BRIDGE CONSTRUCTION
BACKGROUND INFORMATION:
Bridges are structures which carry people and vehicles across natural or man-made
obstacles. As early roads connected villages and towns, people traveled by foot or
with carts and wagons. Although a person carrying a large bundle might be able to
cross a stream by swimming or stepping on stones, as horse drawn vehicles with
heavy loads needed to cross more dangerous terrain, permanent sturdy bridges became
an important part of transportation systems.
Early bridges were made from local materials such as wood, stone and fibers. Today,
most bridges have a concrete, steel, or wood framework with an asphalt or concrete
roadway. Based on the length of the barrier to be crossed, the amount and type of
traffic as well as forces of nature (wind, tide, flood) different materials and shapes of
bridges are used.
There are many types of bridges such as arch bridges, girder bridges, truss bridges,
cantilever bridges, cable-stayed bridges, suspension bridges and moveable bridges.
Many bridges are actually combinations of different types of bridges -- and no two
bridges are identical! Most bridges are held up by at least two supports set in the
ground called abutments. Some bridges have additional supports along the middle of
the bridge called piers. A span is the distance between two supports, either two piers,
a pier and an abutment or two abutments. Many short bridges are supported only by
the abutments and are called single-span bridges. Longer bridges usually have one or
more piers to support them and are known as multi-span bridges.
How Bridges Work
Most machines do some type of work. You know an engine, windmill or other
machine is generating a force or work because you can see the motion. Although a
bridge does not have spinning moving parts like other machines, it is still doing work.
What is a bridge's work? A bridge needs to be strong enough to support its own
weight plus the load of passengers and vehicles travelling on it against the pull of
gravity. A bridge works against the pull of gravity. If you hold a facial tissue between
your hands and place a heavy book on top of a facial tissue, you know the book will
break through the tissue. The tissue is not strong enough to support the weight of the
book.
How do bridge's work? Although there are many types of bridges most bridges work
by balancing compression and tension. Place a flexible object like an eraser, sponge,
or small piece of bread between your thumb and index finger. Press your fingers
together. One side of the object will bend inwards and shorten while the other will
bend outwards and lengthen. The shorter side has been compressed, while the other
side is under tension. Bridge components experience these tension and compression
stresses.
The Upper Side Has Been Compressed, The Lower Side Is Under Tension
Bridge materials, like stone, wood, steel and concrete, all have different strengths. For
example, steel can be much stronger than wood. Some materials are easier to form a
particular shape. Different materials can withstand different amounts of compression
and tension. For example, stone can withstand a lot of compression, but under a lot of
tension will break. Steel is very flexible and can endure a lot more compression and
tension then materials like iron, wood or stone. Engineers will select materials and a
bridge design, based on the strength of the material, amount and type of stress a
material can withstand and other bridge requirements (length, terrain, etc.).
Different Types Of Bridges
Arch bridges are structures in which each span forms an arch. The arch bridge is one
of the oldest types of bridges. Early arch bridges were made from stone. The spans
range up to about 1700 ft.
Arch Bridge
Bixby Creek Bridge, Monterey, California
Photo Courtesy G. Donald Bain, The Geo-Images Project at the University of California,
Berkeley
(http://geogweb.berkeley.edu/GeoImages.html)
Girder bridges are made of beams called girders. The ends of the beams or girders
rest on piers or abutments. The span length of girder bridges ranges up to about 1000
ft.
Girder Overpass Oakland, California
Truss bridges are supported by frameworks called trusses. Trusses are beams
arranged to form triangles.
Truss Bridge Worcester, UK
Photo Courtesy D. Locke, Bridge Building
(www.bb.brantacan.co.uk/bridges.htm)
Cantilever bridges consist of two independent beams, cantilevers, that extend from
opposite banks of a waterway. Cantilever bridges have spans as long at 1800 ft.
Cantilever Section of the Bay Bridge, California
Cable-stayed bridges have roadways that hang from cables. The cables are connected
directly to towers.
Cable-Stayed Bridge
Sabrina Foot-Bridge Worcester, UK
Photo Courtesy D. Locke, Bridge Building
(www.bb.brantacan.co.uk/bridges.htm)
Moveable bridges have roadway that is moved to provide enough clearance for boats
or large ships to pass. An example of a moveable bridge is a drawbridge that tilts the
roadway upward.
Drawbridge and Closeup of Drawbridge Mechanism Larkspur, California
Suspension bridges may be the most impressive type of bridge with their long main
span and beauty. These bridges have a roadway that hangs from steel cables supported
by two high towers. The difference between suspension bridges and cable-stayed
bridges is that suspension bridge cables are not directly connected to the towers. The
cables of a suspension bridge are not connected to the bridge - the cables pass through
a hole in the top of the towers.
A suspension bridge has at least two main cables. These cables extend from one end
of the bridge to the other. Suspender cables hang from these main cables. The other
end of the suspender attaches to the roadway.
SUSPENSION BRIDGES
NAME
LOCATION
MAIN SPAN
LENGTH IN
FEET
TOTAL
LENGTH IN
FEET
Akashi-Kaikyo
Kobe City - Awajishima Island,
Japan
6,532
12,831
Humber
River Humber, England
4,626
7,283
VerranzanoNarrows
Brooklyn-Staten Island, NY
4,260
6,690
Golden Gate
San Francisco, CA
4,200
8,981
Mackinac
Mackinaw City, MI
3,800
8,614
Brooklyn
Brooklyn, NY
1,595
5,989
Suspension bridges have the longest spans in the world and are used to cross great
distances. These types of bridges are used to cross deep water channels, cannons or
gorges, where construction of supporting piers can be difficult. The towers can be
placed far apart eliminating the need for multiple towers and piers. Some suspension
bridges have a main span longer then 4000 ft. The longest suspension bridge in the
world is in Japan (the Akashi-Kaikyo Bridge) and has a span over a mile long. The
largest bridges have cables several feet wide which weigh thousands of pounds per
foot. For that reason the cables are spun in place.
Suspension Bridge - Golden Gate Bridge, San Francisco, California
The cables must be secured and anchored past the ends of the bridge. Why? Look at
Figures 1a and 1b. In Figure 1a the bridge is not anchored to the land. What happens
to the bridge?
Figure 1a -------------- Figure 1b
To feel the difference between a cable that ends at the tower top and one that goes
over it and continues to the ground, grab your head with your right hand and gently
pull (Figure 2a). If you try to keep your head straight, your neck will feel compressed,
but also pulled to the right.
Figure 2a -------------- Figure 2b
Now interlace the fingers of your hands, put them over your head and pull with both
arms (Figure 2b). Your head and neck will feel compressed, but your neck will not
feel a pull to the right or left, since the force of your right arm is balanced by that of
your left arm.
Taken from the following website: http://www.swe.org/iac/lp/bridge_03b.html