Hands-on Activity Cost Comparisons

Learning Objectives

After this activity, students should be able to:

• List several factors that influence the cost of a bridge.
• Estimate the costs of model bridge project.
• Describe the importance for engineers of balancing the cost of the bridge with the design of the bridge.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

Common Core State Standards - Math

International Technology and Engineering Educators Association - Technology

State Standards

Materials List

Each group needs:

• pencil and eraser
• calculator
• Cost Comparisons Worksheet
• Bridge Proposal Worksheet

Worksheets and Attachments

Pre-Req Knowledge

Students should have some previous knowledge of bridges, including a familiarity with bridge types and parts of a bridge, as introduced in other lessons in the Bridges unit.

Introduction/Motivation

Every community has bridges and the stories behind how these bridges get built reveal the creative ways humans come together to accomplish big projects. Who pays for bridges? Do we care how much they cost? When we make plans for a big project, considering costs is one important aspect that engineers consider, but since we're all paying for these bridges, it is important to all of us, too. So today, just like engineers, let's discuss the factors that influence cost, what goes into calculating the total cost of a bridge, and the value of optimizing the cost and design of the bridge.

Several factors influence bridge cost. What comes to mind? (Write student suggestions on the board, before moving on.) First (and sometimes foremost) is the type and amount of material used. With everything else being equal, a bridge with members that are half the size of another bridge is less expensive. Another important factor is the construction labor. In regions where the cost of labor is very expensive, this can become the most important factor in a bridge's cost. In this case, keeping labor costs down by using fewer bridge members can be cost effective. However, using fewer members may require them to be larger, which may affect the safety of the bridge design. More and more, the large steel and concrete members are being pre-fabricated off-site where labor costs are more affordable, and then the bridge is constructed on-site more quickly, with less disruption to the community.

What other factors might influence the total cost of creating a bridge? (Possible answers: Cost to transport materials [trucks, ferries], construction equipment [such as cranes], environmental and site studies, traffic re-routing, administrative expenses [engineers to design the bridge and oversee its construction].)

As you can imagine, the number of expenses that go into creating a real bridge makes a long list. For today's activity, we are going to simplify the process and consider only a few of the many expenses associated with the total cost of building a bridge, enough to do a general cost analysis.

First, let's consider the cost of the material used. How do you think we figure out how much material we are using? Calculating the volume of all of the parts of the bridge gives the total amount of material in the bridge. The cost of the material is usually calculated by its amount or weight. Then, we multiply the amount of material volume by the cost of material per volume to get the entire cost of the material. The cost of labor and equipment are also both major cost components to include. A site investigation must also be included as part of determining the final design. Finally, administrative expenses must be included, too.

Do we care how much bridges cost? Why not build them all the same? Why not build them all to accomplish the same purpose? Would we want that? Probably not. So, every bridge is customized (unique) for its specific purpose and location. Once engineers figure out the known costs for research, materials, labor, equipment, transportation and administration, they must optimize or find the best design to fit the given budget. Optimizing the design produces the most cost-efficient and effective design.

Today, the city of Needabridge issued a request for proposals to create a new transportation bridge connecting their city to another city across the river. Working in engineering teams, we will prepare a proposed budget for building their bridge. We will estimate the amount of materials required to make a bridge member (a girder or beam) out of concrete and again out of steel. From this, we'll calculate the costs to make it for different girder sizes. We will graph the results and prepare a proposal for Needabridge city planners to pitch our design to be the one that is chosen for their new bridge.

Procedure

Before the Activity

• Make copies of the Cost Comparisons Worksheet and Bridge Proposal Worksheet, one per team.
• Make sure each student has a pencil, eraser and calculator.
• Divide the class into teams of two students each.

With the Students

1. Before handing out the worksheets, review with students the key factors that influence the total cost of constructing a bridge. Explain that today they will be solving for the cost of a concrete and steel girder used in a bridge. For two different materials, they will determine how the size of the girder changes the cost of the girder. They will use this information to create a mock budget proposal for the town of Needabridge.
2. Hand out the Cost Comparison Worksheets to student teams. Have them begin by calculating the volume of material for each type of beam.
3. Next, have students complete the charts by filling in the cost per beam of concrete or steel.
4. Next, have students graph their results.
5. Engage the class in a discussion to compare worksheet results. Compare the different costs of the girder and how the cost changes depending on its material and size.
6. Finally, have student teams complete the Bridge Proposal Worksheet. This includes creating an engineering firm name, sketching their bridge design, estimating the cost of materials, equipment and labor for the project, and explaining why their design should be chosen.
7. If time permits, have student teams present their proposals to the class.

Vocabulary/Definitions

budget: The amount of money available for a project, such as creating a bridge.

cost: The amount of money needed to build a project, such as a bridge.

design: (verb) To plan out in systematic, often graphic form. To create for a particular purpose or effect. Design a bridge. (noun) A well thought-out plan.

engineer: A person who applies her/his understanding of science and mathematics to creating things for the benefit of humanity and our world.

engineering: Applying scientific and mathematical principles to practical ends such as the design, manufacture and operation of efficient and economical structures, machines, processes and systems.

estimating: An accurate determination of the cost of the project (such as a bridge) before construction begins.

girder: The "beam" of a bridge; usually a horizontal member.

member: An individual angle, beam, column, plate or built piece intended to become an integral part of an assembled frame or structure.

optimal design: Balancing the cost and design of the bridge so that the least amount of money accomplishes the required task effectively.

pier: The "column" of a bridge; usually a vertical member.

Assessment

Pre-Activity Assessment

Discussion Questions: Ask students to estimate the cost of concrete per unit volume (such as cubic yard or cubic meter). Write their guesses on the board. Ask them to estimate the cost of steel per unit volume (such as per ton). Write their estimates on the board. Give the correct answers (concrete = $65 per cubic yard; steel = $2,000 per ton).

Activity Embedded Assessment

Worksheet: Have students use the attached Cost Comparisons Worksheet to complete the activity. Review their answers to gauge their mastery of the subject.

Discussion Questions: As a class, compare worksheet results. Discuss and compare the different costs of the girders. How did the cost change depending on the material and size? (Answer: Costs are more expensive with steel than concrete. Costs increase with the length of the girder. Sometimes engineers combine steel and concrete (reinforced concrete) to make girders that are strong like steel, but cost less. Engineers also design girders that are not solid rectangular beams, such as I-shaped beams and trusses that use less material and weigh less.)

Post-Activity Assessment

Engineering Proposal: Direct student teams to complete the attached Bridge Proposal Worksheet. If time permits, have each team present their proposals to the class. Have the class act as the city planning commission and vote on the designs, and have the teacher be the mayor of Needabridge, asking questions about the design ideas.

Troubleshooting Tips

If students have difficulty with the worksheet calculations, provide an example problem on the board for the entire class, or pair stronger students with those needing assistance.

Activity Extensions

Have students estimate the cost of a classmate's bridge design and compare the differences in their calculations in the final cost, if any.

Have students model bridge costs for a variety of bridge scenarios. How would the amount of materials change in each scenario? Would the girders, piers and foundations be different? Examples might include:

• A railway bridge across a deep ravine, 300 feet across
• A pedestrian / bicycle bridge over a highway
• A pedestrian bridge over a pond at a local park

Activity Scaling

• For lower grades, complete the worksheet together, as a class.
• For upper grades, have each student research the cost of labor for the use of steel and concrete. Have them determine what percentage of the cost used in the worksheet is for materials cost and what percentage of the cost is for labor cost.

References

Copyright

Contributors

Supporting Program

Acknowledgements

The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0338326. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.