1. Hold a piece of string or thread by grasping one end in each hand, and ask each student to do the same. Demonstrate that the string offers little resistance when you try to bring your hands (and the ends of the string) together. Then show that the string strongly resists your attempts to pull your hands (and the ends) apart. Explain that the resistant force they feel when pulling the string taut is called tension. Tell them that in the following activities they will use strings whenever the instructions call for a tension member.

2. Hand out copies of the diagram Constructing a Compression Member (PDF) and ask teams of two to follow the instructions as they construct their own compression member. (Adjust lengths as necessary if your straws are not 7 3/4 inches long.)

When students have finished constructing their compression members, hold up your model and demonstrate its characteristics. Show students that, in contrast to the tension member, the compression member strongly resists your efforts to push your hands together, yet simply comes apart when you pull on the ends. Explain that the resistance force they feel when pushing on the ends is called compression. Tell students that for this lesson, all compression members must be constructed this way. (For longer beams, they can lengthen the straw and skewer sections; for shorter beams, they should use two of the three straw sections and one skewer section.)

3. Hand out copies of the diagram Constructing a Beam Bridge (PDF) and ask teams to follow the instructions to build their own beam bridge. This bridge should be constructed using 1/2-by-1-inch cardboard rectangles. When students are finished constructing the bridge, have one team member hold it upright with the bridge?s ends supported by books. Have the other team member test the bridge's strength and stiffness by pressing down lightly in the center. Alternatively, have students test their bridge with a load bucket (see Helpful Hints). After testing, have teams construct two additional beam bridges with the larger-size cardboard rectangles and test them, too.

4. After all three beam bridges have been tested, have teams record their observations. Lead a discussion about construction successes and failures and what affect the size of the cardboard rectangles had on beam strength and stiffness.

5. Show students the Clifton Suspension Bridge video.

6. Hand out copies of the diagrams of the Clifton Suspension Bridge Model (PDF) and Attaching Compression Members and Tension Members (PDF). Working in pairs, have students study the drawings and sketch a construction plan before they begin building. Their plan should clearly identify tension and compression members. Explain to students that for the purposes of this activity, the two crosspieces between the towers are non-load-bearing and can each be made of a single piece of soda straw. Also suggest that they use a piece of cardboard for their bridge's roadbed. Have students construct their model, carefully following their sketch and construction plan.

7. When construction is complete, have students test their bridge quantitatively using a load bucket. Ask them to compare their original sketch to their working finished model and record their conclusions about what worked and what had to be changed.

8. Have students watch the Clifton Suspension Bridge video again and discuss any new observations they might have. Point out that the real Clifton Suspension Bridge has cables that descend from the tops of the towers to the cliff faces on both sides of the gorge. Ask students:

• What parts of your bridge work in the same way?
• How is your bridge different from the real bridge?
• How could you make your bridge stronger?

9. Show students the Citigroup Skyscraper Design Problem video and discuss any observations they have.

10. Hand out the Inverted Triangle (PDF) diagram and explain that this shape is the chief structural component of the Citigroup Tower. Based on this diagram, ask students to sketch a construction diagram incorporating compression members of the same dimensions as those constructed in Part I. If students have trouble determining which part should be which type of member, distribute the Constructing an Inverted Triangle (PDF) diagram.

11. Now have students construct an inverted triangle based on their construction diagram. When finished, have students test their construction by pressing down lightly on the ends. After refining their design and recording their conclusions, have students build three additional triangles identical to the first.

12. Hand out the Citigroup Support Unit (PDF) diagram. Have students mount their four inverted triangles on top of a one-square-foot piece of cardboard. When they?re finished, have students test their single supporting units by placing a flat object (like a notebook) on top of the inverted triangles. Have students make any necessary refinements to their design or construction and record their conclusions.

13. For a final test, stack single supporting units one on top of another to replicate the Citigroup Building. (Recommend that students place the most solidly constructed units at the bottom.) Test the skyscraper by stacking notebooks on top.