Project 3 Description Section 19 Groups Section 20 Groups

The purpose of this project is for you to learn qualitative design enhancement through a hands-on implementation. We are going to be building bridge models from balsa wood and glue. Then, we will be testing them in class to see what parameters affect structural performance. There will be two building/testing periods (Initial Design and Final Design) and, hopefully, we will see an improvement in the behavior of the final models.

In addition to building the models your group will submit a brief paper describing the procedures you followed when building the models and your design rationale. Specific points of discussion are included below. The paper should also include detailed drawings (with dimensions) of all the designs and a member contribution page. The report will be due on Thursday, 7 December 2000.

Also, each group will give a brief presentation to the class describing their final design, the rationale behind the design, and a critique of its performance. These presentations will take place on 30 November 2000, 5 December 200, and 7 December 2000. You can view the rules for the presentations here. You can see the schedules here:

Part I. Initial Design

In this hands-on activity, each individual group member will construct a rectangular box out of balsa wood and glue. Each student will design a structure with the dimensions specified in the following figure:

You can include any additional balsa wood structural elements as you see fit to reinforce the structure. However, you can not use more than 5 of the 36" long pieces of balsa wood. The objective is for you to learn basic construction techniques and to observe how the structure's performance is affected by:

After building the models, each of the structures will be tested in the classroom on Tuesday, 14 November 2000. The models will be testing by applying a load at their midspan. A U-bracket will be placed over the model so that the flat part rests on top of the structure at its center. The ends of the model will then be placed on two elevated fulcrum blocks which will support one-inch sections on each end of the structure. A hanging scale will then be placed so that it hangs on the bottom of the curved portion of the U-bracket below the model, and a bucket will be attached to the other end of the scale. Water or sand will be added incrementally to the bucket until the structure breaks, and the weight of the total load should be noted at each step. The final weight the bridge is able to support is known as it's ultimate strength.

Prior to testing, the structure will be weighed. The performance of the structure will be based on the ratio of its ultimate strength to the weight of structure itself.

Recommended Procedures

Drafting

Given the dimensions above you should draw planar (2-dimensional) views of the structure you are going to build. The gradations on the drafting (or graph) paper will help them ensure that the lines representing the structural elements are drawn to the correct 1:1 scale.

Construction

Wax paper should be placed on top of the drawing and you should place the wood on the paper directly over the lines on the drawing representing the corresponding structural elements. Pins may be used to anchor the wood in place. Then, the pieces should be glued together. Any excess glue that flows onto the wax paper can be trimmed later after it has dried. When each of the sides of the box has been constructed, you can then glue the sides together to form the completed box.

The entire model should be given ample time to dry.

Discussion to be included in the report:

Compare the construction and performance of each group member's structure. Do you notice any particular design attributes that contribute to the structure's sturdiness?

Why do you think your structure broke? What kind of breakage occurred? Shearing? Twisting? Rotation? Joint failure? Was it unexpected?

Compared to the rest of the class, do you think you had sturdy designs? If so, why? If not, why?

Include a table showing the weight, ultimate strength and performance ratio of each of the structures.

Part II. Final Design

Each group will design and build one final structure. The structure will have identical length and width characteristics as the initial designs, but the height will be left up to you. However, no structure can be over 4 inches tall to ensure it will fit within the testing apparatus. You will be given nine pieces of 36" long balsa wood to be used as you see fit.

The testing of the final structures will be conducted in class on Tuesday, 28 November 2000. Again, the performance of the structure will be based on the ratio of its ultimate strength to the weight of structure itself.

Discussion to be included in the report:

Describe the process you used to decide upon the design of the final structure. How was the final design influenced by your previous designs? How was it influenced by your classmate's previous designs?

Again, why do you think your structure broke? What kind of breakage occurred? Shearing? Twisting? Rotation? Joint failure? Was it unexpected?

What was the ultimate strength of the structure? What was it's weight? What was its performance ratio?

Compared to the rest of the class, do you think you had a sturdy design? If so, why? If not, why?

How do you think you can improve the design to achieve a better performance?