ENGR 100

ENGR 100: Bridge Design Project

Project Objective:

Design a structure to span a given distance while supporting a maximum load using a minimum of materials.

Project Goals:

The goals for this project are for students to:

Learn the design process.
See practical applications for trigonometry, physics, and engineering courses.
Improve skills associated with collecting data and drawing meaningful conclusions.
Experience the usefulness of prototype testing.
Recognize the necessity of good communication skills for engineers by completing memos, reports, drawings, and presentations.

Project Grading:

The grade for the bridge project is broken down into the following categories:

Group Bridge Report (20%)

Group Bridge Presentation (5%)

*Students will also be responsible for a number of homework assignments including bridge ideas, component test memo, prototype worksheets, bridge software analysis and other related assignments.

Project Constraints:

Size:

Each team needs to design and build a prototype and final bridge which span a distance of 16 in. and hold a minimum weight of 15 lbs. The dimensional constraints are shown in Figure 1:

SIDE VIEW FRONT VEW

Figure 1. Required dimensions for the bridge design.

The length of the bridge should not exceed 19 inches. The width of the bridge should be at least 2.5 inches but not more than 3.5 inches. The height of the bridge should not exceed 8 inches above or 2.5 inches below "ground level".

Materials:

Teams are encouraged to use as much creativity as possible, but are limited to the following materials:

wood tongue depressors (6 in)
hot glue or wood glue
string or fishing line

Teams are limited to a certain total number of tongue depressors (ask instructor).

Testing Procedure:

Each bridge will be tested to failure using the testing apparatus in Wilcox Hall (Materials Science and Engineering Dept) or Mechancial Engineering Bldg. All bridges must accommodate a 2" x 6" plate accessible from the top for loading purposes as shown in Figure 2:

Figure 2. Loading Procedure

Scoring:

Although prototype bridges will be tested, and scores calculated, they will not be counted towards the final score. The purpose of the prototype bridge is to learn how the bridge behaves under actual loading and improve the design based on the performance.

Scoring for final bridge performance will be based on the equation:

SCORE = Load at Failure (lbs)/ Weight (lbs) (Eq. 1)

Teams should strive for outstanding performance and originality.

Design Process:

This project is really about engineering design! Your team should follow the steps described in the engineering design process lecture. Your bridge design process should include the following steps:

1. Problem definition.

· As a team, carefully read the problem statement and put it into your own words.

· Identify the functional requirements of the bridge.

· Identify the constraints.

· Ensure that everyone in the team shares the same vision of the problem before proceeding.

· Also ensure that any questions regarding the project specification are resolved with the instructor.

2. Analysis/synthesis/conceptualization.

· Perform the bridge component strength tests and discuss the results with other members of the team.

· Conceptualize alternative designs with your team.

· With wild abandon, toss around ideas that range from the ridiculous to the doable. Remember that a key element of brainstorming is to not get in the mode of being critical towards ideas. This comes later. You should never hear "That won't work!" at this stage in the planning. Document at least three alternatives with sketches and descriptions. You will need to write about these in the final report.

3. Prototype design and testing.

· Evaluate the proposed (alternative) designs and select the best approach.

· Develop the selected approach into a detailed design. Decide on design details such as materials, construction method, tools and dimensions. This is probably the most critical stage in your design process. If you invest a little more time here, you may save a lot of time in the construction stage.

· Prepare a drawing of your design.

· Build and test a prototype based on this drawing.

4. Redesign.

· Analyze the failure of the prototype.

· Redesign and build your final bridge for testing.

5. Design delivery.

· Write the component test memo as an individual assignment.

· Communicate your team's bridge design by writing and presenting the final report as a group.

Engineering Fundamentals:

Truss bridges are an efficient way of making a small amount of material do a lot of structural work. They are made up of members which are simple beams that are joined with pins. Some common truss bridges are shown in Figure 3:

Figure 3. Examples of Truss Bridges

Trusses work by converting bending of the whole truss into tension and compression of its individual members. To visualize this transfer, imagine taking the whole truss structure in your hands and bending it into a U -shape (the same deformed shape that a heavy truck in the center of a flimsy bridge would cause). Next imagine that the whole truss is wrapped in rubber skin so it looks like a large rubber box. As you bend the box, wrinkles will appear on the top of the box, and the rubber on the bottom will be stretched. The rubber gives an indication of where tension and compression are occurring.

Thus by inspection, the truss members on the top will go into compression (squeezed together) and the members on the bottom with go into tension (stretched apart) as shown in Figure 4. The members in the middle of the truss will also either be in tension or compression, but require force analysis (engineering statics) to figure out. Because truss members are pinned at the joints causing the bending moment to equal zero (equilibrium), all members experience tension or compression, not bending!

(Compression on Top)

(Tension on Bottom)

Figure 4. Truss Analysis