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.

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.

Installation of pre-fabricated concrete structures for the state highway 36 bridge over Lake Belton, TX. click for copyright

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].)

Construction of Carquinez Bridge, Crockett, CA. click for copyright

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.

Before the Activity

With the Students

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.

Notice the steel mesh that that will be used to reinforce the concrete slab about to be poured where the men are standing. click for copyright

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

Ralls, Mary Lou. The Future is Now… Successes in Bridge Construction. Presented January 12, 2005, TRB 2005, Session 628. Highways for Life, Federal Highway Administration, US Department of Transportation. Accessed October 16, 2007. (Photos and information about bridges under construction, the movement of structural members to the site, and efforts to reduce construction impact on traffic and the environment) http://www.fhwa.dot.gov/hfl/ralls.cfm