Teaching Elements and Principles of Bridge Design - International ...

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TEACHING ELEMENTS AND PRINCIPLES OF BRIDGE DESIGN FEATURE ARTICLE Charles Beck Bridge construction is a popular classroom activity. However, the basic principles of tension, compression, and counterbalance are not always clearly represented and defined. The common materials used to construct model bridges, such as straws, toothpicks, Legos, and building blocks, are often too flexible or stationary to demonstrate the principles that keep bridges upright and functional. If the basic principles are not clearly demonstrated, students will fail to understand how bridges are able to support heavy loads. Upper elementary and middle school students can design simple models that demonstrate the essential elements and basic principles of bridge design. ITEA Standards Designing model bridges addresses several of ITEA’s Standards for Technological Literacy (ITEA, 2000/2002). For example, by identifying basic elements and principles of bridge design, students develop an understanding of the “attributes of design” (Standard 8) and “engineering design” (Standard 9). Students “select and use constructive technologies” (Standard 20) when they design model bridges and test their ability to support loads. Most states have Instructional Frameworks or Standards that require classroom teachers to expose their students to the basic physical principles of engineering and technology. Based on the By designing model bridges, students can begin to appreciate how engineers construct elements that produce tremendous force, such as arches and cables. Figure 1: Arch Bridge Construction Principles and Elements constructivist theory to learning, students need to design models to understand these principles. Materials The drawings illustrate two common types of bridges—the arch and the suspension. The arch bridge (see Figure 1) may be constructed from Styrofoam or wooden wedges. Paper clips or cotter pins are used to secure the abutments. To prevent the wedges from wobbling, they should be about two inches in thickness. The suspension bridge (see Figure 2) is constructed from balsa for the towers, deck, and anchorages. The illustration shows short side decks and cable spans. The students may want to lengthen the side decks and their cable spans to resemble an actual suspension bridge. The cables and suspenders are made from paper clips linked together. In order to show tension, the deck should only be 1/16” to 1/8” in thickness. The tower framework is about 1/4” in thickness. The paper clips or pins must pass through the abutments and anchorages and into the baseboard. Students may use glue or paper clips to secure the tower cross beams to the uprights and the foundations to the baseboard. It should not be difficult to push paper clips through balsa. These materials are available in most arts and crafts supply stores. 6 April 2005 • THE TECHNOLOGY TEACHER
FEATURE ARTICLE Figure 2: Suspension Bridge Construction Principles and Elements Students should be encouraged to experiment and problem-solve with the elements. For example, they should experience what happens when loads, such as toy cars or metal weights, are placed on the bridges, and the paper clips are removed from the abutments and anchorages. They will observe how the wedges collapse from the outward compression, and how the towers lean and the deck sags from the tension. If the deck is cut in half, the tension on the cables and towers will become more visible when a load is placed on the deck. The suspension bridge illustration shows how each half of the deck is supported by a pair of cables. Bridge Principles The arch bridge must counterbalance a great deal of compression, whereas the suspension bridge must counterbalance a tremendous amount of tension. Figures 1 and 2 illustrate how the arch and suspension bridges are subject to the following three principles: • Tension—pulling force that tends to stretch an element. In a suspension bridge, the center span deck is pulling downward on the center span cables. • Compression—pushing force that tends to shorten an element. In an arch bridge, one wedge presses the wedge next to it. The abutments must withstand the outward compression or force of all the wedges. In a suspension bridge, the cables press down on the towers. • Counterbalance—connecting elements that provide opposing forces. In the arch bridge, the abutments provide a counterbalance to the outward compression of the wedges. In the suspension bridge, the anchorages provide a counterbalance to the tension pulling on the cables. Bridge Elements The three main elements for the arch bridge include: • Wedges—tapered elements that interlock and compress each other • Abutments—ground level supports to withhold outward compression • Scaffolds—temporary supports for positioning and holding wedges The six main elements for constructing the suspension bridge include: • Cables—staying elements to support the deck and load • Towers—vertical elements for hanging cables • Suspenders—elements for linking the cable to the deck • Anchorages—elements to counterbalance cable tension • Foundations—elements used to hold towers in position • Deck—surface element for load crossing Bridge Construction Given a bridge illustration with measurements, students should be able to construct a model similar to the drawing. Students should be able to cut the materials safely using a coping saw. Construction becomes a cooperative learning experience if the students work in small groups of three or four members, with each member assigned a particular bridge element. The illustration of the arch bridge suggests 12 wedges. The wedges can be colored to represent two adjacent spectrums or rainbows placed end-toend. The two wedges at the top should be red, with one orange wedge on each side, followed by yellow, green, blue and violet at the base. Two or three students working together should be able to position the wedges. The students should try constructing the arch bridge with and without a scaffold. If the scaffold fits the bottom of the arch, it should help to hold the wedges in place until the arch is completed. Learning Assessment Student learning may be assessed based on three skill categories: cooperative, psychomotor, and cognitive. For cooperative skills assessment, the teacher may use an observation sheet to record how well the team members work together and problem-solve to construct a standing bridge. In addition to the teacher’s observation of psychomotor skills, the students should evaluate each team’s bridge construction based on a point system. Points are awarded based on how well each element fulfills its purpose, and how well the bridge demonstrates each principle. In regard to cognitive skills, the following set of questions will help to assess the students’ understanding of THE TECHNOLOGY TEACHER • April 2005 7
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