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By: Terry Simmers and Chris Murphy
What the...?!?
This is not the original design we proposed.
There are two reasons for this. The first was that the original design did not provide an easy way of allowing for the block to which the weight would be attached. The second (and more imposing) reason for the design change was a shortage of time. We started actual construction on the bridge 2 weeks prior to the due date (earlier than some groups and much earlier than others). About halfway through that week, we realized we would not be able to get the proposed design done of a quality for which we were trying. We came to the conclusion we had two choices: (1) try to get the proposed design done in a very low-quality construction, or (2) change the design to one which would accommodate our time schedule and still be competitive. We opted for the latter.
The Bridge of...
In the construction of our bridge, we used approximately 9600 Forsterİ Round w/ Square Centerİ toothpicks (we got tired of counting). The type of glue which we used was Sobo Premium Craft & Fabric Glueİ, a white glue similar in texture to Elmer'sİ with a much higher tack.
The bridge was constructed to cover a span of 123 cm with the actual overall length being 141 cm. Each of the diagonal cross members were 6.5 cm and each of the vertical supports toward the center of the bridge were 4 cm. The width of the bridge was continuously 11 cm and the height of the bridge was continuously 5 cm.
May "The Force" be...
In general, the force of the weight will be carried to the next cross-member through the previous cross-members in a downward direction at the base point of each cross-member. Hence, the thick base deck with the last of the base points resting on the table.
A large amount of force will also be exerted in an outward force on the cross-members in a manner that will try to collapse the bridge through the lengthening of the distance over which the cross-members are spread. Hence, the small "bundle" of toothpicks glued after the last cross-member to prevent slipping.
When Our Bridge is Falling Down...
The way in which we have constructed our bridge leads us to believe that the break will not be able to be linked to any one area of the bridge. As one part breaks, the others will respond by breaking in similar ways. This basically means that as one part of the bridge fails the rest will simultaneously. However, because of the amount and positioning of the toothpicks, it can be assumed that the failure, when it happens, can be attributed to the glue bonds breaking and not to the collapsing of any particular part of the bridge.
Data...
| Position |
Weight Held |
Weight of Bridge |
Ratio Wt. Held/Wt. of Bridge |
| Horizontal * |
9.415 kg. |
1.4429 kg. |
6.525:1 |
| Vertical * |
14.415 kg. |
1.4429 kg. |
9.990:1 |
| Bolted Down ** |
77.09kg. + |
1.4429 kg. |
53.427:1 |
* = test performed over span of approximately 123 cm.
** = test performed over span of approximately 90 cm.
What actually happened...
The bridge did exactly what we were hoping it would, not break. It did, however, fail in the sense that it bowed in a downward direction to the extent that it fell down the space between the tables which we were spanning. Before this unfortunate twist of luck, our bridge held 9.415 kg. (~21.35 lbs.).
Because the bridge never actually broke, we cannot say where it broke. We can, however, say why it failed and what we could do to keep it from failing a second time. If we were to somehow fasten the ends of the bridge to the table or place braces underneath, we could keep it from bowing and, therefore, from failing.
We then put the bridge on its edge (rotated it longitudinally 90 degrees) and tested it in this manner. It held 14.415 kg. (~32.72 lbs.) in this position before it twisted and the length of the string caused the bucket to hit the floor. It still wasn't broken, but, once again, it had failed, this time with noticeably better results.
The bridge still not being broken (or cracked for that matter), we fastened the ends of the bridge to tables which had a shorter span, the bridge held one of our classmates (whose body weight was approximately 175 lbs.=~77 kg.) before said student lost their balance and fell. This loss in balance caused them to wobble back and forth, eventually causing the lower deck on one end of our bridge to crack. If it had not been for this, it would have been interesting to see how much our bridge could have held under ideal conditions.
--May, 1999
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