A4-Weldon/Hidalgo/Almendariz

Bridge Report

Figure 1: Final bridge design.

Background
                The bridge design competition is a lab module that tests different truss designs. The lab brings together computer modeling, physical modeling, forensic analysis, static analysis, and teamwork to build a bridge. The design process starts with computer modeling using programs like West Point Bridge Design and then the design process moves to physical modeling using Knex pieces to build the bridge you had earlier designed. The analysis tactics are used at certain intervals to help increase one’s understanding of how to build the bridge to its full potential. The main goal is to build a bridge with the lowest cost to weight held ratio.

Design Process

            As we started to build our final bridge design we started out by modeling it after our first 24’’ design. It was a simple truss design with two levels of trusses the top one shorter than the other.  One of our main goals was to replace the 360° grooved gusset plates with alternatives that would eliminate the weakness created by the connections between two of the gusset plates. With the help of the analysis of tensile pull out forces of the Knex pieces we wanted to optimize the amount of force each of the gusset plates experienced

(http://www.ce.memphis.edu/3121/projects/Project_1/fall_11/knex_pullout_force_f11.html). As we continued to work on our design our first attempts were failures. We tried several modifications one of them was to use the normal 360° gusset plates buy connecting the members sideways rather than the conventional way. This didn’t work as we planned because we didn’t consider it twisting sideways which it did.

The role of the individual projects such as West Point Bridge Design, Truss Analysis, and Individual Knex Design is that each helped to bring different views and discovers to the group. It allowed us to work off both what we discovered on our own and what others in our group discovered. This made it so many ideas could be brought forth and combined to make the best possible design. For example with the truss analysis it quantified the specific forces that act on a bridge. This helped better our design by purposefully orienting the direction of the grooved gusset plate connections a direction that counteracted the forces acting on the bridge.

In choosing our final design we decided to go two ways and combine them. The first one was to create a very dense truss that was expected to be expensive but also that had a higher resistance. The second was the original “arch” truss design, which had proven to be effective in the first competition. The only component that was constantly being modified was the under truss, this structure wasn’t a complete idea when construction started it was more of an idea of how different gusset plates and connectors could be combined. Eventually we came to the final design by using the 360° gusset plates in any way we could. The load at which we predicted the bridge to fail was 40 lbs, we predicted this because of our previous experiences with the bridges, our first bridge’s load at failure was about 30 lbs and since we used the same set of pieces we didn’t expect our bridge to resist more than 40 lbs. 

Final Design

          Below is the plan view of our design the tilted rectangles and blue circles are 360° gusset plates, all the white circles are two 360° grooved gusset plates connected to each other. 

Figure 2: Plan Drawing

In figure 3 a visual depiction is shown of how the gusset plate was used in the middle of the truss.

Figure 3: Individual truss section

        Our final Design was not very conventional because it included an intermediate truss which helped distribute the downwards force experienced by gravity. The truss helped resist more tension that is felt in the bottom part of any bridge. Below is a separate drawing of the truss that went in between the main structure.

Figure 4: Intermediate truss

       Figure 5 and 6 are elevation views of our bridge. This particular view was challenging to draw because of the intricacy of the design.

Figure 5: Elevation drawing

Figure 6: Elevation view.

Below is the bill of materials.

Figure 7: Bill of Materials 

Testing Results            
           Our bridge was able to hold 102.6 lbs before failure occurred. The bridge failed at the gusset plates at the center of the bridge. The bridge split right down the middle. The failure was not caused by a member slipping out of a gusset plate as was observed earlier. Instead the plastic connector itself cracked from the force created by the weight. The crack of this one gusset plate caused the other gusset plates on the same plane to slide out. As Figures 8 and 9 show the split was right down the middle where most of the force was experienced. 

Figure 8: View of area that broke.

Figure 9: Bridge after failure.

Conclusions

            The group’s final bridge design model behaved completely different than expected.  Originally we predicted the bridge to hold about 40 pounds and not have a good cost to weight ratio.  As stated previously the main focus when designing the final bridge model was it’s strength.  We decided to completely ignore the concern of keeping its cost down and let our creativity focus on the structure and strength of the bridge.  We tried dividing the weight resistance equally throughout the bridge to avoid putting excess tension on a single portion.  This way we were able to build a nonconventional bridge with an interesting structure.  Although we focused on the bridge strength we underestimated its strength by hypothesizing that it wouldn't hold more than 40 pounds.  Our biggest concern was the fact that the bridge had so many members and connections.  We thought that connections would create weak spots that would weaken the bridge.  Therefore we were expecting it to collapse by braking in the middle.  During the competition the bridge had a surprising behavior proving our hypothesis wrong.  The bridge ended up being able to hold 102.6 pounds.  Throughout the competition we didn’t see any bending or twisting in the structure of the bridge.  Although we were able to pull the biggest weight resistance we didn’t expect it to be able to have a good cost to pound ratio.  By ignoring it’s cost during the development of the design we ended surpassing all the other bridges cost by pulling a cost of $826,500.  Due to its high cost we didn’t expect at all to be able to pull a reasonable cost to pound ratio and predicted it to be $21,913.  It is for these reasons that we were surprised to know that our bridge won the competition by pulling the best cost to pound ratio.

Future Work

           Although our bridge design model was successful and won the competition.  If we were given the chance to modify and improve we would make some changes to lower its cost.  We would increase its scale by using bigger members to create the same 36” and not modify its structure and shape at all. This way we would use a less members and connectors reducing its cost significantly and either keeping or improving its strength.  We think that connections create weak points in the bridge and by decreasing their number the bridge would strengthen the bridge.  Further more experimentations and testing would need to be perform to see the effect of this change in its strength.  Also we misused 360 gusset plates in the sides of the bottom level of the bridge. Figure 10 shows the inner design if the white gusset plates had been replaced by yellow 180 plates the cost would have been lowered significantly. We would not make major changes any changes in the design and structure of the bridge model because we think its not necessary.    

Figure 10: View of bottom section before top is added.

Figure 10: Cross section of bridge.