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Background.

Bridges are a technological marvel. Their development has aided travel, led to the growth of communities, and has helped to establish continuing commercial trade routes. The rapid expansion of railways during the last cent challenged engineers to solve a variety of problems and to overcome many obstacles. Foremost among these was how to support the tremendous weight of railroad cars, so they could travel easily across hilly and rugged terrain. The B & O Railroad even established an engineering school to address these difficult problems.

Engineers must decide on the best design and the best materials to use for each bridge they build. For centuries, bridges have been constructed with materials like rope, stone, wood, and iron. The Romans were master bridge-builders and discovered concrete. They used this strong building material to construct hundreds of bridges, many of which are still standing today. Steel is now used for most modem bridges. Even concrete bridges use steel rods for reinforcement.

Types of Bridges

There are many types and styles of bridges. A few are very simple, like a log that has fallen across a creek. Others are far more complex. Some are the result of a combination of designs blended together to provide added strength. Most bridges rely on plers for support. Bridges are designed to be strong and sturdy and to withstand all sorts of weather conditions and natural phenomena. * Beam Bridges. A simple beam bridge is similar in design to a log across a stream, except that the deck (the part of the bridge you can walk or drive on) is supported at both ends. Many highway overpass bridges are made using this design. * Arch Bridges -- Rather than having its weight pushed straight down into its supports like a beam bridge, an arch bridge pushes its weight down and out towards its strong side supports. Both arch and beam bridges are designed to handle the forces of compression caused by the weight of traffic traveling over them. The arch shape results in a stronger bridge design than the beam. * Truss Bridges -- Rigid triangular-shaped trusses add strength to all types of bridges. These different criss-cross truss designs were often named after the engineers who first patented them. * Suspension Bridges -- The longest bridges are suspension bridges. The decks of these bridges are suspended by strong cables hung from high towers and anchored at both ends. The cables of a suspension bridge are under constant tension, much like a fully stretched rubber band. These cables are very thick (e.g., the cables on the Golden Gate Bridge are 921/4 cm [36 1/2 in.] in diameter) and weigh thousands of pounds per foot. Suspension bridges are designed to sway in the wind. * Cantilever Bridges -- A cantilever bridge extends out beyond its supporting piers and usually has a span suspended between two outstretched "arms." These bridges are constructed "up and out" from opposite supporting sides. * Movable Bridges -- To allow for tall boat traffic to pass through, many bridges must open-up or have sections that move apart. The bascule or see-saw bridge is a hinged bridge with counter-weights that draw the bridge open, much like the drawbridges of medieval castles. Some movable bridges have spans that swing out on a swivel base or have spans that move up and down on pulleys like an elevator. * Covered Bridges -- The roofs and covers on these bridges help prevent heavy snow and bad weather from stopping bridge traffic. At one time, it was also believed that livestock would be less afraid to cross wide rivers if the bridges were enclosed.

Many bridges combine several elements of different designs to create the strongest and most durable bridge for the area needed (and with die materials available). In some remote areas, footbridges have been constructed out of wooden planks interlaced with twine or vines. In other places, bridges are constructed across floating boats to create pontoon bridges. Once basic engineering principles have been met, bridge designers are limited only by their imaginations and creativity.

Building a Bridge

When planning a bridge, engineers must consider location, design, cost of materials, height needed for river traffic, and whether the bedrock will be strong enough to support a foundation. If a pier is to be built underwater, a cofferdam is first constructed out of pilings. Then the water is pumped out and concrete poured in. If there is a lot of sand or mud, a "bottomless" caisson is sunk, until solid bedrock is located. Caissons can go deeper than cofferdams and are filled with compressed air to keep water out and workers alive. Scaffolding is sometimes used to help hold up a bridge while under construction. Cranes or helicopters are often used to hoist prebuilt decks into place. For most suspension bridges, towers are constructed first. Because each foot of cable weighs thousands of pounds, the cables are usually spun from the top of these towers, rather than assembled on the ground and raised up. For bridges that cross deep gorges, cantilevering (building out by sections) is an important building technique. The cantilevered sides are held in place by wires, until the sides meet and can support themselves. Not only have bridges led to ease of travel and the connection of different segments of our population, they have also provided us with increased safety, military, and economic opportunities.

1. Ask your students to think about the last time they were on a bridge. Discuss where they were and what they were crossing over (e.g., a bridge over a river, a highway overpass, a walkway in a mall, etc.). Ask about the longest bridge they've crossed over or may have seen.

2. Have them look through magazines, newspapers, and books to find pictures of different bridges. Ask them to bring some bridge pictures to school and create your own class Bridge Book. Main Concepts: Bridges make it easier for us to get across places that are hard to cross. Piers help give bridges support.

