March/April 2017 – Vol. 29 No. 6

President’s Pick for September: Metric Bridge Building

Posted: Thursday, September 1st, 2011

by Rick Pomeroy

Teacher Information Sheet

Overview

Bridge building has become a widely used activity in science classrooms for the past several years.  What has been missing in its use has been an emphasis on the science  and the process of inquiry (discovery learning) behind the project.  One of the most important aspects of the activity regardless of its place in the curriculum is the process students go through to solve the problem of spanning a gap of a specific length in a way that supports the most mass. The planning and testing (hypothesizing and experimentation) that students should do in preparation for constructing a bridge is what makes this a valuable science activity.  To do an adequate job and to incorporate the principles of scientific investigation, students should propose a bridge structure, test various aspects of their construction model and then have adequate time to complete their finished project.  For assessment, students should be able to explain their choices of design types and construction techniques based on the criteria given in the assignment, prior knowledge of bridge structures and the results of their testing and experimentation.  This is truly a problem to be solved and can be utilized as a challenge activity to reinforce or introduce concepts. critical thinking, use of measurement, etc.

The Metric Bridge Building activity attempts to combine the fun and motivation of  building a project for a class competition with a demonstration of student’s understanding of the metric system, forces, and how technology answers common human problems.  The instructions for this activity require students to understand simple metric measurements and use those to construct a bridge that meets minimum criteria.  Each of the metric units of measurement are incorporated into this activity (except temperature).  Students do not have to memorize various units or conversions nor do they have to convert from standard units to metric.  Instead, they must use metric measurements to complete the minimum requirements of the activity.

This activity can be written for different levels of student knowledge of metrics and science terms.  Examples of three different levels of criteria are included in this package.

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Performance Objectives

1. Given materials and class time, students will be able to use metric measurements to build a bridge meeting design criteria.

2. Students will be able to draw on personal experiences with bridges to design a structure that will support a 1 Kg mass over a 30 cm span.

3. Students will use metric mass units to measure and calculate the load capacity  and the load: mass ratio of their bridge

4. Students will use metric units to calculate volume and density of their bridges.

5. Students will predict the structure of their bridge and analyze their bridge structure in light of failure testing data and propose modifications which would make the bridge stronger.

Preparation

Prior to beginning this activity, the teacher should confirm the sizes and masses of locally available materials to prevent student confusion when supplies do not match those described.  The basic instructions are set up for 10 sheets of standard 8.5 x 11 inch 20# bond copy paper (57 ± 2 g) , 15 cm of 1 inch masking tape (1 ± .5 g),  20 cm of parcel string (1 ± .5 g) and  white glue or glue sticks. Note: A 1 inch puddle of white glue is about 2 ±.5 g when dried.  Limiting the amount of glue is a teacher’s choice but students who use large amounts of glue will find that the mass of their finished bridge is greater than the 63 g maximum mass allowed.  One sample bridge constructed under these criteria using three “servings” of glue in 3-4 g lots weighed in at 53 grams using seven sheets of paper.  Students should be encouraged to use limited amounts of glue since otherwise they will tend to build a bridge that is all glue and does not utilize the paper as a structural component

Timing

Given adequate planning time (often as homework), teams of students should be able to construct their bridges in two 50 minute class periods.  Plan ahead for room to store the partially completed bridges or bridge components to insure that they are not tampered with when the builders are not present.  Testing of the bridges, usually a high interest activity, typically takes about half of a period depending on the number of bridges. Testing should never be done on a day that students work on their bridges since allowing the glue to dry overnight greatly enhances its holding power where as a bridges tested before the glue dries may be over weight and weak despite a good design.

Assessment

As stated earlier, the value of the bridge building activity is in the thinking and inquiry that students do in preparing for and constructing their bridges.  The load carrying capacity of the bridge is secondary to the learning that goes on in this project and the teacher should provide ways for students to demonstrate what they have learned about forces or  the concept emphasized by the project.  A page of typical questions has been included which could assist in assessing what students know and or have learned by building their bridge.  One way to assess prior knowledge is to have students give a brief description of their bridge building plans prior to starting the project.  What do they think will work, what do they know about bridges that they might use in the construction of their project.  After their bridge is completed and tested they can revisit this “hypothesis” and evaluate it based on their results and those of other bridges in the class.


Metric Bridge Building

Your task is to build a bridge, given the materials described below, which can span a 30 cm gap between two tables while supporting a 1 Kg mass hanging from the center of the bridge span.  Based on what you know about bridges, meet with your group and discuss possible ways to make a strong bridge with these materials.  You might want to test some of your ideas before beginning construction of the actual bridge.  You may not use any materials other than those described.  Points will be awarded for completion of the bridge and completed bridges which can hold a minimum of 1 Kg.  Additional points will be awarded for  each additional 500 grams that the bridge holds upto 2500 grams.  Bonus points will be awarded for the bridge with the highest  load mass : bridge mass ratio (lightest bridge which holds the most mass).  You must complete the page two of this hand out to receive any points. Begin now by answering question #1 on the handout.

