
of freeze-thaw cycles and hot-dry
cycles within that nine-month
period. The third thing, especially
in the Northeast and Midwest, is
that salt or de-icing agents will get
into the cracks in the deck. They
find a way down to the steel and
break down the steel. We can put
The BEAST is the first machine of its
kind in the world. It “beats up” a bridge
section by simulating weather extremes
and heavy traffic. Those things make a
bridge age and start to break down over
time. The BEAST can make that process
happen quickly, so engineers can see how
the various parts of a bridge will hold up
in the future.
on 50 years of salt in that ninemonth period.
Almost every bridge is built out of
steel and concrete. There are many,
many kinds of steel. But most of the
steels corrode. A lot of new materials
offer ways of coating the steel so it
won’t corrode.
There’s a lot of focus on things
you can add to concrete that will
allow the concrete to be resistant to
cracking and have a low permeability.
You want concrete to be resistant
to cracking. You also want it to
have low permeability so water
and salt can’t get through. And, of
course, any material for a bridge
has to be really cheap because we
use a lot of it.
There are a lot of sealants. If I can
effectively seal the outside, I’m
not going to let anything into the
concrete in the first place. And then
there are overlays. That’s where you
put a high-quality material on the
outside. Some of these things can be
applied to older concrete.
Environmental sustainability is
huge—ensuring that as we move
forward in the future we’re going to
have enough of whatever we need
The wheeled piece of the
BEAST pushes down on
the bridge section with up
to 60,000 pounds. It rolls
back and forth at 20 miles
per hour, 24 hours a day, 7
days a week. The BEAST can
simulate the wear-and-tear
of traffic on a 20-year-old
real-world bridge—after
just 9 months!
Permeability of a
material relates to how
freely it lets fluids enter
or pass through.
Sealants are substances
used to seal a surface
to prevent passage of
a liquid or a gas.
monitor the health of the structure.
I think a lot of bridges can benefit
from short-term applications. You
could put some sensors on the
structure to get a feel for how it’s
carrying the load and what the
stress levels are in some of the
critical members.
CAIT gave tours of the
BEAST when it was unveiled
in October 2015. Visitors are
standing in the lower part
of the weather chamber
that encloses the bridge
section during experiments.
The enclosure simulates
both wet and dry weather
and temperatures from 0°F
to 104°F.
for everyone. A lot of people are
working on coming up with more
environmentally friendly cement.
And we want to make sure that our
bridges and road systems don’t have
an undue negative impact on the
You go to the doctor to get checked
up. Similarly, we’re trying to
We’ve gotten to a point now where
the required investment to bring
our bridges up to good repair is way
more than the money we have to
invest in them. Some bridges are so
far gone that we’ve got to replace
them. We should be replacing them
not with the lowest-cost systems, but
the highest-value systems. We might
spend a little bit more money, but
that bridge can then last 120 years
instead of 50 years.
We’ve got a middle group where
components are falling apart, but
maybe the bridge deck itself or some
of the girders are in good shape.
Maybe we can make an investment
today and get another 50 years out
of that system. Other bridges are in
pretty good shape. We’ve got to do
a better job of understanding what
maintenance is needed to keep them
working well.
To see the
BEAST in action,
watch “Meet the BEAST”
An aerial view of the BEAST. The
two yellow central beams are 120
feet long and weigh 112,000 pounds.
They act like a track for the cart that
simulates traffic.
The Brooklyn Bridge is special in
many ways. It’s almost like an operating historic monument.
Th manufacture
f t
cement contributes to
climate change. The
chemical process of
making cement releases
huge amounts of carbon
dioxide, which is a
greenhouse gas. The
process also burns lots
of fossil fuels, which
add more greenhouse
gases to the air.
Moon’s favorite bridge is
New York City’s Brooklyn
by B
eople around the world have
figured out ways to make bridges
functional, fantastic, and fun!
Living Root Bridges
The Indian state of Meghalaya is one of the wettest places on Earth. During monsoon season, the
rivers deep in its jungles rise and become impossible to
cross. Rather than build bridges across the swollen rivers,
the Khasi people who live there grow bridges. The rubber
tree has roots that grow above the soil. The Khasis train
the roots to reach over the river. They weave them together
with other trees’ roots. It takes years to complete a living root
bridge. Unlike other bridges, though, these
A monsoon is a wind
natural bridges grow stronger with age. The
that comes from
the south and brings
most famous one is Umshiang. It has two levels
heavy rainfall to parts and is more than 180 years old. Some bridges are
of Asia.
even older—more than 500 years old!