Picture Activity

Have your students look at the different bridges on the front page. Ask them what kinds of places bridges help us get across. Explain that bridges must be strong and sturdy. They must be able to carry heavy traffic and stand up to all kinds of wind and weather.

Vocabulary

First, go over the words in the small boxes together. Then have them listen carefully. Tell them to draw a plane in the box over the bridge, a boat under the bridge, and a car on the bridge.

Weekly Lab

You need: large index cards, toilet paper rolls (or rolled and taped index cards), light-weight books. Introduce the term piers to your students. Explain that piers help hold a bridge up and make it strong. Have them look back on the front page and point to the piers. Then ask if they think these index card "piers" can hold up (or support) a book. Have them first guess which of these shapes they think will be the strongest. Show them how to make the "V-shape" by folding their index cards in half. Then let them experiment using the folded shapes and rolled columns to support light-weight books (e.g., placing the cards in the same direction, facing each other, close together, far apart, etc.). Ask which shapes held the most books. (The V-fold is usually less stable than the column. It is most stable when the folded edges are facing each other.)

Weekly Problem

Answers: 3 green, 2 red. Have them first color the upright triangles in the directions green and the downward triangles red. Explain that a truss bridge uses triangle shapes, because they make the bridge very strong -- strong enough to carry trains. Have them look carefully at the bridge and color in all the upright triangles they see green and downward triangles red. After coloring, have them count the number of each type and fill-in the boxes.

Storytelling

Explain that some boats are too tall to fit under some bridges. Special drawbridges are able to open up and let boats pass through. Many castles used drawbridges to help keep their enemies out. Some bridges that let boats pass through use elevators to lift up sections or have parts that swing out sideways.

Challenge

Answers: (left to right) b, a, c. Though falling down, London Bridge actually lasted for 600 years. First built in 1176, it eventually had 19 different sized archways stretching across the Thames River. Boats regularly hit its many piers in the fast currents of the water. Houses were built right on the bridge, which added great pressure to its foundations. A whole row of unstable houses once fell into the river. A replacement bridge built in 1831 was moved to Arizona in 1972.

Home Base

This activity can be done at home or in class using a wide variety of "around-the-house" materials (e.g., assorted boxes, cans, paper towel or toilet paper rolls, magazines, books, blocks, etc. .

Level A Main Concepts and Picture Activity: See TN Level Pre-A.

Vocabulary

First, go over the words in the boxes together. Then have them listen carefully. Tell them to draw a plane in the box over the bridge, a boat under the bridge, and a car on the bridge. Final have them color in the correct word in each box.

Weekly Lab

See TN Level Pre-A -- WEEKLY LAB. In addition, have them experiment by folding other shapes like triangles or boxes.

Weekly Problem

Answers: 4 green, 3 red. Have them first color the upright triangles in the directions green and the downward triangles red. Explain that a truss bridge uses triangle shapes, because they make the bridge very strong -- strong enough to carry trains. Have them look carefully at the bridge and color in all the upright triangles they see green and downward triangles red. After coloring, have them count the number of each type and fill-in the boxes.

Writing for Science

See TN Level Pre-A -- STORYTELLING. In addition, encourage your students to write longer stories on a separate sheet of paper.

Challenge

Answers: (left to right) b, c, a, d. See TN Level Pre-A-CHALLENGE for more information on London Bridge.

Home Base

This activity can be done at home or in class using a wide variety of "around-the-house" materials (e.g., assorted boxes, cans, paper towel or toilet paper rolls, magazines, books, blocks, etc. .

Level B

Main Concepts: Bridges must be strong and sturdy to carry heavy traffic. Piers help give bridges support.

Vocabulary

Answers: LONDON. ACROSS: 1) arch 6) suspension 8) over 9) deck DOWN: 2) cables 3) designs 4) truss 5) bridges 7) piers. See TN Level Pre-A -- CHALLENGE for more on London Bridge. Have them refer back to the front page, if needed.

Weekly Lab

Their book stacks should be flat and equal in height. Have them place the pennies midway between the books. Pennies should be added until the paper sags or the pennies fall. Make sure that the bottom edges of the paper arch flare outward and are not curled under. If they are curled inward, their arch will collapse quickly. Me arch is a stronger shape than the flat beam, because it better distributes the weight of the pennies -- down the sides of the arch. The arch will usually hold about twice as many pennies.

Weekly Problem

Answers: (r = red, bl = blue, g = green, br = brown) Left to right: bl, g, br, g, br, g, br, r, bl, r, b4 r, b4 g. Have them color in the color-key chart first. Point out that all the triangles point in different directions. Triangular shapes make a truss design bridge very strong and sturdy. This bridge is an example of a Burr Truss.

Writing for Science

Bridge builders must work at great heights, at times equivalent to a 100 story building. There are so few people who are able to spin the cables at the tops of suspension towers, that skilled teams fly around the world from one job to another. Painters and maintenance workers also face the same dangers. Ask them to imagine being 1000 feet in the air with the wind gusting and blowing, while trying to remove a rusty bolt or paint a bridge. Tell them to remember that they really want this job, as they write their letters.

