A1 (West Point Bridge Design Bridge) - Bell, Schetley, Staquet
A3 (Method of Joints Analysis) - Bell, Schetley, Staquet
Background:
Assignment 4 of Engineering 101–Bridge Design was for each group to design and construct a truss bridge using K’Nex which could be tested to determine its overall weight capacity. The K’Nex pieces were each given individual prices and the group with the best bridge was determined based off its cost to weight capacity ratio. The bridge had to have a clear span of 36” and be at least 3.5” in width. A continuous clearing, 3” wide by 2” high, had to run through the bridge to simulate a roadway. The bridge was tested by centering an 11” by 6” piece of plywood on top of the bridge; this piece was to act as a washer for a metal rod that ran through the bridge and suspended a bucket to be filled with sand. Understanding how truss bridges are designed and how they react to stress was an important aspect of this project and made it possible to build a cost effective bridge that could also hold a relative amount of weight.Design Process:
In the beginning we did not have any specific goals besides designing and building a bridge with the best cost to weight ratio in the class. We kept this mindset throughout our time of working on the assignment and never once discussed 2nd place. West Point Bridge Designer played a major role in determining a basic design for our bridge; it showed us where an overhead truss bridge experienced tension and compression forces as well as the best way to orient the interior cross members. During lecture we learned how to analyze a truss which is a way of calculating the exact amount of force each truss member experiences when a given load is placed on the bridge. This analysis also aided in how we placed the interior cross members. We did not design individual bridges, instead we came together as a group and designed one bridge. This eliminated having to pick through multiple designs and allowed us work together as a team. Our bridge was designed during the construction process due to the use of fixed length K’Nex members as well as gusset connection pieces that limited angles to increments of 45 degrees. During the open testing time we had in lab, our bridge was able to hold 35 pounds so to be safe we predicted that the bridge would fail at 30 pounds.Description of Final Bridge:
Our bridge is a 38-inch, three levels high bridge utilizing many triangles throughout the bridge. The members are designed to push the force on the bridge outwards towards the edges of the bridge. There is more strength on the center of the bridge to accommodate for more force on the middle of the bridge. We wanted to add additional levels on the bridge because we figured that one tier would leave the bridge too weak in the middle, especially as more weight was added onto the bridge. You can see a hand drawn sketch and a photograph of the bridge (as well as the final cost of the bridge and the number of pieces in the bridge) below. |
| Figure 1: Plan/Elevation Drawing |
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| Figure 2: Bridge Picture During Testing |
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| Figure 3: Bridge Cost and Materials |
Testing Results/Conclusions About Bridge Design:
For the most part, our bridge design was successful. We made
a very accurate prediction as to how much weight the bridge held. Based on our
design and previous test runs, we predicted the bridge to hold a total of 30
lbs. We also made this assumption based on our truss analysis we composed a few
weeks ago. Once the bridge was tested in the final competition, the bridge was
found out to have held a total of 29.8 lbs. We think that having only a 0.2 lb.
difference was a very accurate assumption.
Even though the bridge held almost the exact guessed weight, we still
were partially incorrect in terms of the failure mode. In the precious bridge
designs and the one tested in the competition, we predicted the bridge to fail
by the joints on the bottom of the bridge being disconnected from the members,
usually towards, but not quite at, the ends of the bridge. Now, the bridge did
indeed fail by this method. But, this wasn’t the only factor in the failure of
the bridge. We took a video of the competition, revealing how our bridge might
have failed in case we missed it. We played the video back in slow motion. We
then found out that a major factor in the failure in our bridge was that the
bridge started to contort and twist, meaning one side twisted clockwise and the
other side twisted counter-clockwise. The point in which these two opposites
met was not in the middle, but slightly offset from the middle. So, we were
right in every aspect in terms of load and failure mode except the one aspect
of the failure mode where the bridge started to contort. You can see a video of
the bridge failing below.
Bridge Failure Video
Future Works:
Every group was going to fail at some point; and every
group, including ourselves, could make some changes to improve the bridge
either with weight or the cost to strength ratio. We feel that we could to two
things to our bridge that would put the design to its full potential. We also
would have stuck with the same/similar design if we were to create another
version. We felt that the design we had had an adequate cost to strength ratio.
The first change that we would make would be to add various pieces to the
bridge to help with the tension problem, specifically at the bottom. Throughout
this term, the main problem we have been facing was the tension along the
bottom. If we were to decrease the rate of failure due to tension at the bottom
of the bridge, the next main mode of failure would be contortion and twisting
of the bridge. The easiest and most effective way to fixing this problem would
be to add lateral cross members to the bridge. This would give strength to the
bridge so that contortion would be much more difficult. Even though these two
modifications would be a little costly, they would in turn cause the bridge to
hold much more weight. Ultimately, these modifications should help improve the cost
to strength ratio.
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