Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Prepare the setup for students to visualize on the day that you introduce the project.
   1. Create a testing platform (for example: two desks 24 cm apart or 2 stacks of books 24 cm apart).
   2. Show students the apparatus for testing the bridge. Attach the steel wire to the machine bolt and nut as well as the bucket. The machine bolt will be placed across the bridge roadway with the wire hanging down through the roadway. Weights will be added gradually to the bucket until the bridge breaks (see Figure 7 from the student pages).
   3. Prepare a second setup of a carriage bolt and nut with steel wire attached. Students may use this to make sure their bridges will work with the testing apparatus.
2. Photocopy three “Final Group Design” graph paper worksheets for each group on 8½" x 14" paper. These views will be to scale. One square is equal to 1 cm x 1 cm.
3. Photocopy three copies of the Individual Design Worksheet (8½" x 11") for each student. These views will not be to scale.
4. Count out 15 pieces of balsa wood and 1 bottle of glue for each group.
5. Have newspaper, pliers, scissors, waxed paper and rulers available for each group on each construction day.

Safety Precautions

Students should wear protective eyewear and practice lab safety when working with scissors and other tools. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory.

Lab Hints

• Enough materials are provided in this kit for 30 students working in groups of 5 or for 6 student groups. The prelaboratory assignment may be completed before coming to lab. Plan 4–5 class periods for group bridge scale design and construction and 1–2 class periods for testing and data collection.
• Encourage and remind students not to put the pins into bridge parts with glue. The pins often stick and damage the bridge when students try to remove them.
• Have pliers available for groups to use to remove straight pins from bridges, especially when stuck in glue. If pins are twisted prior to pulling, less damage may occur to the bridge.
• Giving students completion deadlines will help keep the project on schedule. For example:
  • By the end of the first day, students should have their group scale designs drawn in ink and approved by the instructor.
  • By the end of day two, groups should have their side views built and drying.
• Waxed paper is placed over the final group designs to protect them while building. The final group designs are used as blueprints.

Teacher Tips

• Students may want to research different types of bridge designs prior to designing their models.

• An interesting historical connection of bridge design and effects of weather, available on internet videos, is the destruction of the Tacoma Narrows Bridge in Washington when subjected to 40 mph winds.
• Students can calculate the efficiency of their bridge by measuring the weight of their bridge prior to testing. Here is the equation to calculate efficiency.
  {14068_Tips_Equation_1}

• Allow students to check that the machine bolt and nut will fit through the sides of their bridge and that the wire will slide through their roadway and hang directly below the roadway during the construction process. If the setup cannot be placed on the roadway, the bridge will be disqualified. If modifications are quickly made to fit the setup, the bridge may be damaged beyond repair.
• Using gram weights at 100-g increments during testing offers more accuracy than other non-standard weight alternatives.

Answers to Prelab Questions

1. How do tension and compression compromise the structural integrity of a bridge?

   Tension is a pulling force that if left unchecked can cause a bridge to snap, destroying or damaging it. Compression is a pushing force that if left unchecked can causes a bridge to buckle, also destroying or damaging it.

2. What is the primary objective of piers, abutments and anchorages?

   Piers, abutments and anchorages are to support the weight, dissipate forces and serve as attachment points for the bridge.

3. Preview the procedure and predict which type of bridge will be the most successful in holding the largest load.

   Student answers will vary. A beam bridge with a through truss tends to hold the most weight for this competition.

Answers to Questions

1. Sketch your group’s bridge and label where tension and compression occurred.

   Students answers will vary and the location of tension and compression are dependent on the type of bridge drawn.

2. According to the class data, which type of bridge design seemed best for this competition?

   Student answers will vary. The best design for this competition would be a beam bridge with a through truss. However, other bridges often perform well.

3. Describe specifically what occurred with your bridge during the competition. Identify the strengths and weaknesses of your bridge using supporting data evidence.

   Student answers will vary but must include weight supported.

4. Where did your bridge fail? What could you have done to improve the strength of your bridge?

   Student answers will vary. Often bridges have poorly constructed piers, abutments or anchorages that lead to “unlevel” bridges. When the bridge is uneven or “unlevel,” weight is improperly distributed or dissipated which causes pressure to build up resulting in buckling or snapping.

5. Look at the following scenarios and determine which type of bridge should be used at each obstacle.
   1. A bridge is required to connect two sections of a bike path that are separated by a stream. The bridge will need to span a distance of 50 meters and will be used primarily by bicyclists, joggers and rollerbladers.

      Beam bridge

   2. A bridge is required that can span 150 meters in order for railroad tracks to cross a deep river. It will connect two rocky land areas separated by the river.

      Arch bridge/em>

   3. A bridge is required to span a distance of 1,525 meters across a bay. The bridge must be able to accommodate heavy motor vehicle traffic into and out of the city and water traffic must be able to pass under it as the city is a major port/harbor.

      Suspension bridge

References

How Stuff Works. http://science.howstuffworks.com/engineering/civil/bridge.htm. Accessed August 2015.

PBS. http://www.pbs.org/wgbh/buildingbig/bridge/basics.html. Accessed August 2015.

Introduction

How many bridges would you estimate are in the United States? A thousand? Ten thousand? Actually, over half a million bridges are used every day to cross obstacles, such as rivers, railroad tracks or valleys. Learn more about the forces that act on bridges and then design a bridge and test its load capacity.

