BRIDGES
Bridge Basics
 Bridge is a structure built to span a valley, road,
river, body of water, or any other physical
obstacle.
 Designed to carry their own weight, (dead load)
and to carry people and traffic, (live load) and to
resist natural forces, such as wind and
earthquakes.
 Must consider effects of contraction and/or
Expansion due to temperature changes
 Designs of bridges will vary depending on the
function of the bridge and the nature of the area
where the bridge is to be constructed.
Bridge Basics
 Bridges have been used since the dawn of
humankind
 Bridges are based on one or more of three basic
structures that are derived from forms found in
nature.
 Beam: a log fallen across a stream
 Arch: natural rock formations
 Suspension: from a hanging vine
 Though traditionally constructed from wood,
stone, concrete, or steel, new bridges made from
high-performance composite materials such has
reinforced plastic are emerging
Bridge Terminology
 Abutment- A substructure element supporting each

end of a single span or the extreme ends of a multi-span
superstructure and, in general, retaining or supporting
the approach embankment.
Deck- The roadway portion of a bridge that directly
supports vehicular and pedestrian traffic.
 Span- The distance between the main supports
of a bridge.
 The length normally measured when describing the
size of the bridge
 Simple Span- a plank across a stream
 Continuous Span- supported along its length by piers and
abutments
 Cofferdam- A watertight structure allowing underwater
foundations to be built in the dry.
Bridge Terminology
Bridge Forces
 Four types of forces act on a bridge, either
separately or in combination.
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Tension
Compression
Torsion
Shear
 http://www.pbs.org/wgbh/buildingbig/lab/forces.html
Bridges
 Beam Bridge (Girder)
 The simplest form of a bridge
 A rigid horizontal structure that is resting on two piers,
one at each end.
 The weight of the bridge and any traffic on it is directly
supported by the piers. The weight is traveling directly
downward.
 Mostly built from steel and concrete beams, or
girders.
 The size of the beam, and in particular the height of
the beam, controls the distance that the beam can
span.
Truss Bridges
 Truss bridges are a type of
beam bridge made up of
many small beams
attached together in
triangular configuration to
support a large amount of
weight and span great
distances.
 They function by
compression and tension
forces and not by bending
forces.
 They are identified
according to the way the
chords are arranged.
How Truss Bridges Work
 A truss is an
 Trusses must be stable,
interconnected
framework of beams
designed to hold
something up.
 Trusses don’t bend, they
get pulled apart and
pushed together.
 However once the weight
is increased the bridge
will stag in the middle.
This is due to the
individual beams
expanding and
contracting due to the
weight.
and not able freely in any
direction in order for
them to work.
 The beams have to be
placed carefully in the
right angles and in equal
distances away from
each other so they can
distribute the weight
equally.
 They are usually
supported at the ends by
abutments and
sometimes in the middle
by piers.
Arch Bridges
 Arch bridges are one of the
oldest types of bridges and
have great natural strength.
 The load on the roadway of this
bridge is carried by the arch.
The arch is supported at each
end by a support called an
abutment
 The arch is held together by
the main piece in the middle
which is called the “keystone”
 The roadway can be placed
above or below the arch.
Arch Bridges
Arch Bridges consist of compression and tension.
 Compression
 Arch bridges are
continuously under
compression. The force
of compression is pushed
along the curve of the
arch toward abutments.
 Tension
 The greater the degree of
curvature (the larger the
semicircle of the arch), the
greater the effects of tension
on the underside.
 The natural curve of the arch
and its ability to dissipate the
force outward greatly
reduces the effects of
tension on the underside of
the arch.
Suspension Bridges
 Suspension Bridge
 Tall towers on both sides of the
roadway support the main cables.
 The cables run the entire length of
the suspension bridge and are
anchored in the concrete abutments
at each end.
 Smaller cables are suspended from
the main cables and these cables
support the roadway.
 Almost all suspension bridges have,
in addition to the cables, a
supporting truss system beneath
the bridge deck (This helps to
stiffen the deck and reduce the
tendency of the roadway to sway
and ripple.
Suspension Bridges
 Light, and strong,
suspension bridges can
span distances from 2,000 to
7,000 feet far longer than
any other kind of bridge.
 They are ideal for covering
busy waterways.
 This type of bridge is the
only practical type suitable
for very long spans or when
it would be hazardous to
maritime traffic to add
central supports.
Cable-Stayed Bridges
 Similar to the suspension
bridge but the difference lies
in how the cables are
connected to the towers
 This bridge supports the
roadway by cables that run
from towers to the actual
roadway
 Cable-stayed require less
cable, can be constructed out
of identical pre-cast concrete
sections, and are faster to
build.
 Cost-effective bridge that is
undeniably beautiful.
Cable-Stayed Bridges
 A cable stayed bridge is a
bridge with one or more
pillars.
 The two types of cablestayed bridges are parallel
attachment design and radial
attachment design.
 In a parallel attachment
design the cables are
attached at different heights
along the tower and are
parallel to one another
 in a radial attachment design
the cables are attached at a
single point at the top of the
tower and on several places
on the road.
Parallel Attachment Design
Radial Attachment Design
Cantilever Bridges
 A structure or beam that is unsupported at one end
but supported at the other, like diving boards
 A cantilever does not reach all the way across the
bridge so they are connected in the middle by a part
called the suspension span
 The suspended span may be built off-site and lifted
into place, or constructed in place using special
traveling supports.
Cantilever Bridge
Forth Railway Bridge
 The Forth Railway Bridge (or
Firth of Forth Bridge) is made
of a pair of cantilever arms that
extend out from two main
towers.
 The beams are supported by
diagonal steel tubes projecting
from the top and bottom of the
towers.
 These spans support a central
suspended span. Some
structural members of the
bridge are as large as 12 feet
in diameter.
Cantilever Bridges
Movable Bridges
•This bridge is usually used to span
canals and rivers that carry heavy
boat traffic
•Usually powered by electric motors
•In the past they were powered by
steam engines
•original movable bridges were the
drawbridges of medieval times,
raised by ropes and pulleys
• There are three main types:
1.Bascule
2.Vertical lift
3. Swing
Bascule Bridge or Drawbridge
•Used for short
distances
•Have two movable
spans the rise
upward, opening in
the middle
•When open the
weight is supported
by the stationary
section of the bridge
Vertical-lift Bridge
 Used for longer distances
 Straight bridge, held between two towers
 Lifted by steel ropes, attached to counterweights
-as the counterweights go down the bridge goes
up and vise-versa.
 Operate in an elevator like fashion
Swing Bridges
 Mounted on a central pier
 The central pier allows the bridge to rotate to
the side
 Uncommonly used because the central pier is
located in the area where boats like to travel
Moveable Bridges
Bascule Bridge
Swing Bridge
Lift Bridge
Understanding Bridges
 Video
 http://videos.howstuffworks.com/hsw/20829understanding-bridges-video.htm