show #1213

David finds out that a properly designed bridge made from dry pasta can actually support his weight. Segment length: 8:19


Insights & Connections



Main activity

Try this


"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 the eye.

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.


compression the act of pressing or pushing

deck platform extending horizontally that often carries the roadway

engineer person who uses mathematical and scientific principles to design and construct efficient structures and machines

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


Additional sources of information

Design kits such as Structures or The Structures Kit (found in teacher resource catalogs)

Roebling Chapter of the Society for Industrial Archaeology
c/o Bierce Riley
19 Budd Street
Morristown, NJ  07961

Community resources


Civil engineer


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.


  1. 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.
  2. 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, and string.
  3. Present your bridge to the large group. Explain the rationale behind your design.
  4. With all the groups together, test the bridges for length, height, and strength.


For the individual groups:
  1. How did you come up with the initial design for your bridge?
  2. Did your design change as you built your bridge?
  3. Which geometric shapes did you use in your bridge? Why?
  4. How does the strength of the bridge compare to the weight of the bridge?
  5. Would you make any changes in the design of your bridge?

For the large group:
  1. Which bridge was the longest? Tallest? Strongest? Heaviest? Why?
  2. What materials do you envision being used in future bridges?
  3. 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' ) long!


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 concrete?


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 other structures?
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 Association.