Building Bridges
Lesson 5

Subjects:  Language Arts, Math, Science 

Learner Outcomes:  Students will discover that adding the cables to their straw bridge and anchoring the cables on both sides significantly increases the load that the bridge can support. 

Duration of Lesson:  two 45 minute class periods 

Materials:  (per group of two) 

12 drinking straws (non flexible) 
masking tape 
about 4 feet of dental floss or thread 
centimeter ruler 
scissors 
4 large paper clips 
paper cup
books or chairs 

Technology Tools/Courseware:  free West Point Bridge Designer download, Discovery Channel School video "Bridges:  Design and Function (optional) 
 

Teacher Notes: 
 

• Review different types of bridges, noting their benefits and the drawbacks. 
• Groups of two, either assigned or student choice, desirable 
• Students can either be given directions on how to construct a bridge or they can just be given the materials to use and construct a bridge of their own design. 
• A worksheet, Building a Suspension Bridge, is included if students are to construct a bridge of their own design from the given materials. 

Procedure:

•  Cut two short pieces of straw, each 3 centimeters (about 1.25 in.) long. For each tower, tape two straws on either side of a short piece of straw, as shown. Tape the long straws together at the top, too. 
• Tape one tower to the edge of a desk or chair. Tape the second tower to a second desk or chair of the same height. Position the towers 17 cm (about 7 in.) apart. 
• Place another straw between the towers so its ends rest on the short pieces. This straw is the bridge deck.  Now you have a simple beam bridge. 
• Make a load tester by unbending a large paper clip into a V-shape. Poke the ends of the paper clip into  opposite sides of a paper cup, near the rim. Use a second paper clip to hang the load tester over the bridge 
• deck. Record how many pennies the paper cup can hold before the bridge fails. 
• After you test the strength of the beam bridge in Step 4, predict how many pennies your suspension bridge will support. 
• Now change the beam bridge into a suspension bridge. Tie the center of a 100-cm (about 4 ft.) cable around the middle of a new straw. Place the straw between the towers. Pass each end of the cable over a tower 
• and down the other side. 
• To anchor the bridge, wrap each end of the cable around a paper clip. Slide the paper clips away from the tower until the cable pulls tight.  Then tape the paper clips firmly to the desks.  Test it again. 
• Make sure to record data and observations in a chart.  Draw and write a conclusion from your tests.  Make sure you  can  identify the forces acting on the loaded suspension bridge? Which parts of the bridge are in compression? Which parts are in tension? 
• Upon satisfactory completion building a bridge, students will go to West Point Bridge Designer and download free bridge design software. 
• Students are to use the downloaded software to design and test a bridge in virtual simulation. 

• Scale drawings could be done on graph paper.  Various materials could be used to accommodate students with various fine motor skills (i.e. paper towel tubes, popsicle sticks, toothpicks). 
• To help kids understand how forces act in a suspension bridge, have them experiment with attaching the cables from the bridge deck only to the tops of the towers, instead of extending them back down to the surface at the ends of the bridge. Ask: How strong is the bridge this way? Why? (This model is less strong than the  model in which the cables extend back down to the ground on the other sides of the towers. A load on this bridge deck pulls the tips of the towers inward. There is no balancing tension pulling the towers back out toward the ground.) 

Enrichment Activities:

• Challenge students to design and build a straw suspension bridge that spans a gap twice as wide and supports the same amount of weight. 
• Two online challenges will help students understand properties of different kinds of bridges.  Each Web site listed below provides students with several scenarios in which the student must figure out how to solve the problem of building the best type of bridge. 

Evaluation/Assessment: Students will successfully design and construct a model of a bridge.
 
 

References: 

•  NOVA@pbs
• PBS/BuildingBig
• How Stuff Works
• Bridge Types
• West Point Bridge Designer download