David finds out that a properly designed bridge made from dry pasta can actually support
Segment length: 8:19
- Why would you choose one bridge design over another? How do bridges stay
- What different kinds of bridges are built?
- What is a truss?
"London Bridge is falling down, falling down, falling down." We all know the first
verse to that nursery rhyme. But have you ever heard this one: "Set a man to watch all
night, watch all night, watch all night"? People once felt that a bridge required a
human spirit. They sometimes buried a human sacrifice in the bridge's foundation, so
that the spirit could "watch all night." Fortu-nately, we no longer sacrifice a night
watchman when we build these structures. But there is still more to bridges than meets
Bridge builders choose from one of several types of bridges or combine two or more.
Three basic types are the arch, span, and suspension. The structural elements in a
bridge are subjected to various compression and tension forces. Something bearing
weight or being pushed together is under compression while something being pulled apart
is under tension.
In an arch bridge, compression pushes the weight away from the arch and against the
side walls and the stones of the arch itself. The Romans were first to build arch
bridges, and some of their bridges and aqueducts still stand today.
Span bridges consist of beams and/or trusses resting on supports or piers. When the
span length would require a very large or heavy beam to support the loads, a truss
system is often used. Since a truss is composed of triangles (the strongest polygons
because their shapes cannot be distorted), a truss bridge can support heavy loads with
its relatively small weight.
A suspension bridge hangs from cables firmly anchored at each of its ends. Towers
positioned at regular intervals along the span also support it. The main elements of
suspension bridges, the cables, are in tension. The trusses hang from the cables and
the trusses, in turn, support the deck. Trusses in these bridges provide stiffness to
their decks. Suspension bridges are generally used for long spans.
Engineers need to accommodate torsion in their bridge designs. What causes a bridge to
twist? Wind. An example of torsion was the Tacoma Narrows Bridge in the state of
Washington. A suspension bridge without open trusses, this bridge twisted in the breeze.
One day, the bridge twisted so violently in the wind that its center span collapsed.
Since then, engineers test bridge models in wind tunnels prior to construction and
build their decks more stiffly.
When determining what type of bridge to build, engineers evaluate terrain and length of
span. In addition, bridges change as our needs and resources change. Engineers
constantly look for ways to improve bridge designs and materials. During the recent
earthquakes in California, for example, engineers analyzed highway overpasses to find
ways to make them stronger.
- Why is it important to know whether parts of a bridge will be subjected to
tension or compression?
- Why is a triangle the strongest polygon?
compression the act of pressing or
deck platform extending horizontally that often
carries the roadway
engineer person who uses mathematical and
scientific principles to design and construct efficient structures and
girder a horizontal beam used for support
span portion of a bridge between two supports
stress the force acting on a body divided by the body's
cross-sectional area. Force per unit area.
tension the act of stretching or pulling
torsion the act of twisting
truss a rigid triangular framework
- Burns, M. (1982) Math for smarty pants. Boston: Little, Brown and Company.
- Clarke, D. (Ed.) (1979) The encyclopedia of how it's built. New York: A & W Publishers, Inc.
- Corbett, S. (1978) Bridges. New York: Four Winds Press.
- Spangenburg, R. (1991) The story of America's bridges. New York: Facts on File.
- Stephens, J. (1976) Towers, bridges, and other structures. New York: Sterling Publishing
- Stix, G. (1993, Apr) Concrete solutions. Scientific American, pp. 102-112.
- TV Ontario videotape: Trussworthy. Landscape of Geometry series. TV Ontario: (800)
- Whitney, C. (1983) Bridges. New York: Greenwich House.
- Wollomir, R. (1994, Jan) Inside the lab and out, concrete is more than it's cracked up
to be. Smithsonian, pp. 22-31.
Additional sources of information
Design kits such as Structures or The Structures Kit (found in teacher resource
Roebling Chapter of the Society for Industrial Archaeology
c/o Bierce Riley
19 Budd Street
Morristown, NJ 07961
Engineers first create a blueprint and model of a bridge before they begin construction.
Models enable them to test the design of their bridges. Often, engineering companies
must compete to win a contract. For their presentations, they explain features of
their designs with blueprints and models.
- graph paper
- poster board (2' x 3')--one sheet per group
- Each group will design and build a freestanding bridge for a transportation system
of the future. First decide what type of transportation will cross the bridge and what
type of bridge you will build. Create a blueprint of the bridge on graph paper.
- Using your blueprint, create a model of your bridge from the poster board. Each
group is only allowed one sheet of poster board, so measure carefully before you cut.
The only other materials you can use in the construction of your bridge are tape, glue,
- Present your bridge to the large group. Explain the rationale behind your design.
- With all the groups together, test the bridges for length, height, and
For the individual groups:
- How did you come up with the initial design for your bridge?
- Did your design change as you built your bridge?
- Which geometric shapes did you use in your bridge? Why?
- How does the strength of the bridge compare to the weight of the bridge?
- Would you make any changes in the design of your bridge?
For the large group:
- Which bridge was the longest? Tallest? Strongest? Heaviest? Why?
- What materials do you envision being used in future bridges?
- How can computers help design bridges?
The longest cable suspension bridge is 1,410 meters (4,626'). Because its towers stand
exactly perpendicular to Earth's surface, they are 3.49 cm (1 3/8") out of parallel to
allow for the Earth's curvature. In a plane, two lines perpendicular to the same line
are parallel. Why does this change when you work with a curved surface? By the way, t
he Akashi-Kaikyo bridge in Japan, which will open in 1998, will be 1,990 meters (6,528'
Test compression, tension, and torsion on different materials. First, take a strip of
Styrofoam 10 cm x 38 cm (4" x 15"). Ask someone to hold the ends so you can press down
gently on top to test compression. Hold each end and pull it apart to test tension.
Hold the two ends and twist to test torsion. What were the results of your tests? Find
materials that are strong in tension, weak in compression, and vice versa.
Concrete supports many of our bridges and overpasses. How is it holding up? Ice and
road salt affect concrete bridges. Read "Concrete Solutions" by Gary Stix (Scientific
American, April 1993) and "Inside the lab and out, concrete is more than it's cracked
up to be" by Richard Wollomir (Smithsonian, January 1994). What are the pros and cons
of building bridges with concrete? Why is it better than steel? What is reinforced
Invite a civil engineer to talk to your class about bridges. What types of bridges
exist in your area? Which mathematics and science courses did the engineer take to
prepare for a career in engineering? What tools do engineers use to design bridges and
Newton's Apple is a production of KTCA Twin Cities Public Television. Made possible by
a grant from 3M. Educational materials developed with the National Science Teachers