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Roebling and Suspension Bridges: A Thread of Steel
Equipment & Supplies
  • Building the Brooklyn Bridge Activity 5 or 6 small groups. Each group will need: 6 drinking straws a roll of pennies masking tape small Dixie cup 4 large paper clips one ruler one pair of scissors thread or dental floss (3 4-foot long pieces) one copy of Roebling's anchorage patent Teacher Demonstration of Compression small amount of play dough 1 straw 2-foot length of dental floss/string

Day One
1. Ask the class, "Does anybody know what the Allegheny Portage Railroad was?". Explain it was a system of ten inclined planes designed by engineers to get canal boats over the Allegheny Mountains. It was constructed between 1831 and 1833 and connected Hollidaysburg, Pennsylvania to Johnstown, Pennsylvania with 36-miles of railroad. It was considered an engineering marvel of its time. Horses and mules pulled the first trains; later, more efficient steam engines replaced the animal labor. For more information, see the Historical Marker Page markerAllegheny Portage Railroad. Show Inclined Plane Number 8 to the class.

Tell students: In 1841, an engineer named John Roebling noticed a problem. Show Engraving of John Roebling and Bust of John Roebling. He saw that the system was wearing out the thick rope, called hemp hawsers, used to haul the boats up the Allegheny Mountains. This was dangerous because it could cause accidents, and it was also costly to continuously replace.

Ask students, "What do you do think he did?" [See if students can predict the use of different material, other than hemp for rope.]

2. Disseminate Handout 1: Excerpt of The Great Bridge. Have the students read the story about how Roebling created the wire rope for the Allegheny Portage Railroad. As follow-up to this reading, have your students answer questions from Worksheet 1: Questions on Excerpt of The Great Bridge. To facilitate assessment and discussion, you can find the Teacher's Guide to this worksheet in the Teacher Resources section of this lesson plan.

3. Ask the class what uses were suggested in the story for wire rope? What other applications can they think of for wire rope? Make a list on the board.

Here are some possible responses: cranes, elevators, ski lifts, trolleys, communication lines, ship rigging, bridges, airplanes, submarine netting.

Hand out Student Handout 4 - Wire Rope Advertisement in The Iron Age (1875). Note that this was an advertisement Roebling placed in The Iron Age in 1875 for his rope. Ask students to study the advertisement. Did they notice that the words on the advertisements are placed to look like a wire rope? Then ask if they see any more applications they did not think of for wire rope. Add these responses to the list:

Applications mentioned in the advertisement include: inclined planes, standing ship rigging (ropes and chains used by a ship to work the sails and support the mast), suspension bridges, ferries, stays and guys or derricks, cranes and shears, elevators, tillers, etc.
Unknown vocabulary may include:

Stay: Wire-rope used to support a mast of a ship
Guy (wire): Wire used in aiding the stability of a tall structure
Derrick: any of various machines for moving or hoisting heavy weights by means of a long beam fitted with pulleys and cables
Tiller: a lever in a boat used to turn the rudder from side to side

4. Explain that of all the practical uses for wire-rope Roebling is most famous for his work and advancements with one of them. Can your students guess what that is? The suspension bridge. Explain that tomorrow you are going to take a look at bridges and how they work. Before the end of class, pass out Student Handout 2- Timeline of John Augustus Roebling to review his achievements for homework.

Day Two

1. Yesterday, we looked at Roebling's invention of the wire rope. Today we are going to look at how he used wire-rope in bridges by building a model.
2. In order for students to appreciate the structural qualities of a suspension bridge, you will need to lay a foundation of basic bridge types and the forces that they encounter. Refer to Michael Morrissey's article "How Bridges Work" at HowStuffWorks- "How Bridges Work" (
3. Begin by asking your students to define a bridge. (It is a structure built over an obstacle–river, valley, railroad tracks, etc.–so we can get to the other side.)
4. There are different types of bridges. Ask students how they think engineers determine what type of bridge to build. Morrissey explains that the physical characteristics of the obstacle usually determine what type of bridge is best to build. Engineers look at the depth of the valley or the width of a river the bridge is to cross to determine its type.
5. Write on the board three types of bridges: Beam, Arch, Suspension. Now show images of these types of bridges and see if students can guess each type.

Conestoga Bridge Photograph by William H. Rau, 1891 [Arch]
Old Rockville Bridge [Beam]
Stone Bridge at Johnstown [Arch]
Roebling Bridge [Suspension]
Inspectors on Brooklyn Bridge [Suspension]

Explain that a beam bridge is horizontal (flat) structure supported by two piers, a simple arch bridge is a semicircular structure with abutments on either end, and a suspension bridge is where cables are used to support the bridge. Of the three, the suspension bridge can handle the longest span. (A span is the distance between two supports.)

6. Once students understand these basic types, you can explain the forces that a bridge encounters. Write them on the board. These are important concepts to the engineering of a bridge.

Compression: a force that acts to shorten things or press them together
Tension: a force that stretches or lengthens things
Dissipation: to spread over a greater area to the point of disappearing

7. Now you can tell your students, let's see how a suspension bridge handles these forces. You're going to build the Brooklyn Bridge. Explain that the Brooklyn Bridge was remarkable for a number of reasons. The total length of the bridge was more than a mile, it was designed to handle 18,700 tons of load, but one of the most revolutionary features was its use of steel in the stays or wire ropes descending from the cables. No other bridge in the country had used steel in its construction yet. (McCullough, pp.30-31) Divide the class into 5 or 6 small groups. Pass out materials for this activity (listed at the beginning of the procedure section). Disseminate Student Worksheet 2: Building the Brooklyn Bridge Activity.

8. If your students are having difficulty answering the final questions of Student Worksheet 2: Building the Brooklyn Bridge, you can briefly demonstrate the bridge forces in action with a small amount of playdough, 1 straw, and 2 ft. of dental floss or string. Place a small amount of play dough on the desk. Remind students the straw is the tower of the bridge. Lightly place the straw on top. Take the dental floss/string and drape it over the straw. Make it long enough to touch the desk on both sides of the straw. Now ask your students to make a prediction:

If you pull the cable (dental floss/string) tight, what do you think will happen?
[Answer: The straw will press down into the play dough.]

Now show the class the play dough. The ring from the bottom of the straw in the play dough is evidence of a force. What force is this?
[Answer: Compression. The tower is being pressed together or compressed.]

What force is being applied to the cable?
[Answer: Tension. The cables are being stretched and pulled.]

What does the play dough represent in your demonstration?
[Answer: The towers were built on foundations called caissons. However, for this demonstration purpose, if students understand the towers are pushing into the ground/earth, that is sufficient. So, "the earth" would be an acceptable answer.]

Then explain to students that the earth is taking the pressure being applied to the towers and dissipating it. Ask students what "dissipate" means again? [to spread it out to the point of vanishing]
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