Jim Sauer
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[2009-07-02]
Here are some more pictures from another favorite class of mine... this time the task was to build something that fit inside the testing apparatus shown below. (The span of the apparatus is a little over 36 inches, the width of our projects were 4 inches and there's only a half inch difference between the bottom of the triangular weight and the top of the little shelves at the ends where our structures rested.) Hydraulic pressure forced the central weight down onto our construction and part of our grade depended on how much weight our structure held before it failed. On top of the grade, there was subtle and not-so-subtle peer pressure to build the most efficient structure.
I had a great project team for this class -- Alison and Alan. Each of us came up with ideas and then we worked together to come up with a single designs, bought materials together, and built each iteration as a team.
Iteration Number One
Our first attempt, with some encouragement from our professor and TAs, was a pretty straightforward beam. We derived the shape by considering moment forces first and then beefed up the ends to deal with shear forces. We laminated four pieces of 1" thick poplar together to create the beam, hoping to minimize any local weaknesses in the natural material.
This beam performed very well, outperforming many trusses that theoretically would have the advantage. We learned a few important lessons here: (1) our biggest unresolved issue was handling stresses along the outer angled portion of our beam, and (2) more depth would help us resolve the forces.
Iteration Number Two
For our second iteration we decided to use the same general shape but to build a deeper beam and reinforce our beam with a perimeter of poplar. The grain of the poplar would directly oppose the forces in each section of the perimeter. To get greater depth without breaking the bank, we gained permission to use plywood (two 1/2" sheets stuck together) for the main beam (cutting a hole in the middle where there was no shear). We created large overlaps where our poplar supports met each other, laminating multiple pieces of wood to create virtual finger joints.
This structure, a cross between a beam and a truss, performed very well. (It may have been one of the most efficient projects of the semester, but a some of this efficiency came from our use of plywood no doubt.) The structure failed in exactly the same manner as our original beam, however -- at one of the joints along the outer perimeter.
Iteration Number Three
For our final iteration we were determined not to fail in the same manner. We also decided to reduce our use of plywood so that our design would be tested more than our materials. Having just learned how to calculate the exact forces and ratio of forces in each span of a truss, we decided to create a much deeper structure in hopes to minimize the stress that destroyed our previous two iterations. We opened up our window so that it ran from top to bottom. We used the same general shape and the same connection methods we had used previously -- including the carved seats for the weight from above, and the rolling curve of wood where our structure met the testing apparatus.
Our third iteration performed very well. Our professor indicated that the testing apparatus delivered more than 25,000 pounds of force on our structure before it failed. Our team was very happy because the failure point in our previous projects held the weight. This iteration failed along the bottom span -- where two independent spans of poplar, each approximately 2" square, finally gave out. Once this failed, the top span also failed.
I should mention here, before any of my classmates decides to give me a hard time about this, that our projects were the heaviest projects in our class. Our final iteration, weighing in at just over 26 pounds, was by far the heaviest project. The original guidelines for the projects was not to exceed 10 pounds. We came close to that limit with our first project which weighed just over 11 pounds. Our second iteration weighed 16 pounds I think. For our final iteration we did receive permission to go heavier yet in our attempt to create such a deep structure.
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