Challenge

Answers: 1) b 2) c 3) a 4) d. Tell them to choose the name that best fits. Have them look back on the front page, if needed.

Home Base

This activity can be done at home or in-class. Explain that plers help hold up a bridge and make it strong. Have them look back on the front page and find the piers. Then ask if they think these index card "piers" can hold up and support some light-weight books. (Have them try placing the cards in different directions, close together, far apart, etc.) Let them experiment to see how many books each shape can hold and which shape works the best. (The V-fold is usually less stable than the column. It is most stable when the folded edges are facing each other.)

Level C

Main Concepts: Bridges come in many designs and must be strong and sturdy to handle heavy traffic. Piers help give bridges support.

Vocabulary

Answer: Eads Bridge was the first steel bridge in the U.S. It uses arches and trusses. Each set of arrows points to the letters that belong in the blank spaces. Explain that some letters need to be filled-in first, before they can discover others. Some letters even follow a trail. Eads Bridge, built by Jonathan Buchanan Eads, opened in 1874 in St. Louis and spans the Mississippi River.

Weekly Lab

Explain that piers help hold up a bridge and make it strong. Ask if they think these index card "piers" can hold up and support some fight-weight books. Have them first guess which of these shapes they think will be the strongest. Let them experiment placing the cards in different directions, close together, far apart, etc. Have them see how many books each shape can hold and which shape works the best. (The V-fold is usually less stable than the others. It is most stable when the folded edges are facing each other.)

Weekly Problem

Answers: (r = red, bl = blue, g = green, br = brown) Left to right: Warren -- g, br, r, bl (repeated 4 times) and Burr -- bl, g, br, g, br, g, br, r, bl r, bl, r, bl, g. Have them color in the color-key chart first. Point out that all the triangles point in different directions. Triangles are a very rigid geometric shape which adds to the strength of a truss bridge. Many of these criss-cross truss designs are named after the engineers who first patented them.

Writing for Science

See TN Level B -- WRITING FOR SCIENCE.

Challenge

Their book stacks should be flat and equal in height. Have them place the pennies midway between the books. Pennies should be added until the paper sags or the pennies fall. Make sure that the bottom edges of the paper archilare outward and are not curled under. If they are curled inward, their arch will collapse quickly. The arch is a stronger shape than the flat beam, because it better distributes the weight of the pennies -- down the sides of the arch.

Puzzle

Answers: 1) e 2) c 3) a 4) d 5) b. Tell them to choose the name that best fits. Have them refer back to the front page.

Level D

Main Concepts: See TN Level C.

Vocabulary

Answers: a) span B) arch c) design d) truss c) cables f) suspension -- London and Golden Gate. Have them refer back to the words in bold print on the front page.

Weekly Lab

Their book stacks should be flat and equal in height. Have them place the pennies midway between the books. Pennies should be added until the paper sags or the pennies fall. Make sure that the bottom edges of the paper archilare outward and are not curled under. If they are curled inward, their arch will collapse quickly. The arch is a stronger shape, because it better distributes the weight of the pennies -- down the sides of the arch. At 15 cm, the flat beam bridge will hold fewer pennies. Ask how the increased distance affects the arch. Encourage them to try folding the flat paper in different ways to try and strengthen it (e.g., in half, into a "W," with its edges up, etc. . Have them retest these new designs.

Weekly Problem

Answers: (r = red, bl = blue, g = green, br = brown) Left to right: Warren -- g, br, r, bl (repeated 4 times) and Burr -- bl, g, br, g, br, g, br, r, bl, r, bl, r, bl, The Warren truss has more triangles (16) than the Burr (14). Have your students color in the color-key chart first. Triangles are a very rigid geometric shape which adds to the strength of a truss bridge. Many of these crisscross truss designs are named after the engineers who first patented them.

Writing for Science

Bridge builders must work at great heights, at times equivalent to a 100 story building. There are so few people who are able to spin the cables at the tops of suspension towers, that skilled teams fly all around the world from one job site to another. Painters and maintenance workers also face the same dangers. Ask your students to imagine being 1000 feet in the air with the wind gusting and blowing, while trying to remove a rusty bolt or paint a bridge. Have them think about the qualities a person would need to do this job.

Challenge

See TN Level C -- VOCABULARY.

Puzzle

Answers: 1) f 2) c 3) d 4) a 5) e 6) b. Tell your students to choose the bridge that best fits each name. Have them refer back to the front page for descriptions of these bridges.

Level E

Main Concepts: Bridge materials and styles must fit" the place where the bridge is to be built, the "load" it is designed to carry, and the stresses it must handle.

