Bridge Design for Function Student Materials

WHY DOES THIS PROBLEM MATTER?

We are going to attempt a fun and exciting assignment that involves building and testing a bridge. Often when designing structures, there are two very important requirements:

1. to carry the most weight possible

2. to be as light as possible

This project will introduce these design trade-offs to you.

WHAT ARE WE SUPPOSED TO DO?

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

In order for your teacher to test your bridge, you must attach the eye hook provided to the bottom, center of your bridge. Only the hook can extend below the 2 foot level (top of the abutments).

Figure 1

WHAT EQUIPMENT AND MATERIALS CAN WE USE?

Your 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

You may use any tools at your disposal (typically - hammer, pliers, etc.)

HOW WILL WE KNOW HOW WELL WE HAVE SUCCEEDED?

After constructing your bridge, we will attach a basket to the eye hook and add weights gently in small increments, until the basket touches the ground, (whether your bridge fractures or deflects when the basket hits the ground, the test is over).

The Basic Performance Requirement for the bridge is that you build a structure that is able to hold the basket.

The Superior Performance Index (SPI) will be calculated by dividing the weight your bridge supports by the weight of your bridge:

SPI = weight supported (grams) / weight of your bridge (grams)

WHAT ELSE MIGHT BE USEFUL TO KNOW?

How do normal bridges work? People have tried many approaches to building bridges. Some of the more famous ones are suspension bridges, such as the San Francisco Golden Gate Bridge. In this case, the cables are in tension and support the road, while the compression posts support the cables (see figure 2). You may notice this particular configuration will not work in this project because the cables to the outside of the compression posts must attach to the abutments which you are not allowed to do.

Figure 2. Suspension bridge.

Other bridges use trusses, having some members in compression and others in tension. Normally if the weight is carried on the lower portion of the bridge, the top members are in compression and the lower members are in tension.

Many modern bridges can be thought of as multiply supported beams and are normally used for short spans in distance. In order to maintain rigidity, these bridges use reinforced concrete. Here is a tip: we have found that the best bridges for this project are derivations of the suspension and truss configurations. Be sure to use materials in their most effective manner. For example, a piece of string would be useless as a compression member.

SAFETY CONSIDERATIONS UNIQUE TO THIS PROJECT

It is important that no part of your body is under the weights while your bridge is being tested. Also remember to exercise caution with the tools you will be using for constucting your bridge.