Bridges must be built strong enough to safely support their own weight as well as the weight of the people and vehicles that pass over it. The bridge must withstand earthquakes, weathering, strong winds, and freezing and thawing. When determining what type of bridge to build, engineers evaluate terrain, length of the needed span, cost, aesthetics, available materials and technical expertise. Bridges change as our needs and resources change. Engineers are constantly looking for ways to improve bridge designs and materials. During the recent California earthquakes, engineers analyzed overpasses to find ways to make them stronger. A new material that is very strong has glass fibers imbedded in concrete.
Class activities include first completing worksheets checking for understanding and identification of the bridge types. Then hands on activities using guiding handouts and virtual labs on the internet will enable the students to understand and discuss the forces, loads materials and shapes in building bridges.
Pictures of each type of bridge will be cut, pasted and labeled to pages in the students' bridge portfolio. Websites contain a multitude of design examples of each category.
Students will research to compare and contrast various bridges. Dimensions will be recorded on a worksheet table. Lengths of several objects and bridges will be graphed to visually show their differences. Students will write ratios of length to height of a bridge and the ratio that represents one length compared to another.
A NOVA program follows the building of the Clark Bridge in Illinois over the Mississippi River. The newest and best technology is used. There is also an interactive activity where students can test their engineering skills by matching the right bridge to the right location.
The students will investigate the strengths and weaknesses of each bridge type by experiencing the forces and strengths of designs and materials. Use the PBS Web Sites to develop basic bridge science and math knowledge and then show what is learned. PBS has a great introductory interactive lab for simplifying the real-life conditions that affect bridges. The forces lab shows the forces of squeezing (compression), stretching (tension), bending, sliding (shear), and twisting (torsion) and vector analysis.
The materials lab demonstrates and tests the strengths and weaknesses of several structural materials from wood to steel.
Its loads (forces that act on structures) lab illustrates dead, live, dynamic, wind, thermal, earthquake and settlement loads.
The shapes lab shows how different forces affect various geometric shapes. The same amount of material changes its strength and stiffness according to the shape it is.
To experiment, students will place a sheet of paper over two books standing up 8 inches from each other. You can also use file cards and the place the books closer accordingly.
1. Place the paper on top and see how many pennies it can hold. Record your results.
2. Arch the paper so it rests on the surface the two books are on.
3. Arch the paper on top of the books and test it with pennies.
4. Crease the paper in the middle and repeat steps 2 and 3. Test again with a load of pennies.
5. Fold the sheet back and forth to make accordion pleats. Count the amount of pennies after spreading them evenly over the area and again with the pennies loaded in one area.
6. Glue another sheet on the top of the pleats and one on the bottom. Test again. Remember to record and analyze the results.
7. Combine steps 3 and 6, the principles of strength through curvature and also creasing.
8. Finally, an even stronger solution is the creased barrel. A sheet is folded up along diagonal lines and the accordion pleat lines are folded down to form an arched accordion-like form.
Another hands-on activity, "How much can I take?" will teach about forces. Gather at least 6 different materials to test for tension (pull on both ends), compression (push together on both ends) and torsion (twist the two ends in different directions). The students will make up a table to record results, making up a rating scale from 1(very weak/crumbles easily) to 5 (super strong/does not break).
How does shape affect how well a material performs? To test the rigidity of shapes, a quick activity compares the stability of a square to a triangle. Using seven equal size straws or straw pieces and 14 small paper clips, construct the shapes, inserting a paper clip into each end and hooking them together.
Stand each shape up and push down on top. Journal how each bends and twists. Lastly add straws to the unstable shape to make it stable. The solution is to attach diagonals in the square to make rigid triangles. This is the principle behind trusses and also a great geometric concept. To expand on this see the activity included in this unit that stabilizes polygons up to hexagons. Also students can make the most rigid structure possible with 20 straws and 49 paper clips. Another team building activity is to have teams construct these in complete silence and trust their signal to each other.
The West Point Bridge Designer 2006 software is an excellent way to introduce the students to engineering through an authentic, hands-on design experience of modeling, testing and optimizing a steel highway bridge.
http://ww.matsuo-bridge.co.jp/english/ is a site profiling an innovative Japanese bridge company, photos of its bridges and descriptions of bridge construction methods.
http://www.havenet.combridgink/ has a Bridge of the Month Quiz and many photos.