3.4 BRIDGE BUILDING COMPETITION

 

 

Objective: To use simple materials to construct a bridge with the highest strength to weight ratio.

 

Scope:

1. Students will be expected to build a prototype as well as a final bridge design.

2. Students will be expected to produce a report explaining their design strategy.

 

Rules:

A list of acceptable materials and the official rules will be provided in class.

 

Evaluation:

1. All bridges will be checked for conformance to rules.

2. A ratio of failure load to the weight of the bridge will be used to judge performance.

3. Grading will be based on the report.

 

The Test Stand:

The bridge will be mounted between two 1”x6” bars approximately 8” apart.

 

The bridge will be loaded on the upper surface until failure by a ram that is between 2 to 3 inches wide and long. The top loading area must be no larger than 3 inches by 3 inches at the center of the bridge.

A computerized testing machine will be used to load the bridges and produce stress-displacement graphs. The bridges will be loaded until failure. Failure will be measured using engineering judgement. A catastrophic failure will be complete and self evident. A graceful failure will be when the structure is still standing, but will evidently failure. A ratio of failure load to weight ratio of the bridge will be used to rank the performance.

 

Hints:

1. Heavy is not necessarily better (an ant would outperform a human with the strength to weight ratio.

2. Triangles form stable supports in a solid structures. You should be able to identify a visible path for the force to flow to the ground.

3. Try building prototypes first to test the structure.

4. Consider various beam types. Most bridges that win have beams made of special beam sections.

5. Buckling will be a major concern. The buckling load calculation can be found in the textbook. Basically in compression, beams should be shorter, or have stabilizing (zero force) members, or be made thicker.

6. One of the most common points of failure is joints in tension. These require more care in design and construction.

7. Careful construction will increase the strength. When loaded the geometry of the bridge will change. Small distortions will become a source of failure.