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# 12. Students Will Create a Bridge That Supports The Most Amount of Net Weight

## The Bridge Challenge

3-PS2-1 Motion and Stability: Forces and Interactions

Depending on the grade level and depth of understanding required, certain CCSS Math standards about equations are applicable.

Standards for Mathematical Practice
CCSS.MATH.PRACTICE.MP1: Make sense of problems and persevere in solving them.
CCSS.MATH.PRACTICE.MP2: Reason abstractly and quantitatively.
CCSS.MATH.PRACTICE.MP3: Construct viable arguments and critique the reasoning of others.
CCSS.MATH.PRACTICE.MP4: Model with mathematics.
CCSS.MATH.PRACTICE.MP5: Use appropriate tools strategically.
CCSS.MATH.PRACTICE.MP6: Attend to precision.
CCSS.MATH.PRACTICE.MP7: Look for and make use of structure.

Learning Objectives

• Students will create a bridge that supports the most amount of net weight.
• Students will create a bridge that uses no more than 50 grams of filament.
• Students will create a bridge that spans a minimum of 20 cm.
• Students will incorporate properties of bridge design to generate a sustainable structure.
• Students will design a bridge based on models discussed in class and researched within groups.
• Students will design a bridge that allows for at least one figurine (LEGO mini, action figure, doll, etc.) to safely travel from one side to another.

Recommendations
Group Size: 3 to 4 students, depending on how many variations you would like students to compare within the class setting

Class Size: up to 36 students

Materials Required

• Paper and pencil for drafting
• Airwolf 3D Printer
• Pictures of bridges from around the world (see below for reference)
• String, a bucket, weights, and a scale for testing purposes
• Ruler to verify dimensions

Introduction

• Students have a good understanding of 3D modeling. Prior to incorporating this lesson into a unit, it is recommended that students have had training on Google SketchUp.
• Students have a solid understanding of scale factor.
• Students have an understanding of forces.

Video: Creating a 3D Model

Video: Creating a Basic Bridge

Anything that students build needs to be supported. Therefore, some of the more popular bridges (suspension, truss, etc.) are not easily designed on a 3D printer. To ensure that students’ designs will be printed, check to verify that all unsupported structures are designed at an angle less than or equal to 45 degrees.

To introduce the class to the project, put two desks 20 centimeters apart and inform the class that they will be responsible for creating a unique bridge that can support the maximum weight as tested during class. Their product must use less than 50 grams of filament (it is important to note that there should be no way of the bridge touching anything between the two desks during the testing process).

The Challenge
Create a unique bridge that supports the most weight in the class.

The Meat
On paper, students sketch a bridge that will print appropriately. See recommendations above for more details. Have students search for various bridges to draw on inspiration of others. If students google “crazy bridge designs”, the interest should spike immediately. It will be important to remind students that their product must be printable, meaning that all pieces must be built on each other and at less than or equal to 45 degree changes.

Dimensions need to be designated on the drawing and appropriate justification as to why the group feels like their design will support the most weight must be included in their preliminary draft.

Using a 3D modeling program such as Google Sketchup, students create an object that represents their sketch in their notebook. Prior to printing, measurements need to be verified by a partner, then by the instructor. Due to the constraints set for the project, there will (more than likely) be multiple attempts to have a product that is within the specifications. Once verified, student will send the product for printing. Within Matter Control, students will need to generate their layers to determine how much filament will be used, keeping it under the 50 g limit. To support this, direct students to the fill percentage that is being used, typically 0.35 (or 35%). Adjusting this will compensate for groups whose designs are over the limit.

If desired, to take this a step further, have students put together their drawings and Sketchup files as a bid for a bridge project that will be built within the students’ town/city.

To lower the cost and time of printing, making the walls of the bridge 1 layer would be effective.

After printing, have students use a ruler to measure the dimensions.

Discussion
Using a string tied around the center of the bridge and attached to a bucket, weight shall be added to each bridge until it reaches its breaking point. During this process, students are not allowed to touch or hold the bridge while weight is being added or a designated number of seconds thereafter. With a scale, students shall weigh the bucket as it gets heavier. To lower the cost and allow for more variability, using water to weigh down the bridge will be the most efficient way to run the tests. Have students find out the mass of 1 cup of water, adding 1 cup in between each round and marking the number of cups it takes for the bridge to reach its breaking point.

Throughout the testing, students will complete tables that represent the mass of filament used and the weight that it supports prior to reaching failure. A sample of this table is listed below for reference:

Mass of Filament Cups of Water Max Weight Hold
48.6 g 36 18.792 lbs
42.9 g 39 20.358 lbs
49.2 g 43 22.427 lbs

Based on their tables, have students graph the results and compare the relationship between the mass of the filament and the maximum weight that was supported.

Desired Outcomes
Due to the openness of the challenge, there are really no ideal desired outcomes, as long as the bridge meets the specifications laid out in the introduction and is capable of being printed without error. Due to the low quantity of filament in the instructions, it will be crucial for students to lightly fill their designs. Optimal fill will be somewhere in the 20% range.

Some Possible Extensions/Modifications
Increase the filament mass required or, if provided with the right amount of time and resources, lift the limit of filament. However, it is recommended that there is some restriction of the mass required for this project.

Allow for one center support to be built into the design, thus lowering the bridge to near-ground level. If this is a modification that is being applied, keep in mind to restrict the overall volume of the center support.

To account for the variability of student designs, it may be helpful to have a challenge to see who can get closest to exactly 50 grams (or whatever the limit is) while still supporting a certain amount of added weight.

If there are a small number of students in the class, printing time and material won’t be as much of an issue, so the specifications may be adjusted to account for more creativity and a higher level of support.

Possible Application
At this point, it would be of great significance to contact a local civil engineer and ask him/her to speak with the class about bridge design. Due to the fact that this is a model with ABS plastic, the direct translation to bridge design will not be linear. However, students will be able to hear from a professional in the field about what it actually takes to create something on paper and computers that will ensure the safety and stability of the people and environment around it. Contacting the American Society of Civil Engineers would be a great place to start.

Having students play the role of engineer and pitching their bid to a civil engineer or city planner would add the relevance and applicability to the project.

Questions to Ponder
Did the mass of the bridge guarantee the success or failure of its weight limit?

What, if any, correlations were there between the maximum weight supported and the mass of the filament used?

How was the design of the top 3 bridges more effective than the bottom 3?

What were the variables that were involved?

What would happen if you changed the displacement of force?

Content & Instruction Developed by:
John Stevens – Airwolf 3D STEM Consultant
Instructional Coach – Technology
Chaffey Joint Union High School District
CUE Rockstar Faculty & Organizer

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