A2-DURKIN

            The goal for the design of this bridge was to create a bridge with the lowest ratio of the cost of the bridge to the weight of sand in kilograms that the bridge can hold. This means that design of the bridge must be incredibly efficient, only using pieces that are important to stability. This is why I chose to build an above ground bridge with a single layer of members. When originally designing a bridge in WPBD an underneath bridge was most effective however this is not true when working with K’NEX.  With K’NEX it is better to use an above ground design because it is easier to design the bridge in a way that it will be stable when rested on the table. The connections between either side of the bridge were decided on based on the constraints of a flat 8” wide portion at the top of the bridge as well the necessity that the bridge stay together in the middle when pressed under the weight of the sand. This was harder to decide on as it was not possible to decide on the connections on WPBD where the program just made the connections for you. The length of the bridge was based of the constraint of being at least 2’ long combined with cost efficiency which caused the decision to not make it much longer than 2’. 

Figure 1: Elevation of Bridge Design (side view)

Figure 2: Plan of Bridge Design (top view)

Figure 3: Truss Bill of Materials; the spreadsheet calculating the cost of the bridge.

Originally the bridge was designed with the sole goal of stability, this was then found to be way too expensive.  After this initial design a second design with the lowest possible cost was drawn up but after thinking through the execution of the design it was realized that it would be entirely too unstable.  Once the goal of a low cost to failure load ratio was considered more thoroughly many different designs that seemed to be a good compromise were drawn up.  Once the constraints came into consideration that above ground bridge seemed to be the most stable design and since the top of the bridge needed to have a flat top central region it was decided that keeping the entire top flat might be the most effective as it would help distribute the weight.

Designing this bridge taught me a lot about the difference between what works on the computer and what will actually work in the real world.  I also learned the importance of understanding all parameters and factors before creating a design as a design may be very strong or very pretty or very cheap but if it does not meet the constraints it useless.