Background
The goal of this Engineering 103 course was to design and create two bridges of two feet and three feet lengths to span respective gap sizes between two supports(tables and saw horses). These bridges were to have the best(lowest) cost to weight efficiency. Initially, the instructors introduced the class to a program called West Point Bridge Designer(WPBD) and allowed students to virtually create 2-dimensional designs for bridges that were like those in which they would build during the next few weeks of the course. Upon the completion of the two foot-long bridges, students were introduced to Truss Analysis and how forces/weights are distributed throughout the different members in the bridges. While finding the different loads on each component, the class had to determine whether or not the members were in compression or tension. After Truss Analysis was complete for a given 2-dimensional design based on their last name, students were instructed to complete a three foot bridge design within their groups given the resources they have acquired over the past eight weeks.
Design Process
The goals set by the group were exactly that of the design parameters, to create a working bridge of three feet of length with the lowest cost to weight efficiency. Throughout the course, the design of the bridge changed a few times but the goal was set in stone since the beginning. While working with WPBD, Truss Analysis and bridge designing, the group learned quickly as too what would work or not. Each of those three played an integral part in the design of the final project for the group and it was quickly learned that in order to completely understand what goes on when there's a bridge with a dead load (not moving) set upon it there needs to be practice with the computer programs as well as the actual representation.
Before the class began to design truss bridges with KNEX pieces, they started to work within a stimulating tool called West Point Bridge Designer. The WPBD gave a very in-depth understanding on the breaking point of different members of a bridge with variable thicknesses, lengths and materials. The designer could design a bridge from lateral side of view following specific constraints. Designers could also examine the loading of the bridge with a 3-D animation truck test. If the truck safely crossed the bridge, it could be considered as a successful design. After each loading test, the compression force and tension force would be shown in a “box”. The cost of the bridge would also be calculated according to the size and material of members and gusset plates. This data can greatly help the designer improve the design of the truss bridge. The WPBD gave the group a good start to designing a truss bridge. The group has learned how to decrease the cost of the bridge and how to find out an optimal design. The experience that we have learned from the WPBD helped the group to design the KNEX Bridge for the Final Project.
The group members individually designed three different KNEX bridges in A-2. Group members used the Auto-CAD to design the bridge in elevation view and plan view. This process helped the group to have an idea of how to develop a bridge from two dimensions and three dimensions.
The Truss Analyses performed in and out of class were also very helpful in that they incorporated trigonometric calculations in order to find the load distribution on each member of the bridge. This exercise helped the group to estimate the failing point of the bridge. The online Bridge Designer tool inspired the group that the proportions of the triangle greatly benefited design the loading of the bridge. According to the calculation result of the bridge, the group decided to keep the compact design of bridge.
Building the actual KNEX bridges gave the group a real life visual performance of the bridge designs. By testing the KNEX model in reality, the group met some problems that would not happen in idealized simulating tools. The twisting of bridges happened in the loading tests and had a negative impact on the bridge’s performance. The group decided to add more horizontal chords to prevent twisting of the bridge.
The final design was determined by the way the two foot design handled the weight-test. The three foot bridge was built with the same side designs as that of the two foot design. The two foot bridge design was a real extravagant piece of art, and due to the white and blue pieces primarily chosen for the members it was so dubbed, 'The Love Train.' After the first test of the two foot bridge, it held roughly 17 pounds, but after the addition of two more pieces and exchanging two gusset plates, the bridge held 47 pounds. The design of the three foot span changed two times during its design but that was after the top-middle of the span broke due to the weakness of the joints. The changes only made a 1-3 pound difference but any increase of weight is accepted. The predicted load at failure for the three foot bridge was roughly 34 pounds which was significantly lower than the two foot span's 47 held pounds. It might due to the increasing span of the bridge.
Final Bridge
After designing, testing, and analyzing the three-foot bridge in numerous ways, the design was ready to be finalized. The final design for the bridge consisted of twenty-two main squares and each one has an “X” cross through it. Eight squares on the left side of the bridge, eight on the right of the bridge, and six on the top of the bridge. The elevation drawing (Figure 2) and plan drawing (Figure 4) of the bridge could be seen in the figure. There is an angle tilt on the two sides of the bridge which means the bottom part is longer than the top part of the bridge. The bottom part of the bridge is composed by nine 3.375” chords and the top part of the bridge is composed by seven 3.375” chords. One 360 °guest plate and four 1.25” cords consists the “X” cross in the square. In order to satisfy the constraint of leaving enough room for the vehicles to travel across, the height of the bridge is given by the length of 360°guest plate and two 1.25” cords and the width of the bridge is given by 5” long chord. These 5” chords also help to prevent the twisting of the bridge.
The design used a lot of 360°gusset plates and 180°gusset grooved plates. The 360°gusset plates help to extend the length of the bridge and joints to consist triangle of the bridge. The 180°gusset grooved plates greatly increases the possible combinations of the chords in three dimensions. But the fault of the 180°gusset grooved plates is obvious that the cost would apparently increase.
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| Figure 1: The Plan View of the Final Bridge |
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| Figure 2: Plan Drawing of the Final Bridge |
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Figure 3: The Elevation View of the Final Bridge
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| Figure 4: The Elevation Drawing of the Final Bridge |
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Figure 5: The Bill of Cost of the Final Bridge
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Testing Results
The load at failure of the three foot bridge was a sad 28.4 pounds, 5.6 pounds less than that of the group's predicted load of 34 pounds. The bridge failed directly in the middle of the span where it was changed three times. It was as same as our hypothesis. The physical reason of the failure is the weakness of the gusset plates. The amount of designs for the top middle of the bridge was limited due to the placement of the bar in the exact middle of the bridge but the group was satisfied with the results.
Conclusions
The bridge failed exactly where all the members of the group predicted it would. The bridge 'cracked' smack in the middle of the span, which is where the group could not decide on the correct/smart design for loading the weight. The fact that the middle of the span was our downfall and we couldn't think of a way to prevent it was a heartbreaking thing to go through.
Future Work
If the group were to redesign the final bridge, it would be a sure thing that there would be a way found for the middle span of the bridge to hold the weight. Recreating the middle of the bridge would be a priority but touching up the sides would also prove beneficial for the outcome of the new test.
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