In 2004, a small footbridge was installed over the Grand Union
Canal in London, England. The bridge needed to open to let
boats through. Most drawbridges have one or two rigid sections
that lift up. The Rolling Bridge, however, curls up onto itself,
forming an octagon. Heatherwick Studio designed the bridge
to have this unique movement. The designers used the
animatronic tails of the Jurassic Park dinosaurs as inspiration. At about 39 feet long, this bridge consists of eight
triangular segments powered by
Animatronic describes
hydraulics. Tourists and visitors
the use of electronic
stop by each day to watch this
machines to make
something have
fascinating bridge seemingly
lifelike movements.
come to life.
A Dragon Bridge
The growing port city of Da Nang, Vietnam, is considered one of the economic
“dragons” of Asia. That’s why the bridge that opened there in 2013 is so perfect.
The Dragon Bridge, or Cau Rong in Vietnamese, is a six-lane bridge that spans the
Han River. Extending down the middle is a curved, snakelike dragon. The dragon
is painted bright yellow and fitted with 2,500 LED lights. Each night at 9 P.M.,
people gather to watch a light show. And there’s more: The dragon also breathes
fire and blasts out giant plumes of water!
his historic bridge once spanned the River Thames in London, England.
It is not the original London Bridge. (That bridge was built in the early
1200s.) Over the centuries, different bridges were built and then
replaced over the Thames. This one was built in 1831. It lasted more than 130
years. When it started to sink on one side, London officials decided it was time
for a new structure. They offered the old bridge for sale. American developer
Robert P. McCulloch bought it for nearly $2.5 million. Workers carefully numbered the bridge’s stone pieces. The stone were shipped to the United States.
By 1971, the stones were reassembled around a concrete base—in Lake Havasu
City, Arizona. McCulloch owned land there and was trying to establish a city. He
hoped the bridge would attract more people to the desert location. And it did.
Today, Arizona’s London Bridge is a popular attraction.
Bridges come in all shapes and sizes. Long bridges spanning famous
rivers almost always gain notoriety. Bridges such as the Golden Gate
Bridge have achieved worldwide fame for their strength and length.
Others are thrown together, wooden bridges only a few yards
long used to ford tiny streams. You see these bridges at hiking trails
or at camp. Though useful, small bridges seldom achieve fame.
Do not count out the small bridges, though. Some have gained
fame in other ways. The famous artist Claude Monet loved to
paint small bridges. In one painting, he featured a bridge in Paris,
France, called the Pont Neuf. The Pont Neuf is about 760 feet long.
Although Pont Neuf means “new bridge,” it is actually the oldest
bridge in Paris. Another Monet painting features a footbridge in
his private garden. The next time you cross a bridge, remember
the vital role that they play in our lives. Not only do bridges help
people reach their destination, but they also provide beauty.
George Birdsong, age 11
Dallas, Texas
Amelia Earhart Takes Off
Dwight D. Eisenhower
Draw a picture or write a poem or short essay that connects to
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I forgolink!
how to is H., age 9
nd , O r
Ah! It’s
Darth Vader!
let me think.
Eliza Williams, age 8
St. Louis, Missouri
Ari Rabin, age 10
Louisville, Kentucky
September Winners!
. D’sy
illustrated by John Gordon Swogger
his month’s mystery hero’s life was influenced
by two important men. Both her brother and
her husband recognized her amazing intelligence and perseverance.
Our hero was born in 1843. She was the secondyoungest child in a family of 12 children. The
family valued education. She and an older brother
were particularly close. He attended the U.S.
Military Academy at West Point. He paid for her
to attend Georgetown Visitation Convent, a wellknown academy for young women in Washington,
D.C. Our young hero was taught traditional female
pursuits, such as housekeeping, weaving, and playing the piano. But she also learned history and
Our hero’s brother helped shape her
future in another way, too. He was an important Union general during the
Civil War (1861–1865). In
1864, our hero visited
her brother when he was commanding forces in
Virginia. There, she met a civil engineer on her
brother’s staff at an officer’s ball. A year later, she
married that man.
