THE BRIDGE I

"DESIGN FOR FUNCTION"
INSTRUCTOR

OVERVIEW OF THIS PROJECT AND WHAT TO EXPECT

This is the first part of two projects; the projects should be done in order. This project entails designing, building, and testing a bridge. The bridges will be constructed by groups of three to four students. The students will learn that sometimes the specifications given are conflicting. In particular the strength and weight of a bridge are conflicting specifications. When this occurs the designer must often use creative solutions to solve the problem.

SPECIFIC PURPOSE OF THIS PROJECT

The purpose of this project is to give students a exciting way to learn how different materials have vastly different properties. Some materials work only in tension while some materials work best in compression. Also, materials don't have to be heavy to be strong; styrofoam is light and works well in compression. String is very light also, but it only works in tension. This project also, helps students understand that the specifications of products often conflict.

THE REAL-WORLD PROBLEM RELATED TO THE PROJECT

Engineers must understand the properties of materials and how they are most efficiently used. This project gives students the chance to try their hand at solving a real-world problem (making strong light structures) while investigating the ramifications of the materials chosen. This is an excellent chance to watch how structures fail, which is another branch of engineering.

SPACE AND TIME REQUIRED/DESIRABLE

Each team will require a table top area with dimensions approximately three feet by three feet. It should take a team approximately three to fours hours to build a bridge. We recommend doing this project only once with the students before trying the second part.

MATERIALS AND EQUIPMENT NEEDED

A bridge may consist of the following:

5 blocks of styrofoam (.75 x 10 x 5) inches
5 rubber bands
10 paper clips
1 piece of wood (3.75 x 3.75 x 3.75) inches
10 nails
16 inches of clear tape, max width 1 inch
all the spaghetti you want
2-2 liter soda bottles (empty or full of original contents or water)
1 foot wooden dowel rod, diameter _ 1/2 inch
4 feet of string
1 hook that has wood threads on one end

The styrofoam may be cut from a standard 8 foot by 4 foot sheet that is .75 inches thick. We have found that a table saw does a good job of cutting the styrofoam. The wood block can be cut from a 4-by-4. It is best to give each group the materials allowed them, this keeps them from using more then they are allotted. You should allow them the use of any tools at your disposal that seem reasonable (typically - hammer, pliers, drill, cutting tool, etc.)

SUGGESTIONS REGARDING STUDENT TEAMS

The students should be divided into teams of no more than five and no less than three.

PREPARING FOR THIS PROJECT - WHAT TO DO IN ADVANCE

You must construct the stand that the students will test their bridges on. The easiest way is to use two heavy tables that stand the same height. Tape off a 6"x6" square on the top next to the edge of each table and set them two feet apart. Now you must put boxes, chairs, books, or what ever is convenient to build up the "valley" to be only two foot from the top of the table.

Figure 1.

A better method is to build a stand from 4-by-4's, plywood, nails and four threaded rods, and eight nuts. The finished product is shown in figure 1.

You also need to gather the materials for the students bridges, as well as the tools that you will allow them to use. A basket to hang from the bridge must also be acquired as well as weights to put in the basket. Ten kilograms of weight broken up into half kilograms should be adequate to break any bridge.

SUGGESTIONS FOR MANAGING THE PROJECT

Divide the class into design teams.
Distribute the Student Project Description.
Present the project to the students (15-20 minutes).
Let the students read the materials, discuss it among themselves, ask questions, and generate design ideas (30 minutes).
Have students present their ideas or comment on their proposed designs. You should interact to help steer them clear of obviously bad designs eg. filling both 2 liter bottles with water.
Allow students to work on projects (2-4 hours).
Test the final designs (.5-1 hour).
Summarize the results of the project and tie the concepts together in a positive closure (20-40 minutes).

GUIDELINES FOR TESTING THE PRODUCTS

The bridges must touch only the tops of 2 platforms that are 6"x6" and spaced 2' apart (see figure 1). All bridges must rest on the platforms during the entire testing process. A team may not nail, screw, drill, or otherwise modify or attach to the platforms.

Before you may destroy the students bridges you must weigh them and record this weight. In order for you to test the bridges, the students must attach the eye hook provided to the bottom, center of their bridge. Only the hook can extend below the 2 foot level (top of the abutments). You should slowly and carefully load the weights into the basket until the basket hits the two foot mark. Then record this weight. The score is computed as follows:
SPI = weight supported (grams) / weight of your bridge (grams)

PROVIDING SUCCESSFUL CLOSURE

In particular we encourage you in this project to emphasize the problems associated with conflicting specifications. Nearly all products have this problem, therefore creative solutions are always being sought to deal with the design specifications.

This project illustrates that engineering design requires the incorporation of knowledge from multi-disciplinary fields, conflicting objectives, and multiple goals. This is what makes engineering challenging; however, when students loose sight of these overall goals and begin to become obsessed with design details, their designs will become less successful. Even the brightest and most creative students can fall into this trap.

Most designs, even less successful ones, have strong points. Acknowledging those points along with some constructive criticism helps the students learn some of the more difficult points. We have found that group discussion following testing in which each team or individual critiques the strong and weak features of their own design is fruitful.

Finally, please make a special point of reminding students that this kind of creative activity is at the heart of engineering. If they found the project enjoyable, they should consider engineering as a profession.

SPECIAL SAFETY CONSIDERATIONS

It is important that no part of your body or your students bodies be under the weights while a bridge is being tested. Also remember to be sure the students know how to use the tools and that they exercise caution when using them.

This project was developed by Michael I. Hessel, Jr.

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