There, the anchorages are cave-like tunnels of limestone. “We had trouble even getting inside to do the inspection,” said George Zaimes. “In order to see all the wires where they go around the eyebar pins, we had to use fiber optics. No one had been able to see this area since it was built more than a hundred years ago.”
The engineers were alarmed by what they saw. A hundred years of water dripping into this critical space had corroded many of the individual wires, and an earlier, misguided preservation effort had made things worse. “Someone thought they would protect the eyebars and dumped concrete on top of them in some of the anchorage tunnels,” said Zaimes. “The concrete is porous. It absorbed the water and held it right on the wires, greatly increasing the rate of corrosion.”
Of the 152 strand-and-eyebar loops (nineteen at each end of each of the four main cables), at least two and as many as twenty will have to be replaced starting in 1983. Such a task has never been attempted. Once a strand has been identified as needing to be repaired, a special clamp designed by the Steinman firm will be used to hold the strand tightly against the weight of the Bridge itself. The strand will then be cut through, each of its 280-odd wires cleaned of rust and dirt, and a steel socket pulled over the clean wires. The wires will be spread apart to form a cone-shaped brush within the socket, and the socket will be preheated to receive molten zinc.
Until engineers at Columbia University tested the technique on a full-size mockup of the anchorage, no cable of this size had ever been socketed in a horizontal position. “And it has never been done in so confined a space as the anchorage tunnels,” said Dr. Maciej P. Bieniek, the Columbia civil engineering professor who helped direct the project.
The Columbia researchers heated the zinc until it was liquid, then poured it down a twelve-foot funnel and pipe to the socket assembly. This simulated what will actually take place: the zinc will be heated outside the anchorage tunnels, then piped to the new sockets. Inside the anchorages, the sockets will have to be kept heated to at least six hundred degrees Fahrenheit, so that the zinc flows evenly around all the wires in the strands without leaving voids or cracks.
The engineers considered using instead a dense plastic that can be poured as a liquid and allowed to harden in the sockets. The plastic would have removed the need to heat the whole assembly, and tests at Columbia and Lehigh University confirmed that it is more than strong enough to do the job. “But no installation using the plastic has been in service more than six or seven years,” said Bieniek. “We didn’t want to take a chance.”
The engineers did not want to take a chance with the Bridge’s vertical wire-rope suspenders, either. “Superficially they looked fine,” said Birdsall. “But we thought we should take a closer look. Once we did, we found the molten metal used to form the sockets at the ends of the suspenders had not penetrated very far. The metal, probably lead, congealed at the big end of each socket.” Only a little bit of metal is keeping each suspender from pulling loose.
While even this little bit seems enough—no suspender has ever failed— the engineers say the margin of safety is not too great. All the suspenders will be replaced. George Zaimes has suggested selling the old suspenders in short lengths as souvenirs. Some 500,000 pieces could be cut. At $50 each, Zaimes could raise $25 million to pay for more maintenance and repairs. The same thing was done with wire rope from the Golden Gate Bridge’s suspenders when they were replaced in the mid-1970s.
The diagonal stays will also be replaced. They are badly corroded in spots, especially at the tops of the towers, where they pass through in rather untidy tangles. The Steinman firm has designed a neater system to hold the diagonals in place and lessen the chance of rust and of chafing against the stone towers themselves.
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Originally Posted by Ninjahedge
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