Vocabulary

Answer: Eads Bridge was the first steel bridge in the U.S. It uses arches and trusses. Each set of arrows points to the letters that belong in the blank spaces. Explain that some letters need to be filled-in first, before they can discover others. Some letters even follow a trail. Eads Bridge, built by Jonathan Buchanan Eads, opened in 1874 in St. Louis and spans the Mississippi River.

Weekly Lab

See TN Level D -- WEEKLY LAB. In addition, explain that many bridges use a combination of designs to increase their strength. For the arch and beam design in Step 5, they may need to add some more books. This will increase the height of their stacks, so the sheet of paper will lay flat across the top of the arch. It is all right for the flat sheet to be slightly raised above the books, as the arch will compress a bit when the pennies are added. The combined arch and beam design will usually hold about twice the "load" of the arch alone.

Weekly Problem

Answers: (r = red, bl = blue, g = green, br = brown) A) Left to right: Warren -- g, br, r, bl (repeated 4 times) and Burr -- bl, g, br, g, br, g, br, r, bl, r, bl, r, bl, g. The Warren truss has more triangles (16) than the Burr (14). B) The 19 Burr trusses with 266 triangles would have more triangles than the 256 in the 16 Warren trusses. Have your students color in the color-key chart first. Triangles are a very rigid geometric shape which adds to the strength of a truss bridge. Many of these criss-cross truss designs are named after the engineers who first patented them.

Writing for Science

See TN Level D -- WRITING FOR SCIENCE.

Challenge

The number of books your students will need to bridge the open space will depend on the size and weight of the books used. The surface that they are building on should be flat. You can also have them build their cantilever bridges on the floor rather than on chairs or desks. The higher building platform will provide for a more dramatic visual effect, but keep in mind that the books are likely to collapse and be noisy. Have them try this again increasing the distance between chairs.

Puzzle

Answers: 1) g 2) c 3) e 4) a 5) f 6) d 7)b. Tell your students to choose the bridge that best fits each name. Have them refer back to the front page for descriptions of these bridges.

Level F

Main Concepts: See TN Level E.

Weekly Lab

Part A: See TN Level E -- WEEKLY LAB. Part B: The number of books your students will need to bridge the open space will depend on the size and weight of the books used. The surface that they are building on should be flat. You can also have them build their cantilever bridges on the floor rather than on chairs or desks. The higher building platform will provide for a more dramatic visual effect, but keep in mind that the books are likely to collapse and be noisy. Have them try this again increasing the distance.

Weekly Problem

Answers: 1) 16,896,000 pounds (which is 8448 tons) 2) The Quebec Bridge is about 100 feet longer (545.5 x 3.3 = 1800.15 ft. . 3) 72 x 8375 = 603,000 tons 38,390 = 641.390 tons.

Writing for Science

Cable spinners must work at great heights, at times equivalent to a 100 story building. There are so few people who can do this job, that skilled teams fly around the world from one construction site to another. Because each foot of cable weighs thousands of pounds, they are usually spun from the top of the towers, rather than assembled on the ground and raised up. Bridge builders, painters, and maintenance staff all face the same dangers. Ask them to imagine being 1000 feet in the air with the wind gusting and blowing, while trying to handle heavy cable or remove a rusty bolt.

Challenge

Answers: 1) C 2) T 3) T 4) C 5) T 6) C 7) C 8) T.

Puzzle

See TN Level E -- VOCABULARY.

Recommended Resources

* Brown, David J. Bridges. New York: MacMillan Publishing Co., 1993.

* Darling, David. Spiderwebs to Skyscrapers. New York: Dillon Press, 1991.

* Robbins, Ken. Bridges. New York: Dial Books, 1991.

* Salvadori, Mario. The Art of Construction: Projects and Principles for Beginning Engineers and Architects. Chicago: Chicago Review Press, 1990.

Materials Needed for Issue 9 - Sticky Stuff (Adhesives)

Pre-A, A - tablespoons, flour, water, small paper or plastic cups, craft sticks (or spoons), different brands of band-aids[R], pennies, envelopes

B - same as above, plus white "school" glue

C - tablespoons, flour, water, small cups or bowls, craft sticks (or spoons), different types of tape, pennies, plastic sandwich bags

D - same as above, plus raw eggs

E - skim milk, white vinegar, water, baking soda, cups, coffee filters, tablespoons, measuring cups or small bowls, spoons, paper towels, white "school" glue, different types of tape, pennies, plastic sandwich bags

F - same as above, plus flour, raw eggs
COPYRIGHT 1995 Science Weekly, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1995 Gale, Cengage Learning. All rights reserved.

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Title Annotation:historical and background information on bridges, and classroom activities for levels pre-A through F
Publication:Science Weekly
Date:Dec 20, 1995
Words:3871
Previous Article:How strong is your bridge deck?
Next Article:Can you build a cantilever bridge?
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