A4-Group 14

Background

Figure 1

The bridge pictured above was created by three freshman engineering students in order to gain hands on experience with truss design.  Using their knowledge of K’Nex pieces, West Point Bridge Designer software, and the method of joints truss analysis, students were asked to create a bridge to span 36 inches from one flat table top to another using only one support on each end.  The overall goal was to produce a bridge that was able to hold the most weight for the lowest cost.

Design Process 

The initial goal for this bridge was based off of the previous bridge which was designed to span only 24 inches.  Although it covered a shorter distance, the design used for the 24 inch span proved to be fairly sturdy and held 21.2 pounds before the point of failure.  Because the bridge for this assignment was to span 36 inches, a goal of 25 pounds was set based on two factors: the first being that the original bridge had definite room for improvement in terms of support and weight distribution and the second being that the second bridge would span further which is the only reason why the goal was not much higher.

The group determined that by adding more support to the plan, or top view, of the bridge that it would be less likely to succumb to torsion forces as did the first bridge.  After reinforcing the top and bottom connections, work was done to more efficiently distribute the weight.  The initial idea was to create a smaller “secondary” truss portion which was placed on top of the main portion of the truss.  After performing three load tests, the group decided that the secondary truss component was most effective on the under part of the bridge.  The three load tests consisted of testing the original design with the secondary upper truss, then again with no secondary truss, and finally with the secondary under truss.  The upper truss was the least successful in comparison to the other two as it had the highest cost to weight ratio.  It was only able to support 21.0 pounds, which is not only less weight than the initial 24 inch design could hold, but it came at a much higher price.  The bridge which utilized the upper truss had a cost of $490,500.  Similarly the bridge with the under truss also cost much more, but was able to support 33.6 pounds when first tested.  

After testing, the effect of adding more connections to a gusset plate in increasing the force needed for each member to pull out of the gusset plate was taken into account and minor changes were made.  These changes include the addition of the 1.125” white members added to the outermost gusset plates as these were the failure points during testing.  After the three load tests, the prediction of 25 pounds was reevaluated and raised to 35 pounds when the addition of the white pieces was considered.

Bridge Description

The design of the bridge consisted of a main truss and a smaller under truss.  From the results of prior tests, the under truss proved to be the most supportive design.  Figure 2.1 is an elevation drawing of the 36" span bridge.  Figure 2.2 is a plan drawing of the same bridge.  The three different colors represent the three levels.  The red plan view in figure 2.3 is the top level plan view.  This is the view if you removed the bottom two sections.  Figure 2.4, the green plan view is the middle section plan.  The blue plan view is the bottom plan drawing.  They are pictured together in figure 2.2 and separately in figures 2.3, 2.4, and 2.5 to show a more simple view.  The total cost was calculated using the bill of materials in figure 3. The total cost for this bridge is $494,500.  Figure 4 shows a photograph of the bridge built out of K'nex.  

Figure 2.1 - Elevation

Figure 2.2 - Plan

Figure 2.3 - Top Level Plan

Figure 2.4 - Middle Level Plan

Figure 2.5 - Bottom Level Plan

Figure 3 - Bill of Materials

Figure 4

Testing Results

This bridge held a total of 36.6 pounds during the final test.  This was slightly better than the previous prediction of 35 pounds.  Since the design focused on distributing the weight throughout the bridge, the bridge failed at several points as the stress increased. The most noticeable points of failure were the beams at the very ends of the bridge. The tension forces on the bridge caused those two diagonal beams at the ends to bend and snap off the gusset plate. There were also numerous other pieces that slipped off the gusset plates when the bridge failed. The bridge failed in a graceful and spread out manner, which was the intent of the design

Conclusion

After testing the bridge during Week 9, it was predicted that the bridge would be able to support a load of about 35 pounds and the failure would occur at one of the outermost gusset plates.  This prediction proved to be very close to reality as the bridge ended up supporting 36.6 pounds and failing gracefully at the third gusset plate from the end.  The failure most likely did not occur at the predicted gusset plate due to the increased number of pieces in each outermost gusset plate as well as the redistribution of weight.

Future Work

In designing a future bridge, more emphasis would be put in increasing the pull out force needed to pull members from their respective gusset plates.  The current bridge design functioned well in terms of weight distribution, but further testing could have proven beneficial and have allowed for further modification.  Using WPBD and the method of joints, efforts could be made to more efficiently distribute the loads.