The Foundations of Paradise - Arthur C. Clarke [45]
“What’s the displacement?” asked Morgan as his eyes strained to follow the details of the simulation.
“Now about two hundred meters. It gets to three before—”
The thread snapped. In the leisurely slow-motion that represented real speeds of thousands of kilometers an hour, the two segments of the severed tower began to curl away from each other—one bending back to earth, the other whipping upward to space. But Morgan was no longer fully conscious of this imaginary disaster, existing only in the mind of the computer. Superimposed upon it now was the reality that had haunted him for years.
He had seen that two-century-old film at least fifty times, and there were sections that he had examined frame by frame, until he knew every detail by heart. It was, after all, the most expensive movie footage ever shot, at least in peacetime. It had cost the State of Washington several million dollars a minute. . . .
There stood the slim (too slim!) and graceful bridge, spanning the canyon. It bore no traffic, but a single car had been abandoned midway by its driver. And no wonder, for the bridge was behaving as none before in the whole history of engineering.
It seemed impossible that thousands of tons of metal could perform such an aerial ballet. One could more easily believe that the bridge was made of rubber than of steel. Vast, slow undulations, meters in amplitude, were sweeping along the entire width of the span, so that the roadway suspended between the piers twisted back and forth like an angry snake. The wind blowing down the canyon was sounding a note far too low for any human ears to detect, as it hit the natural frequency of the beautiful, doomed structure. For hours, the torsional vibrations had been building up, but no one knew when the end would come. Already, the protracted death throes were a testimonial that the unlucky designers could well have foregone.
Suddenly, the supporting cables snapped, flailing upward like murderous steel whips. Twisting and turning, the roadway pitched into the river, fragments of the structure flying in all directions. Even when projected at normal speed, the final cataclysm looked as if shot in slow motion; the scale of the disaster was so large that the human mind had no basis of comparison. In reality, it lasted perhaps five seconds. At the end of that time, the Tacoma Narrows Bridge had earned an inexpungible place in the history of engineering. Two hundred years later, there was a photograph of its last moments on the wall of Morgan’s office, bearing the caption “One of the more expensive products.”
To Morgan, that was no joke, but a permanent reminder that the unexpected could always strike from ambush. When the Gibraltar Bridge was being designed, he had gone carefully through Theodore von Kármán’s classic analysis of the Tacoma Narrows disaster, learning all he could from one of the most expensive mistakes of the past. There had been no serious vibrational problems even in the worst gales that had come roaring in from the Atlantic, though the roadway had moved a hundred meters from the centerline—precisely as calculated.
But the Space Elevator was such a leap forward into the unknown that some unpleasant surprises were a virtual certainty. Wind forces in the atmospheric section were easy to estimate, but it was also necessary to take into account the vibrations induced by the stopping and starting of the payloads—and even, on so enormous a structure, by the tidal effects of the sun and the moon. And not only individually, but acting all together; with, perhaps, an occasional earthquake to complicate the picture, in the so-called worst-case analysis.
“All the simulations, in this tons-of-payload-per-hour regime, give the same result. The vibrations build up until there’s a fracture at around five hundred kilometers. We’ll have to increase the damping—drastically.”
“I was afraid of that. How much do we need?”
“Another ten megatons.”
Morgan could take some gloomy satisfaction from the