Materials- You may only use the following supplies

10 sheets of 21.5 cm (±2 mm) x 27.9 cm (±2 mm) paper
15 cm (±2 mm) x 25.4 mm(±2 mm) masking tape
200 mm kite string ( for hanging 1 Kg mass only)
White glue or glue sticks

Bridge Specifications

1. Your bridge must span a gap of 30 cm between two desks.
2. The size of your bridge must be 40 ±1 cm long x 2.5 – 10 cm wide x 2.5 – 10 cm high.
3. The mass of your bridge cannot exceed 63 g.
4. You must provide a place to hang a 1 Kg mass from the center of the bridge span. (You may use the kite string for this purpose).  The mass will be a plastic bucket suspended from an S-shaped hook with an interior diameter of 1.5 cm.

Points Rubric

Points              Points
possible          earned

1. Bridge complete but incapable of holding 1 Kg     5                 ______

2. #1 and bridge holds minimum of 1 Kg                      5                  ______

3. #1 & #2 plus 2 points  per 500 g over 1 Kg      ( Max 10)        ______

Bonus Points

4.  Bridge mass: load mass ratio       _______:_______

(5 points for the lowest ratio which meets #2 above) 5                 ______

Total Points                                                                                                  _____


Metric Bridge Building

Your task is to build a bridge, given the materials described below, which can span a 30 cm gap between two tables while supporting a 1 Kg mass hanging from the center of the bridge span.  Based on what you know about bridges, meet with your group and discuss possible ways to make a strong bridge with these materials.  You might want to test some of your ideas before beginning construction of the actual bridge.  You may not use any materials other than those described.  Points will be awarded for completion of the bridge and completed bridges which can hold a minimum of 1 Kg.  Additional points will be awarded for  each additional 500 grams that the bridge holds upto 2500 grams.  Bonus points will be awarded for the bridge with the highest  load mass : bridge mass ratio (lightest bridge which holds the most mass).  You must complete the page two of this hand out to receive any points. Begin now by answering question #1 on the handout.

Materials- You may only use the following supplies

10 sheets of 21.5 cm (±2 mm) x 27.9 cm (±2 mm) paper
15 cm (±2 mm) x 25.4 mm(±2 mm) masking tape
200 mm kite string ( for hanging 1 Kg mass only)
White glue or glue sticks

Bridge Specifications

1. Your bridge must span a gap of 30 cm between two desks.
2. The size of your bridge can have a volume, when calculated by multiplying the length x width x height of no more than 4 liters and no less than .25 liters.
3. The mass of your bridge can be no more than  63 g.
4. You must provide a place to hang a 1 Kg mass from the center of the bridge span. (You may use the kite string for this purpose).  The mass will be a plastic bucket suspended from an S-shaped hook with an interior diameter of 1.5 cm.

Points Rubric

Points              Points
possible          earned

1. Bridge complete but incapable of holding 1 Kg            5                 ______

2. #1 and bridge holds minimum of 1 Kg                             5                 ______

3. #1 & #2 plus 2 points  per 500 g over 1 Kg             (Max 10)        ______

Bonus Points

4.  Bridge mass: load mass ratio       _______:_______

(5 points for the lowest ratio which meets #2 above)   5                 ______

Total Points                                                                                                    _____


Name:(s) ____________________

Students must complete all sections of this sheet before any credit can be awarded.

Do # 1 before beginning work on your bridge.  Have teacher initial before beginning work.______

1. Draw a sketch of your bridge or describe what your bridge will look like.

2. How did you decide on the shape or structure of your bridge.

3. What tests did you do in planning your bridge to insure that it would hold the greatest amount of mass?

4. How does your chosen structure work to support the 1 Kg mass at the center of the span?

5. What is the mass of your bridge?  _________________

6. If you were to create a box with the dimensions of your bridge (length x width x height) what would the volume of the box be? Use metric units ONLY

Length: _______ x Width: _________ x Height: _______ = Volume ________

After testing your bridge

7. Why did your bridge ultimately fail?

8. Compare your bridge to one that held more or less mass than yours.  What are the differences between your two bridges and how might that explain the differences in carrying capacity?

9. How could you modify your bridge structure to prevent the failure described in #7?

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Written by Rick Pomeroy

Rick Pomeroy

Rick Pomeroy is science education lecturer/supervisor in the School of Education, University of California Davis.

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