Week 9

In the past week, we ran a few tests on our bridge design to test our ideas.  We built most of the bridge in week 7 so that we could concentrate on working out the issues in week 8 to prepare for the formal test.  We tested the positioning of a smaller truss by placing it on the top of the bridge, on the bottom of the bridge, and without it all together.  We found that it is worth the cost to have the smaller truss attached to the bottom of the bridge.  When we tested the bridge with the smaller truss connected to the top, it held a little over 20 pounds.  We then decided to remove the small truss because the weight increase was not beneficial in relation to the cost. When we tested the bridge without the small truss it held slightly less than the first test.  The cost to weight ratio was lowered.  Then we expanded the small truss and added it to the bottom of the bridge and tested it.  The results showed that this was the best design.  The cost was increased slightly but it was able to support much more weight.  Overall it held a total of 33 pounds and had the lowest cost to weight ratio.  An issue that we faced this week was the gusset plates on the end were the weak points.  In order to fix this, we added small members to the open spaces to increase the pull out force of the gusset plates.  In the coming week, we plan to test our final design, calculate the cost to weight ratio and analyze the failure.

This course has taught me a lot about the bridge design process.  There are so many possibilities for the design of a simple bridge.  Our task was to create two bridges, one that had a 24" span and one that had a 36" span.  Each group had a design that was very unique to complete the same task.  One major design element that was reinforced was that triangles are the strongest shape because the shape cannot change without the lengths of one member being changed.  Another thing I learned using West Point Bridge Designer is that the force/strength ratio should be as close to one as possible without going over one while still decreasing the cost.  There comes a point where the cost stops decreasing because the materials are custom sizes and they cannot tolerate any more weight for their size.  When designing a bridge, there is a perfect ratio that needs to be found.  There comes a point where the bridge is not beneficial for its purpose.  The design has a huge impact on its stability, cost, and success.  There is a happy medium when designing a bridge.  If the bridge is too tall, much more supports are needed to make it successful.  If the bridge is too short, there are more supports needed to make it work.  There is a height somewhere between the two where the cost is the lowest while still being successful.