Using the appropriate dimensions and the Method of Joints idealogy to figure out the forces associated with each joint and member.
Again inputting the appropriate dimensions specified by the assignment page, the web-based Bridge Designer program was able to give us accurate answers that correspond to what we acheived by hand. Although I decided to use 24 unites within the application that directly correspond to the 24" base specified, it is possible to acheive the same force results so long as the bridge inputted into the program is a scaled version of the specified bridge. For example, a 24" long by 6" high bridge could be scaled down to 12 units by 3, and still produce correct results.
Above: Scaled version of A3 bridge inputted into Bridge Designer Software
Above: Our team's K'Nex bridge inputted into Bridge Designer Software
Using the method of calculating forces through properly scaled down designs in the web-based software, it was incredibly easy to calculate the loads associated with each joint on our K'Nex bridge. Using the information provided in the K'Nex Joints webpage, we are able to edit our design slightly, if not significantly. It had not occured that a gusset plate filled to maximum capacity with cords would withstand more pulling force, but knowing this information remotely early in the game will help us make efficient use out of each plate we incorporate.
A2-GASPERINI
The original idea for this bridge was an arch design, contrary to my A1 post. Creating a perfect arch with K'nex pieces is not really possible, being that there is no piece that can handle bending, and connection points would be under an extrordinary amount of preload. After realizing the reality of the situation, I attempted to create a bridge with gradual steps up in the shape of an arch, but with corners. This played out well and allowed for almost perfect geometry, but some modification will be needed when brought to life. Using this stepped arch design should make for perfect weight distribution, and ideally effecient preformance. Unfortunately however, the design has proved to be costly. At this point there is no reference point in terms of cost, so the decision on whether or not it truly is costly will rest on where the other teams stand. After designing this bridge on paper, I learned how crucial simple geometry is to the success of this bridge.
Fig 1. Top-down and Elevation views
Fig 2. Bill of Materials
A1-GASPERINI
After doing extensive research online, I've decided to use a typical Pratt Truss with a slight modification at each end. Rather than using a typical trapezoid shape, I decided to pull up the first set of supports, in search for a higher load capacity. After experimenting with different size inner triangles, I decided on the below as my final concept. This bridge absolutely needs work; the cost is high and the load capacity (.9) is generally unacceptable. I think returning to a traditional trapezoid shape will remedy the load capacity, and switching build materials will help as well. Currently, I'm using primarily Carbon Steel at the 140mm dimension, but have experimented with adding 200mm CS as well. Although this helped the load capacity, it certainly added weight and cost. Under no circumstance will this be our final design, but working with the rest of the group to remedy its specific problems may yield a very strong bridge. Through my initial attempt at using WPBD, I learned that thickness does not necessarily effectively increase strength. Also, I have learned the cost increases very quickly with variation in thickness and material type. This design in its current state costs a ludicrous $467,000, but with design and research time, I expect our final bridge to settle right around the $300,000 mark.
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