Hands-on Activity: Straw Bridges
Educational Standards :
Learning Objectives (Return to Contents)
After this activity, students should be able to:
Materials List (Return to Contents)
Each group needs:
For the entire class to share:
To make the wooden support structure (see Figure 5; optional; may use two desks instead):
Finished dimensions of the wooden support structure (optional; may use two desks instead). Dimensions may vary from those below, but these particular dimensions can be made by using scrap 2 x 4s. The most important dimension is the inside length or span. The total length should allow for enough space to place the bridge on the "abutments."
Introduction/Motivation (Return to Contents)
After the Industrial Revolution, bridges became more and more sophisticated as iron and steel became more commonly available. By using iron and steel, engineers could design bridges capable of supporting larger loads and spanning greater distances, making it possible to link cities and communities through shorter, more direct routes and crossing obstacles such as waterways or other natural features that had previously blocked passage. Sometimes we take it for granted that bridges provide important links between places. They enable us to get to resources, conduct commerce, travel and visit other people. The design of bridges is important to the transportation networks we depend upon.
We know there are many different types of bridges. Who can name a type of bridge? (Answers include: Beam, truss, arch, suspension, and cable-stayed.) What makes a bridge a beam bridge? (Review these key points: A beam bridge is usually a simple structure made of horizontal, rigid beams. The beam ends rest on two piers or columns. The beam weight (and any other load) is supported by the columns or piers.) Where on a beam do the forces act? (Review these key points: Compressive forces act on the top portion of the beam and bridge deck, shortening these two elements. Tensile forces act on the bottom portion of the beam, stretching this element.)
Beam bridges are the most common type of bridges, and include truss bridges. Truss bridges distribute forces differently than other beam bridges and are often used for heavy car and railroad traffic. In a truss bridge, the beams are substituted by simple trusses, or triangular units, that use fewer materials and are simple to build.
Truss bridge construction rapidly developed during the Industrial Revolution; they were first made of wood, then of iron and finally of steel. During this time, different truss patterns also made great advances. Many truss systems originated in the mid-1800s are still in use today. The Howe Truss, one of the more popular designs, was patented by William Howe in 1840. His innovation was his use of vertical supports in addition to diagonal supports (see Figure 1). The combination of diagonal and vertical members created impressive strength over long spans; this made the truss design ideal for railroad bridges. Howe's truss was similar to the existing Kingpost truss pattern. However, he used iron for the vertical supports and wood for the diagonal supports. Although iron and wood are not used as much today in modern bridges, the Howe Truss pattern is still widely used. See Figures 2-4 for other truss patterns.
Today, we are going to act as teams of engineers making bridge models. We have been hired by a city to create a bridge to cross one of the local rivers. However, the city does not want the bridge to affect the fish population in the river below it. Engineers always consider their "design objective" when creating their models. Our design objective is to make a bridge that spans the river (scaled down to a distance of 10 inches [25 cm], supports the most weight for the cars that will pass over it, and does not disturb the river's fish. To simulate the load of the cars, our bridge must have a place to securely hold a small cup in the center of the span. To demonstrate environmental limitations on the design, no part of the bridge may touch the "water" (or bottom of the wooden support structure) and the bridge cannot be taped to the wooden support structure. Engineers often have many "design constraints" or limitations that are part of their job assignment. Today, our design constraints not only include the environmental and weigh constraints, but also limited budget and materials using straws and tape as our construction materials.
Vocabulary/Definitions (Return to Contents)
Procedure (Return to Contents)
Before the Activity
With the Students
Safety Issues (Return to Contents)
Troubleshooting Tips (Return to Contents)
Use plastic straws that are not the flexible or "bendy neck" type. If only flexible type straws are available, cut off the straw ends that contain the flexible sections. Since this reduces the straw length, give students 25 straws per group.
Using a balance to calculate the weight of the pennies in the cup is a quick method to determine how much weight each straw bridge held before it collapsed. If a balance is not available, count the number of pennies for weight comparison.
If rulers are not available, measure the span by marking its width on another piece of paper as a handy reference. Or, explain how students can obtain simple measurements using full sheets of copy paper (8 ½ x 11 inches). For example, with a 10-inch span, it would be desirable to make the bridge about 11 inches or equal to the longer dimension of the paper.
Assessment (Return to Contents)
Voting & Demo: Ask students to vote by a show of hands their opinion to the following question. Tally the votes and write the totals on the board.
Activity Embedded Assessment
Prediction: Before testing, ask teams to predict how much weight will collapse their bridges. Record predictions on the board.
Re-Engineering: Ask students how they might improve their bridge designs, and have them sketch or test their ideas.
Activity Extensions (Return to Contents)
Ask students if they know about the engineering design process. It is the design, build and test loop used by engineers around the world. The steps of the design process include: 1) Define the problem, 2) Come up with ideas (brainstorming), 3) Select the most promising design, 4) Communicate the design, 5) Create and test the design, and 6) Evaluate and revise the design. Have students reflect upon the bridge-making activity and list what they did for each step of the design process.
Truss patterns are used for more than bridge design. Ask students to note all the real-world applications in which they see truss systems used during one week. Possible examples: the structural members found in roofs (look up into your garage or basement), floors, ceilings and construction of other structures, plus ramps, radio towers, crane arms, and components of other types of bridges. Even a geodesic dome is considered a truss in the shape of a sphere. Can you see triangle geometry in the shape of a bicycle frame? Have students report back to class to share their findings.
Activity Scaling (Return to Contents)
References (Return to Contents)
Dictionary.com. Lexico Publishing Group, LLC. Accessed March 21, 2007. (Source of some vocabulary definitions, with some adaptation) http://www.dictionary.com
ContributorsJonathan S. Goode, Joe Friedrichsen, Natalie Mach, Chris Valenti, Denali Lander, Denise W. Carlson, Malinda Schaefer Zarske
Copyright© 2006 by Regents of the University of Colorado.
Supporting Program (Return to Contents)Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder
Acknowledgements (Return to Contents)
This digital library content was developed by the Integrated Teaching and Learning Program under National Science Foundation grant no. 0338326. However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.