Bridges, Building and Breaking

Goals:
Students will understand the forces that effect bridges.
Apply design and engineering methods.
Construct a scale model and destructively test its capacity.
Concepts include the effects of force on a structure and developing a model to scale.

Vocabulary:
-Load
-Forces
-Mass
-Scale model

Designed for 6th grade science classes, this lesson is easily adaptable to younger students or older ones. This activity is utilized as a stand-alone project designed to introduce engineering and reinforce and further develop modeling skills.

Objective:
o Design, build and test the load bearing capacity of a scale model bridge.

--Bridge Type-show pictures and review
o Trestle
o Beam
o Arch
o Suspension
--Forces at Work
o Gravity
o Normal force
o Strength of material used
--Model Specifications:
o 30 centimeters long
o 5 cm wide
o Bridge may not extend below roadway for last 5 cm on each side

Materials:

o waxed paper
o toothpicks
o white glue
o other materials as approved by instructor
o If you want to use something, ask!

Grading Criteria:
o Specifications within tolerances (0-10 points)
--Is it the right size?
o Load, mass held (0-20 points)
--20 pounds = 20 points
o Difficulty (0-10)
--How challenging was the design
o Aesthetics, how good does it look (0-10 points)
--Neatness, color and quality of work.
o Total (0-50 points)

--Internet Links :
o PBS Super Bridge site http://www.pbs.org/wgbh/nova/bridge/
o PBS Build big http://www.pbs.org/wgbh/buildingbig/bridge/index.html
o Video of bridge collapse http://www.youtube.com/watch?v=P0Fi1VcbpAI
o Minneapolis bridge collapse http://www.youtube.com/watch?v=osocGiofdvc
o High School Physics competition http://www.balsabridge.com/
o Kennesaw State University links http://webtech.kennesaw.edu/jcheek3/bridges.htm Powerpoint for introduction (PowerPoint 636kB Aug21 08)

Teachers Notes:
Provide examples: I usually require a scale drawing of the bridge prior to building but have tried it without drawings. I don't grade the drawing I check it off when it is complete enough to allow building. Two views of the bridge, side view and top view help students appreciate the three dimensional nature of this project. Note-make sure they take into account the thickness of the toothpick in their drawing.
Waxed paper works very well for building on, they can lay it over the drawing, then glue the toothpicks right to the waxed paper and when the glue is dry, peel it away. This allows them to make each side of the bridge and then connect them.
Timeline for this project is variable, if you want, you can spend more time on the building aspect.
I normally require the bridge to be built in class, this minimizes students getting too much help and transportation breakage issues.
I offer extra credit for torture testing of bridges, any bridge that survives 20 pounds can be retested until it reaches its breaking point. I also usually give a prize for the strongest bridge.
To test bridges I built a testing board, out of ½ inch plywood, that was the specifications for the bridge and could be placed between two tables. Weights are placed in a canvas bag hung from the bridge, a ruler can be used to hang the bag from if needed.

This lesson can be adapted for younger students easily by using craft sticks in place of toothpicks.

Students will have their bridge load tested as the final assessment.
Grading Criteria:
o Specifications within tolerances (0-10 points)
Is it the right size?
o Load, mass held (0-20 points)
20 pounds = 20 points
o Difficulty (0-10)
How challenging was the design
o Aesthetics, how good does it look (0-10 points)
Neatness, color and quality of work.
o Total (0-50 points)

GRADE 6
I. HISTORY AND NATURE OF SCIENCE
B. Scientific Inquiry: The student will understand that scientific inquiry is used in systematic ways to investigate the natural world.
II. PHYSICAL SCIENCE
E. Forces of Nature: The student will understand that a variety of forces govern the structure and motion of objects in the universe.