Our hero’s husband became the chief engineer
of a historic U.S. bridge-building project—New
York’s Brooklyn Bridge. The Brooklyn Bridge gave
this month’s hero a place in history, too. While her
husband was working on the bridge, he became
quite ill. To keep the project going, our hero took
charge of it. She did it mostly in secret because
women weren’t recognized as being
as capable as men. But
our hero’s role in the
successful completion
of that famous bridge
is celebrated today.
Can you guess her
name? Answer
on page 48.
r g—
r. De
wn as D s. For more th aking
a n d sp e s 4 u s
“Dr. D”— ry and real he
s, he’s
g book
20 year
eroes a
bout his .
about h
a rd s
.com to ir tue trading c
his Hero
Straws and
great way to understand how bridges work is to build one yourself.
By experimenting with design, construction, and strength, it’s easy
to see why some bridges were built in specific ways or designs.
Y can try building model versions with materials found in your house.
PPBS offers a website with instructions for building a suspension bridge
ffrom straws. You can find the activity at
eeducator/act_suspension_ho.html. Or visit
popsiclebridge.pdf for a greater challenge. Follow the steps to create a
bridge of Popsicle sticks that can support up to 20 pounds of weight! The
Colonel, the squirrels, and I tried it. First, we had to wait for the Colonel to
eat three boxes of frozen treats to get the sticks. Once it was built, though,
the squirrels tested it. We all marveled at its strength!
Our discovery magazines explore science,
history, archaeology, culture, tech,
ge c
Ol w
/ Al
a er
e , Y —All
A Ri ts Reser
Subscribe at
az o
o i ) Flloren
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r geman
man m es; Rawpixe .com
m Shu
Sh tte
ock.c m
p or
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nd sayin
ing the or
the symptoms are real. Sufferers can experience dizziness, shortness of breath,
and extreme anxiety. They go out of their way to avoid having to use a route
that includes a bridge. Some people become so panicked on tall bridges that
they have to be rescued by tow trucks. Some major bridges—such as the Tappan
Zee Bridge in New York, the Chesapeake Bay Bridge in Maryland (BELOW), and
the San Francisco–Oakland Bay Bridge in California, offer a service: They provide
drivers to help gephyrophobia sufferers get their cars across the bridge.
Answer to Dr. D’s Mystery Hero from page 45: Emily Warren Roebling
Picture Credits: cover Gary Crabbe/Alamy Stock Photo; ii (TL), 4 Tomasz Wozniak/; ii (TR), 36 (B), 38 (TL), 39 (T) Drew Noel Photography/Rutgers CAIT; ii (B), 2-3,
11 (B), 13 (T), 20, 26-27, 28 (BR, BL), 30, 33 (both) LOC; 1 (C bridge), 8-9 (bridge) tele52/; 1 (C hwy), 8-9 (hwy) Constantine Pankin/; 1 (B), 22, 23 Patrick
McBriarty; 5 (T) Gary Fowler/; 5-7 (bridge vectors) A7880S/; 5 (B) Simply Photos/; 6 (T) Gregory Johnston/Shutterstock.
com; 6 (B) 560042335/; 7 (T) Felix Mizioznikov/; 7 (B) MarkVanDykePhotography/; 10 (T inset) NEGOVURA/Shutterstock.
com; 10-15 (border) Mallinka1; 10 (T inset), 12 (TL) Alan Stoddard/; 12 (TR) CHAIYA/; 13 (B) Anton Foltin/; 14 Everett Historical/; 15 (T) Avalon/Construction Photography Alamy Stock Photo; 15 (B) Sundry Photography/; 16 (title) Very_Very/; 16-18
(bkgd) netsign33/; 19 Lissandra Melo/; 21 elesi/; 27 (inset) Nip/; 28 (TL) meunierd/;
29 (T) Dan Henson/; 31-34 (bkgd) Involved Channel/; 31 (T) Oleksandr Derevianko/; 31 (B) Felix Mizioznikov/Shutterstock.
com; 32 miker/; 34 (T) Steve Lovegrove/; 34 (B) pleasecat/; 35 (T), 37 (T), 36-39 (bkgd) “The Beast”, 36 (T) A. Thomas/Rutgers
CAIT; 35 (TR) Adam Kenneth Campbell/; 35 (CL) Dean Pennala/; 35 (BR) Richard Pratt/; 38-39 Oscity/; 42
Paul Briden/; 44 (T) Neil Burton/; 44 (B) Real Moment/; 46 mtlapcevic/; 48 Lone Wolf Photography/; 50 Rapin_1981/
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