matthew goulish : lecture in the shape of a bridge collapsing |
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B: 2. collapse On the morning of November 7, 1940, Kenneth Arkin, chairman of the Washington State Toll Bridge Authority, awoke to the noise of wind. He drove to the Tacoma Narrows Bridge at 7:30 A.M. where he read a 38 m.p.h. wind velocity on an anemometer at the midspan, and observed that the bridge was bouncing noticeably, but not exceptionally, and that the tie-down stays on the west side of the span were loose and whipping in a circular arc. Shortly after 10:00 A.M. Arkin again checked the wind velocity and saw that it had increased to 42 m.p.h. and that the movement of the bridge deck had heightened dramatically; by his count the deck rose and fell at the center of the bridge 38 times per minute with a three foot amplitude. Greatly concerned, Arkin halted traffic. The up and down motion of the center span consisted of at least nine vertical undulations while the bridge also deflected laterally by as much as 2 feet. all the words useless B: Suddenly the bridge began twisting violently, and the nine-wave motion changed to a two-wave motion, while the deck near the Tacoma side appeared to twist to an almost 45' angle. The amplitude of the twisting undulations from crest to valley reached 25 feet, and the bridge began to tear itself apart: suspenders flew high, a section of the deck near the quarter point ripped away, the bridge rested a moment, then with a deafening roar, a 600 foot stretch tore away from the suspenders and fell into the water, the tops of the towers tilted almost 12 feet toward each shore, the side spans sagged, and as each section fell shock waves rippled along the remaining sections until what was left of the structure at last came to rest. Relatively modest aerodynamic wind oscillations destroyed the Tacoma because of its weakness in torsion, a weakness which stemmed from two causes: cause #1) the shallowness of its stiffening girders; and cause #2) the narrowness of its roadway in relation to the span length - a ratio of 1:72, from which this lecture takes its shape, echoing that impractical thinness in a form 1 sentence wide by 72 sentences long, or 9 sections of 8 sentences each (with a connecting phrase extracted from each following section and inserted at a transitional point between two sentences of each previous section), section 2 of which now comes to an end with the following question: we understand the collapse of a bridge, but what is the collapse of a thought? |
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