 A swinging cable and wooden plank bridge in a New
Zealand rain forest.
 The many bridges in New York City make
transportation possible for a huge population
 Currently the state of Colorado has more than 8,000
bridges
 The U.S. has more than 500,000 bridges that are more
than 100 feet (30 m) long!
 Compression is a force that acts to compress or
shorten the thing it is acting on.
 Tension is a force that acts to expand or lengthen the
thing it is acting on.
 The job of engineers to design bridges capable of
withstanding these forces without buckling or
snapping.
 Buckling occurs when compressive forces overcome
an object's ability to handle compression
 Snapping occurs when the tensile forces overcome an
object's ability to handle tension.
 Tensile and compressive forces acting on a beam
bridge; in a typical beam bridge tensile forces are
negligible. Compressive forces are indicated by red
arrows and tensile forces are indicated by blue arrows.
 Dissipate force is to spread it out over a greater area,
so that no one spot has to bear the brunt of the
concentrated force.
 Transfer force is to move it from an area of weakness
to an area of strength, an area designed to handle the
force.
An arch bridge is a good example of dissipation, while a
suspension bridge is a good example of transference
Tension and compression forces acting on an arch bridge
(left) and suspension bridge (right). Compressive forces are
indicated by red arrows and tensile forces are indicated by
blue arrows.
 Beam (or truss)Bridge
 Typical span lengths are: up to 200 feet
 Arch Bridge
 Typical span lengths are: 130-500 feet
 Suspension Bridge
 Typical span lengths are: 2,000-7,000 feet
 Typically a simple structure made of horizontal, rigid
beams. The beam ends rest on piers or columns. The
weight of the beams (and any other load) is supported by
the piers or columns.
 Compression force acts on the top portion of the beam and
bridge deck, shortening these two elements.
 Tension force acts on the bottom portion of the beam,
stretching this element
 An arch bridge is easily recognized with its defining
characteristic of a semicircular structure. Not as easily
recognized but extremely important are the abutments at
each end of the semicircular arch.
 Compression force acts outward along the curve of the
arch and into the abutments
 Tension force acts on are small in most arches and usually
negligible.
 Conventional suspension bridges are recognized by the
elongated M shape. In these bridges, parallel sets of large cables
are suspended between at least two towers (with smaller cables
hung vertically from the large cable) and anchor into the earth at
their end points. The smaller cables support the roadway.
 The weight of the bridge deck and any additional load push
down on the bridge deck and create a force of tension in the
cables.
 The cables then transfer their force to the towers. The force
induced in the towers is compressive; the towers dissipate this
force to the earth.
 Beam bridges are the most common type of bridges,
and include truss bridges. Truss bridges distribute
forces differently than other beam bridges and are
often used for heavy car and railroad traffic. In a truss
bridge, the beams are substituted by simple trusses, or
triangular units, that use fewer materials and are
simple to build.
 Today, we are going to act as teams of engineers
making bridge models. We have been hired by a city to
create a bridge to cross one of the local rivers.
However, the city does not want the bridge to affect the
fish population in the river below it. Engineers always
consider their "design objective" when creating their
models.
 Make a bridge that spans the river (scaled down to a
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distance of 10 inches [25 cm]
Supports the most weight for the cars that will pass
over it
Does not disturb the river's fish.
To simulate the load of the cars, our bridge must have
a place to securely hold a small cup in the center of the
span. To demonstrate environmental limitations on
the design, no part of the bridge may touch the "water"
(or bottom of the wooden support structure)
Cannot be taped to the wooden support structure.
1) Developing a complete understanding of the problem
2) Determining potential bridge loads
3) Combining these loads to determine the highest
potential load
4) Computing mathematical relationships to determine
the how much of a particular material is needed to
resist the highest load.
Determining the potential loads or forces that are anticipated
to act on a bridge is related to the bridge location and
purpose. Engineers consider three main types of loads:
 Dead loads include the weight of the bridge itself plus any
other permanent object affixed to the bridge, such as toll
booths, highway signs, guardrails, gates or a concrete road
surface.
 Live loads are temporary loads that act on a bridge, such
as cars, trucks, trains or pedestrians.
 Environmental loads are temporary loads that act on a
bridge and that are due to weather or other environmental
influences, such as wind from hurricanes, tornadoes or
high gusts; snow; and earthquakes. Rainwater collecting
might also be a factor if proper drainage is not provided.
During bridge design, combining the loads for a particular
bridge is an important step. Engineers use several methods
to accomplish this task. The two most popular methods are
the UBC and ASCE methods.
1. Dead Load + Live Load + Snow Load
2. Dead Load + Live Load + Wind Load (Earthquake Load)
3. Dead Load + Live Load + Wind Load + (Snow Load ÷ 2)
4. Dead Load + Live Load + Snow Load + (Wind Load ÷ 2)
5. Dead Load + Live Load + Snow Load + Earthquake Load