Concepts

• Engineering
• Forces
• Model building

Background

Civil engineers are responsible for designing and supervising large construction projects, including roads, bridges and dams. They work with a team of other professionals to determine the feasibility of plans related to the cost and safety of the designs. They are primarily responsible for the basic framework and the implementation of strong, safe and long-lasting structures. When building bridges, civil engineers must take the following into account:

• The bridge span, which is the distance between two bridge supports
• The materials available and associated costs
• The load or weight distributed throughout the structure
• Strength of the riverbed or earth below
• Effects of tides or currents on the supports, which are the physical braces that connect the bridge to the surface below
• Effects of weather conditions
• Time required to build
• Forces that will act on the bridge

Two primary forces that engineers must take into account when building bridges are tension and compression. Tension is a pulling force and can cause a bridge to snap or break if not carefully monitored or accounted for. Compression acts opposite of tension and is a pushing force. If compression puts too much stress onto the bridge, it will buckle (see Figure 1). Other, less common but equally devastating forces that act on bridges include torsion, shear stress and resonance. Torsion specifically affects suspension bridges and causes the roadway to rotate and twist like a rolling wave. This is typically due to high winds. Shear stress can rip a bridge apart when two fastened structures are forced in opposite directions. Finally, resonance occurs when unchecked vibrations increase and travel through the bridge in the form of destructive waves.

Three of the most common bridges are beam bridges, arch bridges and suspension bridges. Each bridge has its own unique characteristics, strengths and weaknesses. A bridge made as the result of a combination of these basic bridge types is called a composite bridge. A beam bridge has a simple design—a horizontal beam with piers at each end (see Figure 2). Rarely will they span over 60–75 meters due to tension and compression. The piers support the downward force of gravity, load and weight of the bridge itself. The further apart the piers, the weaker the bridge is and more likely to succumb to buckling or snapping. Engineers have developed methods to dissipate or spread out the forces. By increasing the height of the beam or adding latticework (a truss system of triangles), tension is dissipated strengthening the bridge.

Arch bridges are some of the oldest bridges, designed by ancient Romans over 2,000 years ago. These bridges will span distances of 60–245 meters. The design allows the forces to act against themselves creating a very stable, long lasting bridge (see Figure 3). By design, the forces are spread outward by the curvature and are concentrated on the abutments—supports for the arch bridge. Tension is not a major concern with an arch bridge unless the curvature is extended too far (i.e., the arch is too large). The greatest challenge to building an arch bridge is maintaining structural integrity during the building process. Until the two converging sides join in the middle, the bridge is very unstable.

The suspension bridge has the ability to span vast distances of 600–2,100 meters. This design suspends the roadway from steel cables that drape over two towers and are ultimately secured to anchorages. A deck truss is built beneath the roadway to avoid torsion. The load on the roadway is transferred through the steel cables via tension force and then toward the towers and the anchorages and lastly to the ground (see Figure 4).

Experiment Overview

The purpose of this activity is to design and build a bridge that will be tested for its ability to carry a load. Small weights will be added to the bridge until the bridge breaks.

Materials

Prelab Questions

1. How do tension and compression compromise the structural integrity of a bridge?
2. What is the primary objective of piers, abutments and anchorages?
3. Preview the procedure and predict which type of bridge will be the most successful in holding the largest load.

Safety Precautions

Wear protective eyewear while cutting balsa wood and testing the bridges. Special precaution should be taken when cutting the wood with scissors or removing pins with the use of pliers. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Part A. Bridge Specifications

1. The final bridge may only be constructed from the balsa wood and wood glue provided. All pins must be removed prior to competition.
2. Bridge dimensions:
   {14068_Procedure_Table_1}

   *The underhang is included in the total bridge height of 10 cm (do not build taller than 10 cm).

3. There must be a clearance of 5 cm above the roadbed to account for traffic/vehicles.
4. The wood cannot be altered in any way to change its strength or appearance.
5. The wood cannot be split or laminated (see Figure 5).
6. Mitering, notching and butting are all permitted (see Figure 6).
   {14068_Procedure_Figure_5and6}
7. Parallel pieces must be at least 1 mm apart.
8. A machine bolt must be able to slide across your bridge and a metal wire must be able to hang down through your road (see Figure 7).
   {14068_Procedure_Figure_7}

1. On the Individual Design Worksheet, design a model of the bridge you believe will be able to hold the largest load.
   1. Draw your bridge design from three different views: side view, end view and top (bird’s eye) view.
   2. Label the view in the space provided above the graph on each sheet.
2. You will be placed into groups and as a team, discuss each of your designs and determine a final bridge model to be built following the bridge specifications from Part A.
3. Create final group scale drawings, in ink, on the Final Group Design graph paper. These will serve as the blueprints for construction.
4. The following final group scale drawings are required and must be approved by your instructor:
   1. Side view
   2. End view
   3. Top view
5. Construction may begin when:
   1. The desk/lab station is covered with newspaper.
   2. Two foam bases have been obtained from the instructor.
   3. The approved scale drawings are placed onto the foam base and covered with waxed paper.
6. Begin constructing your bridge. Use pins to hold the wood pieces in place while the glue dries. Take care to not place any pins in the glue.

1. Place your bridge on the designated platform. The instructor will measure the dimensions of your bridge to ensure all bridge specifications were met. Record dimensions in Data Table A on the Building Bridges Worksheet.
2. Place the machine bolt across the road in the center of your bridge (see Figure 7).
3. Attach the wire to the machine bolt so it hangs down from the road of your bridge (see Figure 7).
4. Attach a bucket to the carabiner at the end of the wire (see Figure 7).
5. Begin placing weight into the bucket as designated by the instructor.
6. Continue adding weight to the bucket until the bridge snaps or buckles and can no longer hold any more weight.
7. Determine the total amount of weight held by the bridge prior to breaking and record in Data Table A.
8. Clean up bridge pieces.

Student Worksheet PDF

14068_Student1.pdf